Spring Integration Reference Manual

Mark Fisher

Marius Bogoevici

Iwein Fuld

Jonas Partner

Oleg Zhurakousky

Spring Integration

1.0.3

© SpringSource Inc., 2009


Table of Contents

1. Spring Integration Overview
1.1. Background
1.2. Goals and Principles
1.3. Main Components
1.4. Message Endpoints
2. Message Construction
2.1. The Message Interface
2.2. Message Headers
2.3. Message Implementations
2.4. The MessageBuilder Helper Class
3. Message Channels
3.1. The MessageChannel Interface
3.2. Message Channel Implementations
3.3. Channel Interceptors
3.4. MessageChannelTemplate
3.5. Configuring Message Channels
4. Message Endpoints
4.1. Message Handler
4.2. Event Driven Consumer
4.3. Polling Consumer
4.4. Namespace Support
5. Service Activator
5.1. Introduction
5.2. The <service-activator/> Element
6. Channel Adapter
6.1. The <inbound-channel-adapter> element
6.2. The <outbound-channel-adapter/> element
7. Router
7.1. Router Implementations
7.2. The <router> element
7.3. The @Router Annotation
8. Filter
8.1. Introduction
8.2. The <filter> Element
9. Transformer
9.1. Introduction
9.2. The <transformer> Element
9.3. The @Transformer Annotation
10. Splitter
10.1. Introduction
10.2. Programming model
10.3. Configuring a Splitter using XML
10.4. Configuring a Splitter with Annotations
11. Aggregator
11.1. Introduction
11.2. Functionality
11.3. Programming model
11.4. Configuring an Aggregator with XML
11.5. Configuring an Aggregator with Annotations
12. Resequencer
12.1. Introduction
12.2. Functionality
12.3. Configuring a Resequencer with XML
13. Delayer
13.1. Introduction
13.2. The <delayer> Element
14. Message Handler Chain
14.1. Introduction
14.2. The <chain> Element
15. Messaging Bridge
15.1. Introduction
15.2. The <bridge> Element
16. Inbound Messaging Gateways
16.1. SimpleMessagingGateway
16.2. GatewayProxyFactoryBean
17. File Support
17.1. Introduction
17.2. Reading Files
17.3. Writing files
17.4. File Transformers
18. JMS Support
18.1. Inbound Channel Adapter
18.2. Message-Driven Channel Adapter
18.3. Outbound Channel Adapter
18.4. Inbound Gateway
18.5. Outbound Gateway
18.6. JMS Samples
19. Web Services Support
19.1. Outbound Web Service Gateways
19.2. Inbound Web Service Gateways
19.3. Web Service Namespace Support
20. RMI Support
20.1. Introduction
20.2. Outbound RMI
20.3. Inbound RMI
20.4. RMI namespace support
21. HttpInvoker Support
21.1. Introduction
21.2. HttpInvoker Inbound Gateway
21.3. HttpInvoker Outbound Gateway
21.4. HttpInvoker Namespace Support
22. HTTP Support
22.1. Introduction
22.2. Http Inbound Gateway
22.3. Http Outbound Gateway
22.4. Http Namespace Support
23. Mail Support
23.1. Mail-Sending Channel Adapter
23.2. Mail-Receiving Channel Adapter
23.3. Mail Namespace Support
24. Stream Support
24.1. Introduction
24.2. Reading from streams
24.3. Writing to streams
24.4. Stream namespace support
25. Spring ApplicationEvent Support
25.1. Receiving Spring ApplicationEvents
25.2. Sending Spring ApplicationEvents
26. Dealing with XML Payloads
26.1. Introduction
26.2. Transforming xml payloads
26.3. Namespace support for xml transformers
26.4. Splitting xml messages
26.5. Routing xml messages using XPath
26.6. Selecting xml messages using XPath
26.7. XPath components namespace support
27. Security in Spring Integration
27.1. Introduction
27.2. Securing channels
A. Spring Integration Samples
A.1. The Cafe Sample
A.2. The XML Messaging Sample
A.3. The OSGi Samples
B. Configuration
B.1. Introduction
B.2. Namespace Support
B.3. Configuring the Task Scheduler
B.4. Error Handling
B.5. Annotation Support
C. Additional Resources
C.1. Spring Integration Home

1. Spring Integration Overview

1.1 Background

One of the key themes of the Spring Framework is inversion of control. In its broadest sense, this means that the framework handles responsibilities on behalf of the components that are managed within its context. The components themselves are simplified since they are relieved of those responsibilities. For example, dependency injection relieves the components of the responsibility of locating or creating their dependencies. Likewise, aspect-oriented programming relieves business components of generic cross-cutting concerns by modularizing them into reusable aspects. In each case, the end result is a system that is easier to test, understand, maintain, and extend.

Furthermore, the Spring framework and portfolio provide a comprehensive programming model for building enterprise applications. Developers benefit from the consistency of this model and especially the fact that it is based upon well-established best practices such as programming to interfaces and favoring composition over inheritance. Spring's simplified abstractions and powerful support libraries boost developer productivity while simultaneously increasing the level of testability and portability.

Spring Integration is a new member of the Spring portfolio motivated by these same goals and principles. It extends the Spring programming model into the messaging domain and builds upon Spring's existing enterprise integration support to provide an even higher level of abstraction. It supports message-driven architectures where inversion of control applies to runtime concerns, such as when certain business logic should execute and where the response should be sent. It supports routing and transformation of messages so that different transports and different data formats can be integrated without impacting testability. In other words, the messaging and integration concerns are handled by the framework, so business components are further isolated from the infrastructure and developers are relieved of complex integration responsibilities.

As an extension of the Spring programming model, Spring Integration provides a wide variety of configuration options including annotations, XML with namespace support, XML with generic "bean" elements, and of course direct usage of the underlying API. That API is based upon well-defined strategy interfaces and non-invasive, delegating adapters. Spring Integration's design is inspired by the recognition of a strong affinity between common patterns within Spring and the well-known Enterprise Integration Patterns as described in the book of the same name by Gregor Hohpe and Bobby Woolf (Addison Wesley, 2004). Developers who have read that book should be immediately comfortable with the Spring Integration concepts and terminology.

1.2 Goals and Principles

Spring Integration is motivated by the following goals:

  • Provide a simple model for implementing complex enterprise integration solutions.

  • Facilitate asynchronous, message-driven behavior within a Spring-based application.

  • Promote intuitive, incremental adoption for existing Spring users.

Spring Integration is guided by the following principles:

  • Components should be loosely coupled for modularity and testability.

  • The framework should enforce separation of concerns between business logic and integration logic.

  • Extension points should be abstract in nature but within well-defined boundaries to promote reuse and portability.

1.3 Main Components

From the vertical perspective, a layered architecture facilitates separation of concerns, and interface-based contracts between layers promote loose coupling. Spring-based applications are typically designed this way, and the Spring framework and portfolio provide a strong foundation for following this best practice for the full-stack of an enterprise application. Message-driven architectures add a horizontal perspective, yet these same goals are still relevant. Just as "layered architecture" is an extremely generic and abstract paradigm, messaging systems typically follow the similarly abstract "pipes-and-filters" model. The "filters" represent any component that is capable of producing and/or consuming messages, and the "pipes" transport the messages between filters so that the components themselves remain loosely-coupled. It is important to note that these two high-level paradigms are not mutually exclusive. The underlying messaging infrastructure that supports the "pipes" should still be encapsulated in a layer whose contracts are defined as interfaces. Likewise, the "filters" themselves would typically be managed within a layer that is logically above the application's service layer, interacting with those services through interfaces much in the same way that a web-tier would.

Message

In Spring Integration, a Message is a generic wrapper for any Java object combined with metadata used by the framework while handling that object. It consists of a payload and headers. The payload can be of any type and the headers hold commonly required information such as id, timestamp, expiration, and return address. Headers are also used for passing values to and from connected transports. For example, when creating a Message from a received File, the file name may be stored in a header to be accessed by downstream components. Likewise, if a Message's content is ultimately going to be sent by an outbound Mail adapter, the various properties (to, from, cc, subject, etc.) may be configured as Message header values by an upstream component. Developers can also store any arbitrary key-value pairs in the headers.

Message Channel

A Message Channel represents the "pipe" of a pipes-and-filters architecture. Producers send Messages to a channel, and consumers receive Messages from a channel. The Message Channel therefore decouples the messaging components, and also provides a convenient point for interception and monitoring of Messages.

A Message Channel may follow either Point-to-Point or Publish/Subscribe semantics. With a Point-to-Point channel, at most one consumer can receive each Message sent to the channel. Publish/Subscribe channels, on the other hand, will attempt to broadcast each Message to all of its subscribers. Spring Integration supports both of these.

Whereas "Point-to-Point" and "Publish/Subscribe" define the two options for how many consumers will ultimately receive each Message, there is another important consideration: should the channel buffer messages? In Spring Integration, Pollable Channels are capable of buffering Messages within a queue. The advantage of buffering is that it allows for throttling the inbound Messages and thereby prevents overloading a consumer. However, as the name suggests, this also adds some complexity, since a consumer can only receive the Messages from such a channel if a poller is configured. On the other hand, a consumer connected to a Subscribable Channel is simply Message-driven. The variety of channel implementations available in Spring Integration will be discussed in detail in Section 3.2, “Message Channel Implementations”.

Message Endpoint

One of the primary goals of Spring Integration is to simplify the development of enterprise integration solutions through inversion of control. This means that you should not have to implement consumers and producers directly, and you should not even have to build Messages and invoke send or receive operations on a Message Channel. Instead, you should be able to focus on your specific domain model with an implementation based on plain Objects. Then, by providing declarative configuration, you can "connect" your domain-specific code to the messaging infrastructure provided by Spring Integration. The components responsible for these connections are Message Endpoints. This does not mean that you will necessarily connect your existing application code directly. Any real-world enterprise integration solution will require some amount of code focused upon integration concerns such as routing and transformation. The important thing is to achieve separation of concerns between such integration logic and business logic. In other words, as with the Model-View-Controller paradigm for web applications, the goal should be to provide a thin but dedicated layer that translates inbound requests into service layer invocations, and then translates service layer return values into outbound replies. The next section will provide an overview of the Message Endpoint types that handle these responsibilities, and in upcoming chapters, you will see how Spring Integration's declarative configuration options provide a non-invasive way to use each of these.

1.4 Message Endpoints

A Message Endpoint represents the "filter" of a pipes-and-filters architecture. As mentioned above, the endpoint's primary role is to connect application code to the messaging framework and to do so in a non-invasive manner. In other words, the application code should ideally have no awareness of the Message objects or the Message Channels. This is similar to the role of a Controller in the MVC paradigm. Just as a Controller handles HTTP requests, the Message Endpoint handles Messages. Just as Controllers are mapped to URL patterns, Message Endpoints are mapped to Message Channels. The goal is the same in both cases: isolate application code from the infrastructure. These concepts are discussed at length along with all of the patterns that follow in the Enterprise Integration Patterns book. Here, we provide only a high-level description of the main endpoint types supported by Spring Integration and their roles. The chapters that follow will elaborate and provide sample code as well as configuration examples.

Transformer

A Message Transformer is responsible for converting a Message's content or structure and returning the modified Message. Probably the most common type of transformer is one that converts the payload of the Message from one format to another (e.g. from XML Document to java.lang.String). Similarly, a transformer may be used to add, remove, or modify the Message's header values.

Filter

A Message Filter determines whether a Message should be passed to an output channel at all. This simply requires a boolean test method that may check for a particular payload content type, a property value, the presence of a header, etc. If the Message is accepted, it is sent to the output channel, but if not it will be dropped (or for a more severe implementation, an Exception could be thrown). Message Filters are often used in conjunction with a Publish Subscribe channel, where multiple consumers may receive the same Message and use the filter to narrow down the set of Messages to be processed based on some criteria.

[Note]Note
Be careful not to confuse the generic use of "filter" within the Pipes-and-Filters architectural pattern with this specific endpoint type that selectively narrows down the Messages flowing between two channels. The Pipes-and-Filters concept of "filter" matches more closely with Spring Integration's Message Endpoint: any component that can be connected to Message Channel(s) in order to send and/or receive Messages.

Router

A Message Router is responsible for deciding what channel or channels should receive the Message next (if any). Typically the decision is based upon the Message's content and/or metadata available in the Message Headers. A Message Router is often used as a dynamic alternative to a statically configured output channel on a Service Activator or other endpoint capable of sending reply Messages. Likewise, a Message Router provides a proactive alternative to the reactive Message Filters used by multiple subscribers as described above.

Splitter

A Splitter is another type of Message Endpoint whose responsibility is to accept a Message from its input channel, split that Message into multiple Messages, and then send each of those to its output channel. This is typically used for dividing a "composite" payload object into a group of Messages containing the sub-divided payloads.

Aggregator

Basically a mirror-image of the Splitter, the Aggregator is a type of Message Endpoint that receives multiple Messages and combines them into a single Message. In fact, Aggregators are often downstream consumers in a pipeline that includes a Splitter. Technically, the Aggregator is more complex than a Splitter, because it is required to maintain state (the Messages to-be-aggregated), to decide when the complete group of Messages is available, and to timeout if necessary. Furthermore, in case of a timeout, the Aggregator needs to know whether to send the partial results or to discard them to a separate channel. Spring Integration provides a CompletionStrategy as well as configurable settings for timeout, whether to send partial results upon timeout, and the discard channel.

Service Activator

A Service Activator is a generic endpoint for connecting a service instance to the messaging system. The input Message Channel must be configured, and if the service method to be invoked is capable of returning a value, an output Message Channel may also be provided.

[Note]Note
The output channel is optional, since each Message may also provide its own 'Return Address' header. This same rule applies for all consumer endpoints.

The Service Activator invokes an operation on some service object to process the request Message, extracting the request Message's payload and converting if necessary (if the method does not expect a Message-typed parameter). Whenever the service object's method returns a value, that return value will likewise be converted to a reply Message if necessary (if it's not already a Message). That reply Message is sent to the output channel. If no output channel has been configured, then the reply will be sent to the channel specified in the Message's "return address" if available.

A request-reply "Service Activator" endpoint connects a target object's method to input and output Message Channels.

Channel Adapter

A Channel Adapter is an endpoint that connects a Message Channel to some other system or transport. Channel Adapters may be either inbound or outbound. Typically, the Channel Adapter will do some mapping between the Message and whatever object or resource is received-from or sent-to the other system (File, HTTP Request, JMS Message, etc). Depending on the transport, the Channel Adapter may also populate or extract Message header values. Spring Integration provides a number of Channel Adapters, and they will be described in upcoming chapters.

An inbound "Channel Adapter" endpoint connects a source system to a MessageChannel.

An outbound "Channel Adapter" endpoint connects a MessageChannel to a target system.

2. Message Construction

The Spring Integration Message is a generic container for data. Any object can be provided as the payload, and each Message also includes headers containing user-extensible properties as key-value pairs.

2.1 The Message Interface

Here is the definition of the Message interface:

public interface Message<T> {

    T getPayload();

    MessageHeaders getHeaders();

}

The Message is obviously a very important part of the API. By encapsulating the data in a generic wrapper, the messaging system can pass it around without any knowledge of the data's type. As an application evolves to support new types, or when the types themselves are modified and/or extended, the messaging system will not be affected by such changes. On the other hand, when some component in the messaging system does require access to information about the Message, such metadata can typically be stored to and retrieved from the metadata in the Message Headers.

2.2 Message Headers

Just as Spring Integration allows any Object to be used as the payload of a Message, it also supports any Object types as header values. In fact, the MessageHeaders class implements the java.util.Map interface:

public final class MessageHeaders implements Map<String, Object>, Serializable {
	...
}

[Note]Note
Even though the MessageHeaders implements Map, it is effectively a read-only implementation. Any attempt to put a value in the Map will result in an UnsupportedOperationException. The same applies for remove and clear. Since Messages may be passed to multiple consumers, the structure of the Map cannot be modified. Likewise, the Message's payload Object can not be set after the initial creation. However, the mutability of the header values themselves (or the payload Object) is intentionally left as a decision for the framework user.

As an implementation of Map, the headers can obviously be retrieved by calling get(..) with the name of the header. Alternatively, you can provide the expected Class as an additional parameter. Even better, when retrieving one of the pre-defined values, convenient getters are available. Here is an example of each of these three options:

 Object someValue = message.getHeaders().get("someKey");

 CustomerId customerId = message.getHeaders().get("customerId", CustomerId.class);

 Long timestamp = message.getHeaders().getTimestamp();
 

The following Message headers are pre-defined:

Table 2.1. Pre-defined Message Headers

Header NameHeader Type
IDjava.util.UUID
TIMESTAMPjava.lang.Long
EXPIRATION_DATEjava.lang.Long
CORRELATION_IDjava.lang.Object
REPLY_CHANNELjava.lang.Object (can be a String or MessageChannel)
ERROR_CHANNELjava.lang.Object (can be a String or MessageChannel)
SEQUENCE_NUMBERjava.lang.Integer
SEQUENCE_SIZEjava.lang.Integer
PRIORITYMessagePriority (an enum)


Many inbound and outbound adapter implementations will also provide and/or expect certain headers, and additional user-defined headers can also be configured.

2.3 Message Implementations

The base implementation of the Message interface is GenericMessage<T>, and it provides two constructors:

new GenericMessage<T>(T payload);

new GenericMessage<T>(T payload, Map<String, Object> headers)

When a Message is created, a random unique id will be generated. The constructor that accepts a Map of headers will copy the provided headers to the newly created Message.

There are also two convenient subclasses available: StringMessage and ErrorMessage. The former accepts a String as its payload:

StringMessage message = new StringMessage("hello world");

String s = message.getPayload();

And, the latter accepts any Throwable object as its payload:

ErrorMessage message = new ErrorMessage(someThrowable);

Throwable t = message.getPayload();

Notice that these implementations take advantage of the fact that the GenericMessage base class is parameterized. Therefore, as shown in both examples, no casting is necessary when retrieving the Message payload Object.

2.4 The MessageBuilder Helper Class

You may notice that the Message interface defines retrieval methods for its payload and headers but no setters. The reason for this is that a Message cannot be modified after its initial creation. Therefore, when a Message instance is sent to multiple consumers (e.g. through a Publish Subscribe Channel), if one of those consumers needs to send a reply with a different payload type, it will need to create a new Message. As a result, the other consumers are not affected by those changes. Keep in mind, that multiple consumers may access the same payload instance or header value, and whether such an instance is itself immutable is a decision left to the developer. In other words, the contract for Messages is similar to that of an unmodifiable Collection, and the MessageHeaders' map further exemplifies that; even though the MessageHeaders class implements java.util.Map, any attempt to invoke a put operation (or 'remove' or 'clear') on the MessageHeaders will result in an UnsupportedOperationException.

Rather than requiring the creation and population of a Map to pass into the GenericMessage constructor, Spring Integration does provide a far more convenient way to construct Messages: MessageBuilder. The MessageBuilder provides two factory methods for creating Messages from either an existing Message or with a payload Object. When building from an existing Message, the headers and payload of that Message will be copied to the new Message:

Message<String> message1 = MessageBuilder.withPayload("test")
        .setHeader("foo", "bar")
        .build();

Message<String> message2 = MessageBuilder.fromMessage(message1).build();

assertEquals("test", message2.getPayload());
assertEquals("bar", message2.getHeaders().get("foo"));

If you need to create a Message with a new payload but still want to copy the headers from an existing Message, you can use one of the 'copy' methods.

Message<String> message3 = MessageBuilder.withPayload("test3")
        .copyHeaders(message1.getHeaders())
        .build();

Message<String> message4 = MessageBuilder.withPayload("test4")
        .setHeader("foo", 123)
        .copyHeadersIfAbsent(message1.getHeaders())
        .build();

assertEquals("bar", message3.getHeaders().get("foo"));
assertEquals(123, message4.getHeaders().get("foo"));

Notice that the copyHeadersIfAbsent does not overwrite existing values. Also, in the second example above, you can see how to set any user-defined header with setHeader. Finally, there are set methods available for the predefined headers as well as a non-destructive method for setting any header (MessageHeaders also defines constants for the pre-defined header names).

Message<Integer> importantMessage = MessageBuilder.withPayload(99)
        .setPriority(MessagePriority.HIGHEST)
        .build();

assertEquals(MessagePriority.HIGHEST, importantMessage.getHeaders().getPriority());

Message<Integer> anotherMessage = MessageBuilder.fromMessage(importantMessage)
        .setHeaderIfAbsent(MessageHeaders.PRIORITY, MessagePriority.LOW)
        .build();

assertEquals(MessagePriority.HIGHEST, anotherMessage.getHeaders().getPriority());

The MessagePriority is only considered when using a PriorityChannel (as described in the next chapter). It is defined as an enum with five possible values:

public enum MessagePriority {
    HIGHEST,
    HIGH,
    NORMAL,
    LOW,
    LOWEST
}

3. Message Channels

While the Message plays the crucial role of encapsulating data, it is the MessageChannel that decouples message producers from message consumers.

3.1 The MessageChannel Interface

Spring Integration's top-level MessageChannel interface is defined as follows.

public interface MessageChannel {

    String getName();

    boolean send(Message message);

    boolean send(Message message, long timeout);
}

When sending a message, the return value will be true if the message is sent successfully. If the send call times out or is interrupted, then it will return false.

PollableChannel

Since Message Channels may or may not buffer Messages (as discussed in the overview), there are two sub-interfaces defining the buffering (pollable) and non-buffering (subscribable) channel behavior. Here is the definition of PollableChannel.

public interface PollableChannel extends MessageChannel {

    Message<?> receive();

    Message<?> receive(long timeout);

    List<Message<?>> clear();

    List<Message<?>> purge(MessageSelector selector);

}

Similar to the send methods, when receiving a message, the return value will be null in the case of a timeout or interrupt.

SubscribableChannel

The SubscribableChannel base interface is implemented by channels that send Messages directly to their subscribed MessageHandlers. Therefore, they do not provide receive methods for polling, but instead define methods for managing those subscribers:

public interface SubscribableChannel extends MessageChannel {

    boolean subscribe(MessageHandler handler);

    boolean unsubscribe(MessageHandler handler);

}

3.2 Message Channel Implementations

Spring Integration provides several different Message Channel implementations. Each is briefly described in the sections below.

PublishSubscribeChannel

The PublishSubscribeChannel implementation broadcasts any Message sent to it to all of its subscribed handlers. This is most often used for sending Event Messages whose primary role is notification as opposed to Document Messages which are generally intended to be processed by a single handler. Note that the PublishSubscribeChannel is intended for sending only. Since it broadcasts to its subscribers directly when its send(Message) method is invoked, consumers cannot poll for Messages (it does not implement PollableChannel and therefore has no receive() method). Instead, any subscriber must be a MessageHandler itself, and the subscriber's handleMessage(Message) method will be invoked in turn.

