For the latest stable version, please use Spring Integration 6.3.2! |
Message
The Spring Integration Message
is a generic container for data.
Any object can be provided as the payload, and each Message
instance includes headers containing user-extensible properties as key-value pairs.
The Message
Interface
The following listing shows the definition of the Message
interface:
public interface Message<T> {
T getPayload();
MessageHeaders getHeaders();
}
The Message
interface is a core 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 or extended, the messaging system is not affected.
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.
Message Headers
Just as Spring Integration lets any Object
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
, as the following class definition shows:
public final class MessageHeaders implements Map<String, Object>, Serializable {
...
}
Even though the MessageHeaders class implements Map , it is effectively a read-only implementation.
Any attempt to put a value in the Map results 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.
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As an implementation of Map
, the headers can 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.
The following example shows 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 table describes the pre-defined message headers:
Header Name | Header Type | Usage |
---|---|---|
MessageHeaders.ID |
java.util.UUID |
An identifier for this message instance. Changes each time a message is mutated. |
MessageHeaders. TIMESTAMP |
java.lang.Long |
The time the message was created. Changes each time a message is mutated. |
MessageHeaders. REPLY_CHANNEL |
java.lang.Object (String or MessageChannel) |
A channel to which a reply (if any) is sent when no explicit output channel is configured and there is no |
MessageHeaders. ERROR_CHANNEL |
java.lang.Object (String or MessageChannel) |
A channel to which errors are sent.
If the value is a |
Many inbound and outbound adapter implementations also provide or expect certain headers, and you can configure additional user-defined headers.
Constants for these headers can be found in those modules where such headers exist — for example.
AmqpHeaders
, JmsHeaders
, and so on.
MessageHeaderAccessor
API
Starting with Spring Framework 4.0 and Spring Integration 4.0, the core messaging abstraction has been moved to the spring-messaging
module, and the MessageHeaderAccessor
API has been introduced to provide additional abstraction over messaging implementations.
All (core) Spring Integration-specific message headers constants are now declared in the IntegrationMessageHeaderAccessor
class.
The following table describes the pre-defined message headers:
Header Name | Header Type | Usage |
---|---|---|
IntegrationMessageHeaderAccessor. CORRELATION_ID |
java.lang.Object |
Used to correlate two or more messages. |
IntegrationMessageHeaderAccessor. SEQUENCE_NUMBER |
java.lang.Integer |
Usually a sequence number with a group of messages with a |
IntegrationMessageHeaderAccessor. SEQUENCE_SIZE |
java.lang.Integer |
The number of messages within a group of correlated messages. |
IntegrationMessageHeaderAccessor. EXPIRATION_DATE |
java.lang.Long |
Indicates when a message is expired.
Not used by the framework directly but can be set with a header enricher and used in a |
IntegrationMessageHeaderAccessor. PRIORITY |
java.lang.Integer |
Message priority — for example, within a |
IntegrationMessageHeaderAccessor. DUPLICATE_MESSAGE |
java.lang.Boolean |
True if a message was detected as a duplicate by an idempotent receiver interceptor. See Idempotent Receiver Enterprise Integration Pattern. |
IntegrationMessageHeaderAccessor. CLOSEABLE_RESOURCE |
java.io.Closeable |
This header is present if the message is associated with a |
IntegrationMessageHeaderAccessor. DELIVERY_ATTEMPT |
java.lang. AtomicInteger |
If a message-driven channel adapter supports the configuration of a |
IntegrationMessageHeaderAccessor. ACKNOWLEDGMENT_CALLBACK |
o.s.i.support. Acknowledgment Callback |
If an inbound endpoint supports it, a call back to accept, reject, or requeue a message. See Deferred Acknowledgment Pollable Message Source and MQTT Manual Acks. |
Convenient typed getters for some of these headers are provided on the IntegrationMessageHeaderAccessor
class, as the following example shows:
IntegrationMessageHeaderAccessor accessor = new IntegrationMessageHeaderAccessor(message);
int sequenceNumber = accessor.getSequenceNumber();
Object correlationId = accessor.getCorrelationId();
...
The following table describes headers that also appear in the IntegrationMessageHeaderAccessor
but are generally not used by user code (that is, they are generally used by internal parts of Spring Integration — their inclusion here is for completeness):
Header Name | Header Type | Usage |
---|---|---|
IntegrationMessageHeaderAccessor. SEQUENCE_DETAILS |
java.util. List<List<Object>> |
A stack of correlation data used when nested correlation is needed (for example,
|
IntegrationMessageHeaderAccessor. ROUTING_SLIP |
java.util. Map<List<Object>, Integer> |
See Routing Slip. |
Message ID Generation
When a message transitions through an application, each time it is mutated (for example,
by a transformer) a new message ID is assigned.
The message ID is a UUID
.
Beginning with Spring Integration 3.0, the default strategy used for IS generation is more efficient than the previous java.util.UUID.randomUUID()
implementation.
It uses simple random numbers based on a secure random seed instead of creating a secure random number each time.
A different UUID generation strategy can be selected by declaring a bean that implements org.springframework.util.IdGenerator
in the application context.
Only one UUID generation strategy can be used in a classloader.
