28. TCP and UDP Support

28. TCP and UDP Support
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28.1 Introduction

Two flavors each of UDP inbound and outbound channel adapters are provided UnicastSendingMessageHandler sends a datagram packet to a single destination. UnicastReceivingChannelAdapter receives incoming datagram packets. MulticastSendingMessageHandler sends (broadcasts) datagram packets to a multicast address. MulticastReceivingChannelAdapter receives incoming datagram packets by joining to a multicast address.

TCP inbound and outbound channel adapters are provided TcpSendingMessageHandler sends messages over TCP. TcpReceivingChannelAdapter receives messages over TCP.

An inbound TCP gateway is provided; this allows for simple request/response processing. While the gateway can support any number of connections, each connection can only process serially. The thread that reads from the socket waits for, and sends, the response before reading again. If the connection factory is configured for single use connections, the connection is closed after the socket times out.

An outbound TCP gateway is provided; this allows for simple request/response processing. If the associated connection factory is configured for single use connections, a new connection is immediately created for each new request. Otherwise, if the connection is in use, the calling thread blocks on the connection until either a response is received or a timeout or I/O error occurs.

The TCP and UDP inbound channel adapters, and the TCP inbound gateway, support the "error-channel" attribute. This provides the same basic functionality as described in Section 7.2.1, “Enter the GatewayProxyFactoryBean”.

28.2 UDP Adapters

<int-ip:udp-outbound-channel-adapter id="udpOut"
    host="somehost"
    port="11111"
    multicast="false"
    channel="exampleChannel"/>

A simple UDP outbound channel adapter.

	Tip
	When setting multicast to true, provide the multicast address in the host attribute.

UDP is an efficient, but unreliable protocol. Two attributes are added to improve reliability. When check-length is set to true, the adapter precedes the message data with a length field (4 bytes in network byte order). This enables the receiving side to verify the length of the packet received. If a receiving system uses a buffer that is too short the contain the packet, the packet can be truncated. The length header provides a mechanism to detect this.

<int-ip:udp-outbound-channel-adapter id="udpOut"
    host="somehost"
    port="11111"
    multicast="false"
    check-length="true"
    channel="exampleChannel"/>

An outbound channel adapter that adds length checking to the datagram packets.

	Tip
	The recipient of the packet must also be configured to expect a length to precede the actual data. For a Spring Integration UDP inbound channel adapter, set its `check-length` attribute.

The second reliability improvement allows an application-level acknowledgment protocol to be used. The receiver must send an acknowledgment to the sender within a specified time.

<int-ip:udp-outbound-channel-adapter id="udpOut"
    host="somehost"
    port="11111"
    multicast="false"
    check-length="true"
    acknowledge="true"
    ack-host="thishost"
    ack-port="22222"
    ack-timeout="10000"
    channel="exampleChannel"/>

An outbound channel adapter that adds length checking to the datagram packets and waits for an acknowledgment.

	Tip
	Setting acknowledge to true implies the recipient of the packet can interpret the header added to the packet containing acknowledgment data (host and port). Most likely, the recipient will be a Spring Integration inbound channel adapter.

	Tip
	When multicast is true, an additional attribute min-acks-for-success specifies how many acknowledgments must be received within the ack-timeout.

For even more reliable networking, TCP can be used.

<int-ip:udp-inbound-channel-adapter id="udpReceiver"
    channel="udpOutChannel"
    port="11111"
    receive-buffer-size="500"
    multicast="false"
    check-length="true"/>

A basic unicast inbound udp channel adapter.

<int-ip:udp-inbound-channel-adapter id="udpReceiver"
    channel="udpOutChannel"
    port="11111"
    receive-buffer-size="500"
    multicast="true"
    multicast-address="225.6.7.8"
    check-length="true"/>

A basic multicast inbound udp channel adapter.

By default, reverse DNS lookups are done on inbound packets to convert IP addresses to hostnames for use in message headers. In environments where DNS is not configured, this can cause delays. This default behavior can be overridden by setting the lookup-host attribute to "false".

28.3 TCP Connection Factories

For TCP, the configuration of the underlying connection is provided using a Connection Factory. Two types of connection factory are provided; a client connection factory and a server connection factory. Client connection factories are used to establish outgoing connections; Server connection factories listen for incoming connections.

A client connection factory is used by an outbound channel adapter but a reference to a client connection factory can also be provided to an inbound channel adapter and that adapter will receive any incoming messages received on connections created by the outbound adapter.

A server connection factory is used by an inbound channel adapter or gateway (in fact the connection factory will not function without one). A reference to a server connection factory can also be provided to an outbound adapter; that adapter can then be used to send replies to incoming messages to the same connection.

	Tip
	Reply messages will only be routed to the connection if the reply contains the header ip_connection_id that was inserted into the original message by the connection factory.

Tip

This is the extent of message correlation performed when sharing connection factories between inbound and outbound adapters. Such sharing allows for asynchronous two-way communication over TCP. By default, only payload information is transferred using TCP; therefore any message correlation must be performed by downstream components such as aggregators or other endpoints. Support for transferring selected headers was introduced in version 3.0. For more information refer to Section 28.8, “TCP Message Correlation”.

A maximum of one adapter of each type may be given a reference to a connection factory.

Connection factories using java.net.Socket and java.nio.channel.SocketChannel are provided.

<int-ip:tcp-connection-factory id="server"
    type="server"
    port="1234"/>

A simple server connection factory that uses java.net.Socket connections.

<int-ip:tcp-connection-factory id="server"
    type="server"
    port="1234"
    using-nio="true"/>

A simple server connection factory that uses java.nio.channel.SocketChannel connections.