QueueChannel

The QueueChannel implementation wraps a queue. Unlike, the PublishSubscribeChannel, the QueueChannel has point-to-point semantics. In other words, even if the channel has multiple consumers, only one of them should receive any Message sent to that channel. It provides a default no-argument constructor (providing an essentially unbounded capacity of Integer.MAX_VALUE) as well as a constructor that accepts the queue capacity:

public QueueChannel(int capacity)

A channel that has not reached its capacity limit will store messages in its internal queue, and the send() method will return immediately even if no receiver is ready to handle the message. If the queue has reached capacity, then the sender will block until room is available. Or, if using the send call that accepts a timeout, it will block until either room is available or the timeout period elapses, whichever occurs first. Likewise, a receive call will return immediately if a message is available on the queue, but if the queue is empty, then a receive call may block until either a message is available or the timeout elapses. In either case, it is possible to force an immediate return regardless of the queue's state by passing a timeout value of 0. The no-argument send and receive methods block indefinitely. Note however, that calling the no-arg versions of send() and receive() will block indefinitely.

PriorityChannel

Whereas the QueueChannel enforces first-in/first-out (FIFO) ordering, the PriorityChannel is an alternative implementation that allows for messages to be ordered within the channel based upon a priority. By default the priority is determined by the 'priority' header within each message. However, for custom priority determination logic, a comparator of type Comparator<Message<?>> can be provided to the PriorityChannel's constructor.

RendezvousChannel

The RendezvousChannel enables a "direct-handoff" scenario where a sender will block until another party invokes the channel's receive() method or vice-versa. Internally, this implementation is quite similar to the QueueChannel except that it uses a SynchronousQueue (a zero-capacity implementation of BlockingQueue). This works well in situations where the sender and receiver are operating in different threads but simply dropping the message in a queue asynchronously is not appropriate. In other words, with a RendezvousChannel at least the sender knows that some receiver has accepted the message, whereas with a QueueChannel, the message would have been stored to the internal queue and potentially never received.

[Tip]Tip

Keep in mind that all of these queue-based channels are storing messages in-memory only. When persistence is required, you can either invoke a database operation within a handler or use Spring Integration's support for JMS-based Channel Adapters. The latter option allows you to take advantage of any JMS provider's implementation for message persistence, and it will be discussed in Chapter 18, JMS Support. However, when buffering in a queue is not necessary, the simplest approach is to rely upon the DirectChannel discussed next.

The RendezvousChannel is also useful for implementing request-reply operations. The sender can create a temporary, anonymous instance of RendezvousChannel which it then sets as the 'replyChannel' header when building a Message. After sending that Message, the sender can immediately call receive (optionally providing a timeout value) in order to block while waiting for a reply Message. This is very similar to the implementation used internally by many of Spring Integration's request-reply components.

DirectChannel

The DirectChannel has point-to-point semantics but otherwise is more similar to the PublishSubscribeChannel than any of the queue-based channel implementations described above. It implements the SubscribableChannel interface instead of the PollableChannel interface, so it dispatches Messages directly to a subscriber. As a point-to-point channel, however, it differs from the PublishSubscribeChannel in that it will only send each Message to a single subscribed MessageHandler.

In addition to being the simplest point-to-point channel option, one of its most important features is that it enables a single thread to perform the operations on "both sides" of the channel. For example, if a handler is subscribed to a DirectChannel, then sending a Message to that channel will trigger invocation of that handler's handleMessage(Message) method directly in the sender's thread, before the send() method invocation can return.

The key motivation for providing a channel implementation with this behavior is to support transactions that must span across the channel while still benefiting from the abstraction and loose coupling that the channel provides. If the send call is invoked within the scope of a transaction, then the outcome of the handler's invocation (e.g. updating a database record) will play a role in determining the ultimate result of that transaction (commit or rollback).

[Note]Note
Since the DirectChannel is the simplest option and does not add any additional overhead that would be required for scheduling and managing the threads of a poller, it is the default channel type within Spring Integration. The general idea is to define the channels for an application and then to consider which of those need to provide buffering or to throttle input, and then modify those to be queue-based PollableChannels. Likewise, if a channel needs to broadcast messages, it should not be a DirectChannel but rather a PublishSubscribeChannel. Below you will see how each of these can be configured.

The DirectChannel internally delegates to a Message Dispatcher to invoke its subscribed Message Handlers, and that dispatcher can have a load-balancing strategy. The load-balancer determines how invocations will be ordered in the case that there are multiple handlers subscribed to the same channel. When using the namespace support described below, the default strategy is "round-robin" which essentially load-balances across the handlers in rotation.

[Note]Note
The "round-robin" strategy is currently the only implementation available out-of-the-box in Spring Integration. Other strategy implementations may be added in future versions.

The load-balancer also works in combination with a boolean failover property. If the "failover" value is true (the default), then the dispatcher will fall back to any subsequent handlers as necessary when preceding handlers throw Exceptions. The order is determined by an optional order value defined on the handlers themselves or, if no such value exists, the order in which the handlers are subscribed.

If a certain situation requires that the dispatcher always try to invoke the first handler, then fallback in the same fixed order sequence every time an error occurs, no load-balancing strategy should be provided. In other words, the dispatcher still supports the failover boolean property even when no load-balancing is enabled. Without load-balancing, however, the invocation of handlers will always begin with the first according to their order. For example, this approach works well when there is a clear definition of primary, secondary, tertiary, and so on. When using the namespace support, the "order" attribute on any endpoint will determine that order.

[Note]Note
Keep in mind that load-balancing and failover only apply when a channel has more than one subscribed Message Handler. When using the namespace support, this means that more than one endpoint shares the same channel reference in the "input-channel" attribute.

ExecutorChannel

The ExecutorChannel is a point-to-point channel that supports the same dispatcher configuration as DirectChannel (load-balancing strategy and the failover boolean property). The key difference between these two dispatching channel types is that the ExecutorChannel delegates to an instance of TaskExecutor to perform the dispatch. This means that the send method typically will not block, but it also means that the handler invocation may not occur in the sender's thread. It therefore does not support transactions spanning the sender and receiving handler.

[Tip]Tip
Note that there are occasions where the sender may block. For example, when using a TaskExecutor with a rejection-policy that throttles back on the client (such as the ThreadPoolExecutor.CallerRunsPolicy), the sender's thread will execute the method directly anytime the thread pool is at its maximum capacity and the executor's work queue is full. Since that situation would only occur in a non-predictable way, that obviously cannot be relied upon for transactions.

ThreadLocalChannel

The final channel implementation type is ThreadLocalChannel. This channel also delegates to a queue internally, but the queue is bound to the current thread. That way the thread that sends to the channel will later be able to receive those same Messages, but no other thread would be able to access them. While probably the least common type of channel, this is useful for situations where DirectChannels are being used to enforce a single thread of operation but any reply Messages should be sent to a "terminal" channel. If that terminal channel is a ThreadLocalChannel, the original sending thread can collect its replies from it.

3.3 Channel Interceptors

One of the advantages of a messaging architecture is the ability to provide common behavior and capture meaningful information about the messages passing through the system in a non-invasive way. Since the Messages are being sent to and received from MessageChannels, those channels provide an opportunity for intercepting the send and receive operations. The ChannelInterceptor strategy interface provides methods for each of those operations:

public interface ChannelInterceptor {

    Message<?> preSend(Message<?> message, MessageChannel channel);

    void postSend(Message<?> message, MessageChannel channel, boolean sent);

    boolean preReceive(MessageChannel channel);

    Message<?> postReceive(Message<?> message, MessageChannel channel);
}

After implementing the interface, registering the interceptor with a channel is just a matter of calling:

channel.addInterceptor(someChannelInterceptor);

The methods that return a Message instance can be used for transforming the Message or can return 'null' to prevent further processing (of course, any of the methods can throw a RuntimeException). Also, the preReceive method can return 'false' to prevent the receive operation from proceeding.

[Note]Note
Keep in mind that receive() calls are only relevant for PollableChannels. In fact the SubscribableChannel interface does not even define a receive() method. The reason for this is that when a Message is sent to a SubscribableChannel it will be sent directly to one or more subscribers depending on the type of channel (e.g. a PublishSubscribeChannel sends to all of its subscribers). Therefore, the preReceive(..) and postReceive(..) interceptor methods are only invoked when the interceptor is applied to a PollableChannel.

Spring Integration also provides an implementation of the Wire Tap pattern. It is a simple interceptor that sends the Message to another channel without otherwise altering the existing flow. It can be very useful for debugging and monitoring. An example is shown in the section called “Wire Tap”.

Because it is rarely necessary to implement all of the interceptor methods, a ChannelInterceptorAdapter class is also available for sub-classing. It provides no-op methods (the void method is empty, the Message returning methods return the Message as-is, and the boolean method returns true). Therefore, it is often easiest to extend that class and just implement the method(s) that you need as in the following example.

public class CountingChannelInterceptor extends ChannelInterceptorAdapter {

    private final AtomicInteger sendCount = new AtomicInteger();

    @Override
    public Message<?> preSend(Message<?> message, MessageChannel channel) {
        sendCount.incrementAndGet();
        return message;
    }
}

[Tip]Tip
The order of invocation for the interceptor methods depends on the type of channel. As described above, the queue-based channels are the only ones where the receive method is intercepted in the first place. Additionally, the relationship between send and receive interception depends on the timing of separate sender and receiver threads. For example, if a receiver is already blocked while waiting for a message the order could be: preSend, preReceive, postReceive, postSend. However, if a receiver polls after the sender has placed a message on the channel and already returned, the order would be: preSend, postSend, (some-time-elapses) preReceive, postReceive. The time that elapses in such a case depends on a number of factors and is therefore generally unpredictable (in fact, the receive may never happen!). Obviously, the type of queue also plays a role (e.g. rendezvous vs. priority). The bottom line is that you cannot rely on the order beyond the fact that preSend will precede postSend and preReceive will precede postReceive.

3.4 MessageChannelTemplate

As you will see when the endpoints and their various configuration options are introduced, Spring Integration provides a foundation for messaging components that enables non-invasive invocation of your application code from the messaging system. However, sometimes it is necessary to invoke the messaging system from your application code. For convenience when implementing such use-cases, Spring Integration provides a MessageChannelTemplate that supports a variety of operations across the Message Channels, including request/reply scenarios. For example, it is possible to send a request and wait for a reply.

MessageChannelTemplate template = new MessageChannelTemplate();

Message reply = template.sendAndReceive(new StringMessage("test"), someChannel);

In that example, a temporary anonymous channel would be created internally by the template. The 'sendTimeout' and 'receiveTimeout' properties may also be set on the template, and other exchange types are also supported.

public boolean send(final Message<?> message, final MessageChannel channel) { ... }

public Message<?> sendAndReceive(final Message<?> request, final MessageChannel channel) { .. }

public Message<?> receive(final PollableChannel<?> channel) { ... }

[Note]Note

A less invasive approach that allows you to invoke simple interfaces with payload and/or header values instead of Message instances is described in Section 16.2, “GatewayProxyFactoryBean”.

3.5 Configuring Message Channels

To create a Message Channel instance, you can use the 'channel' element:

<channel id="exampleChannel"/>

The default channel type is Point to Point. To create a Publish Subscribe channel, use the "publish-subscribe-channel" element:

<publish-subscribe-channel id="exampleChannel"/>

To create a Datatype Channel that only accepts messages containing a certain payload type, provide the fully-qualified class name in the channel element's datatype attribute:

<channel id="numberChannel" datatype="java.lang.Number"/>

Note that the type check passes for any type that is assignable to the channel's datatype. In other words, the "numberChannel" above would accept messages whose payload is java.lang.Integer or java.lang.Double. Multiple types can be provided as a comma-delimited list:

<channel id="stringOrNumberChannel" datatype="java.lang.String,java.lang.Number"/>

When using the "channel" element without any sub-elements, it will create a DirectChannel instance (a SubscribableChannel).

However, you can alternatively provide a variety of "queue" sub-elements to create any of the pollable channel types (as described in Section 3.2, “Message Channel Implementations”). Examples of each are shown below.

DirectChannel Configuration

As mentioned above, DirectChannel is the default type.

<channel id="directChannel"/>

A default channel will have a round-robin load-balancer and will also have failover enabled (See the discussion in the section called “DirectChannel” for more detail). To disable one or both of these, add a <dispatcher/> sub-element and configure the attributes:

<channel id="failFastChannel">
    <dispatcher failover="false"/>
</channel>

<channel id="channelWithFixedOrderSequenceFailover">
    <dispatcher load-balancer="none"/>
</channel>

QueueChannel Configuration

To create a QueueChannel, use the "queue" sub-element. You may specify the channel's capacity:

<channel id="queueChannel">
    <queue capacity="25"/>
</channel>

[Note]Note
If you do not provide a value for the 'capacity' attribute on this <queue/> sub-element, the resulting queue will be unbounded. To avoid issues such as OutOfMemoryErrors, it is highly recommended to set an explicit value for a bounded queue.

PublishSubscribeChannel Configuration

To create a PublishSubscribeChannel, use the "publish-subscribe-channel" element. When using this element, you can also specify the "task-executor" used for publishing Messages (if none is specified it simply publishes in the sender's thread):

<publish-subscribe-channel id="pubsubChannel" task-executor="someExecutor"/>

If you are providing a Resequencer or Aggregator downstream from a PublishSubscribeChannel, then you can set the 'apply-sequence' property on the channel to true. That will indicate that the channel should set the sequence-size and sequence-number Message headers as well as the correlation id prior to passing the Messages along. For example, if there are 5 subscribers, the sequence-size would be set to 5, and the Messages would have sequence-number header values ranging from 1 to 5.

<publish-subscribe-channel id="pubsubChannel" apply-sequence="true"/>

[Note]Note
The 'apply-sequence' value is false by default so that a Publish Subscribe Channel can send the exact same Message instances to multiple outbound channels. Since Spring Integration enforces immutability of the payload and header references, the channel creates new Message instances with the same payload reference but different header values when the flag is set to true.

ExecutorChannel

To create an ExecutorChannel, add the <dispatcher> sub-element along with a 'task-executor' attribute. Its value can reference any TaskExecutor within the context. For example, this enables configuration of a thread-pool for dispatching messages to subscribed handlers. As mentioned above, this does break the "single-threaded" execution context between sender and receiver so that any active transaction context will not be shared by the invocation of the handler (i.e. the handler may throw an Exception, but the send invocation has already returned successfully).

<channel id="executorChannel">
    <dispatcher task-executor="someExecutor"/>
</channel>

[Note]Note
The "load-balancer" and "failover" options are also both available on the dispatcher sub-element as described above in the section called “DirectChannel Configuration”. The same defaults apply as well. So, the channel will have a round-robin load-balancing strategy with failover enabled unless explicit configuration is provided for one or both of those attributes.
<channel id="executorChannelWithoutFailover">
    <dispatcher task-executor="someExecutor" failover="false"/>
</channel>

PriorityChannel Configuration

To create a PriorityChannel, use the "priority-queue" sub-element:

<channel id="priorityChannel">
    <priority-queue capacity="20"/>
</channel>

By default, the channel will consult the MessagePriority header of the message. However, a custom Comparator reference may be provided instead. Also, note that the PriorityChannel (like the other types) does support the "datatype" attribute. As with the QueueChannel, it also supports a "capacity" attribute. The following example demonstrates all of these:

<channel id="priorityChannel" datatype="example.Widget">
    <priority-queue comparator="widgetComparator"
                    capacity="10"/>
</channel>

RendezvousChannel Configuration

A RendezvousChannel is created when the queue sub-element is a <rendezvous-queue>. It does not provide any additional configuration options to those described above, and its queue does not accept any capacity value since it is a 0-capacity direct handoff queue.

<channel id="rendezvousChannel"/>
    <rendezvous-queue/>
</channel>

ThreadLocalChannel Configuration

The ThreadLocalChannel does not provide any additional configuration options.

<thread-local-channel id="threadLocalChannel"/>

Channel Interceptor Configuration

Message channels may also have interceptors as described in Section 3.3, “Channel Interceptors”. The <interceptors> sub-element can be added within <channel> (or the more specific element types). Provide the "ref" attribute to reference any Spring-managed object that implements the ChannelInterceptor interface:

<channel id="exampleChannel">
    <interceptors>
        <ref bean="trafficMonitoringInterceptor"/>
    </interceptors>
</channel>

In general, it is a good idea to define the interceptor implementations in a separate location since they usually provide common behavior that can be reused across multiple channels.

Wire Tap

As mentioned above, Spring Integration provides a simple Wire Tap interceptor out of the box. You can configure a Wire Tap on any channel within an 'interceptors' element. This is especially useful for debugging, and can be used in conjunction with Spring Integration's logging Channel Adapter as follows:

 <channel id="in">
     <interceptors>
         <wire-tap channel="logger"/>
     </interceptors>
 </channel>

 <logging-channel-adapter id="logger" level="DEBUG"/>

[Tip]Tip
The 'logging-channel-adapter' also accepts a boolean attribute: 'log-full-message'. That is false by default so that only the payload is logged. Setting that to true enables logging of all headers in addition to the payload.

[Note]Note

If namespace support is enabled, there are also two special channels defined within the context by default: errorChannel and nullChannel. The 'nullChannel' acts like /dev/null, simply logging any Message sent to it at DEBUG level and returning immediately. Any time you face channel resolution errors for a reply that you don't care about, you can set the affected component's 'output-channel' to reference 'nullChannel' (the name 'nullChannel' is reserved within the context). The 'errorChannel' is used internally for sending error messages, and it can be overridden with a custom configuration. It is discussed in greater detail in Section B.4, “Error Handling”.

4. Message Endpoints

The first part of this chapter covers some background theory and reveals quite a bit about the underlying API that drives Spring Integration's various messaging components. This information can be helpful if you want to really understand what's going on behind the scenes. However, if you want to get up and running with the simplified namespace-based configuration of the various elements, feel free to skip ahead to Section 4.4, “Namespace Support” for now.

As mentioned in the overview, Message Endpoints are responsible for connecting the various messaging components to channels. Over the next several chapters, you will see a number of different components that consume Messages. Some of these are also capable of sending reply Messages. Sending Messages is quite straightforward. As shown above in Chapter 3, Message Channels, it's easy to send a Message to a Message Channel. However, receiving is a bit more complicated. The main reason is that there are two types of consumers: Polling Consumers and Event Driven Consumers.

Of the two, Event Driven Consumers are much simpler. Without any need to manage and schedule a separate poller thread, they are essentially just listeners with a callback method. When connecting to one of Spring Integration's subscribable Message Channels, this simple option works great. However, when connecting to a buffering, pollable Message Channel, some component has to schedule and manage the polling thread(s). Spring Integration provides two different endpoint implementations to accommodate these two types of consumers. Therefore, the consumers themselves can simply implement the callback interface. When polling is required, the endpoint acts as a "container" for the consumer instance. The benefit is similar to that of using a container for hosting Message Driven Beans, but since these consumers are simply Spring-managed Objects running within an ApplicationContext, it more closely resembles Spring's own MessageListener containers.

4.1 Message Handler

Spring Integration's MessageHandler interface is implemented by many of the components within the framework. In other words, this is not part of the public API, and a developer would not typically implement MessageHandler directly. Nevertheless, it is used by a Message Consumer for actually handling the consumed Messages, and so being aware of this strategy interface does help in terms of understanding the overall role of a consumer. The interface is defined as follows:

public interface MessageHandler {

    void handleMessage(Message<?> message);

}

Despite its simplicity, this provides the foundation for most of the components that will be covered in the following chapters (Routers, Transformers, Splitters, Aggregators, Service Activators, etc). Those components each perform very different functionality with the Messages they handle, but the requirements for actually receiving a Message are the same, and the choice between polling and event-driven behavior is also the same. Spring Integration provides two endpoint implementations that "host" these callback-based handlers and allow them to be connected to Message Channels.

4.2 Event Driven Consumer

Because it is the simpler of the two, we will cover the Event Driven Consumer endpoint first. You may recall that the SubscribableChannel interface provides a subscribe() method and that the method accepts a MessageHandler parameter (as shown in the section called “SubscribableChannel”):

subscribableChannel.subscribe(messageHandler);

Since a handler that is subscribed to a channel does not have to actively poll that channel, this is an Event Driven Consumer, and the implementation provided by Spring Integration accepts a a SubscribableChannel and a MessageHandler:

SubscribableChannel channel = (SubscribableChannel) context.getBean("subscribableChannel");

EventDrivenConsumer consumer = new EventDrivenConsumer(channel, exampleHandler);

4.3 Polling Consumer

Spring Integration also provides a PollingConsumer, and it can be instantiated in the same way except that the channel must implement PollableChannel:

PollableChannel channel = (PollableChannel) context.getBean("pollableChannel");

PollingConsumer consumer = new PollingConsumer(channel, exampleHandler);

There are many other configuration options for the Polling Consumer. For example, the trigger is a required property:

PollingConsumer consumer = new PollingConsumer(channel, handler);

consumer.setTrigger(new IntervalTrigger(30, TimeUnit.SECONDS));

Spring Integration currently provides two implementations of the Trigger interface: IntervalTrigger and CronTrigger. The IntervalTrigger is typically defined with a simple interval (in milliseconds), but also supports an 'initialDelay' property and a boolean 'fixedRate' property (the default is false, i.e. fixed delay):

IntervalTrigger trigger = new IntervalTrigger(1000);
trigger.setInitialDelay(5000);
trigger.setFixedRate(true);

The CronTrigger simply requires a valid cron expression (see the Javadoc for details):

CronTrigger trigger = new CronTrigger("*/10 * * * * MON-FRI");

In addition to the trigger, several other polling-related configuration properties may be specified:

PollingConsumer consumer = new PollingConsumer(channel, handler);

consumer.setMaxMessagesPerPoll(10);

consumer.setReceiveTimeout(5000);

The 'maxMessagesPerPoll' property specifies the maximum number of messages to receive within a given poll operation. This means that the poller will continue calling receive() without waiting until either null is returned or that max is reached. For example, if a poller has a 10 second interval trigger and a 'maxMessagesPerPoll' setting of 25, and it is polling a channel that has 100 messages in its queue, all 100 messages can be retrieved within 40 seconds. It grabs 25, waits 10 seconds, grabs the next 25, and so on.

The 'receiveTimeout' property specifies the amount of time the poller should wait if no messages are available when it invokes the receive operation. For example, consider two options that seem similar on the surface but are actually quite different: the first has an interval trigger of 5 seconds and a receive timeout of 50 milliseconds while the second has an interval trigger of 50 milliseconds and a receive timeout of 5 seconds. The first one may receive a message up to 4950 milliseconds later than it arrived on the channel (if that message arrived immediately after one of its poll calls returned). On the other hand, the second configuration will never miss a message by more than 50 milliseconds. The difference is that the second option requires a thread to wait, but as a result it is able to respond much more quickly to arriving messages. This technique, known as "long polling", can be used to emulate event-driven behavior on a polled source.

A Polling Consumer may also delegate to a Spring TaskExecutor, and it can be configured to participate in Spring-managed transactions. The following example shows the configuration of both:

PollingConsumer consumer = new PollingConsumer(channel, handler);

TaskExecutor taskExecutor = (TaskExecutor) context.getBean("exampleExecutor");
consumer.setTaskExecutor(taskExecutor);

PlatformTransactionManager txManager = (PlatformTransationManager) context.getBean("exampleTxManager");
consumer.setTransactionManager(txManager);

The examples above show dependency lookups, but keep in mind that these consumers will most often be configured as Spring bean definitions. In fact, Spring Integration also provides a FactoryBean that creates the appropriate consumer type based on the type of channel, and there is full XML namespace support to even further hide those details. The namespace-based configuration will be featured as each component type is introduced.

