实现自定义MessageBodyWriter

Question

我在控制台上收到以下消息：

`MessageBodyWriter not found for media type=application/json, type=class java.util.HashMap, genericType=class java.util.HashMap.`

调用以下ReST服务：

@GET
@Path("/m1")
@Produces(MediaType.APPLICATION_JSON)
public Map<String, ArrayList<EntityClass>> m1(){
    return new Definition().webMethod1();
}

所以我实现了MessageBodyWriter类，如下所示：

package web;

import java.io.IOException;
import java.io.OutputStream;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Map;

import javax.ws.rs.Produces;
import javax.ws.rs.WebApplicationException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.MultivaluedMap;
import javax.ws.rs.ext.MessageBodyWriter;
import javax.ws.rs.ext.Provider;

@Provider
@Produces(MediaType.APPLICATION_JSON)
public class Writer implements MessageBodyWriter<Map<String, ArrayList<EntityClass>>>{

    @Override
    public long getSize(Map<String, ArrayList<EntityClass>> arg0, Class<?> arg1, Type arg2, Annotation[] arg3, MediaType arg4) {
        return arg0.size();
    }

    @Override
    public boolean isWriteable(Class<?> arg0, Type arg1, Annotation[] arg2, MediaType arg3) {
        return  arg0 == EntityClass.class;
    }

    @Override
    public void writeTo(Map<String, ArrayList<EntityClass>> arg0, Class<?> arg1, Type arg2, Annotation[] arg3,
            MediaType arg4, MultivaluedMap<String, Object> arg5, OutputStream arg6)
            throws IOException, WebApplicationException {
        System.out.println("Exe started..............................................................................");
        StringBuffer sb = new StringBuffer("{");
        ArrayList<EntityClass> entityObj = arg0.get("Entity");
        for(EntityClass eobj: entityObj){
            sb.append("[{");
            sb.append("\"id\":\""+eobj.getId()+"\",");
            sb.append("\"name\":\""+eobj.getName()+"\"");
            sb.append("}],");
        }
        sb.replace(sb.length()-1, sb.length(), "}");
        System.out.println(sb);
        arg6.write(sb.toString().getBytes());
    }

}

我仍然收到同样的错误，并且控制台上没有显示system.out.println类writeTo类的writer行。

可能是什么错误，可能是一个可能的解决方案？