<int-ip:tcp-connection-factory id="client"
    type="client"
    host="localhost"
    port="1234"
    single-use="true"
    so-timeout="10000"/>

A client connection factory that uses java.net.Socket connections and creates a new connection for each message.

<int-ip:tcp-connection-factory id="client"
    type="client"
    host="localhost"
    port="1234"
    single-use="true"
    so-timeout="10000"
    using-nio=true/>

A client connection factory that uses java.nio.channel.Socket connections and creates a new connection for each message.

TCP is a streaming protocol; this means that some structure has to be provided to data transported over TCP, so the receiver can demarcate the data into discrete messages. Connection factories are configured to use (de)serializers to convert between the message payload and the bits that are sent over TCP. This is accomplished by providing a deserializer and serializer for inbound and outbound messages respectively. A number of standard (de)serializers are provided.

The ByteArrayCrlfSerializer, converts a byte array to a stream of bytes followed by carriage return and linefeed characters (\r\n). This is the default (de)serializer and can be used with telnet as a client, for example.

The ByteArraySingleTerminatorSerializer, converts a byte array to a stream of bytes followed by a single termination character (default 0x00).

The ByteArrayLfSerializer, converts a byte array to a stream of bytes followed by a single linefeed character (0x0a).

The ByteArrayStxEtxSerializer, converts a byte array to a stream of bytes preceded by an STX (0x02) and followed by an ETX (0x03).

The ByteArrayLengthHeaderSerializer, converts a byte array to a stream of bytes preceded by a binary length in network byte order (big endian). This a very efficient deserializer because it does not have to parse every byte looking for a termination character sequence. It can also be used for payloads containing binary data; the above serializers only support text in the payload. The default size of the length header is 4 bytes (Integer), allowing for messages up to 2**31-1 bytes. However, the length header can be a single byte (unsigned) for messages up to 255 bytes, or an unsigned short (2 bytes) for messages up to 2**16 bytes. If you need any other format for the header, you can subclass this class and provide implementations for the readHeader and writeHeader methods. The absolute maximum data size supported is 2**31-1 bytes.

The ByteArrayRawSerializer, converts a byte array to a stream of bytes and adds no additional message demarcation data; with this (de)serializer, the end of a message is indicated by the client closing the socket in an orderly fashion. When using this serializer, message reception will hang until the client closes the socket, or a timeout occurs; a timeout will NOT result in a message. When this serializer is being used, and the client is a Spring Integration application, the client must use a connection factory that is configured with single-use=true - this causes the adapter to close the socket after sending the message; the serializer will not, itself, close the connection. This serializer should only be used with connection factories used by channel adapters (not gateways), and the connection factories should be used by either an inbound or outbound adapter, and not both.

Each of these is a subclass of AbstractByteArraySerializer which implements both org.springframework.core.serializer.Serializer and org.springframework.core.serializer.Deserializer. For backwards compatibility, connections using any subclass of AbstractByteArraySerializer for serialization will also accept a String which will be converted to a byte array first. Each of these (de)serializers converts an input stream containing the corresponding format to a byte array payload.

To avoid memory exhaustion due to a badly behaved client (one that does not adhere to the protocol of the configured serializer), these serializers impose a maximum message size. If the size is exceeded by an incoming message, an exception will be thrown. The default maximum message size is 2048 bytes, and can be increased by setting the maxMessageSize property. If you are using the default (de)serializer and wish to increase the maximum message size, you must declare it as an explicit bean with the property set and configure the connection factory to use that bean.

The MapJsonSerializer uses a Jackson ObjectMapper to convert between a Map and JSON. This can be used in conjunction with a MessageConvertingTcpMessageMapper and a MapMessageConverter to transfer selected headers and the payload in a JSON format.

	Note
	The Jackson `ObjectMapper` cannot demarcate messages in the stream. Therefore, the `MapJsonSerializer` needs to delegate to another (de)serializer to handle message demarcation. By default, a `ByteArrayLfSerializer` is used, resulting in messages with the format `<json><LF>` on the wire, but you can configure it to use others instead.

The final standard serializer is org.springframework.core.serializer.DefaultSerializer which can be used to convert Serializable objects using java serialization. org.springframework.core.serializer.DefaultDeserializer is provided for inbound deserialization of streams containing Serializable objects.

To implement a custom (de)serializer pair, implement the org.springframework.core.serializer.Deserializer and org.springframework.core.serializer.Serializer interfaces.

If you do not wish to use the default (de)serializer (ByteArrayCrLfSerializer), you must supply serializer and deserializer attributes on the connection factory (example below).

<bean id="javaSerializer"
      class="org.springframework.core.serializer.DefaultSerializer" />
<bean id="javaDeserializer"
      class="org.springframework.core.serializer.DefaultDeserializer" />

<int-ip:tcp-connection-factory id="server"
    type="server"
    port="1234"
    deserializer="javaDeserializer"
    serializer="javaSerializer"/>

A server connection factory that uses java.net.Socket connections and uses Java serialization on the wire.

For full details of the attributes available on connection factories, see the reference at the end of this section.

By default, reverse DNS lookups are done on inbound packets to convert IP addresses to hostnames for use in message headers. In environments where DNS is not configured, this can cause connection delays. This default behavior can be overridden by setting the lookup-host attribute to "false".

	Note
	It is possible to modify the creation of and/or attributes of sockets - see Section 28.10, “SSL/TLS Support”. As is noted there, such modifications are possible whether or not SSL is being used.

28.3.1 TCP Caching Client Connection Factory

As noted above, TCP sockets cam be 'single-use' (one request/response) or shared. Shared sockets do not perform well with outbound gateways, in high-volume environments, because the socket can only process one request/response at a time.

To improve performance, users could use collaborating channel adapters instead of gateways, but that requires application-level message correlation. See Section 28.8, “TCP Message Correlation”for more information.