[Note]Note
Many of the MessageHandler implementations are also capable of generating reply Messages. As mentioned above, sending Messages is trivial when compared to the Message reception. Nevertheless, when and how many reply Messages are sent depends on the handler type. For example, an Aggregator waits for a number of Messages to arrive and is often configured as a downstream consumer for a Splitter which may generate multiple replies for each Message it handles. When using the namespace configuration, you do not strictly need to know all of the details, but it still might be worth knowing that several of these components share a common base class, the AbstractReplyProducingMessageHandler, and it provides a setOutputChannel(..) method.

4.4 Namespace Support

Throughout the reference manual, you will see specific configuration examples for endpoint elements, such as router, transformer, service-activator, and so on. Most of these will support an "input-channel" attribute and many will support an "output-channel" attribute. After being parsed, these endpoint elements produce an instance of either the PollingConsumer or the EventDrivenConsumer depending on the type of the "input-channel" that is referenced: PollableChannel or SubscribableChannel respectively. When the channel is pollable, then the polling behavior is determined based on the endpoint element's "poller" sub-element. For example, a simple interval-based poller with a 1-second interval would be configured like this:

 <transformer input-channel="pollable"
              ref="transformer"
              output-channel="output">
     <poller>
         <interval-trigger interval="1000"/>
     </poller>
</transformer>

For a poller based on a Cron expression, use the "cron-trigger" child element instead:

 <transformer input-channel="pollable"
              ref="transformer"
              output-channel="output">
     <poller>
         <cron-trigger expression="*/10 * * * * MON-FRI"/>
     </poller>
 </transformer>

If the input channel is a PollableChannel, then the poller configuration is required. Specifically, as mentioned above, the 'trigger' is a required property of the PollingConsumer class. Therefore, if you omit the "poller" sub-element for a Polling Consumer endpoint's configuration, an Exception may be thrown. However, it is also possible to create top-level pollers in which case only a "ref" is required:

 <poller id="weekdayPoller">
     <cron-trigger expression="*/10 * * * * MON-FRI"/>
 </poller>

 <transformer input-channel="pollable"
              ref="transformer"
              output-channel="output">
    <poller ref="weekdayPoller"/>
 </transformer>

In fact, to simplify the configuration, you can define a global default poller. A single top-level poller within an ApplicationContext may have the default attribute with a value of "true". In that case, any endpoint with a PollableChannel for its input-channel that is defined within the same ApplicationContext and has no explicitly configured 'poller' sub-element will use that default.

 <poller id="defaultPoller" default="true" max-messages-per-poll="5">
     <interval-trigger interval="3" time-unit="SECONDS"/>
 </poller>

 <!-- No <poller/> sub-element is necessary since there is a default -->
 <transformer input-channel="pollable"
              ref="transformer"
              output-channel="output"/>

Spring Integration also provides transaction support for the pollers so that each receive-and-forward operation can be performed as an atomic unit-of-work. To configure transactions for a poller, simply add the <transactional/> sub-element. The attributes for this element should be familiar to anyone who has experience with Spring's Transaction management:

<poller>
    <interval-trigger interval="1000"/>
    <transactional transaction-manager="txManager"
                   propagation="REQUIRED"
                   isolation="REPEATABLE_READ"
                   timeout="10000"
                   read-only="false"/>
</poller>

The polling threads may be executed by any instance of Spring's TaskExecutor abstraction. This enables concurrency for an endpoint or group of endpoints. As a convenience, there is also namespace support for creating a simple thread pool executor. The <thread-pool-task-executor/> element defines attributes for common concurrency settings such as core-size, max-size, and queue-capacity. Configuring a thread-pooling executor can make a substantial difference in how the endpoint performs under load. These settings are available per-endpoint since the performance of an endpoint is one of the major factors to consider (the other major factor being the expected volume on the channel to which the endpoint subscribes). To enable concurrency for a polling endpoint that is configured with the XML namespace support, provide the 'task-executor' reference on its <poller/> element and then provide one or more of the properties shown below:

 <poller task-executor="pool"/>
     <interval-trigger interval="5" time-unit="SECONDS"/>
 </poller>

 <thread-pool-task-executor id="pool"
                            core-size="5"
                            max-size="25"
                            queue-capacity="20"
                            keep-alive-seconds="120"/>

If no 'task-executor' is provided, the consumer's handler will be invoked in the caller's thread. Note that the "caller" is usually the default TaskScheduler (see Section B.3, “Configuring the Task Scheduler”). Also, keep in mind that the 'task-executor' attribute can provide a reference to any implementation of Spring's TaskExecutor interface by specifying the bean name. The thread pool element is simply provided for convenience.

As mentioned in the background section for Polling Consumers above, you can also configure a Polling Consumer in such a way as to emulate event-driven behavior. With a long receive-timeout and a short interval-trigger, you can ensure a very timely reaction to arriving messages even on a polled message source. Note that this will only apply to sources that have a blocking wait call with a timeout. For example, the File poller does not block, each receive() call returns immediately and either contains new files or not. Therefore, even if a poller contains a long receive-timeout, that value would never be usable in such a scenario. On the other hand when using Spring Integration's own queue-based channels, the timeout value does have a chance to participate. The following example demonstrates how a Polling Consumer will receive Messages nearly instantaneously.

 <service-activator input-channel="someQueueChannel"
        output-channel="output">  
     <poller receive-timeout="30000">
         <interval-trigger interval="10"/>
     </poller>
 </service-activator>

Using this approach does not carry much overhead since internally it is nothing more then a timed-wait thread which does not require nearly as much CPU resource usage as a thrashing, infinite while loop for example.

5. Service Activator

5.1 Introduction

The Service Activator is the endpoint type for connecting any Spring-managed Object to an input channel so that it may play the role of a service. If the service produces output, it may also be connected to an output channel. Alternatively, an output producing service may be located at the end of a processing pipeline or message flow in which case, the inbound Message's "replyChannel" header can be used. This is the default behavior if no output channel is defined, and as with most of the configuration options you'll see here, the same behavior actually applies for most of the other components we have seen.

5.2 The <service-activator/> Element

To create a Service Activator, use the 'service-activator' element with the 'input-channel' and 'ref' attributes:

<service-activator input-channel="exampleChannel" ref="exampleHandler"/>

The configuration above assumes that "exampleHandler" either contains a single method annotated with the @ServiceActivator annotation or that it contains only one public method at all. To delegate to an explicitly defined method of any object, simply add the "method" attribute.

<service-activator input-channel="exampleChannel" ref="somePojo" method="someMethod"/>

In either case, when the service method returns a non-null value, the endpoint will attempt to send the reply message to an appropriate reply channel. To determine the reply channel, it will first check if an "output-channel" was provided in the endpoint configuration:

<service-activator input-channel="exampleChannel" output-channel="replyChannel"
                   ref="somePojo" method="someMethod"/>

If no "output-channel" is available, it will then check the Message's REPLY_CHANNEL header value. If that value is available, it will then check its type. If it is a MessageChannel, the reply message will be sent to that channel. If it is a String, then the endpoint will attempt to resolve the channel name to a channel instance. If the channel cannot be resolved, then a ChannelResolutionException will be thrown.

The argument in the service method could be either a Message or an arbitrary type. If the latter, then it will be assumed that it is a Message payload, which will be extracted from the message and injected into such service method. This is generally the recommended approach as it follows and promotes a POJO model when working with Spring Integration. Arguments may also have @Header or @Headers annotations as described in Section B.5, “Annotation Support”

[Note]Note
Since v1.0.3 of Spring Integration, the service method is not required to have an argument at all, which means you can now implement event-style Service Activators, where all you care about is an invocation of the service method, not worrying about the contents of the message. Think of it as a NULL JMS message. An example use-case for such an implementation could be a simple counter/monitor of messages deposited on the input channel.

Using a "ref" attribute is generally recommended if the custom Service Activator handler implementation can be reused in other <service-activator> definitions. However if the custom Service Activator handler implementation should be scoped to a single definition of the <service-activator>, you can use an inner bean definition:

<service-activator id="exampleServiceActivator" input-channel="inChannel" 
						output-channel = "outChannel" method="foo">
		<beans:bean class="org.foo.ExampleServiceActivator"/>
</service-activator>

[Note]Note

Using both the "ref" attribute and an inner handler definition in the same <service-activator> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

6. Channel Adapter

A Channel Adapter is a Message Endpoint that enables connecting a single sender or receiver to a Message Channel. Spring Integration provides a number of adapters out of the box to support various transports, such as JMS, File, HTTP, Web Services, and Mail. Those will be discussed in upcoming chapters of this reference guide. However, this chapter focuses on the simple but flexible Method-invoking Channel Adapter support. There are both inbound and outbound adapters, and each may be configured with XML elements provided in the core namespace.

6.1 The <inbound-channel-adapter> element

An "inbound-channel-adapter" element can invoke any method on a Spring-managed Object and send a non-null return value to a MessageChannel after converting it to a Message. When the adapter's subscription is activated, a poller will attempt to receive messages from the source. The poller will be scheduled with the TaskScheduler according to the provided configuration. To configure the polling interval or cron expression for an individual channel-adapter, provide a 'poller' element with either an 'interval-trigger' (in milliseconds) or 'cron-trigger' sub-element.

<inbound-channel-adapter ref="source1" method="method1" channel="channel1">
    <poller>
        <interval-trigger interval="5000"/>
    </poller>
</inbound-channel-adapter>

<inbound-channel-adapter ref="source2" method="method2" channel="channel2">
    <poller>
        <cron-trigger expression="30 * 9-17 * * MON-FRI"/>
    </poller>
</channel-adapter>

[Note]Note

If no poller is provided, then a single default poller must be registered within the context. See Section 4.4, “Namespace Support” for more detail.

6.2 The <outbound-channel-adapter/> element

An "outbound-channel-adapter" element can also connect a MessageChannel to any method that should be invoked with the payload of Messages sent to that channel.

<outbound-channel-adapter channel="channel1" ref="target1" method="method1"/>

If the channel being adapted is a PollableChannel, provide a poller sub-element:

<outbound-channel-adapter channel="channel2" ref="target2" method="method2">
    <poller>
        <interval-trigger interval="3000"/>
    </poller>
</outbound-channel-adapter>

Any Channel Adapter can be created without a "channel" reference in which case it will implicitly create an instance of DirectChannel. The created channel's name will match the "id" attribute of the <inbound-channel-adapter/> or <outbound-channel-adapter element. Therefore, if the "channel" is not provided, the "id" is required.

7. Router

7.1 Router Implementations

Since content-based routing often requires some domain-specific logic, most use-cases will require Spring Integration's options for delegating to POJOs using the XML namespace support and/or Annotations. Both of these are discussed below, but first we present a couple implementations that are available out-of-the-box since they fulfill generic, but common, requirements.

PayloadTypeRouter

A PayloadTypeRouter will send Messages to the channel as defined by payload-type mappings.

<bean id="payloadTypeRouter" class="org.springframework.integration.router.PayloadTypeRouter">
    <property name="payloadTypeChannelMap">
        <map>
            <entry key="java.lang.String" value-ref="stringChannel"/>
            <entry key="java.lang.Integer" value-ref="integerChannel"/>
        </map>
    </property>
</bean>

Configuration of PayloadTypeRouter is also supported via the namespace provided by Spring Integration (see Section B.2, “Namespace Support”), which essentially simplifies configuration by combining <router/> configuration and its corresponding implementation defined using <bean/> element into a single and more concise configuration element. The example below demonstrates PayloadTypeRouter configuration which is equivalent to the one above using Spring Integration's namespace support:

<payload-type-router input-channel="routingChannel">
	<mapping type="java.lang.String" channel="stringChannel" />
	<mapping type="java.lang.Integer" channel="integerChannel" />
</payload-type-router>

HeaderValueRouter

A HeaderValueRouter will send Messages to the channel based on the individual header value mappings. When HeaderValueRouter is created it is initialized with the name of the header to be evaluated, using constructor-arg. The value of the header could be one of two things:

1. Arbitrary value

2. Channel name

If arbitrary value, then a channelResolver should be provided to map header values to channel names. The example below uses MapBasedChannelResolver to set up a map of header values to channel names.

 <bean id="myHeaderValueRouter"
		class="org.springframework.integration.router.HeaderValueRouter">
  <constructor-arg value="someHeaderName" /> 
  <property name="channelResolver">
    <bean class="org.springframework.integration.channel.MapBasedChannelResolver">
      <property name="channelMap">
        <map>
          <entry key="someHeaderValue" value-ref="channelA" />
          <entry key="someOtherHeaderValue" value-ref="channelB" />
        </map>
      </property>
    </bean>
  </property>
</bean>

If channelResolver is not specified, then the header value will be treated as a channel name making configuration much simpler, where no channelResolver needs to be specified.

 
<bean id="myHeaderValueRouter"
	class="org.springframework.integration.router.HeaderValueRouter">
	<constructor-arg value="someHeaderName" /> 
</bean>

Similar to the PayloadTypeRouter, configuration of HeaderValueRouter is also supported via namespace support provided by Spring Integration (see Section B.2, “Namespace Support”). The example below demonstrates two types of namespace-based configuration of HeaderValueRouter which are equivalent to the ones above using Spring Integration namespace support:

1. Configuration where mapping of header values to channels is required

<header-value-router input-channel="routingChannel" header-name="testHeader">
	<mapping value="someHeaderValue" channel="channelA" />
	<mapping value="someOtherHeaderValue" channel="channelB" />
</header-value-router>

2. Configuration where mapping of header values is not required if header values themselves represent the channel names

<header-value-router input-channel="routingChannel" header-name="testHeader"/>

[Note]Note
The two router implementations shown above share some common properties, such as "defaultOutputChannel" and "resolutionRequired". If "resolutionRequired" is set to "true", and the router is unable to determine a target channel (e.g. there is no matching payload for a PayloadTypeRouter and no "defaultOutputChannel" has been specified), then an Exception will be thrown.

RecipientListRouter

A RecipientListRouter will send each received Message to a statically-defined list of Message Channels:

<bean id="recipientListRouter" class="org.springframework.integration.router.RecipientListRouter">
    <property name="channels">
        <list>
            <ref bean="channel1"/>
            <ref bean="channel2"/>
            <ref bean="channel3"/>
        </list>
    </property>
</bean>

Configuration for RecipientListRouter is also supported via namespace support provided by Spring Integration (see Section B.2, “Namespace Support”). The example below demonstrates namespace-based configuration of RecipientListRouter and all the supported attributes using Spring Integration namespace support:

<recipient-list-router id="customRouter" input-channel="routingChannel"
				timeout="1234"
				ignore-send-failures="true"
				apply-sequence="true">
	<recipient channel="channel1"/>
	<recipient channel="channel2"/>
</recipient-list-router>

[Note]Note
The 'apply-sequence' flag here has the same affect as it does for a publish-subscribe-channel, and like publish-subscribe-channel it is disabled by default on the recipient-list-router. Refer to the section called “PublishSubscribeChannel Configuration” for more information.

7.2 The <router> element

The "router" element provides a simple way to connect a router to an input channel, and also accepts the optional default output channel. The "ref" may provide the bean name of a custom Router implementation (extending AbstractMessageRouter):

<router ref="payloadTypeRouter" input-channel="input1" default-output-channel="defaultOutput1"/>

<router ref="recipientListRouter" input-channel="input2" default-output-channel="defaultOutput2"/>

<router ref="customRouter" input-channel="input3" default-output-channel="defaultOutput3"/>

<beans:bean id="customRouterBean class="org.foo.MyCustomRouter"/>

Alternatively, the "ref" may point to a simple Object that contains the @Router annotation (see below), or the "ref" may be combined with an explicit "method" name. When specifying a "method", the same behavior applies as described in the @Router annotation section below.

<router input-channel="input" ref="somePojo" method="someMethod"/>

Using a "ref" attribute is generally recommended if the custom router implementation can be reused in other <router> definitions. However if the custom router implementation should be scoped to a concrete definition of the <router>, you can provide an inner bean definition:

<router method="someMethod" input-channel="input3" default-output-channel="defaultOutput3">
	<beans:bean class="org.foo.MyCustomRouter"/>
</router>

[Note]Note

Using both the "ref" attribute and an inner handler definition in the same <router> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

7.3 The @Router Annotation

When using the @Router annotation, the annotated method can return either the MessageChannel or String type. In the case of the latter, the endpoint will resolve the channel name as it does for the default output. Additionally, the method can return either a single value or a collection. When a collection is returned, the reply message will be sent to multiple channels. To summarize, the following method signatures are all valid.

@Router
public MessageChannel route(Message message) {...}

@Router
public List<MessageChannel> route(Message message) {...}

@Router
public String route(Foo payload) {...}

@Router
public List<String> route(Foo payload) {...}

In addition to payload-based routing, a common requirement is to route based on metadata available within the message header as either a property or attribute. Rather than requiring use of the Message type as the method parameter, the @Router annotation may also use the @Header parameter annotation that is documented in Section B.5, “Annotation Support”.

@Router
public List<String> route(@Header("orderStatus") OrderStatus status)

[Note]Note
For routing of XML-based Messages, including XPath support, see Chapter 26, Dealing with XML Payloads.

8. Filter

8.1 Introduction

Message Filters are used to decide whether a Message should be passed along or dropped based on some criteria such as a Message Header value or even content within the Message itself. Therefore, a Message Filter is similar to a router, except that for each Message received from the filter's input channel, that same Message may or may not be sent to the filter's output channel. Unlike the router, it makes no decision regarding which Message Channel to send to but only decides whether to send.

[Note]Note
As you will see momentarily, the Filter does also support a discard channel, so in certain cases it can play the role of a very simple router (or "switch") based on a boolean condition.

In Spring Integration, a Message Filter may be configured as a Message Endpoint that delegates to some implementation of the MessageSelector interface. That interface is itself quite simple:

 public interface MessageSelector {

     boolean accept(Message<?> message);

 }

The MessageFilter constructor accepts a selector instance:

 MessageFilter filter = new MessageFilter(someSelector);

8.2 The <filter> Element

The <filter> element is used to create a Message-selecting endpoint. In addition to "input-channel" and "output-channel" attributes, it requires a "ref". The "ref" may point to a MessageSelector implementation:

 <filter input-channel="input" ref="selector" output-channel="output"/>

 <bean id="selector" class="example.MessageSelectorImpl"/>

Alternatively, the "method" attribute can be added at which point the "ref" may refer to any object. The referenced method may expect either the Message type or the payload type of inbound Messages. The return value of the method must be a boolean value. Any time the method returns 'true', the Message will be passed along to the output-channel.

 <filter input-channel="input" output-channel="output"
         ref="exampleObject" method="someBooleanReturningMethod"/>

 <bean id="exampleObject" class="example.SomeObject"/>

If the selector or adapted POJO method returns false, there are a few settings that control the fate of the rejected Message. By default (if configured like the example above), the rejected Messages will be silently dropped. If rejection should instead indicate an error condition, then set the 'throw-exception-on-rejection' flag to true:

 <filter input-channel="input" ref="selector"
      output-channel="output" throw-exception-on-rejection="true"/> 

If you want the rejected messages to go to a specific channel, provide that reference as the 'discard-channel':

 <filter input-channel="input" ref="selector"
      output-channel="output" discard-channel="rejectedMessages"/> 

[Note]Note
A common usage for Message Filters is in conjunction with a Publish Subscribe Channel. Many filter endpoints may be subscribed to the same channel, and they decide whether or not to pass the Message for the next endpoint which could be any of the supported types (e.g. Service Activator). This provides a reactive alternative to the more proactive approach of using a Message Router with a single Point-to-Point input channel and multiple output channels.

Using a "ref" attribute is generally recommended if the custom filter implementation can be reused in other <filter> definitions. However if the custom filter implementation should be scoped to a single <filter> element, provide an inner bean definition:

<filter method="someMethod" input-channel="inChannel" output-channel="outChannel">
	<beans:bean class="org.foo.MyCustomFilter"/>
</filter>

[Note]Note

Using both the "ref" attribute and an inner handler definition in the same <filter> configuration is not allowed, as it creates an ambiguous condition, and it will therefore result in an Exception being thrown.

9. Transformer

9.1 Introduction

Message Transformers play a very important role in enabling the loose-coupling of Message Producers and Message Consumers. Rather than requiring every Message-producing component to know what type is expected by the next consumer, Transformers can be added between those components. Generic transformers, such as one that converts a String to an XML Document, are also highly reusable.

For some systems, it may be best to provide a Canonical Data Model, but Spring Integration's general philosophy is not to require any particular format. Rather, for maximum flexibility, Spring Integration aims to provide the simplest possible model for extension. As with the other endpoint types, the use of declarative configuration in XML and/or Annotations enables simple POJOs to be adapted for the role of Message Transformers. These configuration options will be described below.

[Note]Note
For the same reason of maximizing flexibility, Spring does not require XML-based Message payloads. Nevertheless, the framework does provide some convenient Transformers for dealing with XML-based payloads if that is indeed the right choice for your application. For more information on those transformers, see Chapter 26, Dealing with XML Payloads.

9.2 The <transformer> Element

The <transformer> element is used to create a Message-transforming endpoint. In addition to "input-channel" and "output-channel" attributes, it requires a "ref". The "ref" may either point to an Object that contains the @Transformer annotation on a single method (see below) or it may be combined with an explicit method name value provided via the "method" attribute.

<transformer id="testTransformer" ref="testTransformerBean" input-channel="inChannel" 
					   method="transform" output-channel="outChannel"/>
<beans:bean id="testTransformerBean" class="org.foo.TestTransformer" />

Using a "ref" attribute is generally recommended if the custom transformer handler implementation can be reused in other <transformer> definitions. However if the custom transformer handler implementation should be scoped to a single definition of the <transformer>, you can define an inner bean definition:

<transformer id="testTransformer" input-channel="inChannel" method="transform"
								output-channel="outChannel">
	<beans:bean class="org.foo.TestTransformer"/>
</transformer>

[Note]Note

Using both the "ref" attribute and an inner handler definition in the same <transformer> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

The method that is used for transformation may expect either the Message type or the payload type of inbound Messages. It may also accept Message header values either individually or as a full map by using the @Header and @Headers parameter annotations respectively. The return value of the method can be any type. If the return value is itself a Message, that will be passed along to the transformer's output channel. If the return type is a Map, and the original Message payload was not a Map, the entries in that Map will be added to the Message headers of the original Message (the keys must be Strings). If the return value is null, then no reply Message will be sent (effectively the same behavior as a Message Filter returning false). Otherwise, the return value will be sent as the payload of an outbound reply Message.

There are a also a few Transformer implementations available out of the box. Because, it is fairly common to use the toString() representation of an Object, Spring Integration provides an ObjectToStringTransformer whose output is a Message with a String payload. That String is the result of invoking the toString operation on the inbound Message's payload.

