[CONF] Apache Camel: Bean Integration (page edited)

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Page Edited : CAMEL : Bean Integration Bean Integration has been edited by James Strachan (Oct 01, 2008). (View changes) Content: Bean Integration Camel supports the integration of beans with Camel message exchanges in a number of ways. Annotations If a bean is defined in Spring XML or scanned using the Spring 2.5 component scanning mechanism and a <camelContext> is used or a CamelBeanPostProcessor then we process a number of Camel annotations to do various things such as injecting resources or producing, consuming or routing messages. Bean Injection to inject Camel related resources into your POJOs POJO Consuming to consume and possibly route messages from Camel POJO Producing to make it easy to produce camel messages from your POJOs RecipientList Annotation for creating a Recipient List from a POJO method Using Exchange Pattern Annotations describes how the pattern annotations can be used to change the behaviour of method invocations with Spring Remoting or POJO Producing Spring Remoting We support a Spring Remoting provider which uses Camel as the underlying transport mechanism. The nice thing about this approach is we can use any of the Camel transport Components to communicate between beans. It also means we can use Content Based Router and the other Enterprise Integration Patterns in between the beans; in particular we can use Message Translator to be able to convert what the on-the-wire messages look like in addition to adding various headers and so forth. Bean Component The Bean component supports the creation of a proxy via ProxyHelper to a Java interface; which the implementation just sends a message containing a BeanInvocation to some Camel endpoint. Then there is a server side implementation which consumes a message and uses the Bean Binding to bind the message to invoke a method passing in its parameters. Bean binding The binding of a Camel Message to a bean method call can occur in different ways if the bean can be converted to a Processor using the Type Converter mechanism then this is used to process the message. This mechanism is used by the ActiveMQ component to allow any MessageListener to be invoked by the Bean component if the body of the message can be converted to a BeanInvocation (the default payload used by the ProxyHelper) - then that its used to invoke the method and pass the arguments if the message contains the header org.apache.camel.MethodName then that method is invoked, converting the body to whatever the argument is to the method otherwise the type of the method body is used to try find a method which matches; an error is thrown if a single method cannot be chosen unambiguously. you can also use Exchange as the parameter itself, but then the return type must be void. By default the return value is set on the outbound message body. For example a POJO such as: public class Bar { public String doSomething(String body) { // process the in body and return whatever you want return "Bye World"; } Or the Exchange example. Notice that the return type must be void: public class Bar { public void doSomething(Exchange exchange) { // process the exchange exchange.getIn().setBody("Bye World"); } For example you could write a method like this (showing also a feature in Camel, the @MessageDriven annotation): @MessageDriven requires camel-spring Using the @MessageDriven annotations requires camel-spring that uses the org.apache.camel.spring.CamelBeanPostProcessor to perform the setup for this consumer and the needed bean bindings. public class Foo { @MessageDriven(uri = "activemq:my.queue") public void doSomething(String body) { // process the inbound message here } } Here Camel with subscribe to an ActiveMQ queue, then convert the message payload to a String (so dealing with TextMessage, ObjectMessage and BytesMessage in JMS), then process this method. Using Annotations to bind parameters to the Exchange Bean component The annotations below is part of camel-core and its Bean component and thus does not require camel-spring. These annotations is to be used with the Bean component. The annotations can be used to bind in situations where traditional methods would result in ambiguous methods. So by adding annotations you can decorate your bean to help Camel invoke the correct method. You can also use the following annotations to bind parameters to different kinds of _expression_ Annotation Meaning @Body To bind to an inbound message body @Header To bind to an inbound message header @Headers To bind to the Map of the inbound message headers @OutHeaders To bind to the Map of the outbound message headers @Property To bind to a named property on the exchange @Properties To bind to the property map on the exchange For example: public class Foo { @MessageDriven(uri = "activemq:my.queue") public void doSomething(@Header('JMSCorrelationID') String correlationID, @Body String body) { // process the inbound message here } } In the above you can now pass the Message.getJMSCorrelationID() as a parameter to the method (using the Type Converter to adapt the value to the type of the parameter). Finally you don't need the @MessageDriven annotation; as the Camel route could describe which method to invoke. e.g. a route could look like from("activemq:someQueue"). to("bean:myBean"); Here myBean would be looked up in the Registry (such as JNDI or the Spring ApplicationContext), then the body of the message would be used to try figure out what method to call. If you want to be explicit you can use from("activemq:someQueue"). to("bean:myBean?methodName=doSomething"); And here we have a nifty example for you to show some great power in Camel. You can mix and match the annotations with the normal parameters, so we can have this example with annotations and the Exchange also: public void doSomething(@Header(name = "user") String user, @Body String body, Exchange exchange) { exchange.getIn().setBody(body + "MyBean"); } Using _expression_ Languages You can also use any of the Languages supported in Camel to bind expressions to method parameters when using bean integration. For example you can use any of these annotations: Annotation Description @Bean Inject a Bean _expression_ @BeanShell Inject a BeanShell _expression_ @Constant Inject a Constant _expression_ @EL Inject an EL _expression_ @Groovy Inject a Groovy _expression_ @Header Inject a Header _expression_ @_javascript_ Inject a _javascript_ _expression_ @OGNL Inject an OGNL _expression_ @PHP Inject a PHP _expression_ @Python Inject a Python _expression_ @Ruby Inject a Ruby _expression_ @Simple Inject an Simple _expression_ @XPath Inject an XPath _expression_ @XQuery Inject an XQuery _expression_ For example: public class Foo { @MessageDriven(uri = "activemq:my.queue") public void doSomething(@XPath("/foo/bar/text()") String correlationID, @Body String body) { // process the inbound message here } } Advanced example using @Bean And an example of using the the @Bean binding annotation, where you can use a POJO where you can do whatever java code you like: public class Foo { @MessageDriven(uri = "activemq:my.queue") public void doSomething(@Bean("myCorrelationIdGenerator") String correlationID, @Body String body) { // process the inbound message here } } And then we can have a spring bean with the id myCorrelationIdGenerator where we can compute the id. public class MyIdGenerator { private UserManager userManager; public String generate(@Header(name = "user") String user, @Body String payload) throws Exception { User user = userManager.lookupUser(user); String userId = user.getPrimaryId(); String id = userId + generateHashCodeForPayload(payload); return id; } } The POJO MyIdGenerator has one public method that accepts two parameters. However we have also annotated this one with the @Header and @Body annotation to help Camel know what to bind here from the Message from the Exchange being processed. Of course this could be simplified a lot if you for instance just have a simple id generator. But we wanted to demonstrate that you can use the Bean Binding annotations anywhere. public class MySimpleIdGenerator { public static int generate() { // generate a unique id return 123; } } And finally we just need to remember to have our bean registered in the Spring Registry: <bean id="myCorrelationIdGenerator" class="com.mycompany.MyIdGenerator"/>

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