This means that, if two or more application contexts run in the same classloader, they share the same strategy.
If one of the contexts changes the strategy, it is used by all contexts.
If two or more contexts in the same classloader declare a bean of type org.springframework.util.IdGenerator , they must all be an instance of the same class.
Otherwise, the context attempting to replace a custom strategy fails to initialize.
If the strategy is the same, but parameterized, the strategy in the first context to be initialized is used.
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In addition to the default strategy, two additional IdGenerators
are provided.
org.springframework.util.JdkIdGenerator
uses the previous UUID.randomUUID()
mechanism.
You can use o.s.i.support.IdGenerators.SimpleIncrementingIdGenerator
when a UUID is not really needed and a simple incrementing value is sufficient.
Read-only Headers
The MessageHeaders.ID
and MessageHeaders.TIMESTAMP
are read-only headers and cannot be overridden.
Since version 4.3.2, the MessageBuilder
provides the readOnlyHeaders(String… readOnlyHeaders)
API to customize a list of headers that should not be copied from an upstream Message
.
Only the MessageHeaders.ID
and MessageHeaders.TIMESTAMP
are read only by default.
The global spring.integration.readOnly.headers
property (see Global Properties) is provided to customize DefaultMessageBuilderFactory
for framework components.
This can be useful when you would like do not populate some out-of-the-box headers, such as contentType
by the ObjectToJsonTransformer
(see JSON Transformers).
When you try to build a new message using MessageBuilder
, this kind of header is ignored and a particular INFO
message is emitted to logs.
Starting with version 5.0, Messaging Gateway, Header Enricher, Content Enricher and Header Filter do not let you configure the MessageHeaders.ID
and MessageHeaders.TIMESTAMP
header names when DefaultMessageBuilderFactory
is used, and they throw BeanInitializationException
.
Header Propagation
When messages are processed (and modified) by message-producing endpoints (such as a service activator), in general, inbound headers are propagated to the outbound message. One exception to this is a transformer, when a complete message is returned to the framework. In that case, the user code is responsible for the entire outbound message. When a transformer just returns the payload, the inbound headers are propagated. Also, a header is only propagated if it does not already exist in the outbound message, letting you change header values as needed.
Starting with version 4.3.10, you can configure message handlers (that modify messages and produce output) to suppress the propagation of specific headers.
To configure the header(s) you do not want to be copied, call the setNotPropagatedHeaders()
or addNotPropagatedHeaders()
methods on the MessageProducingMessageHandler
abstract class.
You can also globally suppress propagation of specific message headers by setting the readOnlyHeaders
property in META-INF/spring.integration.properties
to a comma-delimited list of headers.
Starting with version 5.0, the setNotPropagatedHeaders()
implementation on the AbstractMessageProducingHandler
applies simple patterns (xxx*
, xxx
, *xxx
, or xxx*yyy
) to allow filtering headers with a common suffix or prefix.
See PatternMatchUtils
Javadoc for more information.
When one of the patterns is *
(asterisk), no headers are propagated.
All other patterns are ignored.
In that case, the service activator behaves the same way as a transformer and any required headers must be supplied in the Message
returned from the service method.
The notPropagatedHeaders()
option is available in the ConsumerEndpointSpec
for the Java DSL
It is also available for XML configuration of the <service-activator>
component as a not-propagated-headers
attribute.
Message Implementations
The base implementation of the Message
interface is GenericMessage<T>
, and it provides two constructors, shown in the following listing:
new GenericMessage<T>(T payload);
new GenericMessage<T>(T payload, Map<String, Object> headers)
When a Message
is created, a random unique ID is generated.
The constructor that accepts a Map
of headers copies the provided headers to the newly created Message
.
There is also a convenient implementation of Message
designed to communicate error conditions.
This implementation takes a Throwable
object as its payload, as the following example shows:
ErrorMessage message = new ErrorMessage(someThrowable);
Throwable t = message.getPayload();
Note that this implementation takes 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
.
The MessageBuilder
Helper Class
You may notice that the Message
interface defines retrieval methods for its payload and headers but provides 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 (for example,
through a publish-subscribe Channel), if one of those consumers needs to send a reply with a different payload type, it must 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 you.
In other words, the contract for Message
instances 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 a MessageHeaders
instance results 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 Message
instances from either an existing Message
or with a payload Object
.
When building from an existing Message
, the headers and payload of that Message
are copied to the new Message
, as the following example shows:
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, as the following example shows:
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"));
Note that the copyHeadersIfAbsent
method does not overwrite existing values.
Also, in the preceding example, 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).
You can also use MessageBuilder
to set the priority of messages, as the following example shows:
Message<Integer> importantMessage = MessageBuilder.withPayload(99)
.setPriority(5)
.build();
assertEquals(5, importantMessage.getHeaders().getPriority());
Message<Integer> lessImportantMessage = MessageBuilder.fromMessage(importantMessage)
.setHeaderIfAbsent(IntegrationMessageHeaderAccessor.PRIORITY, 2)
.build();
assertEquals(2, lessImportantMessage.getHeaders().getPriority());
The priority
header is considered only when using a PriorityChannel
(as described in the next chapter).
It is defined as a java.lang.Integer
.