 <object-to-string-transformer input-channel="in" output-channel="out"/>

A potential example for this would be sending some arbitrary object to the 'outbound-channel-adapter' in the file namespace. Whereas that Channel Adapter only supports String, byte-array, or java.io.File payloads by default, adding this transformer immediately before the adapter will handle the necessary conversion. Of course, that works fine as long as the result of the toString() call is what you want to be written to the File. Otherwise, you can just provide a custom POJO-based Transformer via the generic 'transformer' element shown previously.

[Tip]Tip
When debugging, this transformer is not typically necessary since the 'logging-channel-adapter' is capable of logging the Message payload. Refer to the section called “Wire Tap” for more detail.

If you need to serialize an Object to a byte array or deserialize a byte array back into an Object, Spring Integration provides symmetrical serialization transformers.

 <payload-serializing-transformer input-channel="objectsIn" output-channel="bytesOut"/>

 <payload-deserializing-transformer input-channel="bytesIn" output-channel="objectsOut"/>

If you only need to add headers to a Message, and they are not dynamically determined by Message content, then referencing a custom implementation may be overkill. For that reason, Spring Integration provides the 'header-enricher' element.

 <header-enricher input-channel="in" output-channel="out">
     <header name="foo" value="123"/>
     <header name="bar" ref="someBean"/>
 </header-enricher>

9.3 The @Transformer Annotation

The @Transformer annotation can also be added to methods that expect either the Message type or the message payload type. The return value will be handled in the exact same way as described above in the section describing the <transformer> element.

@Transformer
Order generateOrder(String productId) {
    return new Order(productId);
}

Transformer methods may also accept the @Header and @Headers annotations that is documented in Section B.5, “Annotation Support”

@Transformer
Order generateOrder(String productId, @Header("customerName") String customer) {
    return new Order(productId, customer);
}

10. Splitter

10.1 Introduction

The Splitter is a component whose role is to partition a message in several parts, and send the resulting messages to be processed independently. Very often, they are upstream producers in a pipeline that includes an Aggregator.

10.2 Programming model

The API for performing splitting consists from one base class, AbstractMessageSplitter, which is a MessageHandler implementation, encapsulating features which are common to splitters, such as filling in the appropriate message headers CORRELATION_ID, SEQUENCE_SIZE, and SEQUENCE_NUMBER on the messages that are produced. This allows to track down the messages and the results of their processing (in a typical scenario, these headers would be copied over to the messages that are produced by the various transforming endpoints), and use them, for example, in a Composed Message Processor scenario.

An excerpt from AbstractMessageSplitter can be seen below:

public abstract class AbstractMessageSplitter
                extends AbstractReplyProducingMessageConsumer {
  ...
  protected abstract Object splitMessage(Message<?> message);

}

For implementing a specific Splitter in an application, a developer can extend AbstractMessageSplitter and implement the splitMessage method, thus defining the actual logic for splitting the messages. The return value can be one of the following:

  • a Collection (or subclass thereof) or an array of Message objects - in this case the messages will be sent as such (after the CORRELATION_ID, SEQUENCE_SIZE and SEQUENCE_NUMBER are populated). Using this approach gives more control to the developer, for example for populating custom message headers as part of the splitting process.

  • a Collection (or subclass thereof) or an array of non-Message objects - works like the prior case, except that each collection element will be used as a Message payload. Using this approach allows developers to focus on the domain objects without having to consider the Messaging system and produces code that is easier to test.

  • a Message or non-Message object (but not a Collection or an Array) - it works like the previous cases, except that there is a single message to be sent out.

In Spring Integration, any POJO can implement the splitting algorithm, provided that it defines a method that accepts a single argument and has a return value. In this case, the return value of the method will be interpreted as described above. The input argument might either be a Message or a simple POJO. In the latter case, the splitter will receive the payload of the incoming message. Since this decouples the code from the Spring Integration API and will typically be easier to test, it is the recommended approach.

10.3 Configuring a Splitter using XML

A splitter can be configured through XML as follows:

<channel id="inputChannel"/>

<splitter id="splitter" 1
  ref="splitterBean" 2
  method="split" 3
  input-channel="inputChannel" 4
  output-channel="outputChannel" 5/>

<channel id="outputChannel"/>

<beans:bean id="splitterBean" class="sample.PojoSplitter"/>

1

The id of the splitter is optional.

2

A reference to a bean defined in the application context. The bean must implement the splitting logic as described in the section above. Optional. If reference to a bean is not provided, then it is assumed that the payload of the Message that arrived on the input-channel is an implementation of java.util.Collection and the default splitting logic will be applied on such Collection, incorporating each individual element into a Message and depositing it on the output-channel.

3

The method (defined on the bean specified above) that implements the splitting logic. Optional.

4

The input channel of the splitter. Required.

5

The channel where the splitter will send the results of splitting the incoming message. Optional (because incoming messages can specify a reply channel themselves).

Using a "ref" attribute is generally recommended if the custom splitter handler implementation can be reused in other <splitter> definitions. However if the custom splitter handler implementation should be scoped to a single definition of the <splitter>, you can configure an inner bean definition:

<splitter id="testSplitter" input-channel="inChannel" method="split"
								output-channel="outChannel">
	<beans:bean class="org.foo.TestSplitter"/>
</spliter>

[Note]Note

Using both a "ref" attribute and an inner handler definition in the same <splitter> configuration is not allowed, as it creates an ambiguous condition and will result in an Exception being thrown.

10.4 Configuring a Splitter with Annotations

The @Splitter annotation is applicable to methods that expect either the Message type or the message payload type, and the return values of the method should be a collection of any type. If the returned values are not actual Message objects, then each of them will be sent as the payload of a message. Those messages will be sent to the output channel as designated for the endpoint on which the @Splitter is defined.

@Splitter
List<LineItem> extractItems(Order order) {
    return order.getItems()
}

11. Aggregator

11.1 Introduction

Basically a mirror-image of the Splitter, the Aggregator is a type of Message Handler that receives multiple Messages and combines them into a single Message. In fact, Aggregators are often downstream consumers in a pipeline that includes a Splitter.

Technically, the Aggregator is more complex than a Splitter, because it is required to maintain state (the Messages to-be-aggregated), to decide when the complete group of Messages is available, and to timeout if necessary. Furthermore, in case of a timeout, the Aggregator needs to know whether to send the partial results or to discard them to a separate channel.

11.2 Functionality

The Aggregator combines a group of related messages, by correlating and storing them, until the group is deemed complete. At that point, the Aggregator will create a single message by processing the whole group, and will send that aggregated message as output.

As messages might arrive with a certain delay (or certain messages from the group might not arrive at all), the Aggregator can specify a timeout (counted from the moment when the first message in the group has arrived), and whether, in the case of a timeout, the group should be discarded, or the Aggregator should merely attempt to create a single message out of what has arrived so far. An important aspect of implementing an Aggregator is providing the logic that has to be executed when the aggregation (creation of a single message out of many) takes place.

In Spring Integration, the grouping of the messages for aggregation is done by default based on their CORRELATION_ID message header (i.e. the messages with the same CORRELATION_ID will be grouped together). However, this can be customized, and the users can opt for other ways of specifying how the messages should be grouped together, by using a CorrelationStrategy (see below).

An important concern with respect to the timeout is, what happens if late messages arrive after the aggregation has taken place? In this case, a configuration option allows the user to decide whether they should be discarded or not.

11.3 Programming model

The Aggregation API consists of a number of classes:

  • The base class AbstractMessageAggregator and its subclass MethodInvokingMessageAggregator

  • The CompletionStrategy interface and its default implementation SequenceSizeCompletionStrategy

  • The CorrelationStrategy interface and its default implementation HeaderAttributeCorrelationStrategy

AbstractMessageAggregator

The AbstractMessageAggregator is a MessageHandler implementation, encapsulating the common functionalities of an Aggregator, which are:

  • correlating messages into a group to be aggregated

  • maintaining those messages until the group is complete

  • deciding when the group is in fact complete

  • processing the completed group into a single aggregated message

  • recognizing and responding to a timed-out completion attempt

The responsibility of deciding how the messages should be grouped together is delegated to a CorrelationStrategy instance. The responsibility of deciding whether the message group is complete is delegated to a CompletionStrategy instance.

Here is a brief highlight of the base AbstractMessageAggregator (the responsibility of implementing the aggregateMessages method is left to the developer):

public abstract class AbstractMessageAggregator 
              extends AbstractMessageBarrierHandler {

  private volatile CompletionStrategy completionStrategy
                            = new SequenceSizeCompletionStrategy();
  ....

  protected abstract Message<?> aggregateMessages(List<Message<?>> messages);

}
It also inherits the following default CorrelationStrategy:
private volatile CorrelationStrategy correlationStrategy =
          new HeaderAttributeCorrelationStrategy(MessageHeaders.CORRELATION_ID);

When appropriate, the simplest option is the DefaultMessageAggregator. It creates a single Message whose payload is a List of the payloads received for a given group. It uses the default CorrelationStrategy and CompletionStrategy as shown above. This works well for simple Scatter Gather implementations with either a Splitter, Publish Subscribe Channel, or Recipient List Router upstream.

[Note]Note

When using a Publish Subscribe Channel or Recipient List Router in this type of scenario, be sure to enable the flag to apply sequence. That will add the necessary headers (correlation id, sequence number and sequence size). That behavior is enabled by default for Splitters in Spring Integration, but it is not enabled for the Publish Subscribe Channel or Recipient List Router because those components may be used in a variety of contexts where those headers are not necessary.

When implementing a specific aggregator object for an application, a developer can extend AbstractMessageAggregator and implement the aggregateMessages method. However, there are better suited (which reads, less coupled to the API) solutions for implementing the aggregation logic, which can be configured easily either through XML or through annotations.

In general, any ordinary Java class (i.e. POJO) can implement the aggregation algorithm. For doing so, it must provide a method that accepts as an argument a single java.util.List (parametrized lists are supported as well). This method will be invoked for aggregating messages, as follows:

  • if the argument is a parametrized java.util.List, and the parameter type is assignable to Message, then the whole list of messages accumulated for aggregation will be sent to the aggregator

  • if the argument is a non-parametrized java.util.List or the parameter type is not assignable to Message, then the method will receive the payloads of the accumulated messages

  • if the return type is not assignable to Message, then it will be treated as the payload for a Message that will be created automatically by the framework.

[Note]Note

In the interest of code simplicity, and promoting best practices such as low coupling, testability, etc., the preferred way of implementing the aggregation logic is through a POJO, and using the XML or annotation support for setting it up in the application.

CompletionStrategy

The CompletionStrategy interface is defined as follows:

public interface CompletionStrategy {

  boolean isComplete(List<Message<?>> messages);

}

In general, any ordinary Java class (i.e. POJO) can implement the completion decision mechanism. For doing so, it must provide a method that accepts as an argument a single java.util.List (parametrized lists are supported as well), and returns a boolean value. This method will be invoked after the arrival of a new message, to decide whether the group is complete or not, as follows:

  • if the argument is a parametrized java.util.List, and the parameter type is assignable to Message, then the whole list of messages accumulated in the group will be sent to the method

  • if the argument is a non-parametrized java.util.List or the parameter type is not assignable to Message, then the method will receive the payloads of the accumulated messages

  • the method must return true if the message group is complete and ready for aggregation, and false otherwise.

Spring Integration provides an out-of-the box implementation for CompletionStrategy, the SequenceSizeCompletionStrategy. This implementation uses the SEQUENCE_NUMBER and SEQUENCE_SIZE of the arriving messages for deciding when a message group is complete and ready to be aggregated. As shown above, it is also the default strategy.

CorrelationStrategy

The CorrelationStrategy interface is defined as follows:

public interface CorrelationStrategy {

  Object getCorrelationKey(Message<?> message);

}

The method shall return an Object which represents the correlation key used for grouping messages together. The key must satisfy the criteria used for a key in a Map with respect to the implementation of equals() and hashCode().

In general, any ordinary Java class (i.e. POJO) can implement the correlation decision mechanism, and the rules for mapping a message to a method's argument (or arguments) are the same as for a ServiceActivator (including support for @Header annotations). The method must return a value, and the value must not be null.

Spring Integration provides an out-of-the box implementation for CorrelationStrategy, the HeaderAttributeCorrelationStrategy. This implementation returns the value of one of the message headers (whose name is specified by a constructor argument) as the correlation key. By default, the correlation strategy is a HeaderAttributeCorrelationStrategy returning the value of the CORRELATION_ID header attribute.

11.4 Configuring an Aggregator with XML

Spring Integration supports the configuration of an aggregator via XML through the <aggregator/> element. Below you can see an example of an aggregator with all optional parameters defined.

<channel id="inputChannel"/>

<aggregator id="completelyDefinedAggregator" 1
    input-channel="inputChannel" 2
    output-channel="outputChannel"  3
    discard-channel="discardChannel"  4
    ref="aggregatorBean" 5
    method="add" 6
    completion-strategy="completionStrategyBean"  7
    completion-strategy-method="checkCompleteness" 8
    correlation-strategy="correlationStrategyBean" 9
    correlation-strategy-method="groupNumbersByLastDigit" 10
    timeout="42" 11
    send-partial-result-on-timeout="true" 12
    reaper-interval="135" 13
    tracked-correlation-id-capacity="99" 14
    send-timeout="86420000" 15 /> 

<channel id="outputChannel"/>

<bean id="aggregatorBean" class="sample.PojoAggregator"/>

<bean id="completionStrategyBean" class="sample.PojoCompletionStrategy"/>

<bean id="correlationStrategyBean" class="sample.PojoCorrelationStrategy"/>

1

The id of the aggregator is optional.

2

The input channel of the aggregator. Required.

3

The channel where the aggregator will send the aggregation results. Optional (because incoming messages can specify a reply channel themselves).

4

The channel where the aggregator will send the messages that timed out (if send-partial-results-on-timeout is false). Optional.

5

A reference to a bean defined in the application context. The bean must implement the aggregation logic as described above. Required.

6

A method defined on the bean referenced by ref, that implements the message aggregation algorithm. Optional, with restrictions (see above).

7

A reference to a bean that implements the decision algorithm as to whether a given message group is complete. The bean can be an implementation of the CompletionStrategy interface or a POJO. In the latter case the completion-strategy-method attribute must be defined as well. Optional (by default, the aggregator will use sequence size) .

8

A method defined on the bean referenced by completion-strategy, that implements the completion decision algorithm. Optional, with restrictions (requires completion-strategy to be present).

9

A reference to a bean that implements the correlation strategy. The bean can be an implementation of the CorrelationStrategy interface or a POJO. In the latter case the correlation-strategy-method attribute must be defined as well. Optional (by default, the aggregator will use the correlation id header attribute) .

10

A method defined on the bean referenced by correlation-strategy, that implements the correlation key algorithm. Optional, with restrictions (requires correlation-strategy to be present).

11

The timeout (in milliseconds) for aggregating messages (counted from the arrival of the first message). Optional.

12

Whether upon the expiration of the timeout, the aggregator shall try to aggregate the messages that have already arrived. Optional (false by default).

13

The interval (in milliseconds) at which a reaper task is executed, checking if there are any timed out groups. Optional.

14

The capacity of the correlation id tracker. Remembers the already processed correlation ids, preventing the formation of new groups for messages that arrive after their group has been already processed (aggregated or discarded). Set this value to 0 if you do not want the messages to be discarded in such a scenario. Optional.

15

The timeout for sending the aggregated messages to the output or reply channel. Optional.

Using a "ref" attribute is generally recommended if a custom aggregator handler implementation can be reused in other <aggregator> definitions. However if a custom aggregator handler implementation should be scoped to a concrete definition of the <aggregator>, you can use an inner bean definition (starting with version 1.0.3) for custom aggregator handlers within the <aggregator> element:

<aggregator input-channel="input" method="sum" output-channel="output">
    <beans:bean class="org.foo.ExampleAggregator"/>
</aggregator>

[Note]Note

Using both a "ref" attribute and an inner bean definition in the same <aggregator> configuration is not allowed, as it creates an ambiguous condition. In such cases, an Exception will be thrown.

An example implementation of the aggregator bean looks as follows:

public class PojoAggregator {

  public Long add(List<Long> results) {
    long total = 0l;
    for (long partialResult: results) {
      total += partialResult;
    }
    return total;
  }

}

An implementation of the completion strategy bean for the example above may be as follows:

public class PojoCompletionStrategy {
...
  public boolean checkCompleteness(List<Long> numbers) {
    int sum = 0;
    for (long number: numbers) {
      sum += number;
    }
    return sum >= maxValue;
  }
}

[Note]Note

Wherever it makes sense, the completion strategy method and the aggregator method can be combined in a single bean.

An implementation of the correlation strategy bean for the example above may be as follows:

public class PojoCorrelationStrategy {
...
  public Long groupNumbersByLastDigit(Long number) {
    return number % 10;
  }
}

For example, this aggregator would group numbers by some criterion (in our case the remainder after dividing by 10) and will hold the group until the sum of the numbers which represents the payload exceeds a certain value.

[Note]Note

Wherever it makes sense, the completion strategy method, correlation strategy method and the aggregator method can be combined in a single bean (all of them or any two).

11.5 Configuring an Aggregator with Annotations

An aggregator configured using annotations can look like this.

public class Waiter {
  ... 

  @Aggregator  
  public Delivery aggregatingMethod(List<OrderItem> items) {
    ...
  }

  @CompletionStrategy 
  public boolean completionChecker(List<Message<?>> messages) {
    ...
  }

  @CorrelationStrategy 
  public String correlateBy(OrderItem item) {
    ...
  }

}

1

An annotation indicating that this method shall be used as an aggregator. Must be specified if this class will be used as an aggregator.

2

An annotation indicating that this method shall be used as the completion strategy of an aggregator. If not present on any method, the aggregator will use the SequenceSizeCompletionStrategy.

3

An annotation indicating that this method shall be used as the correlation strategy of an aggregator. If no correlation strategy is indicated, the aggregator will use the HeaderAttributeCorrelationStrategy based on CORRELATION_ID.

All of the configuration options provided by the xml element are also available for the @Aggregator annotation.

The aggregator can be either referenced explicitly from XML or, if the @MessageEndpoint is defined on the class, detected automatically through classpath scanning.

12. Resequencer

12.1 Introduction

Related to the Aggregator, albeit different from a functional standpoint, is the Resequencer.

12.2 Functionality

The Resequencer works in a similar way to the Aggregator, in the sense that it uses the CORRELATION_ID to store messages in groups, the difference being that the Resequencer does not process the messages in any way. It simply releases them in the order of their SEQUENCE_NUMBER header values.

With respect to that, the user might opt to release all messages at once (after the whole sequence, according to the SEQUENCE_SIZE, has been released), or as soon as a valid sequence is available. Another option is to set a timeout, deciding whether to drop the whole sequence if the timeout has expired, and not all messages have arrived, or to release the messages accumulated so far, in the appropriate order.

12.3 Configuring a Resequencer with XML

Configuring a resequencer requires only including the appropriate element in XML.

A sample resequencer configuration is shown below.

<channel id="inputChannel"/>

<channel id="outputChannel"/>

<resequencer id="completelyDefinedResequencer" 
  input-channel="inputChannel" 
  output-channel="outputChannel" 
  discard-channel="discardChannel" 
  release-partial-sequences="true" 
  timeout="42" 
  send-partial-result-on-timeout="true" 
  reaper-interval="135" 
  tracked-correlation-id-capacity="99" 
  send-timeout="86420000"  /> 

1

The id of the resequencer is optional.

2

The input channel of the resequencer. Required.

3

The channel where the resequencer will send the reordered messages. Optional.

4

The channel where the resequencer will send the messages that timed out (if send-partial-result-on-timeout is false). Optional.

5

Whether to send out ordered sequences as soon as they are available, or only after the whole message group arrives. Optional (true by default).

6

The timeout (in milliseconds) for reordering message sequences (counted from the arrival of the first message). Optional.

7

Whether, upon the expiration of the timeout, the ordered group shall be sent out (even if some of the messages are missing). Optional (false by default).

8

The interval (in milliseconds) at which a reaper task is executed, checking if there are any timed out groups. Optional.

9

The capacity of the correlation id tracker. Remembers the already processed correlation ids, preventing the formation of new groups for messages that arrive after their group has been already processed (reordered or discarded). Optional.

10

The timeout for sending out messages. Optional.

[Note]Note
Since there is no custom behavior to be implemented in Java classes for resequencers, there is no annotation support for it.

13. Delayer

13.1 Introduction

A Delayer is a simple endpoint that allows a Message flow to be delayed by a certain interval. When a Message is delayed, the original sender will not block. Instead, the delayed Messages will be scheduled with an instance of java.util.concurrent.ScheduledExecutorService to be sent to the output channel after the delay has passed. This approach is scalable even for rather long delays, since it does not result in a large number of blocked sender Threads. On the contrary, in the typical case a thread pool will be used for the actual execution of releasing the Messages. Below you will find several examples of configuring a Delayer.

13.2 The <delayer> Element

The <delayer> element is used to delay the Message flow between two Message Channels. As with the other endpoints, you can provide the "input-channel" and "output-channel" attributes, but the delayer also requires at least the 'default-delay' attribute with the number of milliseconds that each Message should be delayed.

 <delayer input-channel="input" default-delay="3000" output-channel="output"/>

If you need per-Message determination of the delay, then you can also provide the name of a header within the 'delay-header-name' attribute:

 <delayer input-channel="input" output-channel="output"
          default-delay="3000" delay-header-name="delay"/>

In the example above the 3 second delay would only apply in the case that the header value is not present for a given inbound Message. If you only want to apply a delay to Messages that have an explicit header value, then you can set the 'default-delay' to 0. For any Message that has a delay of 0 (or less), the Message will be sent directly. In fact, if there is not a positive delay value for a Message, it will be sent to the output channel on the calling Thread.

[Tip]Tip
The delay handler actually supports header values that represent an interval in milliseconds (any Object whose toString() method produces a value that can be parsed into a Long) as well as java.util.Date instances representing an absolute time. In the former case, the milliseconds will be counted from the current time (e.g. a value of 5000 would delay the Message for at least 5 seconds from the time it is received by the Delayer). In the latter case, with an actual Date instance, the Message will not be released until that Date occurs. In either case, a value that equates to a non-positive delay, or a Date in the past, will not result in any delay. Instead, it will be sent directly to the output channel in the original sender's Thread.

The default scheduler will have a thread pool of size 1. If you want to configure and provide a different scheduler, you can provide the reference through the 'scheduler' attribute:

 <delayer input-channel="input" output-channel="output"
          default-delay="0" delay-header-name="delay"
          scheduler="someScheduledExecutorService"/>

14. Message Handler Chain

14.1 Introduction

The MessageHandlerChain is an implementation of MessageHandler that can be configured as a single Message Endpoint while actually delegating to a chain of other handlers, such as Filters, Transformers, Splitters, and so on. This can lead to a much simpler configuration when several handlers need to be connected in a fixed, linear progression. For example, it is fairly common to provide a Transformer before other components. Similarly, when providing a Filter before some other component in a chain, you are essentially creating a Selective Consumer. In either case, the chain only requires a single input-channel and a single output-channel as opposed to the configuration of channels for each individual component.

[Tip]Tip
Spring Integration's Filter provides a boolean property 'throwExceptionOnRejection'. When providing multiple Selective Consumers on the same point-to-point channel with different acceptance criteria, this value should be set to 'true' (the default is false) so that the dispatcher will know that the Message was rejected and as a result will attempt to pass the Message on to other subscribers. If the Exception were not thrown, then it would appear to the dispatcher as if the Message had been passed on successfully even though the Filter had dropped the Message to prevent further processing.

