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58.7. Entity Writer Interceptor

Overview
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This section explains how to implement and register an entity writer interceptor, which enables you to intercept the output stream when writing a message body either on the client side or on the server side. This is typically useful for generic transformations of the request body, such as encryption and decryption, or compressing and decompressing.

WriterInterceptor interface
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The javax.ws.rs.ext.WriterInterceptor interface is defined as follows: 
// Java
...
package javax.ws.rs.ext;

public interface WriterInterceptor {
    void aroundWriteTo(WriterInterceptorContext context)
            throws java.io.IOException, javax.ws.rs.WebApplicationException;
}
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By implementing the WriterInterceptor interface, you can intercept the message body (Entity object) as it is being written either on the server side or the client side. You can use an entity writer interceptor in either of the following contexts:
  • Server side—if bound as a server-side interceptor, the entity writer interceptor intercepts the response message body just before it is marshalled and sent back to the client.
  • Client side—if bound as a client-side interceptor, the entity writer interceptor intercepts the request message body just before it is marshalled and sent out to the server.

WriterInterceptorContext interface
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The aroundWriteTo method of WriterInterceptor receives one argument of type javax.ws.rs.ext.WriterInterceptorContext, which can be used to access both the message body (Entity object) and message metadata.
The WriterInterceptorContext interface is defined as follows: 
// Java
...
package javax.ws.rs.ext;

import java.io.IOException;
import java.io.OutputStream;

import javax.ws.rs.WebApplicationException;
import javax.ws.rs.core.MultivaluedMap;

public interface WriterInterceptorContext extends InterceptorContext {

    void proceed() throws IOException, WebApplicationException;

    Object getEntity();

    void setEntity(Object entity);

    OutputStream getOutputStream();

    public void setOutputStream(OutputStream os);

    MultivaluedMap<String, Object> getHeaders();
}
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InterceptorContext interface
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The WriterInterceptorContext interface also supports the methods inherited from the base InterceptorContext interface. For the definition of InterceptorContext, see the section called “InterceptorContext interface”.

Sample implementation on the client side
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To implement an entity writer interceptor for the client side, define a class that implements the WriterInterceptor interface.
For example, the following code shows an example of an entity writer interceptor for the client side (with a priority of 10), which appends an extra line of text to the message body of the outgoing request: 
// Java
package org.jboss.fuse.example;
 
import java.io.IOException;
import java.io.OutputStream;

import javax.ws.rs.WebApplicationException;
import javax.ws.rs.ext.WriterInterceptor;
import javax.ws.rs.ext.WriterInterceptorContext;
import javax.annotation.Priority;
 
@Priority(value = 10)
public class SampleClientWriterInterceptor implements WriterInterceptor {

  @Override
  public void aroundWriteTo(WriterInterceptorContext interceptorContext)
          throws IOException, WebApplicationException {
    OutputStream outputStream = interceptorContext.getOutputStream();
    String appendedContent = "\nInterceptors always get the last word in.";
    outputStream.write(appendedContent.getBytes());
    interceptorContext.setOutputStream(outputStream);

    interceptorContext.proceed();
  }
}
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Sample implementation on the server side
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To implement an entity writer interceptor for the server side, define a class that implements the WriterInterceptor interface and annotate it with the @Provider annotation.
For example, the following code shows an example of an entity writer interceptor for the server side (with a priority of 10), which appends an extra line of text to the message body of the outgoing request: 
// Java
package org.jboss.fuse.example;
 
import java.io.IOException;
import java.io.OutputStream;

import javax.ws.rs.WebApplicationException;
import javax.ws.rs.ext.Provider;
import javax.ws.rs.ext.WriterInterceptor;
import javax.ws.rs.ext.WriterInterceptorContext;
import javax.annotation.Priority;
 
@Priority(value = 10)
@Provider
public class SampleServerWriterInterceptor implements WriterInterceptor {

  @Override
  public void aroundWriteTo(WriterInterceptorContext interceptorContext)
          throws IOException, WebApplicationException {
    OutputStream outputStream = interceptorContext.getOutputStream();
    String appendedContent = "\nInterceptors always get the last word in.";
    outputStream.write(appendedContent.getBytes());
    interceptorContext.setOutputStream(outputStream);

    interceptorContext.proceed();
  }
}
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Binding a writer interceptor on the client side
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Using the JAX-RS 2.0 client API, you can register an entity writer interceptor directly on a javax.ws.rs.client.Client object or on a javax.ws.rs.client.WebTarget object. Effectively, this means that the writer interceptor can optionally be applied to different scopes, so that only certain URI paths are affected by the interceptor.
For example, the following code shows how to register the SampleClientReaderInterceptor interceptor so that it applies to all invocations made using the client object: 
// Java
... 
import javax.ws.rs.client.Client;
import javax.ws.rs.client.ClientBuilder;
import javax.ws.rs.client.Invocation;
import javax.ws.rs.client.WebTarget;
import javax.ws.rs.core.Response;
...
Client client = ClientBuilder.newClient();
client.register(SampleClientReaderInterceptor.class);
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For more details about registering interceptors with a JAX-RS 2.0 client, see Section 46.5, “Configuring the Client Endpoint”.

Binding a writer interceptor on the server side
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To bind a writer interceptor on the server side (that is, to install it into the Apache CXF runtime), perform the following steps:
  1. Add the @Provider annotation to the writer interceptor class, as shown in the following code fragment: 
    // Java
package org.jboss.fuse.example;
...
import javax.ws.rs.WebApplicationException;
import javax.ws.rs.ext.Provider;
import javax.ws.rs.ext.WriterInterceptor;
import javax.ws.rs.ext.WriterInterceptorContext;
import javax.annotation.Priority;
 
@Priority(value = 10)
@Provider
public class SampleServerWriterInterceptor implements WriterInterceptor {
  ...
}
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    When the writer interceptor implementation is loaded into the Apache CXF runtime, the REST implementation automatically scans the loaded classes to search for the classes marked with the @Provider annotation (the scanning phase).
  2. When defining a JAX-RS server endpoint in XML (for example, see Section 16.1, “Configuring JAX-RS Server Endpoints”), add the writer interceptor to the list of providers in the jaxrs:providers element. 
    <blueprint xmlns="http://www.osgi.org/xmlns/blueprint/v1.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:jaxrs="http://cxf.apache.org/blueprint/jaxrs"
    xmlns:cxf="http://cxf.apache.org/blueprint/core"
    ...
>
    ...
    <jaxrs:server id="customerService" address="/customers">
      ...
      <jaxrs:providers>
        <ref bean="interceptorProvider" />
      </jaxrs:providers>
      <bean id="interceptorProvider" class="org.jboss.fuse.example.SampleServerWriterInterceptor"/>

    </jaxrs:server>

</blueprint>
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    Note
    This step is a non-standard requirement of Apache CXF. Strictly speaking, according to the JAX-RS standard, the @Provider annotation should be all that is required to bind the interceptor. But in practice, the standard approach is somewhat inflexible and can lead to clashing providers when many libraries are included in a large project.
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