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.

A PayloadTypeRouter will send Messages to the channel as defined by payload-type mappings. ]]> Configuration of PayloadTypeRouter is also supported via the namespace provided by Spring Integration (see ), 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: ]]>

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. The value of the header could be one of two things: 1. Arbitrary value 2. Channel name If arbitrary then additional mapping of these header values to channel names is required, otherwise no additional configuration is needed. Spring Integration provides a simple namespace-based XML configuration to configure HeaderValueRouter. The example below demonstrates two types of namespace-based configuration of HeaderValueRouter. 1. Configuration where mapping of header values to channels is required ]]> Obviously during the resolution process this router may encounter resolution failures, thus resulting in exception. If you want to suppress such resolution failures and send all messages to the default output channel (i.e., identified with default-output-channel attribute) you may do so by setting ignore-channel-name-resolution-failures to true. This particular attribute may sound confusing when compared with the default-output-channel. So just to clarify, the default-output-channel handles messages where routing information can not be determined. However, in the above scenario route has determined successfully but the resolution of that route failed and that is why we need an extra attribute to let Spring Integration know that it is OK to suppress resolution failures and treat such route as undetermined thus routing messages to the default-output-channel 2. Configuration where mapping of header values to channel names is not required since header values themselves represent channel names ]]> 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.

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

Configuration for RecipientListRouter is also supported via namespace support provided by Spring Integration (see ). The example below demonstrates namespace-based configuration of RecipientListRouter and all the supported attributes using Spring Integration namespace support: ]]> 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 for more information. Another convenient option to configure Recipient List Router is to use Spring Expression Language (SpEL) support ]]> In the above configuration a SpEL expression identified by selector-expression attribute will be evaluated to determine if this recipient should be included in the recipient list for a given input Message. The evaluation result of the expression must be a boolean. If this attribute is not defined, the channel will always be among the list of recipients.

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): ]]> 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. ]]> 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: ]]> 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. Routers and Spring Expression Language (SpEL) Some times the routing logic may be simple and writing a separate class for it and configuring it as a bean may seem like an overkill. Since Spring Integration 2.0 we offer an alternative where you can now use SpEL (http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/expressions.html) to implement simple computations that otherwise were implemented in a custom POJO router. ]]> In the above configuration the result channel will be computed by the SpEL expression which simply concatenates the value of the payload with the literal 'Channel' Another value of SpEL when it comes to configuring routers is that expression can actually return a Collection, thus making every <router> a Recipient List Router. Whenever expression returns multiple channel values Message will be forwarded to all such channels. ]]> In the above configuration lets assume that you have a message header with the name 'channels' and the value being the List of channel names. Now, message will be sent to all channels in this list. You ,ay also fine Collection Projection and Collection Selection expressions usefull to select multiple channels. See (http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/expressions.html#d0e12084)

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

For routing of XML-based Messages, including XPath support, see .

