MessageBodyWriter vs. WriterInterceptor

Question

I am a bit confused about the exact intention of the WriterInterceptor Interface of the JAX-RS 2.0 specification.

What I know so far:

• WriterInterceptor is called before MessageBodyWriter
• Both Interfaces grant access to the same variables
• MessageBodyWriter is concerned with translating an object into a stream
• Arun Gupta states in his Java EE 7 book that Writer/Reader Inteceptors are mainly concerned with marshalling and unmarshalling of HTTP bodies.

My questionis: For which intent should the Reader/Writer Interceptor be used for?

Answer 1

On the server-side you can let the Message Body Writers do the entity marshalling and while the Interceptors can take care of:

• performing GZIP compression of the marshalled entity,
• wrapping the marshalled entity in a Digital Signature enveloppe (reader interceptor could verify a signature before further processing),
• adding Cache control headers

For example:

@Override
void aroundWriteTo(WriterInterceptorContext ctx) ... {
    OutputStream old = ctx.getOutputStream();
    GZIPOutputStream gzipOutputStream = new GZIPOutputStream(old);
    ctx.setOutputStream(gzipOutputStream);
    updateHeaders(ctx);
    try {
        ctx.proceed();
    } finally {
        gzipOutputStream.finish();
        ctx.setOutputStream(old);
    }
}

(code above comes from the JAX-RS 2.0 spec, section 6.3)

HTH.