Spring Integration 2.2 introduced a caching client connection factory, where a pool of shared sockets is used, allowing a gateway to process multiple concurrent requests with a pool of shared connections.

28.3.2 TCP Failover Client Connection Factory

It is now possible to configure a TCP connection factory that supports failover to one or more other servers. When sending a message, the factory will iterate over all its configured factories until either the message can be sent, or no connection can be found. Initially, the first factory in the configured list is used; if a connection subsequently fails the next factory will become the current factory.

<bean id="failCF" class="o.s.i.ip.tcp.connection.FailoverClientConnectionFactory">
    <constructor-arg>
        <list>
            <ref bean="clientFactory1"/>
            <ref bean="clientFactory2"/>
        </list>
    </constructor-arg>
</bean>

	Note
	When using the failover connection factory, the singleUse property must be consistent between the factory itself and the list of factories it is configured to use.

28.4 TCP Connection Interceptors

Connection factories can be configured with a reference to a TcpConnectionInterceptorFactoryChain. Interceptors can be used to add behavior to connections, such as negotiation, security, and other setup. No interceptors are currently provided by the framework but, for an example, see the InterceptedSharedConnectionTests in the source repository.

The HelloWorldInterceptor used in the test case works as follows:

When configured with a client connection factory, when the first message is sent over a connection that is intercepted, the interceptor sends 'Hello' over the connection, and expects to receive 'world!'. When that occurs, the negotiation is complete and the original message is sent; further messages that use the same connection are sent without any additional negotiation.

When configured with a server connection factory, the interceptor requires the first message to be 'Hello' and, if it is, returns 'world!'. Otherwise it throws an exception causing the connection to be closed.

All TcpConnection methods are intercepted. Interceptor instances are created for each connection by an interceptor factory. If an interceptor is stateful, the factory should create a new instance for each connection. Interceptor factories are added to the configuration of an interceptor factory chain, which is provided to a connection factory using the interceptor-factory attribute. Interceptors must implement the TcpConnectionInterceptor interface; factories must implement the TcpConnectionInterceptorFactory interface. A convenience class AbstractTcpConnectionInterceptor is provided with passthrough methods; by extending this class, you only need to implement those methods you wish to intercept.

<bean id="helloWorldInterceptorFactory"
    class="o.s.i.ip.tcp.connection.TcpConnectionInterceptorFactoryChain">
    <property name="interceptors">
        <array>
            <bean class="o.s.i.ip.tcp.connection.HelloWorldInterceptorFactory"/>
        </array>
    </property>
</bean>

<int-ip:tcp-connection-factory id="server"
    type="server"
    port="12345"
    using-nio="true"
    single-use="true"
    interceptor-factory-chain="helloWorldInterceptorFactory"/>

<int-ip:tcp-connection-factory id="client"
    type="client"
    host="localhost"
    port="12345"
    single-use="true"
    so-timeout="100000"
    using-nio="true"
    interceptor-factory-chain="helloWorldInterceptorFactory"/>

Configuring a connection interceptor factory chain.

28.5 TCP Connection Events

Beginning with version 3.0, changes to TcpConnections are reported by TcpConnectionEvents. TcpConnectionEvent is a subclass of ApplicationEvent and thus can be received by any ApplicationListener defined in the ApplicationContext.

For convenience, a <int-ip:tcp-connection-event-inbound-channel-adapter/> is provided. This adapter will receive all TcpConnectionEvents (by default), and send them to its channel. The adapter accepts an event-type attribute, which is a list of class names for events that should be sent. This can be used if an application subclasses TcpConnectionEvent for some reason, and wishes to only receive those events. Omitting this attribute will mean that all TcpConnectionEvents will be sent. You can also use this to limit which TcpConnectionEvents you are interested in ( TcpConnectionOpenEvent, TcpConnectionCloseEvent, or TcpConnectionExceptionEvent).

TcpConnectionEvents have the following properties:

connectionId - the connection identifier which can be used in a message header to send data to the connection
connectionFactoryName - the bean name of the connection factory the connection belongs to
throwable - the Throwable (for TcpConnectionExceptionEvent events only)
source - the TcpConnection; this can be used, for example, to determine the remote IP Address with getHostAddress() (cast required)

28.6 TCP Adapters

TCP inbound and outbound channel adapters that utilize the above connection factories are provided. These adapters have attributes connection-factory and channel. The channel attribute specifies the channel on which messages arrive at an outbound adapter and on which messages are placed by an inbound adapter. The connection-factory attribute indicates which connection factory is to be used to manage connections for the adapter. While both inbound and outbound adapters can share a connection factory, server connection factories are always 'owned' by an inbound adapter; client connection factories are always 'owned' by an outbound adapter. One, and only one, adapter of each type may get a reference to a connection factory.

<bean id="javaSerializer"
      class="org.springframework.core.serializer.DefaultSerializer"/>
<bean id="javaDeserializer"
      class="org.springframework.core.serializer.DefaultDeserializer"/>

<int-ip:tcp-connection-factory id="server"
    type="server"
    port="1234"
    deserializer="javaDeserializer"
    serializer="javaSerializer"
    using-nio="true"
    single-use="true"/>

<int-ip:tcp-connection-factory id="client"
    type="client"
    host="localhost"
    port="#{server.port}"
    single-use="true"
    so-timeout="10000"
    deserializer="javaDeserializer"
    serializer="javaSerializer"/>

<int:channel id="input" />

<int:channel id="replies">
    <int:queue/>
</int:channel>

<int-ip:tcp-outbound-channel-adapter id="outboundClient"
    channel="input"
    connection-factory="client"/>

<int-ip:tcp-inbound-channel-adapter id="inboundClient"
    channel="replies"
    connection-factory="client"/>

<int-ip:tcp-inbound-channel-adapter id="inboundServer"
    channel="loop"
    connection-factory="server"/>

<int-ip:tcp-outbound-channel-adapter id="outboundServer"
    channel="loop"
    connection-factory="server"/>

<int:channel id="loop"/>

In this configuration, messages arriving in channel 'input' are serialized over connections created by 'client' received at the server and placed on channel 'loop'. Since 'loop' is the input channel for 'outboundServer' the message is simply looped back over the same connection and received by 'inboundClient' and deposited in channel 'replies'. Java serialization is used on the wire.

