如何在java中创建一个动态代理,保留方法上的参数注释？

Question

我目前正在尝试代理一些现有的JAX/RS资源,以便允许我使用Hibernate Validator的方法验证支持.但是,当我代理我的类(当前使用cglib 2.2)时,FormParam注释不存在于代理类中的参数上,因此JAX/RS运行时(apache wink)不填充参数.这里有一些测试代码显示:

import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;

import java.lang.annotation.Annotation;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

import net.sf.cglib.proxy.Enhancer;
import net.sf.cglib.proxy.MethodInterceptor;
import net.sf.cglib.proxy.MethodProxy;

import javassist.util.proxy.ProxyFactory;

public class ProxyTester {

    @Target( { PARAMETER })
    @Retention(RUNTIME)
    public static @interface TestAnnotation {
    }

    public static interface IProxyMe {
        void aMethod(@TestAnnotation int param);
    }

    public static class ProxyMe implements IProxyMe {
            public void aMethod(@TestAnnotation int param) {
        }
    }

    static void dumpAnnotations(String type, Object proxy, Object forObject,
            String forMethod) {
        String className = forObject.getClass().getName();

        System.err.println(type + " proxy for Class: " + className);

        for (Method method : proxy.getClass().getMethods()) {
            if (method.getName().equals(forMethod)) {
                final int paramCount = method.getParameterTypes().length;
                System.err.println(" Method: " + method.getName() + " has "
                        + paramCount + " parameters");
                int i = 0;
                for (Annotation[] paramAnnotations : method
                        .getParameterAnnotations()) {
                    System.err.println("  Param " + (i++) + " has "
                            + paramAnnotations.length + " annotations");
                    for (Annotation annotation : paramAnnotations) {
                        System.err.println("   Annotation "
                                + annotation.toString());
                    }
                }
            }
        }
    }

    static Object javassistProxy(IProxyMe in) throws Exception {
        ProxyFactory pf = new ProxyFactory();
        pf.setSuperclass(in.getClass());
        Class c = pf.createClass();
        return c.newInstance();
    }

    static Object cglibProxy(IProxyMe in) throws Exception {
        Object p2 = Enhancer.create(in.getClass(), in.getClass()
                .getInterfaces(), new MethodInterceptor() {
            public Object intercept(Object arg0, Method arg1, Object[] arg2,
                    MethodProxy arg3) throws Throwable {
                return arg3.invokeSuper(arg0, arg2);
            }
        });
        return p2;

    }

    static Object jdkProxy(final IProxyMe in) throws Exception {
        return java.lang.reflect.Proxy.newProxyInstance(in.getClass()
                .getClassLoader(), in.getClass().getInterfaces(),
                new InvocationHandler() {
                    public Object invoke(Object proxy, Method method,
                            Object[] args) throws Throwable {
                        return method.invoke(in, args);
                    }
                });
    }

    public static void main(String[] args) throws Exception {
        IProxyMe proxyMe = new ProxyMe();
        dumpAnnotations("no", proxyMe, proxyMe, "aMethod");
        dumpAnnotations("javassist", javassistProxy(proxyMe), proxyMe,
            "aMethod");
        dumpAnnotations("cglib", cglibProxy(proxyMe), proxyMe, "aMethod");

        dumpAnnotations("jdk", jdkProxy(proxyMe), proxyMe, "aMethod");
    }
}

这给了我以下输出:

no proxy for Class: ProxyTester$ProxyMe
 Method: aMethod has 1 parameters
  Param 0 has 1 annotations
   Annotation @ProxyTester.TestAnnotation()
javassist proxy for Class: ProxyTester$ProxyMe
 Method: aMethod has 1 parameters
  Param 0 has 0 annotations
cglib proxy for Class: ProxyTester$ProxyMe
 Method: aMethod has 1 parameters
  Param 0 has 0 annotations
jdk proxy for Class: ProxyTester$ProxyMe
 Method: aMethod has 1 parameters
  Param 0 has 0 annotations

还有其他选择吗？

Answer 1

从技术上讲，CGLib增强器只是从您的类扩展而来。我不知道这对你来说是否可行（对象的数量），但是暴露一个接口而不是类怎么样？

Object p2 = Enhancer.create(resource.getClass(),
    new Class[] { IResource.class },
    new MethodInterceptor() {
      public Object intercept(Object arg0, Method arg1, Object[] arg2, MethodProxy arg3)
          throws Throwable {
            return arg3.invokeSuper(arg0, arg2);
           }
    });

将增强类中的注解去掉，放入接口中。然后针对接口进行验证。对于许多此类资源来说，这可能是大量的样板接口，但仍然比将所有内容映射到形成支持对象或 DTO 更好。