The handler chain simplifies configuration while internally maintaining the same degree of loose coupling between components, and it is trivial to modify the configuration if at some point a non-linear arrangement is required.

Internally, the chain will be expanded into a linear setup of the listed endpoints, separated by direct channels. The reply channel header will not be taken into account within the chain: only after the last handler is invoked will the resulting message be forwarded on to the reply channel or the chain's output channel. Because of this setup all handlers except the last require a setOutputChannel implementation. The last handler only needs an output channel if the outputChannel on the MessageHandlerChain is set.

[Note]Note

As with other endpoints, the output-channel is optional. If there is a reply Message at the end of the chain, the output-channel takes precedence, but if not available, the chain handler will check for a reply channel header on the inbound Message.

In most cases there is no need to implement MessageHandlers yourself. The next section will focus on namespace support for the chain element. Most Spring Integration endpoints, like Service Activators and Transformers, are suitable for use within a MessageHandlerChain.

14.2 The <chain> Element

The <chain> element provides an 'input-channel' attribute, and if the last element in the chain is capable of producing reply messages (optional), it also supports an 'output-channel' attribute. The sub-elements are then filters, transformers, splitters, and service-activators. The last element may also be a router.

 <chain input-channel="input" output-channel="output">
     <filter ref="someSelector" throw-exception-on-rejection="true"/>
     <header-enricher error-channel="customErrorChannel">
         <header name="foo" value="bar"/>
     </header-enricher>
     <service-activator ref="someService" method="someMethod"/>
 </chain>

The <header-enricher> element used in the above example will set a message header with name "foo" and value "bar" on the message. A header enricher is a specialization of Transformer that touches only header values. You could obtain the same result by implementing a MessageHandler that did the header modifications and wiring that as a bean.

15. Messaging Bridge

15.1 Introduction

A Messaging Bridge is a relatively trivial endpoint that simply connects two Message Channels or Channel Adapters. For example, you may want to connect a PollableChannel to a SubscribableChannel so that the subscribing endpoints do not have to worry about any polling configuration. Instead, the Messaging Bridge provides the polling configuration.

By providing an intermediary poller between two channels, a Messaging Bridge can be used to throttle inbound Messages. The poller's trigger will determine the rate at which messages arrive on the second channel, and the poller's "maxMessagesPerPoll" property will enforce a limit on the throughput.

Another valid use for a Messaging Bridge is to connect two different systems. In such a scenario, Spring Integration's role would be limited to making the connection between these systems and managing a poller if necessary. It is probably more common to have at least a Transformer between the two systems to translate between their formats, and in that case, the channels would be provided as the 'input-channel' and 'output-channel' of a Transformer endpoint. If data format translation is not required, the Messaging Bridge may indeed be sufficient.

15.2 The <bridge> Element

The <bridge> element is used to create a Messaging Bridge between two Message Channels or Channel Adapters. Simply provide the "input-channel" and "output-channel" attributes:

 <bridge input-channel="input" output-channel="output"/>

As mentioned above, a common use case for the Messaging Bridge is to connect a PollableChannel to a SubscribableChannel, and when performing this role, the Messaging Bridge may also serve as a throttler:

 <bridge input-channel="pollable" output-channel="subscribable">
     <poller max-messages-per-poll="10">
         <interval-trigger interval="5" time-unit="SECONDS"/>
     </poller>
 </bridge>

Connecting Channel Adapters is just as easy. Here is a simple echo example between the "stdin" and "stdout" adapters from Spring Integration's "stream" namespace.

 <stream:stdin-channel-adapter id="stdin"/>

 <stream:stdout-channel-adapter id="stdout"/>

 <bridge id="echo" input-channel="stdin" output-channel="stdout"/>

Of course, the configuration would be similar for other (potentially more useful) Channel Adapter bridges, such as File to JMS, or Mail to File. The various Channel Adapters will be discussed in upcoming chapters.

[Note]Note

If no 'output-channel' is defined on a bridge, the reply channel provided by the inbound Message will be used, if available. If neither output or reply channel is available, an Exception will be thrown.

16. Inbound Messaging Gateways

16.1 SimpleMessagingGateway

Even though the MessageChannelTemplate is fairly straightforward, it does not hide the details of messaging from your application code. To support working with plain Objects instead of messages, Spring Integration provides SimpleMessagingGateway with the following methods:

 public void send(Object object) { ... }

 public Object receive() { ... }

 public Object sendAndReceive(Object object) { ... }

 Message<?> sendAndReceiveMessage(Object object);

It enables configuration of a request and/or reply channel and delegates to instances of the InboundMessageMapper and OutboundMessageMapper strategy interfaces.

 SimpleMessagingGateway gateway = new SimpleMessagingGateway(inboundMapper, outboundMapper);
 gateway.setRequestChannel(requestChannel);
 gateway.setReplyChannel(replyChannel);
 Object result = gateway.sendAndReceive("test");

16.2 GatewayProxyFactoryBean

Working with Objects instead of Messages is an improvement. However, it would be even better to have no dependency on the Spring Integration API at all - including the gateway class. For that reason, Spring Integration also provides a GatewayProxyFactoryBean that generates a proxy for any interface and internally invokes the gateway methods shown above. Namespace support is also provided as demonstrated by the following example.

<gateway id="fooService"
         service-interface="org.example.FooService"
         default-request-channel="requestChannel"
         default-reply-channel="replyChannel"/>

Then, the "fooService" can be injected into other beans, and the code that invokes the methods on that proxied instance of the FooService interface has no awareness of the Spring Integration API. The general approach is similar to that of Spring Remoting (RMI, HttpInvoker, etc.). See the "Samples" Appendix for an example that uses this "gateway" element (in the Cafe demo).

The reason that the attributes on the 'gateway' element are named 'default-request-channel' and 'default-reply-channel' is that you may also provide per-method channel references by using the @Gateway annotation.

 public interface Cafe {

     @Gateway(requestChannel="orders")
     void placeOrder(Order order);

 }

It is also possible to pass values to be interpreted as Message headers on the Message that is created and sent to the request channel by using the @Header annotation:

 public interface FileWriter {

     @Gateway(requestChannel="filesOut")
     void write(byte[] content, @Header(FileHeaders.FILENAME) String filename);

 }

17. File Support

17.1 Introduction

Spring Integration's File support extends the Spring Integration Core with a dedicated vocabulary to deal with reading, writing, and transforming files. It provides a namespace that enables elements defining Channel Adapters dedicated to files and support for Transformers that can read file contents into strings or byte arrays.

This section will explain the workings of FileReadingMessageSource and FileWritingMessageHandler and how to configure them as beans. Also the support for dealing with files through file specific implementations of Transformer will be discussed. Finally the file specific namespace will be explained.

17.2 Reading Files

A FileReadingMessageSource can be used to consume files from the filesystem. This is an implementation of MessageSource that creates messages from a file system directory.

<bean id="pollableFileSource"
      class="org.springframework.integration.file.FileReadingMessageSource"
        p:inputDirectory="file:${input.directory}"/>

To prevent creating messages for certain files, you may supply a FileListFilter. By default, an AcceptOnceFileListFilter is used. This filter ensures files are picked up only once from the directory.

<bean id="pollableFileSource"
      class="org.springframework.integration.file.FileReadingMessageSource"
        p:inputDirectory="file:${input.directory}"
        p:filter-ref="customFilterBean"/>

A common problem with reading files is that a file may be detected before it is ready. The default AcceptOnceFileListFilter does not prevent this. In most cases, this can be prevented if the file-writing process renames each file as soon as it is ready for reading. A pattern-matching filter that accepts only files that are ready (e.g. based on a known suffix), composed with the default AcceptOnceFileListFilter allows for this. The CompositeFileListFilter enables the composition.

<bean id="pollableFileSource"
      class="org.springframework.integration.file.FileReadingMessageSource"
        p:inputDirectory="file:${input.directory}"
        p:filter-ref="compositeFilter"/>
<bean id="compositeFilter" class="org.springframework.integration.file.CompositeFileListFilter">
    <constructor-arg>
        <list>
            <bean class="org.springframework.integration.file.AcceptOnceFileListFilter" />
            <bean class="org.springframework.integration.file.PatternMatchingFileListFilter">
                <constructor-arg value="^test.*$"/>
            </bean>
        </list>
    </constructor-arg>
</bean>

The configuration can be simplified using the file specific namespace. To do this use the following template.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:integration="http://www.springframework.org/schema/integration"
	xmlns:file="http://www.springframework.org/schema/integration/file"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
			http://www.springframework.org/schema/integration
			http://www.springframework.org/schema/integration/spring-integration-1.0.xsd
			http://www.springframework.org/schema/integration/file
			http://www.springframework.org/schema/integration/file/spring-integration-file-1.0.xsd">
</beans>

Within this namespace you can reduce the FileReadingMessageSource and wrap it in an inbound Channel Adapter like this:

 <file:inbound-channel-adapter id="filesIn"
       directory="file:${input.directory}"/>
 					
 <file:inbound-channel-adapter id="filesIn"
        directory="file:${input.directory}" 
        filter="customFilterBean" />							

 <file:inbound-channel-adapter id="filesIn"
        directory="file:${input.directory}" 
        filename-pattern="^test.*$" /> 							

The first channel adapter is relying on the default filter that just prevents duplication, the second is using a custom filter, and the third is using the filename-pattern attribute to add a Pattern based filter to the FileReadingMessageSource.

17.3 Writing files

To write messages to the file system you can use a FileWritingMessageHandler. This class can deal with File, String, or byte array payloads. In its simplest form the FileWritingMessageHandler only requires a destination directory for writing the files. The name of the file to be written is determined by the handler's FileNameGenerator. The default implementation looks for a Message header whose key matches the constant defined as FileHeaders.FILENAME.

Additionally, you can configure the encoding and the charset that will be used in case of a String payload.

To make things easier you can configure the FileWritingMessageHandler as part of an outbound channel adapter using the namespace.

 <file:outbound-channel-adapter id="filesOut" directory="file:${input.directory.property}"/>

The namespace based configuration also supports a delete-source-files attribute. If set to true, it will trigger deletion of the original source files after writing to a destination. The default value for that flag is false.

 <file:outbound-channel-adapter id="filesOut"
             directory="file:${output.directory}" 
             delete-source-files="true"/>

[Note]Note

The delete-source-files attribute will only have an effect if the inbound Message has a File payload or if the FileHeaders.ORIGINAL_FILE header value contains either the source File instance or a String representing the original file path.

In cases where you want to continue processing messages based on the written File you can use the outbound-gateway instead. It plays a very similar role as the outbound-channel-adapter. However after writing the File, it will also send it to the reply channel as the payload of a Message.

 <file:outbound-gateway id="mover" request-channel="moveInput" 
            reply-channel="output"
            directory="${output.directory}" 
            delete-source-files="true"/>

[Note]Note
The 'outbound-gateway' works well in cases where you want to first move a File and then send it through a processing pipeline. In such cases, you may connect the file namespace's 'inbound-channel-adapter' element to the 'outbound-gateway' and then connect that gateway's reply-channel to the beginning of the pipeline.

If you have more elaborate requirements or need to support additional payload types as input to be converted to file content you could extend the FileWritingMessageHandler, but a much better option is to rely on a Transformer.

17.4 File Transformers

To transform data read from the file system to objects and the other way around you need to do some work. Contrary to FileReadingMessageSource and to a lesser extent FileWritingMessageHandler, it is very likely that you will need your own mechanism to get the job done. For this you can implement the Transformer interface. Or extend the AbstractFilePayloadTransformer for inbound messages. Some obvious implementations have been provided.

FileToByteArrayTransformer transforms Files into byte[]s using Spring's FileCopyUtils. It is often better to use a sequence of transformers than to put all transformations in a single class. In that case the File to byte[] conversion might be a logical first step.

FileToStringTransformer will convert Files to Strings as the name suggests. If nothing else, this can be useful for debugging (consider using with a Wire Tap).

To configure File specific transformers you can use the appropriate elements from the file namespace.

 <file-to-bytes-transformer input-channel="input" output-channel="output"
             delete-files="true"/>

 <file:file-to-string-transformer input-channel="input" output-channel="output
             delete-files="true" charset="UTF-8"/>

The delete-files option signals to the transformer that it should delete the inbound File after the transformation is complete. This is in no way a replacement for using the AcceptOnceFileListFilter when the FileReadingMessageSource is being used in a multi-threaded environment (e.g. Spring Integration in general).

18. JMS Support

Spring Integration provides Channel Adapters for receiving and sending JMS messages. There are actually two JMS-based inbound Channel Adapters. The first uses Spring's JmsTemplate to receive based on a polling period. The second is "message-driven" and relies upon a Spring MessageListener container. There is also an outbound Channel Adapter which uses the JmsTemplate to convert and send a JMS Message on demand.

Whereas the JMS Channel Adapters are intended for unidirectional Messaging (send-only or receive-only), Spring Integration also provides inbound and outbound JMS Gateways for request/reply operations. The inbound gateway relies on one of Spring's MessageListener container implementations for Message-driven reception that is also capable of sending a return value to the "reply-to" Destination as provided by the received Message. The outbound Gateway sends a JMS Message to a "request-destination" and then receives a reply Message. The "reply-destination" reference (or "reply-destination-name") can be configured explicitly or else the outbound gateway will use a JMS TemporaryQueue.

18.1 Inbound Channel Adapter

The inbound Channel Adapter requires a reference to either a single JmsTemplate instance or both ConnectionFactory and Destination (a 'destinationName' can be provided in place of the 'destination' reference). The following example defines an inbound Channel Adapter with a Destination reference.

 <jms:inbound-channel-adapter id="jmsIn" destination="inQueue" channel="exampleChannel">
     <integration:poller>
         <integration:interval-trigger interval="30" time-unit="SECONDS"/>
     </integration:poller>
 </jms:inbound-channel-adapter>

[Tip]Tip
Notice from the configuration that the inbound-channel-adapter is a Polling Consumer. That means that it invokes receive() when triggered. This should only be used in situations where polling is done relatively infrequently and timeliness is not important. For all other situations (a vast majority of JMS-based use-cases), the message-driven-channel-adapter described below is a better option.

[Note]Note
All of the JMS adapters that require a reference to the ConnectionFactory will automatically look for a bean named "connectionFactory" by default. That is why you don't see a "connection-factory" attribute in many of the examples. However, if your JMS ConnectionFactory has a different bean name, then you will need to provide that attribute.

If 'extract-payload' is set to true (which is the default), the received JMS Message will be passed through the MessageConverter. When relying on the default SimpleMessageConverter, this means that the resulting Spring Integration Message will have the JMS Message's body as its payload. A JMS TextMessage will produce a String-based payload, a JMS BytesMessage will produce a byte array payload, and a JMS ObjectMessage's Serializable instance will become the Spring Integration Message's payload. If instead you prefer to have the raw JMS Message as the Spring Integration Message's payload, then set 'extract-payload' to false.

 <jms:inbound-channel-adapter id="jmsIn"
                              destination="inQueue"
                              channel="exampleChannel"
                              extract-payload="false"/>
     <integration:poller>
         <integration:interval-trigger interval="30" time-unit="SECONDS"/>
     </integration:poller>
 </jms:inbound-channel-adapter>

18.2 Message-Driven Channel Adapter

The "message-driven-channel-adapter" requires a reference to either an instance of a Spring MessageListener container (any subclass of AbstractMessageListenerContainer) or both ConnectionFactory and Destination (a 'destinationName' can be provided in place of the 'destination' reference). The following example defines a message-driven Channel Adapter with a Destination reference.

 <jms:message-driven-channel-adapter id="jmsIn" destination="inQueue" channel="exampleChannel"/>

[Note]Note
The Message-Driven adapter also accepts several properties that pertain to the MessageListener container. These values are only considered if you do not provide an actual 'container' reference. In that case, an instance of DefaultMessageListenerContainer will be created and configured based on these properties. For example, you can specify the "transaction-manager" reference, the "concurrent-consumers" value, and several other property references and values. Refer to the JavaDoc and Spring Integration's JMS Schema (spring-integration-jms-1.0.xsd) for more detail.

The 'extract-payload' property has the same effect as described above, and once again its default value is 'true'. The poller sub-element is not applicable for a message-driven Channel Adapter, as it will be actively invoked. For most usage scenarios, the message-driven approach is better since the Messages will be passed along to the MessageChannel as soon as they are received from the underlying JMS consumer.

18.3 Outbound Channel Adapter

The JmsSendingMessageHandler implements the MessageHandler interface and is capable of converting Spring Integration Messages to JMS messages and then sending to a JMS destination. It requires either a 'jmsTemplate' reference or both 'connectionFactory' and 'destination' references (again, the 'destinationName' may be provided in place of the 'destination'). As with the inbound Channel Adapter, the easiest way to configure this adapter is with the namespace support. The following configuration will produce an adapter that receives Spring Integration Messages from the "exampleChannel" and then converts those into JMS Messages and sends them to the JMS Destination reference whose bean name is "outQueue".

<jms:outbound-channel-adapter id="jmsOut" destination="outQueue" channel="exampleChannel"/>

As with the inbound Channel Adapters, there is an 'extract-payload' property. However, the meaning is reversed for the outbound adapter. Rather than applying to the JMS Message, the boolean property applies to the Spring Integration Message payload. In other words, the decision is whether to pass the Spring Integration Message itself as the JMS Message body or whether to pass the Spring Integration Message's payload as the JMS Message body. The default value is once again 'true'. Therefore, if you pass a Spring Integration Message whose payload is a String, a JMS TextMessage will be created. If on the other hand you want to send the actual Spring Integration Message to another system via JMS, then simply set this to 'false'.

[Note]Note
Regardless of the boolean value for payload extraction, the Spring Integration MessageHeaders will map to JMS properties as long as you are relying on the default converter or provide a reference to another instance of HeaderMappingMessageConverter (the same holds true for 'inbound' adapters except that in those cases, it's the JMS properties mapping to Spring Integration MessageHeaders).

18.4 Inbound Gateway

Spring Integration's message-driven JMS inbound-gateway delegates to a MessageListener container, supports dynamically adjusting concurrent consumers, and can also handle replies. The inbound gateway requires references to a ConnectionFactory, and a request Destination (or 'requestDestinationName'). The following example defines a JMS "inbound-gateway" that receives from the JMS queue referenced by the bean id "inQueue" and sends to the Spring Integration channel named "exampleChannel".

 <jms:inbound-gateway id="jmsInGateway"
                      request-destination="inQueue"
                      request-channel="exampleChannel"/>

Since the gateways provide request/reply behavior instead of unidirectional send or receive, they also have two distinct properties for the "payload extraction" (as discussed above for the Channel Adapters' 'extract-payload' setting). For an inbound-gateway, the 'extract-request-payload' property determines whether the received JMS Message body will be extracted. If 'false', the JMS Message itself will become the Spring Integration Message payload. The default is 'true'.

Similarly, for an inbound-gateway the 'extract-reply-payload' property applies to the Spring Integration Message that is going to be converted into a reply JMS Message. If you want to pass the whole Spring Integration Message (as the body of a JMS ObjectMessage) then set this to 'false'. By default, it is also 'true' such that the Spring Integration Message payload will be converted into a JMS Message (e.g. String payload becomes a JMS TextMessage).

18.5 Outbound Gateway

The outbound Gateway creates JMS Messages from Spring Integration Messages and then sends to a 'request-destination'. It will then handle the JMS reply Message either by using a selector to receive from the 'reply-destination' that you configure, or if no 'reply-destination' is provided, it will create JMS TemporaryQueues. Notice that the "reply-channel" is also provided.

 <jms:outbound-gateway id="jmsOutGateway"
                       request-destination="outQueue"
                       request-channel="outboundJmsRequests"
                       reply-channel="jmsReplies"/>

The 'outbound-gateway' payload extraction properties are inversely related to those of the 'inbound-gateway' (see the discussion above). That means that the 'extract-request-payload' property value applies to the Spring Integration Message that is being converted into a JMS Message to be sent as a request, and the 'extract-reply-payload' property value applies to the JMS Message that is received as a reply and then converted into a Spring Integration Message to be subsequently sent to the 'reply-channel' as shown in the example configuration above.

[Note]Note
For all of these JMS adapters, you can also specify your own "message-converter" reference. Simply provide the bean name of an instance of MessageConverter that is available within the same ApplicationContext. Note, however, that when you provide your own MessageConverter instance, it will still be wrapped within the HeaderMappingMessageConverter. This means that the 'extract-request-payload' and 'extract-reply-payload' properties may effect what actual objects are passed to your converter. The HeaderMappingMessageConverter itself simply delegates to a target MessageConverter while also mapping the Spring Integration MessageHeaders to JMS Message properties and vice-versa.

18.6 JMS Samples

To experiment with these JMS adapters, check out the samples available within the "samples/jms" directory in the distribution. There are two samples included. One provides inbound and outbound Channel Adapters, and the other provides inbound and outbound Gateways. They are configured to run with an embedded ActiveMQ process, but the "common.xml" file can easily be modified to support either a different JMS provider or a standalone ActiveMQ process. In other words, you can split the configuration so that the inbound and outbound adapters are running in separate JVMs. If you have ActiveMQ installed, simply modify the "brokerURL" property within the configuration to use "tcp://localhost:61616" for example (instead of "vm://localhost"). Both of the samples accept input via stdin and then echo back to stdout. Look at the configuration to see how these messages are routed over JMS.

19. Web Services Support

19.1 Outbound Web Service Gateways

To invoke a Web Service upon sending a message to a channel, there are two options - both of which build upon the Spring Web Services project: SimpleWebServiceOutboundGateway and MarshallingWebServiceOutboundGateway. The former will accept either a String or javax.xml.transform.Source as the message payload. The latter provides support for any implementation of the Marshaller and Unmarshaller interfaces. Both require a Spring Web Services DestinationProvider for determining the URI of the Web Service to be called.

 simpleGateway = new SimpleWebServiceOutboundGateway(destinationProvider);

 marshallingGateway = new MarshallingWebServiceOutboundGateway(destinationProvider, marshaller);

[Note]Note
When using the namespace support described below, you will only need to set a URI. Internally, the parser will configure a fixed URI DestinationProvider implementation. If you do need dynamic resolution of the URI at runtime, however, then the DestinationProvider can provide such behavior as looking up the URI from a registry. See the Spring Web Services javadoc for more information about the DestinationProvider strategy.

For more detail on the inner workings, see the Spring Web Services reference guide's chapter covering client access as well as the chapter covering Object/XML mapping.

19.2 Inbound Web Service Gateways

To send a message to a channel upon receiving a Web Service invocation, there are two options again: SimpleWebServiceInboundGateway and MarshallingWebServiceInboundGateway. The former will extract a javax.xml.transform.Source from the WebServiceMessage and set it as the message payload. The latter provides support for implementation of the Marshaller and Unmarshaller interfaces. If the incoming web service message is a SOAP message the SOAP Action header will be added to the headers of the Message that is forwarded onto the request channel.

 simpleGateway = new SimpleWebServiceInboundGateway();
 simpleGateway.setRequestChannel(forwardOntoThisChannel);
 simpleGateway.setReplyChannel(listenForResponseHere); //Optional

 marshallingGateway = new MarshallingWebServiceInboundGateway(marshaller);
 //set request and optionally reply channel

Both gateways implement the Spring Web Services MessageEndpoint interface, so they can be configured with a MessageDispatcherServlet as per standard Spring Web Services configuration.