Normally, inbound adapters use a type="server" connection factory, which listens for incoming connection requests. In some cases, it is desireable to establish the connection in reverse, whereby the inbound adapter connects to an external server and then waits for inbound messages on that connection.

This topology is supported by using client-mode="true" on the inbound adapter. In this case, the connection factory must be of type 'client' and must have single-use set to false.

Two additional attributes are used to support this mechanism: retry-interval specifies (in milliseconds) how often the framework will attempt to reconnect after a connection failure. scheduler is used to supply a TaskScheduler used to schedule the connection attempts, and to test that the connection is still active.

For an outbound adapter, the connection is normally established when the first message is sent. client-mode="true" on an outbound adapter will cause the connection to be established when the adapter is started. Adapters are automatically started by default. Again, the connection factory must be of type client and have single-use set to false and retry-interval and scheduler are also supported. If a connection fails, it will be re-established either by the scheduler or when the next message is sent.

For both inbound and outbound, if the adapter is started, you may force the adapter to establish a connection by sending a <control-bus /> command: @adapter_id.retryConnection() and examine the current state with @adapter_id.isConnected().

28.7 TCP Gateways

The inbound TCP gateway TcpInboundGateway and outbound TCP gateway TcpOutboundGateway use a server and client connection factory respectively. Each connection can process a single request/response at a time.

The inbound gateway, after constructing a message with the incoming payload and sending it to the requestChannel, waits for a response and sends the payload from the response message by writing it to the connection.

	Note
	For the inbound gateway, care must be taken to retain, or populate, the ip_connectionId header because it is used to correlate the message to a connection. Messages that originate at the gateway will automatically have the header set. If the reply is constructed as a new message, you will need to set the header. The header value can be captured from the incoming message.

As with inbound adapters, inbound gateways normally use a type="server" connection factory, which listens for incoming connection requests. In some cases, it is desireable to establish the connection in reverse, whereby the inbound gateway connects to an external server and then waits for, and replies to, inbound messages on that connection.

This topology is supported by using client-mode="true" on the inbound gateway. In this case, the connection factory must be of type 'client' and must have single-use set to false.

Two additional attributes are used to support this mechanism: retry-interval specifies (in milliseconds) how often the framework will attempt to reconnect after a connection failure. scheduler is used to supply a TaskScheduler used to schedule the connection attempts, and to test that the connection is still active.

If the gateway is started, you may force the gateway to establish a connection by sending a <control-bus /> command: @adapter_id.retryConnection() and examine the current state with @adapter_id.isConnected().

The outbound gateway, after sending a message over the connection, waits for a response and constructs a response message and puts in on the reply channel. Communications over the connections are single-threaded. Users should be aware that only one message can be handled at a time and, if another thread attempts to send a message before the current response has been received, it will block until any previous requests are complete (or time out). If, however, the client connection factory is configured for single-use connections each new request gets its own connection and is processed immediately.

<int-ip:tcp-inbound-gateway id="inGateway"
    request-channel="tcpChannel"
    reply-channel="replyChannel"
    connection-factory="cfServer"
    reply-timeout="10000"/>

A simple inbound TCP gateway; if a connection factory configured with the default (de)serializer is used, messages will be \r\n delimited data and the gateway can be used by a simple client such as telnet.

<int-ip:tcp-outbound-gateway id="outGateway"
    request-channel="tcpChannel"
    reply-channel="replyChannel"
    connection-factory="cfClient"
    request-timeout="10000"
    remote-timeout="10000"/>

A simple outbound TCP gateway.

28.8 TCP Message Correlation

28.8.1 Overview

One goal of the IP Endpoints is to provide communication with systems other than another Spring Integration application. For this reason, only message payloads are sent and received, by default. Since 3.0, headers can be transferred, using JSON, Java serialization, or with custom Serializers and Deserializers; see Section 28.8.4, “Transferring Headers” for more information. No message correlation is provided by the framework, except when using the gateways, or collaborating channel adapters on the server side. In the paragraphs below we discuss the various correlation techniques available to applications. In most cases, this requires specific application-level correlation of messages, even when message payloads contain some natural correlation data (such as an order number).

28.8.2 Gateways

The gateways will automatically correlate messages. However, an outbound gateway should only be used for relatively low-volume use. When the connection factory is configured for a single shared connection to be used for all message pairs ('single-use="false"'), only one message can be processed at a time. A new message will have to wait until the reply to the previous message has been received. When a connection factory is configured for each new message to use a new connection ('single-use="true"'), the above restriction does not apply. While this may give higher throughput than a shared connection environment, it comes with the overhead of opening and closing a new connection for each message pair.

Therefore, for high-volume messages, consider using a collaborating pair of channel adapters. However, you will need to provide collaboration logic.

Another solution, introduced in Spring Integration 2.2, is to use a CachingClientConnectionFactory, which allows the use of a pool of shared connections.

