Examples for Chapter 12 - RESTful Java with JAX-RS 2.0 - page 322

To implement a WriterInterceptor , we must define an aroundWriteTo() method. We
start off in this method by creating a java.security.MessageDigest . We'll use this class
to create an MD5 hash of the entity we're marshalling.

ByteArrayOutputStream buffer = new new ByteArrayOutputStream ();

DigestOutputStream digestStream = new new DigestOutputStream ( buffer , digest );

OutputStream old = context . getOutputStream ();
context . setOutputStream ( digestStream );

Next we create a java.io.ByteArrayOutputStream and wrap it with a
java.security.DigestOutputStream . The MD5 hash is created from the marshalled bytes
of the entity. We need to buffer this marshalling in memory, as we need to set the Content-
MD5 before the entity is sent across the wire. We override the OutputStream of the Contain-
erRequestContext so that the MessageBodyWriter that performs the marshalling uses the
DigestOutputStream .

try try

{

context . proceed ();

byte byte [] hash = digest . digest ();

String encodedHash = Base64 . encodeBytes ( hash );
context . getHeaders (). putSingle ( "Content-MD5" , encodedHash );

Next, context.proceed() is invoked. This continues with the interceptor chain and until
the underlying MessageBodyWriter is invoked. After proceed() finishes, we obtain the
hash from the MessageDigest and Base-64-encode it using a RESTEasy utility class. We
then set the Content-MD5 header value with this encoded string.

byte byte [] content = buffer . toByteArray ();

old . write ( content );

}

After the header is set, we write the buffered content to the real OutputStream .

finally finally

{

context . setOutputStream ( old );

}

}

}

Finally, if you override the context's OutputStream it is always best practice to revert it after
you finish intercepting. We do this in the finally block.