For more detail on how to use these components, see the Spring Web Services reference guide's chapter covering creating a Web Service. The chapter covering Object/XML mapping is also applicable again.

19.3 Web Service Namespace Support

To configure an outbound Web Service Gateway, use the "outbound-gateway" element from the "ws" namespace:

<ws:outbound-gateway id="simpleGateway"
                     request-channel="inputChannel"
                     uri="http://example.org"/>

[Note]Note
Notice that this example does not provide a 'reply-channel'. If the Web Service were to return a non-empty response, the Message containing that response would be sent to the reply channel provided in the request Message's REPLY_CHANNEL header, and if that were not available a channel resolution Exception would be thrown. If you want to send the reply to another channel instead, then provide a 'reply-channel' attribute on the 'outbound-gateway' element.

[Tip]Tip
When invoking a Web Service that returns an empty response after using a String payload for the request Message, no reply Message will be sent by default. Therefore you don't need to set a 'reply-channel' or have a REPLY_CHANNEL header in the request Message. If for any reason you actually do want to receive the empty response as a Message, then provide the 'ignore-empty-responses' attribute with a value of false (this only applies for Strings, because using a Source or Document object simply leads to a NULL response and will therefore never generate a reply Message).

To set up an inbound Web Service Gateway, use the "inbound-gateway":

<ws:inbound-gateway id="simpleGateway"
                    request-channel="inputChannel"/>

To use Spring OXM Marshallers and/or Unmarshallers, provide bean references. For outbound:

<ws:outbound-gateway id="marshallingGateway"
                     request-channel="requestChannel"
                     uri="http://example.org"
                     marshaller="someMarshaller"
                     unmarshaller="someUnmarshaller"/>

And for inbound:

<ws:inbound-gateway id="marshallingGateway"
                    request-channel="requestChannel"
                    marshaller="someMarshaller"
                    unmarshaller="someUnmarshaller"/>

[Note]Note
Most Marshaller implementations also implement the Unmarshaller interface. When using such a Marshaller, only the "marshaller" attribute is necessary. Even when using a Marshaller, you may also provide a reference for the "request-callback" on the outbound gateways.

For either outbound gateway type, a "destination-provider" attribute can be specified instead of the "uri" (exactly one of them is required). You can then reference any Spring Web Services DestinationProvider implementation (e.g. to lookup the URI at runtime from a registry).

For either outbound gateway type, the "message-factory" attribute can also be configured with a reference to any Spring Web Services WebServiceMessageFactory implementation.

For the simple inbound gateway type, the "extract-payload" attribute can be set to false to forward the entire WebServiceMessage instead of just its payload as a Message to the request channel. This might be useful, for example, when a custom Transformer works against the WebServiceMessage directly.

20. RMI Support

20.1 Introduction

This Chapter explains how to use RMI specific channel adapters to distribute a system over multiple JVMs. The first section will deal with sending messages over RMI. The second section shows how to receive messages over RMI. The last section shows how to define rmi channel adapters through the namespace support.

20.2 Outbound RMI

To send messages from a channel over RMI, simply define an RmiOutboundGateway. This gateway will use Spring's RmiProxyFactoryBean internally to create a proxy for a remote gateway. Note that to invoke a remote interface that doesn't use Spring Integration you should use a service activator in combination with Spring's RmiProxyFactoryBean.

To configure the outbound gateway write a bean definition like this:

	<bean id="rmiOutGateway" class=org.spf.integration.rmi.RmiOutboundGateway>
		<constructor-arg value="rmi://host"/>
		<property name="replyChannel" value="replies"/>
	</bean>
	

20.3 Inbound RMI

To receive messages over RMI you need to use a RmiInboundGateway. This gateway can be configured like this

 <bean id="rmiOutGateway" class=org.spf.integration.rmi.RmiInboundGateway>
     <property name="requestChannel" value="requests"/>
 </bean>
		

20.4 RMI namespace support

To configure the inbound gateway you can choose to use the namespace support for it. The following code snippet shows the different configuration options that are supported.

 <rmi:inbound-gateway id="gatewayWithDefaults" request-channel="testChannel"/>

 <rmi:inbound-gateway id="gatewayWithCustomProperties" request-channel="testChannel"
                      expect-reply="false" request-timeout="123" reply-timeout="456"/>

 <rmi:inbound-gateway id="gatewayWithHost" request-channel="testChannel"
                      registry-host="localhost"/>

 <rmi:inbound-gateway id="gatewayWithPort" request-channel="testChannel"
                      registry-port="1234"/>

 <rmi:inbound-gateway id="gatewayWithExecutorRef" request-channel="testChannel"
                      remote-invocation-executor="invocationExecutor"/>

To configure the outbound gateway you can use the namespace support as well. The following code snippet shows the different configuration for an outbound rmi gateway.

 <rmi:outbound-gateway id="gateway"
                       request-channel="localChannel"
                       remote-channel="testChannel"
                       host="localhost"/>

21. HttpInvoker Support

21.1 Introduction

HttpInvoker is a Spring-specific remoting option that essentially enables Remote Procedure Calls (RPC) over HTTP. In order to accomplish this, an outbound representation of a method invocation is serialized using standard Java serialization and then passed within an HTTP POST request. After being invoked on the target system, the method's return value is then serialized and written to the HTTP response. There are two main requirements. First, you must be using Spring on both sides since the marshalling to and from HTTP requests and responses is handled by the client-side invoker and server-side exporter. Second, the Objects that you are passing must implement Serializable and be available on both the client and server.

While traditional RPC provides physical decoupling, it does not offer nearly the same degree of logical decoupling as a messaging-based system. In other words, both participants in an RPC-based invocation must be aware of a specific interface and specific argument types. Interestingly, in Spring Integration, the "parameter" being sent is a Spring Integration Message, and the interface is an internal detail of Spring Integration's implementation. Therefore, the RPC mechanism is being used as a transport so that from the end user's perspective, it is not necessary to consider the interface and argument types. It's just another adapter to enable messaging between two systems.

21.2 HttpInvoker Inbound Gateway

To receive messages over http you can use an HttpInvokerInboundGateway. Here is an example bean definition:

<bean id="inboundGateway"
      class="org.springframework.integration.httpinvoker.HttpInvokerInboundGateway">
    <property name="requestChannel" ref="requestChannel"/>
    <property name="replyChannel" ref="replyChannel"/>
    <property name="requestTimeout" value="30000"/>
    <property name="replyTimeout" value="10000"/>
</bean>

Because the inbound gateway must be able to receive HTTP requests, it must be configured within a Servlet container. The easiest way to do this is to provide a servlet definition in web.xml:

<servlet>
    <servlet-name>inboundGateway</servlet-name>
    <servlet-class>org.springframework.web.context.support.HttpRequestHandlerServlet</servlet-class>
</servlet>

Notice that the servlet name matches the bean name.

[Note]Note
If you are running within a Spring MVC application and using the BeanNameHandlerMapping, then the servlet definition is not necessary. In that case, the bean name for your gateway can be matched against the URL path just like a Spring MVC Controller bean.

21.3 HttpInvoker Outbound Gateway

To configure the HttpInvokerOutboundGateway write a bean definition like this:

<bean id="outboundGateway"
      class="org.springframework.integration.httpinvoker.HttpInvokerOutboundGateway">
    <property name="replyChannel" ref="replyChannel"/>
</bean>

The outbound gateway is a MessageHandler and can therefore be registered with either a PollingConsumer or EventDrivenConsumer. The URL must match that defined by an inbound HttpInvoker Gateway as described in the previous section.

21.4 HttpInvoker Namespace Support

Spring Integration provides an "httpinvoker" namespace and schema definition. To include it in your configuration, simply provide the following URI within a namespace declaration: 'http://www.springframework.org/schema/integration/httpinvoker'. The schema location should then map to 'http://www.springframework.org/schema/integration/httpinvoker/spring-integration-httpinvoker-1.0.xsd'.

To configure the inbound gateway you can choose to use the namespace support for it. The following code snippet shows the different configuration options that are supported.

<httpinvoker:inbound-gateway id="inboundGateway"
                             request-channel="requestChannel"
                             request-timeout="10000" 
                             expect-reply="false"
                             reply-timeout="30000"/>

[Note]Note
A 'reply-channel' may also be provided, but it is recommended to rely on the temporary anonymous channel that will be created automatically for handling replies.

To configure the outbound gateway you can use the namespace support as well. The following code snippet shows the different configuration for an outbound HttpInvoker gateway. Only the 'url' and 'request-channel' are required.

<httpinvoker:outbound-gateway id="outboundGateway"
                              url="http://localhost:8080/example"
                              request-channel="requestChannel"
                              request-timeout="5000"
                              reply-channel="replyChannel"
                              reply-timeout="10000"/>

22. HTTP Support

22.1 Introduction

The HTTP support allows for the making of HTTP requests and the processing of inbound Http requests. Because interaction over HTTP is always synchronous, even if all that is returned is a 200 status code the Http support consists of two gateway implementations HttpInboundEndpoint and HttpOutboundEndpoint.

22.2 Http Inbound Gateway

To receive messages over http you need to use an HttpInboundEndpoint. In common with the HttpInvoker support the Http Inbound Gateway needs to be deployed within a servlet container. The easiest way to do this is to provide a servlet definition in web.xml, see Section 21.2, “HttpInvoker Inbound Gateway” for further details. Below is an example bean definition for a simple HttpInboundEndpoint

<bean id="httpInbound" class="org.springframework.integration.http.HttpInboundEndpoint">
	<property name="requestChannel" ref="httpRequestChannel" />
	<property name="replyChannel" ref="httpReplyChannel" />	
</bean>

The HttpInboundEndpoint accepts an instance of InboundRequestMapper which allows customisation of the mapping from HttpServletRequest to Message. If none is provided an instance of DefaultInboundRequestMapper will be used. This encapsulates a simple strategy, which for example will create a String message for a POST request where the content type starts with "text", see the Javadoc for full details.

Starting with this release MultiPart File support was implemented. If the request has been wrapped as a MultipartHttpServletRequest, then the 'content type' can be checked. If it is known, and begins with "text", then the MultipartFile will be copied to a String in the parameter map. If the content type does not begin with "text", then the MultipartFile will be copied to a byte array within the parameter map instead.

[Note]Note
The HttpInboundEndpoint will locate a MultipartResolver in the context if one exists with the bean name "multipartResolver" (the same name expected by Spring's DispatcherServlet). If it does in fact locate that bean, then the support for MultipartFiles will be enabled on the inbound request mapper. Otherwise, it will fail when trying to map a multipart-file request to a Spring Integration Message. For more on Spring's support for MultipartResolvers, refer to the Spring Reference Manual.

In sending a response to the client there are a number of ways to customise the behaviour of the gateway. By default the gateway will simply acknowledge that the request was received by sending a 200 status code back. It is possible to customise this response by providing an implementation of the Spring MVC View which will be invoked with the created Message. In the case that the gateway should expect a reply to the Message then setting the expectReply flag will cause the gateway to wait for a response Message before creating an Http response. Below is an example of a gateway configured to use a custom view and to wait for a response. It also shows how to customise the Http methods accepted by the gateway, which are POST and GET by default.

<bean id="httpInbound" class="org.springframework.integration.http.HttpInboundEndpoint">
	<property name="requestChannel" ref="httpRequestChannel" />
	<property name="replyChannel" ref="httpReplyChannel" />
	<property name="view" ref="jsonView" />
	<property name="supportedMethods" >
		<list>
			<value>GET</value>
			<value>DELETE</value>
		</list>
	</property>
	<property name="expectReply" value="true" />
	<property name="requestMapper" ref="customRequestMapper" />
</bean>

The message created from the request will be available in the Model map. The key that is used for that map entry by default is 'requestMessage', but this can be overridden by setting the 'requestKey' property on the endpoint's configuration.

22.3 Http Outbound Gateway

To configure the HttpOutboundEndpoint write a bean definition like this:

<bean id="httpOutbound" class="org.springframework.integration.http.HttpOutboundEndpoint" >
	<property name="outputChannel" ref="responseChannel" />
</bean>

This bean definition will execute Http requests by first converting the message to the Http request using an instance of DefaultOutboundRequestMapper. This will expect to find the request URL in the message header under the key HttpHeaders.REQUEST_URL. It is also possible to set a default target URL as a constructor argument along with other options as shown below.

<bean id="httpOutbound" class="org.springframework.integration.http.HttpOutboundEndpoint" >
	<constructor-arg value="http://localhost:8080/example" />
	<property name="outputChannel" ref="responseChannel" />
	<property name="sendTimeout" value="5000" />
	<property name="requestMapper" ref="customRequestMapper" />
</bean>

By default the Http request will be made using an instance of SimpleHttpRequestExecutor which uses the JDK HttpURLConnection. Use of the Apache Commons Http Client is also supported through the provided CommonsHttpRequestExecutor which can be injected into the outbound gateway.

22.4 Http Namespace Support

Spring Integration provides an "http" namespace and schema definition. To include it in your configuration, simply provide the following URI within a namespace declaration: 'http://www.springframework.org/schema/integration/http'. The schema location should then map to 'http://www.springframework.org/schema/integration/http/spring-integration-http-1.0.xsd'.

To configure an inbound http channel adapter which is an instance of HttpInboundEndpoint configured not to expect a response.

 <http:inbound-channel-adapter id="httpChannelAdapter" channel="requests" supported-methods="PUT, DELETE"/>

To configure an inbound http gateway which expects a response.

 <http:inbound-gateway id="inboundGateway" request-channel="requests" reply-channel="responses"/>

To configure the outbound gateway you can use the namespace support as well. The following code snippet shows the different configuration options for an outbound Http gateway.

<http:outbound-gateway id="fullConfigWithoutMapper"
		request-channel="requests"
		default-url="http://localhost/test"
		extract-request-payload="false"
		charset="UTF-8"
		request-executor="executor"
		request-timeout="1234"
		reply-channel="replies"/>

If you want to provide a custom OutboundRequestMapper, then a reference may be supplied to the 'request-mapper' attribute. In that case however you will not be allowed to set the default URL, charset, and 'extract-request-payload' properties since those are all properties of the default mapper (see the JavaDoc for DefaultOutboundRequestMapper for more information).

23. Mail Support

23.1 Mail-Sending Channel Adapter

Spring Integration provides support for outbound email with the MailSendingMessageHandler. It delegates to a configured instance of Spring's JavaMailSender:

 JavaMailSender mailSender = (JavaMailSender) context.getBean("mailSender");

 MailSendingMessageHandler mailSendingHandler = new MailSendingMessageHandler(mailSender);

MailSendingMessageHandler has various mapping strategies that use Spring's MailMessage abstraction. If the received Message's payload is already a MailMessage instance, it will be sent directly. Therefore, it is generally recommended to precede this consumer with a Transformer for non-trivial MailMessage construction requirements. However, a few simple Message mapping strategies are supported out-of-the-box. For example, if the message payload is a byte array, then that will be mapped to an attachment. If the payload is neither a MailMessage or byte array, then a MailMessage will be created with text content corresponding to the value returned from the Spring Integration Message payload's toString() method. For simple text-based emails, simply provide a String-based Message payload.

The outbound MailMessage may also be configured with certain values from the MessageHeaders. If available, values will be mapped to the outbound mail's properties, such as the recipients (TO, CC, and BCC), the from/reply-to, and the subject. The header names are defined by the following constants:

 MailHeaders.SUBJECT
 MailHeaders.TO
 MailHeaders.CC
 MailHeaders.BCC
 MailHeaders.FROM
 MailHeaders.REPLY_TO

23.2 Mail-Receiving Channel Adapter

Spring Integration also provides support for inbound email with the MailReceivingMessageSource. It delegates to a configured instance of Spring Integration's own MailReceiver interface, and there are two implementations: Pop3MailReceiver and ImapMailReceiver. The easiest way to instantiate either of these is by passing the 'uri' for a Mail store to the receiver's constructor. For example:

 MailReceiver receiver = new Pop3MailReceiver("pop3://usr:pwd@localhost/INBOX");

Another option for receiving mail is the IMAP "idle" command (if supported by the mail server you are using). Spring Integration provides the ImapIdleChannelAdapter which is itself a Message-producing endpoint. It delegates to an instance of the ImapMailReceiver but enables asynchronous reception of Mail Messages. There are examples in the next section of configuring both types of inbound Channel Adapter with Spring Integration's namespace support in the 'mail' schema.

23.3 Mail Namespace Support

Spring Integration provides a namespace for mail-related configuration. To use it, configure the following schema locations.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:mail="http://www.springframework.org/schema/integration/mail"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
            http://www.springframework.org/schema/integration/mail
            http://www.springframework.org/schema/integration/mail/spring-integration-mail-1.0.xsd">

To configure an outbound Channel Adapter, provide the channel to receive from, and the MailSender:

<mail:outbound-channel-adapter channel="outboundMail"
                              mail-sender="mailSender"/>

Alternatively, provide the host, username, and password:

<mail:outbound-channel-adapter channel="outboundMail"
                              host="somehost" username="someuser" password="somepassword"/>

[Note]Note
Keep in mind, as with any outbound Channel Adapter, if the referenced channel is a PollableChannel, a <poller> sub-element should be provided with either an interval-trigger or cron-trigger.

To configure an inbound Channel Adapter, you have the choice between polling or event-driven (assuming your mail server supports IMAP IDLE - if not, then polling is the only option). A polling Channel Adapter simply requires the store URI and the channel to send inbound Messages to. The URI may begin with "pop3" or "imap":

 <mail:inbound-channel-adapter channel="mailIn"
                               store-uri="imap://usr:pwd@imap.example.com/INBOX">
     <poller max-messages-per-poll="3">
         <interval-trigger interval="30" time-unit="SECONDS"/>
     </poller>
 </mail:inbound-channel-adapter>

If you do have IMAP idle support, then you may want to configure the "imap-idle-channel-adapter" element instead. Since the "idle" command enables event-driven notifications, no poller is necessary for this adapter. It will send a Message to the specified channel as soon as it receives the notification that new mail is available:

 <mail:imap-idle-channel-adapter channel="mailIn"
                                 store-uri="imaps://usr:pwd@imap.example.com:993/INBOX"/>

When using the namespace support, a header-enricher Message Transformer is also available. This simplifies the application of the headers mentioned above to any Message prior to sending to the Mail-sending Channel Adapter.

<mail:header-enricher subject="Example Mail"
                     to="to@example.org"
                     cc="cc@example.org"
                     bcc="bcc@example.org"
                     from="from@example.org"
                     reply-to="replyTo@example.org"
                     overwrite="false"/>

24. Stream Support

24.1 Introduction

In many cases application data is obtained from a stream. It is not recommended to send a reference to a Stream as a message payload to a consumer. Instead messages are created from data that is read from an input stream and message payloads are written to an output stream one by one.

24.2 Reading from streams

Spring Integration provides two adapters for streams. Both ByteStreamReadingMessageSource and CharacterStreamReadingMessageSource implement MessageSource. By configuring one of these within a channel-adapter element, the polling period can be configured, and the Message Bus can automatically detect and schedule them. The byte stream version requires an InputStream, and the character stream version requires a Reader as the single constructor argument. The ByteStreamReadingMessageSource also accepts the 'bytesPerMessage' property to determine how many bytes it will attempt to read into each Message. The default value is 1024

<bean class="org.springframework.integration.stream.ByteStreamReadingMessageSource">
	<constructor-arg ref="someInputStream"/>
	<property name="bytesPerMessage" value="2048"/>
</bean>

<bean class="org.springframework.integration.stream.CharacterStreamReadingMessageSource">
	<constructor-arg ref="someReader"/>
</bean>
		

24.3 Writing to streams

For target streams, there are also two implementations: ByteStreamWritingMessageHandler and CharacterStreamWritingMessageHandler. Each requires a single constructor argument - OutputStream for byte streams or Writer for character streams, and each provides a second constructor that adds the optional 'bufferSize'. Since both of these ultimately implement the MessageHandler interface, they can be referenced from a channel-adapter configuration as described in more detail in Chapter 6, Channel Adapter.

<bean class="org.springframework.integration.stream.ByteStreamWritingMessageHandler">
	<constructor-arg ref="someOutputStream"/>
	<constructor-arg value="1024"/>
</bean>

<bean class="org.springframework.integration.stream.CharacterStreamWritingMessageHandler">
	<constructor-arg ref="someWriter"/>
</bean>
	

24.4 Stream namespace support

To reduce the configuration needed for stream related channel adapters there is a namespace defined. The following schema locations are needed to use it.

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns="http://www.springframework.org/schema/integration/stream"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:beans="http://www.springframework.org/schema/beans"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
			http://www.springframework.org/schema/integration/stream
			http://www.springframework.org/schema/integration/stream/spring-integration-stream-1.0.xsd">

To configure the inbound channel adapter the following code snippet shows the different configuration options that are supported.

<stdin-channel-adapter id="adapterWithDefaultCharset"/>

<stdin-channel-adapter id="adapterWithProvidedCharset" charset="UTF-8"/>

To configure the outbound channel adapter you can use the namespace support as well. The following code snippet shows the different configuration for an outbound channel adapters.

<stdout-channel-adapter id="stdoutAdapterWithDefaultCharset" channel="testChannel"/>

<stdout-channel-adapter id="stdoutAdapterWithProvidedCharset" charset="UTF-8" channel="testChannel"/>

<stderr-channel-adapter id="stderrAdapter" channel="testChannel"/>

<stdout-channel-adapter id="newlineAdapter" append-newline="true" channel="testChannel"/>
		

25. Spring ApplicationEvent Support

Spring Integration provides support for inbound and outbound ApplicationEvents as defined by the underlying Spring Framework. For more information about the events and listeners, refer to the Spring Reference Manual.

25.1 Receiving Spring ApplicationEvents

To receive events and send them to a channel, simply define an instance of Spring Integration's ApplicationEventListeningChannelAdapter. This class is an implementation of Spring's ApplicationListener interface. By default it will pass all received events as Spring Integration Messages. To limit based on the type of event, configure the list of event types that you want to receive with the 'eventTypes' property.

25.2 Sending Spring ApplicationEvents

To send Spring ApplicationEvents, create an instance of the ApplicationEventPublishingMessageHandler and register it within an endpoint. This implementation of the MessageHandler interface also implements Spring's ApplicationEventPublisherAware interface and thus acts as a bridge between Spring Integration Messages and ApplicationEvents.

26. Dealing with XML Payloads

26.1 Introduction

Spring Integration's XML support extends the Spring Integration Core with implementations of splitter, transformer, selector and router designed to make working with xml messages in Spring Integration simple. The provided messaging components are designed to work with xml represented in a range of formats including instances of java.lang.String, org.w3c.dom.Document and javax.xml.transform.Source. It should be noted however that where a DOM representation is required, for example in order to evaluate an XPath expression, the String payload will be converted into the required type and then converted back again to String. Components that require an instance of DocumentBuilder will create a namespace aware instance if one is not provided. Where greater control of the document being created is required an appropriately configured instance of DocumentBuilder should be provided.