28.8.3 Collaborating Outbound and Inbound Channel Adapters

To achieve high-volume throughput (avoiding the pitfalls of using gateways as mentioned above) you may consider configuring a pair of collaborating outbound and inbound channel adapters. Collaborating adapters can also be used (server-side or client-side) for totally asynchronous communication (rather than with request/reply semantics). On the server side, message correlation is automatically handled by the adapters because the inbound adapter adds a header allowing the outbound adapter to determine which connection to use to send the reply message.

	Note
	On the server side, care must be taken to populate the ip_connectionId header because it is used to correlate the message to a connection. Messages that originate at the inbound adapter will automatically have the header set. If you wish to construct other messages to send, you will need to set the header. The header value can be captured from an incoming message.

On the client side, the application will have to provide its own correlation logic, if needed. This can be done in a number of ways.

If the message payload has some natural correlation data, such as a transaction id or an order number, AND there is no need to retain any information (such as a reply channel header) from the original outbound message, the correlation is simple and would done at the application level in any case.

If the message payload has some natural correlation data, such as a transaction id or an order number, but there is a need to retain some information (such as a reply channel header) from the original outbound message, you may need to retain a copy of the original outbound message (perhaps by using a publish-subscribe channel) and use an aggregator to recombine the necessary data.

For either of the previous two paragraphs, if the payload has no natural correlation data, you may need to provide a transformer upstream of the outbound channel adapter to enhance the payload with such data. Such a transformer may transform the original payload to a new object containing both the original payload and some subset of the message headers. Of course, live objects (such as reply channels) from the headers can not be included in the transformed payload.

If such a strategy is chosen you will need to ensure the connection factory has an appropriate serializer/deserializer pair to handle such a payload, such as the DefaultSerializer/Deserializer which use java serialization, or a custom serializer and deserializer. The ByteArray*Serializer options mentioned in Section 28.3, “TCP Connection Factories”, including the default ByteArrayCrLfSerializer, do not support such payloads, unless the transformed payload is a String or byte[],

Note

Before the 2.2 release, when a client connection factory was used by collaborating channel adapters, the so-timeout attribute defaulted to the default reply timeout (10 seconds). This meant that if no data were received by the inbound adapter for this period of time, the socket was closed.

This default behavior was not appropriate in a truly asynchronous environment, so it now defaults to an infinite timeout. You can reinstate the previous default behavior by setting the so-timeout attribute on the client connection factory to 10000 milliseconds.

28.8.4 Transferring Headers

TCP is a streaming protocol; Serializers and Deserializers are used to demarcate messages within the stream. Prior to 3.0, only message payloads (String or byte[]) could be transferred over TCP. Beginning with 3.0, you can now transfer selected headers as well as the payload. It is important to understand, though, that "live" objects, such as the replyChannel header cannot be serialized.

Sending header information over TCP requires some additional configuration.

The first step is to provide the ConnectionFactory with a MessageConvertingTcpMessageMapper using the mapper attribute. This mapper delegates to any MessageConverter implementation to convert the message to/from some object that can be (de)serialized by the configured serializer and deserializer.

A MapMessageConverter is provided, which allows the specification of a list of headers that will be added to a Map object, along with the payload. The generated Map has two entries: payload and headers. The headers entry is itself a Map containing the selected headers.

The second step is to provide a (de)serializer that can convert between a Map and some wire format. This can be a custom (de)Serializer, which would typically be needed if the peer system is not a Spring Integration application.

A MapJsonSerializer is provided that will convert a Map to/from JSON. This uses a Spring Integration JsonObjectMapper to perform this function. You can provide a custom JsonObjectMapper if needed. By default, the serializer inserts a linefeed 0x0a character between objects. See the JavaDocs for more information.

	Note
	At the time of writing, the `JsonObjectMapper` uses whichever version of `Jackson` is on the classpath.

You can also use standard Java serialization of the Map, using the DefaultSerializer and DefaultDeserializer.

The following example shows the configuration of a connection factory that transfers the correlationId, sequenceNumber, and sequenceSize headers using JSON.

<int-ip:tcp-connection-factory id="client"
    type="client"
    host="localhost"
    port="12345"
    mapper="mapper"
    serializer="jsonSerializer"
    deserializer="jsonSerializer"/>

<bean id="mapper"
      class="o.sf.integration.ip.tcp.connection.MessageConvertingTcpMessageMapper">
    <constructor-arg name="messageConverter">
        <bean class="o.sf.integration.support.converter.MapMessageConverter">
            <property name="headerNames">
                <list>
                    <value>correlationId</value>
                    <value>sequenceNumber</value>
                    <value>sequenceSize</value>
                </list>
            </property>
        </bean>
    </constructor-arg>
</bean>

<bean id="jsonSerializer" class="o.sf.integration.ip.tcp.serializer.MapJsonSerializer" />

A message sent with the above configuration, with payload 'foo' would appear on the wire like so:

{"headers":{"correlationId":"bar","sequenceSize":5,"sequenceNumber":1},"payload":"foo"}

28.9 A Note About NIO

Using NIO (see using-nio in Section 28.11, “IP Configuration Attributes”) avoids dedicating a thread to read from each socket. For a small number of sockets, you will likely find that not using NIO, together with an async handoff (e.g. to a QueueChannel), will perform as well as, or better than, using NIO.

Consider using NIO when handling a large number of connections. However, the use of NIO has some other ramifications. A pool of threads (in the task executor) is shared across all the sockets; each incoming message is assembled and sent to the configured channel as a separate unit of work on a thread selected from that pool. Two sequential messages arriving on the same socket might be processed by different threads. This means that the order in which the messages are sent to the channel is indeterminate; the strict ordering of the messages arriving on the socket is not maintained.

For some applications, this is not an issue; for others it is. If strict ordering is required, consider setting using-nio to false and using async handoff.

