When processing JAX-RS deployments, RESTEasy relies on ResourceBuilder to create metadata for each JAX-RS resource. Such metadata is defined using the metadata SPI in package org.jboss.resteasy.spi.metadata, in particular the ResourceClass interface:

package org.jboss.resteasy.spi.metadata;

public interface ResourceClass
{
  String getPath();

  Class<?> getClazz();

  ResourceConstructor getConstructor();

  FieldParameter[] getFields();

  SetterParameter[] getSetters();

  ResourceMethod[] getResourceMethods();

  ResourceLocator[] getResourceLocators();
}

Among the other classes and interfaces defining metadata SPI, the following interfaces are worth a mention here:

public interface ResourceConstructor
{
  ResourceClass getResourceClass();

  Constructor getConstructor();

  ConstructorParameter[] getParams();
}

public interface ResourceMethod extends ResourceLocator
{
  Set<String> getHttpMethods();

  MediaType[] getProduces();

  MediaType[] getConsumes();

  boolean isAsynchronous();

  void markAsynchronous();
}

public interface ResourceLocator
{
  ResourceClass getResourceClass();

  Class<?> getReturnType();

  Type getGenericReturnType();

  Method getMethod();

  Method getAnnotatedMethod();

  MethodParameter[] getParams();

  String getFullpath();

  String getPath();

}

Now, the interesting point is that RESTEasy allows tuning the metadata generation by providing implementations of the ResourceClassProcessor interface:

package org.jboss.resteasy.spi.metadata;

public interface ResourceClassProcessor
{

  /**
   * Allows the implementation of this method to modify the resource metadata represented by
   * the supplied {@link ResourceClass} instance. Implementation will typically create
   * wrappers which modify only certain aspects of the metadata.
   *
   * @param clazz The original metadata
   * @return the (potentially modified) metadata (never null)
   */
  ResourceClass process(ResourceClass clazz);

}

The processors are meant to be, and are resolved as, regular JAX-RS annotated providers. They allow for wrapping resource metadata classes with custom versions that can be used for various advanced scenarios like