docs/user-manual/modules/ROOT/pages/parameter-binding-annotations.adoc

= Parameter Binding Annotations

The bean parameter binding annotations from Camel are as follows:

[width="100%",cols="34%,33%,33%",options="header",]
|=======================================================================
|Annotation |Meaning |Parameter
|`org.apache.camel.Body`
|To bind to an inbound message body | 

|`org.apache.camel.Header`
|To bind to a message header |String name of the header

|`org.apache.camel.Headers`
|To bind to the Map of the message headers |

|`org.apache.camel.Variable`
|To bind to a named variable |String name of the
variable

|`org.apache.camel.Variables`
|To bind to the variables map |

|`org.apache.camel.ExchangeProperty`
|To bind to a named property on the exchange |String name of the
property

|`org.apache.camel.ExchangeProperties`
|To bind to the exchange property map on the exchange |

|`org.apache.camel.ExchangeException`
|To bind to an Exception set on the exchange |

|=======================================================================

These annotations can be used with the xref:components::bean-component.adoc[Bean]
component or when invoking beans in the xref:dsl.adoc[DSL]

Annotations can be used to define an xref:expression.adoc[Expression] or
to extract various headers, properties or payloads from a
xref:components:eips:message.adoc[Message] when invoking a bean method (see
xref:bean-integration.adoc[Bean Integration] for more detail of how to
invoke bean methods) together with being useful to help disambiguate
which method to invoke.

If no annotations are used then Camel assumes that a single parameter is
the body of the message. Camel will then use the
xref:type-converter.adoc[Type Converter] mechanism to convert from the
expression value to the actual type of the parameter.

== Using bean parameter binding annotations

In this example below we have a `@Consume` consumer (like message driven)
that consumes JMS messages from the activemq queue. We use the `@Header`
and `@Body` parameter binding annotations to bind from the JMSMessage to
the method parameters.

[source,java]
----
public class MyBean {

    @Consume("activemq:my.queue")
    public void doSomething(@Header("JMSCorrelationID") String correlationID, @Body String body) {
        // process the inbound message here
    }

}
----

In the above Camel will extract the value of
`Message.getJMSCorrelationID()`, then using the
xref:type-converter.adoc[Type Converter] to adapt the value to the type
of the parameter if required - it will inject the parameter value for
the *correlationID* parameter. Then the payload of the message will be
converted to a String and injected into the *body* parameter.

TIP: You don't necessarily need to use the `@Consume` annotation if you don't
want to as you could also make use of the Camel xref:dsl.adoc[DSL] to
route to the bean's method as well.

=== Using the DSL to invoke the bean method

Here is another example which does not use xref:pojo-consuming.adoc[POJO
Consuming] annotations but instead uses the xref:dsl.adoc[DSL] to route
messages to the bean method

[source,java]
----
public class MyBean {

    public void doSomething(@Header("JMSCorrelationID") String correlationID, @Body String body) {
        // process the inbound message here
    }

}
----

The routing DSL then looks like this

[source,java]
----
from("activemq:someQueue").
  to("bean:myBean");
----

Here *myBean* would be looked up in the xref:registry.adoc[Registry]
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:

[source,java]
----
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:

[source,java]
----
public class MyBean {

    public void doSomething(@Header("user") String user, @Body String body, Exchange exchange) {
        exchange.getIn().setBody(body + "MyBean");
    }

}
----

=== Annotation Based Expression Language

You can also use any of the xref:languages.adoc[Languages] supported in
Camel to bind expressions to method parameters when using
xref:bean-integration.adoc[Bean Integration]. For example, you can use
any of these annotations:

[width="100%",cols="50%,50%",options="header",]
|=======================================================================
|Annotation |Description
|`@Bean`
|Inject a xref:components:languages:bean-language.adoc[Bean] expression

|`@Constant`
|Inject a xref:components:languages:constant-language.adoc[Constant] expression

|`@Groovy`
|Inject a  xref:components:languages:groovy-language.adoc[Groovy] expression

|`@Header`
|Inject a xref:components:languages:header-language.adoc[Header] expression

|`@Simple`
|Inject an xref:components:languages:simple-language.adoc[Simple] expression

|`@XPath`
|Inject an xref:components:languages:xpath-language.adoc[XPath] expression

|=======================================================================

The table above only list some of the commonly used languages. You can find
a list of all supported xref:components:languages:index.adoc[Languages]
which each have their own annotation that can be used.

It is required to include the JAR of the language, for example `camel-groovy`,
or `camel-jsonpath` to use the `@JSonPath` annotation.

Here is an example how to use `@XPath`:

[source,java]
----
public class Foo {

    @Consume("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 call a xref:components::bean-component.adoc[POJO] to supply
the parameter value:

[source,java]
----
public class MyBean {
    
    @Consume("activemq:my.queue")
    public void doSomething(@Bean("myCorrelationIdGenerator") String correlationID, @Body String body) {
        // process the inbound message here
    }
}
----

When a message is consumed from the activemq queue, then Camel will invoke the `doSomething`
method. The parameter with `@Bean` is telling Camel to call yet another bean that
computes the correlation id parameter:

[source,java]
----
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 xref:components::bean-component.adoc[POJO] MyIdGenerator has one public method that
accepts two parameters. We have also annotated this one with the
`@Header` and `@Body` annotations to help Camel know what to bind here 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
xref:bean-binding.adoc[Bean Binding] annotations anywhere.

[source,java]
----
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 xref:registry.adoc[Registry]:

For example in Spring XML:

[source,xml]
----
<bean id="myCorrelationIdGenerator" class="com.mycompany.MySimpleIdGenerator"/>
----

==== Example using Groovy

In this example we have an Exchange that has a User object stored in the
in header. This User object has methods to get some user information. We
want to use xref:components:languages:groovy-language.adoc[Groovy] to inject an expression that
extracts and concats the fullname of the user into the fullName parameter.

[source,java]
----
public class MyBean {

    public void doSomething(@Groovy("$request.header['user'].firstName $request.header['user'].familyName") String fullName, @Body String body) {
        // process the inbound message here
    }

}
----

Groovy supports _GStrings_ that is like a template where we can insert `$`
placeholders that will be evaluated by Groovy.