Alternatively, you may choose to insert a resequencer downstream of the inbound endpoint to return the messages to their proper sequence. Set apply-sequence to true on the connection factory, and messages arriving on a TCP connection will have sequenceNumber and correlationId headers set. The resequencer uses these headers to return the messages to their proper sequence.

Pool Size

The pool size attribute is no longer used; previously, it specified the size of the default thread pool when a task-executor was not specified. It was also used to set the connection backlog on server sockets. The first function is no longer needed (see below); the second function is replaced by the backlog attribute.

Previously, when using a fixed thread pool task executor (which was the default), with NIO, it was possible to get a deadlock and processing would stop. The problem occurred when a buffer was full, a thread reading from the socket was trying to add more data to the buffer, and there were no threads available to make space in the buffer. This only occurred with a very small pool size, but it could be possible under extreme conditions. Since 2.2, two changes have eliminated this problem. First, the default task executor is a cached thread pool executor. Second, deadlock detection logic has been added such that if thread starvation occurs, instead of deadlocking, an exception is thrown, thus releasing the deadlocked resources.

	Note
	Now that the default task executor is unbounded, it is possible that an out of memory condition might occur with high rates of incoming messages, if message processing takes extended time. If your application exhibits this type of behavior, you are advised to use a pooled task executor with an appropriate pool size.

28.10 SSL/TLS Support

28.10.1 Overview

Secure Sockets Layer/Transport Layer Security is supported. When using NIO, the JDK 5+ SSLEngine feature is used to handle handshaking after the connection is established. When not using NIO, standard SSLSocketFactory and SSLServerSocketFactory objects are used to create connections. A number of strategy interfaces are provided to allow significant customization; default implementations of these interfaces provide for the simplest way to get started with secure communications.

28.10.2 Getting Started

Regardless of whether NIO is being used, you need to configure the ssl-context-support attribute on the connection factory. This attribute references a <bean/> definition that describes the location and passwords for the required key stores.

SSL/TLS peers require two keystores each; a keystore containing private/public key pairs identifying the peer; a truststore, containing the public keys for peers that are trusted. See the documentation for the keytool utility provided with the JDK. The essential steps are

Create a new key pair and store in a keystore.
Export the public key.
Import the public key into the peer's truststore.

Repeat for the other peer.

	Note
	It is common in test cases to use the same key stores on both peers, but this should be avoided for production.

After establishing the key stores, the next step is to indicate their locations to the TcpSSLContextSupport bean, and provide a reference to that bean to the connection factory.

<bean id="sslContextSupport"
    class="o.sf.integration.ip.tcp.connection.support.DefaultTcpSSLContextSupport">
    <constructor-arg value="client.ks"/>
    <constructor-arg value="client.truststore.ks"/>
    <constructor-arg value="secret"/>
    <constructor-arg value="secret"/>
</bean>

<ip:tcp-connection-factory id="clientFactory"
    type="client"
    host="localhost"
    port="1234"
    ssl-context-support="sslContextSupport"

The DefaulTcpSSLContextSupport class also has an optional 'protocol' property, which can be 'SSL' or 'TLS' (default).

The keystore file names (first two constructor arguments) use the Spring Resource abstraction; by default the files will be located on the classpath, but this can be overridden by using the file: prefix, to find the files on the filesystem instead.

28.10.3 Advanced Techniques

In many cases, the configuration described above is all that is needed to enable secure communication over TCP/IP. However, a number of strategy interfaces are provided to allow customization and modification of socket factories and sockets.

TcpSSLContextSupport
TcpSocketFactorySupport
TcpSocketSupport

public interface TcpSSLContextSupport {

	SSLContext getSSLContext() throws Exception;

}

Implementations of this interface are responsible for creating an SSLContext. The sole implementation provided by the framework is the DefaultTcpSSLContextSupport described above. If you require different behavior, implement this interface and provide the connection factory with a reference to a bean of your class' implementation.

public interface TcpSocketFactorySupport {

    ServerSocketFactory getServerSocketFactory();

    SocketFactory getSocketFactory();

}

Implementations of this interface are responsible for obtaining references to ServerSocketFactory and SocketFactory. Two implementations are provided; the first is DefaultTcpNetSocketFactorySupport for non-SSL sockets (when no 'ssl-context-support' attribute is defined); this simply uses the JDK's default factories. The second implementation is DefaultTcpNetSSLSocketFactorySupport; this is used, by default, when an 'ssl-context-support' attribute is defined; it uses the SSLContext created by that bean to create the socket factories.

	Note
	This interface only applies if `using-nio` is "false"; socket factories are not used by NIO.

public interface TcpSocketSupport {

    void postProcessServerSocket(ServerSocket serverSocket);

    void postProcessSocket(Socket socket);

Implementations of this interface can modify sockets after they are created, and after all configured attributes have been applied, but before the sockets are used. This applies whether or not NIO is being used. For example, you could use an implementation of this interface to modify the supported cipher suites on an SSL socket, or you could add a listener that gets notified after SSL handshaking is complete. The sole implementation provided by the framework is the DefaultTcpSocketSupport which does not modify the sockets in any way

To supply your own implementation of TcpSocketFactorySupport or TcpSocketSupport, provide the connection factory with references to beans of your custom type using the socket-factory-support and socket-support attributes, respectively.