26.2 Transforming xml payloads

This section will explain the workings of XmlPayloadUnmarshallingTransformer, XmlPayloadMarshallingTransformer, XsltPayloadTransformer and how to configure them as beans. All of the provided xml transformers extend AbstractPayloadTransformer and therefore implement Transformer. When configuring xml transformers as beans in Spring Integration you would normally configure the transformer in conjunction with either a MessageTransformingChannelInterceptor or a MessageTransformingHandler. This allows the transformer to be used as either an interceptor, which transforms the message as it is sent or received to the channel, or as an endpoint. Finally the namespace support will be discussed which allows for the simple configuration of the transformers as elements in XML.

XmlPayloadUnmarshallingTransformer allows an xml Source to be unmarshalled using implementations of Spring OXM Unmarshaller. Spring OXM provides several implementations supporting marshalling and unmarshalling using JAXB, Castor and JiBX amongst others. Since the unmarshaller requires an instance of Source where the message payload is not currently an instance of Source, conversion will be attempted. Currently String and org.w3c.dom.Document payloads are supported. Custom conversion to a Source is also supported by injecting an implementation of SourceFactory.

<bean id="unmarshallingTransformer"
      class="org.springframework.integration.xml.transformer.XmlPayloadUnmarshallingTransformer">
    <constructor-arg>
        <bean class="org.springframework.oxm.jaxb.Jaxb1Marshaller">
		    <property name="contextPath" value="org.example" />
        </bean>
    </constructor-arg>
</bean>

The XmlPayloadMarshallingTransformer allows an object graph to be converted into xml using a Spring OXM Marshaller. By default the XmlPayloadMarshallingTransformer will return a DomResult. However the type of result can be controlled by configuring an alternative ResultFactory such as StringResultFactory. In many cases it will be more convenient to transform the payload into an alternative xml format. To achieve this configure a ResultTransformer. Two implementations are provided, one which converts to String and another which converts to Document.

<bean id="marshallingTransformer"
      class="org.springframework.integration.xml.transformer.XmlPayloadMarshallingTransformer">
    <constructor-arg>
        <bean class="org.springframework.oxm.jaxb.Jaxb1Marshaller">
            <property name="contextPath" value="org.example" />
        </bean>
    </constructor-arg>
    <constructor-arg>
        <bean class="org.springframework.integration.xml.transformer.ResultToDocumentTransformer" />
    </constructor-arg>
</bean>

XsltPayloadTransformer transforms xml payloads using xsl. The transformer requires an instance of either Resource or Templates. Passing in a Templates instance allows for greater configuration of the TransformerFactory used to create the template instance. As in the case of XmlPayloadMarshallingTransformer by default XsltPayloadTransformer will create a message with a Result payload. This can be customised by providing a ResultFactory and/or a ResultTransformer.

<bean id="xsltPayloadTransformer"
      class="org.springframework.integration.xml.transformer.XsltPayloadTransformer">
    <constructor-arg value="classpath:org/example/xsl/transform.xsl" />
    <constructor-arg>
        <bean class="org.springframework.integration.xml.transformer.ResultToDocumentTransformer" />
    </constructor-arg>
</bean>

26.3 Namespace support for xml transformers

Namespace support for all xml transformers is provided in the Spring Integration xml namespace, a template for which can be seen below. The namespace support for transformers creates an instance of either EventDrivenConsumer or PollingConsumer according to the type of the provided input channel. The namespace support is designed to reduce the amount of xml configuration by allowing the creation of an endpoint and transformer using one element.

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:integration="http://www.springframework.org/schema/integration"
       xmlns:si-xml="http://www.springframework.org/schema/integration/xml"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/integration
           http://www.springframework.org/schema/integration/spring-integration-1.0.xsd
           http://www.springframework.org/schema/integration/xml
           http://www.springframework.org/schema/integration/xml/spring-integration-xml-1.0.xsd">
</beans>

The namespace support for XmlPayloadUnmarshallingTransformer is shown below. Since the namespace is now creating an instance of MessageEndpoint rather than a transformer a poller can also be nested within the element to control the polling of the input channel.

<si-xml:unmarshalling-transformer id="defaultUnmarshaller"
    input-channel="input"
    output-channel="output"
    unmarshaller="unmarshaller"/>

<si-xml:unmarshalling-transformer id="unmarshallerWithPoller"
    input-channel="input"
    output-channel="output"
    unmarshaller="unmarshaller">
    <si:poller>
        <si:interval-trigger interval="2000"/>
    </si:poller>
<si-xml:unmarshalling-transformer/>    
    

The namespace support for the marshalling transformer requires an input channel, output channel and a reference to a marshaller. The optional result-type attribute can be used to control the type of result created, valid values are StringResult or DomResult (the default). Where the provided result types are not sufficient a reference to a custom implementation of ResultFactory can be provided as an alternative to setting the result-type attribute using the result-factory attribute. An optional result-transformer can also be specified in order to convert the created Result after marshalling.

<si-xml:marshalling-transformer
     input-channel="marshallingTransformerStringResultFactory"
     output-channel="output"
     marshaller="marshaller"
     result-type="StringResult" />
     
<si-xml:marshalling-transformer
    input-channel="marshallingTransformerWithResultTransformer"
    output-channel="output"
    marshaller="marshaller"
    result-transformer="resultTransformer" />
     
<bean id="resultTransformer"
      class="org.springframework.integration.xml.transformer.ResultToStringTransformer"/>

Namespace support for the XsltPayloadTransformer allows either a resource to be passed in in order to create the Templates instance or alternatively a precreated Templates instance can be passed in as a reference. In common with the marshalling transformer the type of the result output can be controlled by specifying either the result-factory or result-type attribute. A result-transfomer attribute can also be used to reference an implementation of ResultTransfomer where conversion of the result is required before sending.

<si-xml:xslt-transformer id="xsltTransformerWithResource"
    input-channel="withResourceIn"
    output-channel="output"
    xsl-resource="org/springframework/integration/xml/config/test.xsl"/>
<si-xml:xslt-transformer id="xsltTransformerWithTemplatesAndResultTransformer"
    input-channel="withTemplatesAndResultTransformerIn"
    output-channel="output"
    xsl-templates="templates"
    result-transformer="resultTransformer"/>

26.4 Splitting xml messages

XPathMessageSplitter supports messages with either String or Document payloads. The splitter uses the provided XPath expression to split the payload into a number of nodes. By default this will result in each Node instance becoming the payload of a new message. Where it is preferred that each message be a Document the createDocuments flag can be set. Where a String payload is passed in the payload will be converted then split before being converted back to a number of String messages. The XPath splitter implements MessageHandler and should therefore be configured in conjunction with an appropriate endpoint (see the namespace support below for a simpler configuration alternative).

<bean id="splittingEndpoint"
      class="org.springframework.integration.endpoint.SubscribingConsumerEndpoint">
    <constructor-arg>
        <bean class="org.springframework.integration.xml.splitter.XPathMessageSplitter">
            <constructor-arg value="/order/items" />
            <property name="documentBuilder" ref="customisedDocumentBuilder" />
            <property name="outputChannel" ref="orderItemsChannel" />
        </bean>
    </constructor-arg>
    <constructor-arg ref="orderChannel" />
</bean>

26.5 Routing xml messages using XPath

Two Router implementations based on XPath are provided XPathSingleChannelRouter and XPathMultiChannelRouter. The implementations differ in respect to how many channels any given message may be routed to, exactly one in the case of the single channel version or zero or more in the case of the multichannel router. Both evaluate an XPath expression against the xml payload of the message, supported payload types by default are Node, Document and String. For other payload types a custom implementation of XmlPayloadConverter can be provided. The router implementations use ChannelResolver to convert the result(s) of the XPath expression to a channel name. By default a BeanFactoryChannelResolver strategy will be used, this means that the string returned by the XPath evaluation should correspond directly to the name of a channel. Where this is not the case an alternative implementation of ChannelResolver can be used. Where there is a simple mapping from Xpath result to channel name the provided MapBasedChannelResolver can be used.

<!-- Expects a channel for each value of order type to exist  -->
<bean id="singleChannelRoutingEndpoint"
      class="org.springframework.integration.endpoint.SubscribingConsumerEndpoint">
    <constructor-arg>
        <bean class="org.springframework.integration.xml.router.XPathSingleChannelRouter">
            <constructor-arg value="/order/@type" />
        </bean>
    </constructor-arg>
	<constructor-arg ref="orderChannel" />
</bean>
	
	
<!-- Multi channel router which uses a map channel resolver to resolve the channel name
     based on the XPath evaluation result Since the router is multi channel it may deliver
     message to one or both of the configured channels -->
<bean id="multiChannelRoutingEndpoint"
      class="org.springframework.integration.endpoint.SubscribingConsumerEndpoint">
    <constructor-arg>
        <bean class="org.springframework.integration.xml.router.XPathMultiChannelRouter">
            <constructor-arg value="/order/recipient" />
            <property name="channelResolver">
                <bean class="org.springframework.integration.channel.MapBasedChannelResolver">
                    <constructor-arg>
                        <map>
                            <entry key="accounts"
                                   value-ref="accountConfirmationChannel" />
                            <entry key="humanResources"
                                   value-ref="humanResourcesConfirmationChannel" />
                        </map>
                     </constructor-arg>
	             </bean>
            </property>
        </bean>
    </constructor-arg>
    <constructor-arg ref="orderChannel" />
</bean>

26.6 Selecting xml messages using XPath

Two MessageSelector implementations are provided, BooleanTestXPathMessageSelector and StringValueTestXPathMessageSelector. BooleanTestXPathMessageSelector requires an XPathExpression which evaluates to a boolean, for example boolean(/one/two) which will only select messages which have an element named two which is a child of a root element named one. StringValueTestXPathMessageSelector evaluates any XPath expression as a String and compares the result with the provided value.

<!-- Interceptor which rejects messages that do not have a root element order  -->
<bean id="orderSelectingInterceptor"
      class="org.springframework.integration.channel.interceptor.MessageSelectingInterceptor">
    <constructor-arg>
        <bean class="org.springframework.integration.xml.selector.BooleanTestXPathMessageSelector">
            <constructor-arg value="boolean(/order)" />
        </bean>
    </constructor-arg>
</bean>
	
<!--  Interceptor which rejects messages that are not version one orders --> 
<bean id="versionOneOrderSelectingInterceptor"
      class="org.springframework.integration.channel.interceptor.MessageSelectingInterceptor">
    <constructor-arg>
        <bean class="org.springframework.integration.xml.selector.StringValueTestXPathMessageSelector">
            <constructor-arg value="/order/@version" index="0"/>
            <constructor-arg value="1" index="1"/>
        </bean>
    </constructor-arg>
</bean>

26.7 XPath components namespace support

All XPath based components have namespace support allowing them to be configured as Message Endpoints with the exception of the XPath selectors which are not designed to act as endpoints. Each component allows the XPath to either be referenced at the top level or configured via a nested xpath-expression element. So the following configurations of an xpath-selector are all valid and represent the general form of XPath namespace support. All forms of XPath expression result in the creation of an XPathExpression using the Spring XPathExpressionFactory

<si-xml:xpath-selector id="xpathRefSelector"
                       xpath-expression="refToXpathExpression"
                       evaluation-result-type="boolean" />

<si-xml:xpath-selector id="selectorWithNoNS" evaluation-result-type="boolean" >
    <si-xml:xpath-expression expression="/name"/>
</si-xml:xpath-selector>	
	
<si-xml:xpath-selector id="selectorWithOneNS" evaluation-result-type="boolean" >
    <si-xml:xpath-expression expression="/ns1:name"
                             ns-prefix="ns1" ns-uri="www.example.org" />
</si-xml:xpath-selector>
	
<si-xml:xpath-selector id="selectorWithTwoNS" evaluation-result-type="boolean" >
    <si-xml:xpath-expression expression="/ns1:name/ns2:type">
        <map>
            <entry key="ns1" value="www.example.org/one" />
            <entry key="ns2" value="www.example.org/two" />
        </map>
    </si-xml:xpath-expression>
</si-xml:xpath-selector>
	
<si-xml:xpath-selector id="selectorWithNamespaceMapRef" evaluation-result-type="boolean" >
    <si-xml:xpath-expression expression="/ns1:name/ns2:type"
                             namespace-map="defaultNamespaces"/>
</si-xml:xpath-selector>
	
<util:map id="defaultNamespaces">
    <util:entry key="ns1" value="www.example.org/one" />
    <util:entry key="ns2" value="www.example.org/two" />
</util:map>

XPath splitter namespace support allows the creation of a Message Endpoint with an input channel and output channel.

<!-- Split the order into items creating a new message for each item node -->
<si-xml:xpath-splitter id="orderItemSplitter"
                       input-channel="orderChannel"
                       output-channel="orderItemsChannel">
    <si-xml:xpath-expression expression="/order/items"/>
</si-xml:xpath-splitter>

<!-- Split the order into items creating a new document for each item-->
<si-xml:xpath-splitter id="orderItemDocumentSplitter"
                       input-channel="orderChannel" 
                       output-channel="orderItemsChannel"
                       create-documents="true">
    <si-xml:xpath-expression expression="/order/items"/>
    <si:poller>
        <si:interval-trigger interval="2000"/>
    </si:poller>
</si-xml:xpath-splitter>

XPath router namespace support allows for the creation of a Message Endpoint with an input channel but no output channel since the output channel is determined dynamically. The multi-channel attribute causes the creation of a multi channel router capable of routing a single message to many channels when true and a single channel router when false.

<!-- route the message according to exactly one order type channel -->
<si-xml:xpath-router id="orderTypeRouter" input-channel="orderChannel" multi-channel="false">
    <si-xml:xpath-expression expression="/order/type"/>
</si-xml:xpath-router>

<!-- route the order to all responders-->
<si-xml:xpath-router id="responderRouter" input-channel="orderChannel" multi-channel="true">
    <si-xml:xpath-expression expression="/request/responders"/>
    <si:poller>
        <si:interval-trigger interval="2000"/>
    </si:poller>
</si-xml:xpath-router>

27. Security in Spring Integration

27.1 Introduction

Spring Integration provides integration with the Spring Security project to allow role based security checks to be applied to channel send and receive invocations.

27.2 Securing channels

Spring Integration provides the interceptor ChannelSecurityInterceptor, which extends AbstractSecurityInterceptor and intercepts send and receive calls on the channel. Access decisions are then made with reference to ChannelInvocationDefinitionSource which provides the definition of the send and receive security constraints. The interceptor requires that a valid SecurityContext has been established by authenticating with Spring Security, see the Spring Security reference documentation for details.

Namespace support is provided to allow easy configuration of security constraints. This consists of the secured channels tag which allows definition of one or more channel name patterns in conjunction with a definition of the security configuration for send and receive. The pattern is a java.util.regexp.Pattern.

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns="http://www.springframework.org/schema/integration"
 	xmlns:si-security="http://www.springframework.org/schema/integration/security"
	xmlns:beans="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:security="http://www.springframework.org/schema/security"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
			http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
			http://www.springframework.org/schema/security
			http://www.springframework.org/schema/security/spring-security-2.0.xsd
			http://www.springframework.org/schema/integration
			http://www.springframework.org/schema/integration/spring-integration-1.0.xsd
			http://www.springframework.org/schema/integration/security
			http://www.springframework.org/schema/integration/security/spring-integration-security-1.0.xsd">

<si-security:secured-channels>
    <si-security:access-policy pattern="admin.*" send-access="ROLE_ADMIN"/>
    <si-security:access-policy pattern="user.*" receive-access="ROLE_USER"/>
</si-security:secured-channels>
	

By default the secured-channels namespace element expects a bean named authenticationManager which implements AuthenticationManager and a bean named accessDecisionManager which implements AccessDecisionManager. Where this is not the case references to the appropriate beans can be configured as attributes of the secured-channels element as below.

<si-security:secured-channels access-decision-manager="customAccessDecisionManager"
                              authentication-manager="customAuthenticationManager">
    <si-security:access-policy pattern="admin.*" send-access="ROLE_ADMIN"/>
    <si-security:access-policy pattern="user.*" receive-access="ROLE_USER"/>
</si-security:secured-channels>
	

Appendix A. Spring Integration Samples

[Note]Note
Starting with the current release of Spring Integration the samples are distributed as independent Maven-based projects (http://maven.apache.org/) to minimize the setup time. Since each project is also an Eclipse-based project, they can be imported as is using the Eclipse Import wizard. If you prefer another IDE, configuration should be very trivial, since a special Maven profile was setup to download all of the required dependencies for all samples. Detailed instructions on how to build and run the samples are provided in the README.txt file located in the samples directory of the main distribution.

A.1 The Cafe Sample

In this section, we will review a sample application that is included in the Spring Integration distribution. This sample is inspired by one of the samples featured in Gregor Hohpe's Ramblings.

The domain is that of a Cafe, and the basic flow is depicted in the following diagram:

The Order object may contain multiple OrderItems. Once the order is placed, a Splitter will break the composite order message into a single message per drink. Each of these is then processed by a Router that determines whether the drink is hot or cold (checking the OrderItem object's 'isIced' property). The Barista prepares each drink, but hot and cold drink preparation are handled by two distinct methods: 'prepareHotDrink' and 'prepareColdDrink'. The prepared drinks are then sent to the Waiter where they are aggregated into a Delivery object.

Here is the XML configuration:

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans xmlns="http://www.springframework.org/schema/integration"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:beans="http://www.springframework.org/schema/beans"
    xmlns:stream="http://www.springframework.org/schema/integration/stream"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
            http://www.springframework.org/schema/integration
            http://www.springframework.org/schema/integration/spring-integration-1.0.xsd
            http://www.springframework.org/schema/integration/stream
            http://www.springframework.org/schema/integration/stream/spring-integration-stream-1.0.xsd">

    <gateway id="cafe" service-interface="org.springframework.integration.samples.cafe.Cafe"/>

    <channel id="orders"/>
    <splitter input-channel="orders" ref="orderSplitter" method="split" output-channel="drinks"/>

    <channel id="drinks"/>
    <router input-channel="drinks" ref="drinkRouter" method="resolveOrderItemChannel"/>

    <channel id="coldDrinks">
        <queue capacity="10"/>
    </channel>
    <service-activator input-channel="coldDrinks" ref="barista"
                       method="prepareColdDrink" output-channel="preparedDrinks"/>

    <channel id="hotDrinks">
        <queue capacity="10"/>
    </channel>
    <service-activator input-channel="hotDrinks" ref="barista"
                       method="prepareHotDrink" output-channel="preparedDrinks"/>

    <channel id="preparedDrinks"/>
    <aggregator input-channel="preparedDrinks" ref="waiter"
                method="prepareDelivery" output-channel="deliveries"/>

    <stream:stdout-channel-adapter id="deliveries"/>

    <beans:bean id="orderSplitter"
                class="org.springframework.integration.samples.cafe.xml.OrderSplitter"/>

    <beans:bean id="drinkRouter"
                class="org.springframework.integration.samples.cafe.xml.DrinkRouter"/>

    <beans:bean id="barista" class="org.springframework.integration.samples.cafe.xml.Barista"/>

    <beans:bean id="waiter" class="org.springframework.integration.samples.cafe.xml.Waiter"/>

    <poller id="poller" default="true">
        <interval-trigger interval="1000"/>
    </poller>

</beans:beans>

As you can see, each Message Endpoint is connected to input and/or output channels. Each endpoint will manage its own Lifecycle (by default endpoints start automatically upon initialization - to prevent that add the "auto-startup" attribute with a value of "false"). Most importantly, notice that the objects are simple POJOs with strongly typed method arguments. For example, here is the Splitter:

public class OrderSplitter {

    public List<OrderItem> split(Order order) {
        return order.getItems();
    }
}

In the case of the Router, the return value does not have to be a MessageChannel instance (although it can be). As you see in this example, a String-value representing the channel name is returned instead.

public class DrinkRouter {

    public String resolveOrderItemChannel(OrderItem orderItem) {
        return (orderItem.isIced()) ? "coldDrinks" : "hotDrinks";
    }
}

Now turning back to the XML, you see that there are two <service-activator> elements. Each of these is delegating to the same Barista instance but different methods: 'prepareHotDrink' or 'prepareColdDrink' corresponding to the two channels where order items have been routed.

public class Barista {

    private long hotDrinkDelay = 5000;
    private long coldDrinkDelay = 1000;

    private AtomicInteger hotDrinkCounter = new AtomicInteger();
    private AtomicInteger coldDrinkCounter = new AtomicInteger();

    public void setHotDrinkDelay(long hotDrinkDelay) {
        this.hotDrinkDelay = hotDrinkDelay;
    }

    public void setColdDrinkDelay(long coldDrinkDelay) {
        this.coldDrinkDelay = coldDrinkDelay;
    }

    public Drink prepareHotDrink(OrderItem orderItem) {
        try {
            Thread.sleep(this.hotDrinkDelay);
            System.out.println(Thread.currentThread().getName()
                    + " prepared hot drink #" + hotDrinkCounter.incrementAndGet()
                    + " for order #" + orderItem.getOrder().getNumber() + ": " + orderItem);
            return new Drink(orderItem.getOrder().getNumber(), orderItem.getDrinkType(),
                    orderItem.isIced(), orderItem.getShots());
        }
        catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            return null;
        }
    }

    public Drink prepareColdDrink(OrderItem orderItem) {
        try {
            Thread.sleep(this.coldDrinkDelay);
            System.out.println(Thread.currentThread().getName()
                    + " prepared cold drink #" + coldDrinkCounter.incrementAndGet()
                    + " for order #" + orderItem.getOrder().getNumber() + ": " + orderItem);
            return new Drink(orderItem.getOrder().getNumber(), orderItem.getDrinkType(),
                    orderItem.isIced(), orderItem.getShots());
        }
        catch (InterruptedException e) {
            Thread.currentThread().interrupt();
            return null;
        }
    }

}

As you can see from the code excerpt above, the barista methods have different delays (the hot drinks take 5 times as long to prepare). This simulates work being completed at different rates. When the CafeDemo 'main' method runs, it will loop 100 times sending a single hot drink and a single cold drink each time. It actually sends the messages by invoking the 'placeOrder' method on the Cafe interface. Above, you will see that the <gateway> element is specified in the configuration file. This triggers the creation of a proxy that implements the given 'service-interface' and connects it to a channel. The channel name is provided on the @Gateway annotation of the Cafe interface.

public interface Cafe {

    @Gateway(requestChannel="orders")
    void placeOrder(Order order);

}

Finally, have a look at the main() method of the CafeDemo itself.

public static void main(String[] args) {
    AbstractApplicationContext context = null;
    if (args.length > 0) {
        context = new FileSystemXmlApplicationContext(args);
    }
    else {
        context = new ClassPathXmlApplicationContext("cafeDemo.xml", CafeDemo.class);
    }
    Cafe cafe = (Cafe) context.getBean("cafe");
    for (int i = 1; i <= 100; i++) {
        Order order = new Order(i);
        order.addItem(DrinkType.LATTE, 2, false);
        order.addItem(DrinkType.MOCHA, 3, true);
        cafe.placeOrder(order);
    }
}

[Tip]Tip
To run this sample as well as 8 others, refer to the README.txt within the "samples" directory of the main distribution as described at the beginning of this chapter.

