Exception handling components

[fcamblor]

This is very frequent that some RestX projects implements a RestxFilter for exception handling, because there is no extension point on StdRestxMainRouter exception handling

I would introduce a new RestxExceptionHandler interface for this purpose :

public interface RestxExceptionHandler<E extends Throwable> extends Predicate<Throwable> {
    void handle(E exception, RestxRequest restxRequest, RestxResponse restxResponse);
}
public abstract class RestxExceptionHandlerBase<E extends Throwable> implements RestxExceptionHandler<E> {
    private Class<E> exceptionClass;
    public RestxExceptionHandlerBase(Class<E> exceptionClass) {
        this.exceptionClass = exceptionClass;
    }

    public boolean apply(Throwable t) {
        return exceptionClass.isAssignableFrom(t);
    }

    // Maybe some more utility methods such as the one for filtering stacktraces (see #281)
}

with a bunch of default exception handlers allowing to replicate existing exception handlings in StdRestxMainRouter, for instance :

@Module(priority = 1000)
public class ErrorsModule {
    @Provide @Named("WebException")
    public RestxExceptionHandler<? extends Throwable> webException() {
        return new RestxExceptionHandlerBase(WebException.class) {
            void handle(WebException ex, RestxRequest restxRequest, RestxResponse restxResponse) {
                ex.writeTo(restxRequest, restxResponse);
            }
        });
    }

    @Provide @Named("RestxException")
    public RestxExceptionHandler<? extends Throwable> webException() {
        return new RestxExceptionHandlerBase(RestxError.RestxException.class) {
            void handle(RestxError.RestxException ex, RestxRequest restxRequest, RestxResponse restxResponse) {
                logger.debug("request raised RestxException", ex);
                restxResponse.setStatus(ex.getErrorStatus());
                restxResponse.setContentType("application/json");
                PrintWriter out = restxResponse.getWriter();
                out.println(ex.toJSON());
            }
        });
    }
    // etc..
}

WDYT ?