28.11 IP Configuration Attributes

Table 28.1. Connection Factory Attributes

Attribute Name	Client?	Server?	Allowed Values	Attribute Description
type	Y	Y	client, server	Determines whether the connection factory is a client or server.
host	Y	N		The host name or ip address of the destination.
port	Y	Y		The port.
serializer	Y	Y		An implementation of `Serializer` used to serialize the payload. Defaults to `ByteArrayCrLfSerializer`
deserializer	Y	Y		An implementation of `Deserializer` used to deserialize the payload. Defaults to `ByteArrayCrLfSerializer`
using-nio	Y	Y	true, false	Whether or not connection uses NIO. Refer to the java.nio package for more information. See Section 28.9, “A Note About NIO”. Default false.
using-direct-buffers	Y	N	true, false	When using NIO, whether or not the connection uses direct buffers. Refer to `java.nio.ByteBuffer` documentation for more information. Must be false if using-nio is false.
apply-sequence	Y	Y	true, false	When using NIO, it may be necessary to resequence messages. When this attribute is set to true, correlationId and sequenceNumber headers will be added to received messages. See Section 28.9, “A Note About NIO”. Default false.
so-timeout	Y	Y		Defaults to 0 (infinity), except for server connection factories with single-use="true". In that case, it defaults to the default reply timeout (10 seconds).
so-send-buffer-size	Y	Y		See `java.net.Socket. setSendBufferSize()`.
so-receive-buffer- size	Y	Y		See `java.net.Socket. setReceiveBufferSize()`.
so-keep-alive	Y	Y	true, false	See `java.net.Socket. setKeepAlive()`.
so-linger	Y	Y		Sets linger to true with supplied value. See `java.net.Socket. setSoLinger()`.
so-tcp-no-delay	Y	Y	true, false	See `java.net.Socket. setTcpNoDelay()`.
so-traffic-class	Y	Y		See `java.net.Socket. setTrafficClass()`.
local-address	N	Y		On a multi-homed system, specifies an IP address for the interface to which the socket will be bound.
task-executor	Y	Y		Specifies a specific Executor to be used for socket handling. If not supplied, an internal cached thread executor will be used. Needed on some platforms that require the use of specific task executors such as a WorkManagerTaskExecutor.
single-use	Y	Y	true, false	Specifies whether a connection can be used for multiple messages. If true, a new connection will be used for each message.
pool-size	N	N		This attribute is no longer used. For backward compatibility, it sets the backlog but users should use backlog to specify the connection backlog in server factories
backlog	N	Y		Sets the connection backlog for server factories.
lookup-host	Y	Y	true, false	Specifies whether reverse lookups are done on IP addresses to convert to host names for use in message headers. If false, the IP address is used instead. Defaults to true.
interceptor-factory-chain	Y	Y		See Section 28.4, “TCP Connection Interceptors”
ssl-context-support	Y	Y		See Section 28.10, “SSL/TLS Support”
socket-factory-support	Y	Y		See Section 28.10, “SSL/TLS Support”
socket-support	Y	Y		See Section 28.10, “SSL/TLS Support”

Table 28.2. UDP Inbound Channel Adapter Attributes

Attribute Name	Allowed Values	Attribute Description
port		The port on which the adapter listens.
multicast	true, false	Whether or not the udp adapter uses multicast.
multicast-address		When multicast is true, the multicast address to which the adapter joins.
pool-size		Specifies the concurrency. Specifies how many packets can be handled concurrently. It only applies if task-executor is not configured. Defaults to 5.
task-executor		Specifies a specific Executor to be used for socket handling. If not supplied, an internal pooled executor will be used. Needed on some platforms that require the use of specific task executors such as a WorkManagerTaskExecutor. See pool-size for thread requirements.
receive-buffer-size		The size of the buffer used to receive DatagramPackets. Usually set to the MTU size. If a smaller buffer is used than the size of the sent packet, truncation can occur. This can be detected by means of the check-length attribute..
check-length	true, false	Whether or not a udp adapter expects a data length field in the packet received. Used to detect packet truncation.
so-timeout		See `java.net.DatagramSocket` setSoTimeout() methods for more information.
so-send-buffer-size		Used for udp acknowledgment packets. See `java.net.DatagramSocket` setSendBufferSize() methods for more information.
so-receive-buffer- size		See `java.net.DatagramSocket` setReceiveBufferSize() for more information.
local-address		On a multi-homed system, specifies an IP address for the interface to which the socket will be bound.
error-channel		If an Exception is thrown by a downstream component, the MessagingException message containing the exception and failed message is sent to this channel.
lookup-host	true, false	Specifies whether reverse lookups are done on IP addresses to convert to host names for use in message headers. If false, the IP address is used instead. Defaults to true.

Table 28.3. UDP Outbound Channel Adapter Attributes

Attribute Name	Allowed Values	Attribute Description
host		The host name or ip address of the destination. For multicast udp adapters, the multicast address.
port		The port on the destination.
multicast	true, false	Whether or not the udp adapter uses multicast.
acknowledge	true, false	Whether or not a udp adapter requires an acknowledgment from the destination. when enabled, requires setting the following 4 attributes.
ack-host		When acknowledge is true, indicates the host or ip address to which the acknowledgment should be sent. Usually the current host, but may be different, for example when Network Address Transaction (NAT) is being used.
ack-port		When acknowledge is true, indicates the port to which the acknowledgment should be sent. The adapter listens on this port for acknowledgments.
ack-timeout		When acknowledge is true, indicates the time in milliseconds that the adapter will wait for an acknowledgment. If an acknowledgment is not received in time, the adapter will throw an exception.
min-acks-for- success		Defaults to 1. For multicast adapters, you can set this to a larger value, requiring acknowledgments from multiple destinations.
check-length	true, false	Whether or not a udp adapter includes a data length field in the packet sent to the destination.
time-to-live		For multicast adapters, specifies the time to live attribute for the `MulticastSocket`; controls the scope of the multicasts. Refer to the Java API documentation for more information.
so-timeout		See `java.net.DatagramSocket` setSoTimeout() methods for more information.
so-send-buffer-size		See `java.net.DatagramSocket` setSendBufferSize() methods for more information.
so-receive-buffer- size		Used for udp acknowledgment packets. See `java.net.DatagramSocket` setReceiveBufferSize() methods for more information.
local-address		On a multi-homed system, for the UDP adapter, specifies an IP address for the interface to which the socket will be bound for reply messages. For a multicast adapter it is also used to determine which interface the multicast packets will be sent over.
task-executor		Specifies a specific Executor to be used for acknowledgment handling. If not supplied, an internal single threaded executor will be used. Needed on some platforms that require the use of specific task executors such as a WorkManagerTaskExecutor. One thread will be dedicated to handling acknowledgments (if the acknowledge option is true).