So as you can see, Spring Integration provides quite a few different router configurations for most common content-based routing use cases as well as the option of implementing custom routers as POJOs. For example; Payload Type Router provides a simple way to configure a router which computes channels based on the payload type of the incoming Message while Header Value Router provides the same convenience in configuring a router which computes channels based on evaluating the value of a particular Message Header. There is also an expression-based (SpEL) routers where the channel is determined based on evaluating an expression which gives these type of routers some dynamic characteristics. However these routers share one common attribute - static configuration. Even in the case of expression-based routers, the expression itself is defined as part of the router configuration which means that the same expression operating on the same value will always result in the computation of the same channel. This is good in most cases since such routes are well defined and therefore predictable. But there are times when we need to change router configurations dynamically so message flows could be routed to a different channel. For example: You might want to bring down some part of your system for maintenance. So, temporarily you want to re-reroute messages to a different message flow. Or you may want to introduce more granularity to your message flow by adding another route to handle a more concrete type of java.lang.Number (in cases of Payload Type Router). Unfortunately with static router configuration to accomplish this you'd have to bring down your entire application, change the configuration of the router (change routes) and bring it back up. This is obviously not the solution. Dynamic Router pattern describes the mechanisms by which one can change/configure routers dynamically without bringing down your system or individual routers.  Before we get into the specifics of how it is accomplished in Spring Integration lets quickly summarize the typical flow of the router, which consists of 3 simple steps: Step 1 - Compute channel identifier which is a value calculated by the router once it receives the Message. Typically it is a String or and instance of the actual MessageChannel. Step 2 - Resolve channel identifier to channel name. We'll describe specifics of this process in a moment. Step 3 - Resolve channel name to the actual MessageChannel There is not much that could be done with regard to router dynamics if Step 1 results in the actual instance of the MessageChannel simply because MessageChannel is the final product of any router's job. However, if Step 1 results in channel identifier that is not and instance of MessageChannel, then there are quite a few possibilities to influence the process of calculating what will be the final instance of the Message Channel. Lets look at couple of the examples in the context of the 3 steps mentioned above:  Payload Type Router ]]> Within the context of the Payload Type Router the 3 steps mentioned above would be realized as: Step 1 - Compute channel identifier which is the fully qualified name of the payload type (e.g., java.lang.String). Step 2 - Resolve channel identifier to channel name where the result of the previous step is used to select the appropriate value from the payload type mapping defined via mapping element. Step 3 - Resolve channel name to the actual instance of the MessageChannel where using ChannelResolver router will obtain a reference to a bean (which is hopefully a MessageChannel) identified by the result of the previous step. In other words each step feeds the next step until thr process completes. Header Value Router ]]> Similar to the PayloadTypeRouter: Step 1 - Compute channel identifier which is the value of the header identified by the header-name attribute. Step 2 - Resolve channel identifier to channel name where the result of the previous step is used to select the appropriate value from the general mapping defined via mapping element. Step 3 - Resolve channel name to the actual instance of the MessageChannel where using ChannelResolver router will obtain a reference to a bean (which is hopefully a MessageChannel) identified by the result of the previous step. The above two configurations of two different router types look almost identical. However if we look at the different configuration of the HeaderValueRouter we clearly see that there is no mapping sub element: ]]> But configuration is still perfectly valid. So the natural question is what about the maping in the Step 2? What this means is that Step 2 is now an optional step. If mapping is not defined then the channel identifier value computed in Step 1 will automatically be treated as the channel name which will now be resolved to the actual MessageChannel in the Step 3. What it also means is that Step 2 is one of the key steps to provide dynamic characteristics to the routers, since it introduces a process which allows you to change the way 'channel identifier' resolves to 'channel name', thus influencing the process of determining the final instance of the MessageChannel from the initial channel identifier.  For Example: In the above configuration lets assume that the testHeader value is 'kermit' which is now a channel identifier (Step 1). Since there is no mapping in this router, resolving this channel identifier to a channel name (Step 2) is impossible and this channel identifier is now treated as channel name. However what if there was mapping but for a different value, the end result would still be the same and that is: if new value can not be determined through the process of resolving 'channel identifier' to a 'channel name', such 'channel identifier' becomes 'channel name' So all that is left is for Step 3 to resolve channel name ('kermit') to an actual instance of the MessageChannel identified by this name. That will be done via default ChannelResolver implementation which is BeanFactoryChannelResolver which basically does a bean lookup by the name provided. So now all messages which contain the header/value pair as testHeader=kermit are going to be routed to a 'kermit' MessageChannel. But what if you want to route these messages to 'simpson' channel? Obviously changing static configuration would work, but would also require bringing your system down. However if you had access to channel identifier map, then you could just introduce a new mapping where header/value pair is now kermit=simpson, thus allowing Step 2 to treat 'kermit' as channel identifier while resolving it to 'simpson' as channel name . The same obviously applies for PayloadTypeRouter where you can now remap or remove a particular payload type mapping, and every other router including expression-based routers since their computed value will now have a chance to go through Step 2 to be aditionally resolved to the actual channel name. In Spring Integration 2.0 routers hierarchy underwent major refactoring and now any router that is a subclass of the AbstractMessageRouter (all framework defined routers) is a Dynamic Router simply because channelIdentiferMap is defined at the AbstractMessageRouter with convenient accessors and modifiers exposed as public methods allowing you to change/add/remove router mapping at runtime via JMX (see section section 29) or ControlBus (see section section 29.7) functionality.  Control Bus One of the way to manage the router mappings is through the Control Bus which exposes a Control Channel where you can send control messages to manage and monitor Spring Integration components which includes routers. For more information about the Control Bus see section 29.7. Typically you would send a control message asking to invoke a particular JMX operation on a particular managed component (e.g., router). The two managed operations (methods) that are specific to changing router resolution process are: public void setChannelMapping(String channelIdentifier, String channelName) - will allow you to add new or modify existing mapping of channel identifier to channel name public void removeChannelMapping(String channelIdentifier) - will allow you to remove a particular channel mapping, thus disconnecting the relationship between channel identifier and channel name There are obviously other managed operations, so please refer to an AbstractMessageRouter for more detail You can also use your favorite JMX client (e.g., JConsole) and use those operations (methods) to change router configuration. For more information on Spring Integration management and monitoring please visit section 29 of this manual.