When you run cafeDemo, you will see that the cold drinks are initially prepared more quickly than the hot drinks. Because there is an aggregator, the cold drinks are effectively limited by the rate of the hot drink preparation. This is to be expected based on their respective delays of 1000 and 5000 milliseconds. However, by configuring a poller with a concurrent task executor, you can dramatically change the results. For example, you could use a thread pool executor with 5 workers for the hot drink barista while keeping the cold drink barista as it is:

<service-activator input-channel="hotDrinks"
                   ref="barista"
                   method="prepareHotDrink"
                   output-channel="preparedDrinks"/>

<service-activator input-channel="hotDrinks"
                   ref="barista"
                   method="prepareHotDrink"
                   output-channel="preparedDrinks">
    <poller task-executor="pool">
        <interval-trigger interval="1000"/>
    </poller>
</service-activator>

<thread-pool-task-executor id="pool" core-size="5"/>

Also, notice that the worker thread name is displayed with each invocation. You will see that the hot drinks are prepared by the task-executor threads. If you provide a much shorter poller interval (such as 100 milliseconds), then you will notice that occasionally it throttles the input by forcing the task-scheduler (the caller) to invoke the operation.

[Note]Note
In addition to experimenting with the poller's concurrency settings, you can also add the 'transactional' sub-element and then refer to any PlatformTransactionManager instance within the context.

A.2 The XML Messaging Sample

The xml messaging sample in the org.springframework.integration.samples.xml illustrates how to use some of the provided components which deal with xml payloads. The sample uses the idea of processing an order for books represented as xml.

First the order is split into a number of messages, each one representing a single order item using the XPath splitter component.

<si-xml:xpath-splitter id="orderItemSplitter" input-channel="ordersChannel" 
            output-channel="stockCheckerChannel" create-documents="true">
    <si-xml:xpath-expression expression="/orderNs:order/orderNs:orderItem" namespace-map="orderNamespaceMap" />
</si-xml:xpath-splitter>

A service activator is then used to pass the message into a stock checker POJO. The order item document is enriched with information from the stock checker about order item stock level. This enriched order item message is then used to route the message. In the case where the order item is in stock the message is routed to the warehouse. The XPath router makes use of a MapBasedChannelResolver which maps the XPath evaluation result to a channel reference.

<si-xml:xpath-router id="instockRouter" channel-resolver="mapChannelResolver" 
        input-channel="orderRoutingChannel" resolution-required="true">
    <si-xml:xpath-expression expression="/orderNs:orderItem/@in-stock" namespace-map="orderNamespaceMap"  />
</si-xml:xpath-router>
	
<bean id="mapChannelResolver"
    class="org.springframework.integration.channel.MapBasedChannelResolver">
    <property name="channelMap">
        <map>
            <entry key="true" value-ref="warehouseDispatchChannel" />
            <entry key="false" value-ref="outOfStockChannel" />
        </map>
    </property>
</bean>

Where the order item is not in stock the message is transformed using xslt into a format suitable for sending to the supplier.

<si-xml:xslt-transformer input-channel="outOfStockChannel" output-channel="resupplyOrderChannel" 
     xsl-resource="classpath:org/springframework/integration/samples/xml/bigBooksSupplierTransformer.xsl"/>

A.3 The OSGi Samples

This release of Spring Integration includes several samples that are OSGi enabled as well as samples that were specifically designed to show some of the other benefits of OSGi and Spring Integration when used together. First lets look at the two familiar examples that are also configured to be valid OSGi bundles. These are Hello World and Cafe. All you need to do to see these samples work in an OSGi environment is deploy the generated JAR into such an environment.

Use Maven to generate the JAR by executing the 'mvn install' command on either of these projects. This will generate the JAR file in the target directory. Now you can simply drop that JAR file into the deployment directory of your OSGi platform. For example, if you are using SpringSource dm Server, drop the files into the 'pickup' directory within the dm Server home directory.

[Note]Note
Prior to deploying and testing Spring Integration samples in the dm Server or any other OSGi server platform, you must have the Spring Integration and Spring bundles installed on that platform. For example, to install Spring Integration into SpringSource dm Server, copy all JAR files that are located in the 'dist' directory of your Spring Integration distribution into the 'repository/bundles/usr' directory of your dm Server instance (see the dm Server User Guide for more detail on how to install bundles).

The Spring Integration samples require a few other bundles to be installed. For the 1.0.3 release, the full list including transitive dependencies is:

  • org.apache.commons.codec-1.3.0.jar
  • org.apache.commons.collections-3.2.0.jar
  • org.apache.commons.httpclient-3.1.0.jar
  • org.apache.ws.commons.schema-1.3.2.jar
  • org.springframework.oxm-1.5.5.A.jar
  • org.springframework.security-2.0.4.A.jar
  • org.springframework.ws-1.5.5.A.jar
  • org.springframework.xml-1.5.5.A.jar

These are all located within the 'lib' directory of the Spring Integration distribution. So, you can simply copy those JARs into the dm Server 'repository/bundles/usr' directory as well.

[Note]Note
The Spring Framework bundles (aop, beans, context, etc.) are also included in the 'lib' directory of the Spring Integration distribution, but they do not need to be installed since they are already part of the dm Server infrastructure. Also, note that the versions listed above are those included with the Spring Integration 1.0.3 release. Newer versions of individual JARs may be used as long as they are within the range specified in the MANIFEST.MF files of those bundles that depend upon them.

[Tip]Tip
The bundles listed above are appropriate for a SpringSource dm Server 1.0.x deployment environment with a Spring Framework 2.5.x foundation. That is the version against which Spring Integration 1.0.3 has been developed and tested. However, as of the time of the Spring Integration 1.0.3 release, the Spring Framework 3.0 release candidates are about to be available, and the dm Server 2.0.x milestones are available. If you want to try running these samples in that environment, then you will need to replace the Spring Security and Spring Web Services bundles with versions that support Spring 3.0. The OXM functionality is moving into the Spring Framework itself for the 3.0 release. Otherwise, Spring Integration 1.0.3 has been superficially tested against the Spring 3.0 snapshots available at the time of its release. In fact, some internal changes were made in the 1.0.3 release specifically to support Spring 3.0 (whereas 1.0.2 does not). Spring Integration 2.0 will be built upon a Spring 3.0 foundation.

To demonstrate some of the benefits of running Spring Integration projects in an OSGi environment (e.g. modularity, OSGi service dynamics, etc.), we have included a couple new samples that are dedicated to highlighting those benefits. In the 'samples' directory, you will find the following two projects:

  • osgi-inbound (producer bundle)
  • osgi-outbound (consumer bundle)

Unlike the other samples in the distribution, these are not Maven enabled. Instead, we have simply configured them as valid dm Server Bundle projects. That means you can import these projects directly into an STS workspace using the "Existing Projects into Workspace" option from the Eclipse Import wizard. Then, you can take advantage of the STS dm Server tools to deploy them into a SpringSource dm Server instance.

[Note]Note
A simple Ant 'build.xml' file has been included within each of these projects as well. The build files contain a single 'jar' target. Therefore, after these projects have been built within Eclipse/STS, you can generate the bundle (JAR) directly and deploy it manually.

The structure of these projects is very simple, yet the concepts they showcase are quite powerful. The 'osgi-inbound' module enables sending a Message to a Publish-Subscribe Channel using a Spring Integration Gateway proxy. The interesting part, however, is that the Publish-Subscribe Channel is exported as an OSGi service via the <osgi:service/> element. As a result, any other bundles can be developed, deployed, and maintained independently yet still subscribe to that channel.

The 'osgi-outbound' module is an example of such a subscribing consumer bundle. It uses the corresponding <osgi:reference/> element to locate the channel exported by the 'osgi-inbound' bundle. It also contains configuration for a <file:outbound-gateway/> which is a subscriber to that channel and will write the Message content to a file once it arrives. It then sends a response Message with the name of the file and its location.

To make it easy to run, we've exposed a command-line interface where you can type in the command, the message, and the file name to execute the demo. This is exposed through the OSGi console. Likewise, the response that provides the name and location of the resulting file will also be visible within the OSGi console.

To run these samples, make sure your OSGi environment is properly configured to host Spring Integration bundles (as described in the note above). Deploy the producer bundle (osgi-inbound) first, and then deploy the consumer bundle (osgi-outbound). After you have deployed these bundles, open the OSGi console and type the following command:

 osgi> help 

You will see the following amidst the output:

 ---Spring Integration CLI-based OSGi Demo---
         siSend <message> <filename> - send text to be written to a file

As you can see, that describes the command that you will be able to use to send messages. If you are interested in how it is implemented or want to customize message sending logic or even create a new command look at InboundDemoBundleActivator.java in the consumer bundle.

[Tip]Tip
By default, when using dm Server, you can open the OSGi console by connecting to port 2401 via telnet:
 telnet localhost 2401

Now send a message by typing:

 osgi> siSend "Hello World" hello.txt

You will see something similar to the following in the OSGi console:

 Sending message: 'Hello World'
 Message sent and its contents were written to:   
 /usr/local/dm-server/work/tmp/spring-integration-samples/output/hello.txt

[Note]Note
It is not necessary to wrap the message in quotes if it does not contain spaces. Go ahead and open up the file and verify that the message content was written to it.

Let's assume you wanted to change the directory to which the files are written or make any other change to the consumer bundle (osgi-outboud). You only need to update the consumer bundle and not the producer bundle. So, go ahead and change the directory in the 'osgi-outbound.xml' file located within 'src/META-INF/spring' and refresh the consumer bundle.

[Tip]Tip
If using STS to deploy to dm Server, the refresh will happen automatically. If replacing bundles manually, you can issue the command 'refresh n' in the OSGi console (where n would be the ID of the bundle as displayed at any point after issuing the 'ss' command to see the short status output).

You will see that the change takes affect immediately. Not only that, you could even start developing and deploying new bundles that subscribe to the messages produced by the producer bundle the same way as the existing consumer bundle (osgi-outbound) does. With a publish-subscribe-channel any newly deployed bundles would start receiving each Message as well.

[Note]Note
If you also want to modify and refresh the producer bundle, be sure to refresh the consumer bundle afterwards as well. This is necessary because the consumer's subscription must be explicitly re-enabled after the producer's channel disappears. You could alternatively deploy a relatively static bundle that defines channels so that producers and consumers can be completely dynamic without affecting each other at all. In Spring Integration 2.0, we plan to support automatic re-subscription and more through the use of a Control Bus.

That pretty much wraps up this very simple example. Hopefully it has successfully demonstrated the benefits of modularity and OSGi service dynamics while working with Spring Integration. Feel free to experiment by following some of the suggestions mentioned above. For deeper coverage of the applicability of OSGi when used with Spring Integration, read this blog by Spring Integration team member Iwein Fuld.

Appendix B. Configuration

B.1 Introduction

Spring Integration offers a number of configuration options. Which option you choose depends upon your particular needs and at what level you prefer to work. As with the Spring framework in general, it is also possible to mix and match the various techniques according to the particular problem at hand. For example, you may choose the XSD-based namespace for the majority of configuration combined with a handful of objects that are configured with annotations. As much as possible, the two provide consistent naming. XML elements defined by the XSD schema will match the names of annotations, and the attributes of those XML elements will match the names of annotation properties. Direct usage of the API is of course always an option, but we expect that most users will choose one of the higher-level options, or a combination of the namespace-based and annotation-driven configuration.

B.2 Namespace Support

Spring Integration components can be configured with XML elements that map directly to the terminology and concepts of enterprise integration. In many cases, the element names match those of the Enterprise Integration Patterns.

To enable Spring Integration's core namespace support within your Spring configuration files, add the following namespace reference and schema mapping in your top-level 'beans' element:

<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:integration="http://www.springframework.org/schema/integration"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/integration
           http://www.springframework.org/schema/integration/spring-integration-1.0.xsd">

You can choose any name after "xmlns:"; integration is used here for clarity, but you might prefer a shorter abbreviation. Of course if you are using an XML-editor or IDE support, then the availability of auto-completion may convince you to keep the longer name for clarity. Alternatively, you can create configuration files that use the Spring Integration schema as the primary namespace:

<beans:beans xmlns="http://www.springframework.org/schema/integration"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:beans="http://www.springframework.org/schema/beans"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
           http://www.springframework.org/schema/integration
           http://www.springframework.org/schema/integration/spring-integration-1.0.xsd">

When using this alternative, no prefix is necessary for the Spring Integration elements. On the other hand, if you want to define a generic Spring "bean" within the same configuration file, then a prefix would be required for the bean element (<beans:bean ... />). Since it is generally a good idea to modularize the configuration files themselves based on responsibility and/or architectural layer, you may find it appropriate to use the latter approach in the integration-focused configuration files, since generic beans are seldom necessary within those same files. For purposes of this documentation, we will assume the "integration" namespace is primary.

Many other namespaces are provided within the Spring Integration distribution. In fact, each adapter type (JMS, File, etc.) that provides namespace support defines its elements within a separate schema. In order to use these elements, simply add the necessary namespaces with an "xmlns" entry and the corresponding "schemaLocation" mapping. For example, the following root element shows several of these namespace declarations:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:integration="http://www.springframework.org/schema/integration"
    xmlns:file="http://www.springframework.org/schema/integration/file"
    xmlns:jms="http://www.springframework.org/schema/integration/jms"
    xmlns:mail="http://www.springframework.org/schema/integration/mail"
    xmlns:rmi="http://www.springframework.org/schema/integration/rmi"
    xmlns:ws="http://www.springframework.org/schema/integration/ws"
    xsi:schemaLocation="http://www.springframework.org/schema/beans
            http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
            http://www.springframework.org/schema/integration
            http://www.springframework.org/schema/integration/spring-integration-1.0.xsd
            http://www.springframework.org/schema/integration/file
            http://www.springframework.org/schema/integration/file/spring-integration-file-1.0.xsd
            http://www.springframework.org/schema/integration/jms
            http://www.springframework.org/schema/integration/jms/spring-integration-jms-1.0.xsd
            http://www.springframework.org/schema/integration/mail
            http://www.springframework.org/schema/integration/mail/spring-integration-mail-1.0.xsd
            http://www.springframework.org/schema/integration/rmi
            http://www.springframework.org/schema/integration/rmi/spring-integration-rmi-1.0.xsd
            http://www.springframework.org/schema/integration/ws
            http://www.springframework.org/schema/integration/ws/spring-integration-ws-1.0.xsd">
 ...
</beans>

The reference manual provides specific examples of the various elements in their corresponding chapters. Here, the main thing to recognize is the consistency of the naming for each namespace URI and schema location.

B.3 Configuring the Task Scheduler

In Spring Integration, the ApplicationContext plays the central role of a Message Bus, and there are only a couple configuration options to be aware of. First, you may want to control the central TaskScheduler instance. You can do so by providing a single bean with the name "taskScheduler". This is also defined as a constant:

 IntegrationContextUtils.TASK_SCHEDULER_BEAN_NAME 

By default Spring Integration uses the SimpleTaskScheduler implementation. That in turn just delegates to any instance of Spring's TaskExecutor abstraction. Therefore, it's rather trivial to supply your own configuration. The "taskScheduler" bean is then responsible for managing all pollers. The TaskScheduler will startup automatically by default. If you provide your own instance of SimpleTaskScheduler however, you can set the 'autoStartup' property to false instead.

When Polling Consumers provide an explicit task-executor reference in their configuration, the invocation of the handler methods will happen within that executor's thread pool and not the main scheduler pool. However, when no task-executor is provided for an endpoint's poller, it will be invoked by one of the main scheduler's threads.

[Note]Note
An endpoint is a Polling Consumer if its input channel is one of the queue-based (i.e. pollable) channels. On the other hand, Event Driven Consumers are those whose input channels have dispatchers instead of queues (i.e. they are subscribable). Such endpoints have no poller configuration since their handlers will be invoked directly.

The next section will describe what happens if Exceptions occur within the asynchronous invocations.

B.4 Error Handling

As described in the overview at the very beginning of this manual, one of the main motivations behind a Message-oriented framework like Spring Integration is to promote loose-coupling between components. The Message Channel plays an important role in that producers and consumers do not have to know about each other. However, the advantages also have some drawbacks. Some things become more complicated in a very loosely coupled environment, and one example is error handling.

When sending a Message to a channel, the component that ultimately handles that Message may or may not be operating within the same thread as the sender. If using a simple default DirectChannel (with the <channel> element that has no <queue> sub-element and no 'task-executor' attribute), the Message-handling will occur in the same thread as the Message-sending. In that case, if an Exception is thrown, it can be caught by the sender (or it may propagate past the sender if it is an uncaught RuntimeException). So far, everything is fine. This is the same behavior as an Exception-throwing operation in a normal call stack. However, when adding the asynchronous aspect, things become much more complicated. For instance, if the 'channel' element does provide a 'queue' sub-element, then the component that handles the Message will be operating in a different thread than the sender. The sender may have dropped the Message into the channel and moved on to other things. There is no way for the Exception to be thrown directly back to that sender using standard Exception throwing techniques. Instead, to handle errors for asynchronous processes requires an asynchronous error-handling mechanism as well.

Spring Integration supports error handling for its components by publishing errors to a Message Channel. Specifically, the Exception will become the payload of a Spring Integration Message. That Message will then be sent to a Message Channel that is resolved in a way that is similar to the 'replyChannel' resolution. First, if the request Message being handled at the time the Exception occurred contains an 'errorChannel' header (the header name is defined in the constant: MessageHeaders.ERROR_CHANNEL), the ErrorMessage will be sent to that channel. Otherwise, the error handler will send to a "global" channel whose bean name is "errorChannel" (this is also defined as a constant: IntegrationContextUtils.ERROR_CHANNEL_BEAN_NAME).

Whenever relying on Spring Integration's XML namespace support, a default "errorChannel" bean will be created behind the scenes. However, you can just as easily define your own if you want to control the settings.

 <channel id="errorChannel">
     <queue capacity="500"/>
 </channel>

[Note]Note
The default "errorChannel" is a PublishSubscribeChannel.

The most important thing to understand here is that the messaging-based error handling will only apply to Exceptions that are thrown by a Spring Integration task that is executing within a TaskExecutor. This does not apply to Exceptions thrown by a handler that is operating within the same thread as the sender (e.g. through a DirectChannel as described above).

[Note]Note
When Exceptions occur in a scheduled poller task's execution, those exceptions will be wrapped in ErrorMessages and sent to the 'errorChannel' as well.

To enable global error handling, simply register a handler on that channel. For example, you can configure Spring Integration's ErrorMessageExceptionTypeRouter as the handler of an endpoint that is subscribed to the 'errorChannel'. That router can then spread the error messages across multiple channels based on Exception type.

B.5 Annotation Support

In addition to the XML namespace support for configuring Message Endpoints, it is also possible to use annotations. First, Spring Integration provides the class-level @MessageEndpoint as a stereotype annotation meaning that is itself annotated with Spring's @Component annotation and therefore is recognized automatically as a bean definition when using Spring component-scanning.

Even more importantly are the various Method-level annotations that indicate the annotated method is capable of handling a message. The following example demonstrates both:

@MessageEndpoint
public class FooService {

    @ServiceActivator
    public void processMessage(Message message) {
        ...
    }
}

Exactly what it means for the method to "handle" the Message depends on the particular annotation. The following are available with Spring Integration, and the behavior of each is described in its own chapter or section within this reference: @Transformer, @Router, @Splitter, @Aggregator, @ServiceActivator, and @ChannelAdapter.

[Note]Note
The @MessageEndpoint is not required if using XML configuration in combination with annotations. If you want to configure a POJO reference from the "ref" attribute of a <service-activator/> element, it is sufficient to provide the method-level annotations. In that case, the annotation prevents ambiguity even when no "method" attribute exists on the <service-activator/> element.

In most cases, the annotated handler method should not require the Message type as its parameter. Instead, the method parameter type can match the message's payload type.

public class FooService {

    @Handler
    public void bar(Foo foo) {
        ...
    }

}

When the method parameter should be mapped from a value in the MessageHeaders, another option is to use the parameter-level @Header annotation. In general, methods annotated with the Spring Integration annotations can either accept the Message itself, the message payload, or a header value (with @Header) as the parameter. In fact, the method can accept a combination, such as:

public class FooService {

    @ServiceActivator
    public void bar(String payload, @Header("x") int valueX, @Header("y") int valueY) {
        ...
    }

}

There is also a @Headers annotation that provides all of the Message headers as a Map:

public class FooService {

    @ServiceActivator
    public void bar(String payload, @Headers Map<String, Object> headerMap) {
        ...
    }

}

[Tip]Tip
A Map-typed argument does not strictly require the use of the @Headers annotation. In other words the following is also valid:
public void bar(String payload, Map<String, Object> headerMap)
However this can lead to unresolvable ambiguities if the payload is itself a Map. For that reason, we highly recommend using the annotation whenever expecting the headers. For a much more detailed description, see the javadoc for MethodParameterMessageMapper.

For several of these annotations, when a Message-handling method returns a non-null value, the endpoint will attempt to send a reply. This is consistent across both configuration options (namespace and annotations) in that such an endpoint's output channel will be used if available, and the REPLY_CHANNEL message header value will be used as a fallback.

[Tip]Tip
The combination of output channels on endpoints and the reply channel message header enables a pipeline approach where multiple components have an output channel, and the final component simply allows the reply message to be forwarded to the reply channel as specified in the original request message. In other words, the final component depends on the information provided by the original sender and can dynamically support any number of clients as a result. This is an example of Return Address.

In addition to the examples shown here, these annotations also support inputChannel and outputChannel properties.

public class FooService {

    @ServiceActivator(inputChannel="input", outputChannel="output")
    public void bar(String payload, @Headers Map<String, Object> headerMap) {
        ...
    }

}

That provides a pure annotation-driven alternative to the XML configuration. However, it is generally recommended to use XML for the endpoints, since it is easier to keep track of the overall configuration in a single, external location (and besides the namespace-based XML configuration is not very verbose). If you do prefer to provide channels with the annotations however, you just need to enable a BeanPostProcessor. The following element should be added:

 <annotation-config/> 

[Note]Note
When configuring the "inputChannel" and "outputChannel" with annotations, the "inputChannel" must be a reference to a SubscribableChannel instance. Otherwise, it would be necessary to also provide the full poller configuration via annotations, and those settings (e.g. the trigger for scheduling the poller) should be externalized rather than hard-coded within an annotation. If the input channel that you want to receive Messages from is indeed a PollableChannel instance, one option to consider is the Messaging Bridge. Spring Integration's "bridge" element can be used to connect a PollableChannel directly to a SubscribableChannel. Then, the polling metadata is externally configured, but the annotation option is still available. For more detail see Chapter 15, Messaging Bridge.

Appendix C. Additional Resources

C.1 Spring Integration Home

The definitive source of information about Spring Integration is the Spring Integration Home at http://www.springsource.org. That site serves as a hub of information and is the best place to find up-to-date announcements about the project as well as links to articles, blogs, and new sample applications.