Table 28.4. TCP Inbound Channel Adapter Attributes

Attribute Name	Allowed Values	Attribute Description
channel		The channel to which inbound messages will be sent.
connection-factory		If the connection factory has a type 'server', the factory is 'owned' by this adapter. If it has a type 'client', it is 'owned' by an outbound channel adapter and this adapter will receive any incoming messages on the connection created by the outbound adapter.
error-channel		If an Exception is thrown by a downstream component, the MessagingException message containing the exception and failed message is sent to this channel.
client-mode	true, false	When true, the inbound adapter will act as a client, with respect to establishing the connection and then receive incoming messages on that connection. Default = false. Also see retry-interval and scheduler. The connection factory must be of type 'client' and have single-use set to false.
retry-interval		When in client-mode, specifies the number of milliseconds to wait between connection attempts, or after a connection failure. Default 60,000 (60 seconds).
scheduler	true, false	Specifies a `TaskScheduler` to use for managing the client-mode connection. Defaults to a `ThreadPoolTaskScheduler` with a pool size of `.

Table 28.5. TCP Outbound Channel Adapter Attributes

Attribute Name	Allowed Values	Attribute Description
channel		The channel on which outbound messages arrive.
connection-factory		If the connection factory has a type 'client', the factory is 'owned' by this adapter. If it has a type 'server', it is 'owned' by an inbound channel adapter and this adapter will attempt to correlate messages to the connection on which an original inbound message was received.
client-mode	true, false	When true, the outbound adapter will attempt to establish the connection as soon as it is started. When false, the connection is established when the first message is sent. Default = false. Also see retry-interval and scheduler. The connection factory must be of type 'client' and have single-use set to false.
retry-interval		When in client-mode, specifies the number of milliseconds to wait between connection attempts, or after a connection failure. Default 60,000 (60 seconds).
scheduler	true, false	Specifies a `TaskScheduler` to use for managing the client-mode connection. Defaults to a `ThreadPoolTaskScheduler` with a pool size of `.

Table 28.6. TCP Inbound Gateway Attributes

Attribute Name	Allowed Values	Attribute Description
connection-factory		The connection factory must be of type server.
request-channel		The channel to which incoming messages will be sent.
reply-channel		The channel on which reply messages may arrive. Usually replies will arrive on a temporary reply channel added to the inbound message header
reply-timeout		The time in milliseconds for which the gateway will wait for a reply. Default 1000 (1 second).
error-channel		If an Exception is thrown by a downstream component, the MessagingException message containing the exception and failed message is sent to this channel; any reply from that flow will then be returned as a response by the gateway.
client-mode	true, false	When true, the inbound gateway will act as a client, with respect to establishing the connection and then receive (and reply to) incoming messages on that connection. Default = false. Also see retry-interval and scheduler. The connection factory must be of type 'client' and have single-use set to false.
retry-interval		When in client-mode, specifies the number of milliseconds to wait between connection attempts, or after a connection failure. Default 60,000 (60 seconds).
scheduler	true, false	Specifies a `TaskScheduler` to use for managing the client-mode connection. Defaults to a `ThreadPoolTaskScheduler` with a pool size of `.

Table 28.7. TCP Outbound Gateway Attributes

Attribute Name	Allowed Values	Attribute Description
connection-factory		The connection factory must be of type client.
request-channel		The channel on which outgoing messages will arrive.
reply-channel		Optional. The channel to which reply messages may be sent if the original outbound message did not contain a reply channel header.
remote-timeout		The time in milliseconds for which the gateway will wait for a reply from the remote system. Default: Same value as reply-timeout, if specified, or 10000 (10 seconds) otherwise.
request-timeout		If a single-use connection factory is not being used, The time in milliseconds for which the gateway will wait to get access to the shared connection.
reply-timeout		The time in milliseconds for which the gateway will wait when sending the reply to the reply-channel. Only applies if the reply-channel might block, such as a bounded QueueChannel that is currently full.

28.12 IP Message Headers

The following MessageHeaders are used by this module:

Table 28.8.

Header Name	IpHeaders Constant	Description
ip_hostname	HOSTNAME	The host name from which a TCP message or UDP packet was received. If `lookupHost` is `false`, this will contain the ip address.
ip_address	IP_ADDRESS	The ip address from which a TCP message or UDP packet was received.
ip_port	PORT	The remote port for a UDP packet.
ip_ackTo	ACKADDRESS	The remote ip address to which UDP application-level acks will be sent. The framework includes acknowledgment information in the data packet.
ip_ackId	ACK_ID	A correlation id for UDP application-level acks. The framework includes acknowledgment information in the data packet.
ip_tcp_remotePort	REMOTE_PORT	The remote port for a UDP packet.
ip_connectionId	CONNECTION_ID	A unique identifier for a TCP connection; set by the framework for inbound messages; when sending to a server-side inbound channel adapter, or replying to an inbound gateway, this header is required so the endpoint can determine which connection to send the message to.
ip_actualConnectionId	ACTUAL_CONNECTION_ID	For information only - when using a cached or failover client connection factory, contains the actual underlying connection id.

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