Java反射：從接口名稱調用方法

Question

我有一個接口的名稱，我想調用由其具體實現的類定義的方法。所以我接受了Java Reflection的幫助。

接口：

package tsb.learning.reflection; 

public interface IAnyThing { 

    void doSomething(); 
} 

它的實現類：

package tsb.learning.reflection; 

public class AnyThing implements IAnyThing { 

    public void doSomething() { 
     System.out.println("JYM"); 
    } 
} 

的InvocationHandler實施：

package tsb.learning.reflection; 

import java.lang.reflect.InvocationHandler; 
import java.lang.reflect.Method; 

public class AnyInvocationHandler implements InvocationHandler { 

    @Override 
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
     return method.invoke(proxy, args); 
    } 
} 

和控制器：

package tsb.learning.reflection; 

import java.lang.reflect.InvocationHandler; 
import java.lang.reflect.Proxy; 

public class Controller { 

    /** 
    * @param args 
    * @throws ClassNotFoundException 
    */ 
    public static void main(String[] args) throws ClassNotFoundException { 
     String interfaceName = "tsb.learning.reflection.IAnyThing"; 
     ClassLoader classLoader = Class.forName(interfaceName).getClassLoader(); 
     Class<?>[] interfaces = new Class<?>[] { Class.forName(interfaceName) }; 
     InvocationHandler handler = new AnyInvocationHandler(); 
     IAnyThing anyThing = (IAnyThing) Proxy.newProxyInstance(classLoader, interfaces, handler); 
     anyThing.doSomething(); 
    } 
} 

但它不工作，我得到以下異常：

Caused by: java.lang.reflect.InvocationTargetException 
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
    at java.lang.reflect.Method.invoke(Unknown Source) 
    at tsb.learning.reflection.AnyInvocationHandler.invoke(AnyInvocationHandler.java:10) 
    at $Proxy0.doSomething(Unknown Source) 
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
    at java.lang.reflect.Method.invoke(Unknown Source) 
    at tsb.learning.reflection.AnyInvocationHandler.invoke(AnyInvocationHandler.java:10) 
    at $Proxy0.doSomething(Unknown Source) 
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
    at java.lang.reflect.Method.invoke(Unknown Source) 
    at tsb.learning.reflection.AnyInvocationHandler.invoke(AnyInvocationHandler.java:10) 
    at $Proxy0.doSomething(Unknown Source) 
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source) 
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) 
    at java.lang.reflect.Method.invoke(Unknown Source) 
    at tsb.learning.reflection.AnyInvocationHandler.invoke(AnyInvocationHandler.java:10) 

唯一的例外是在控制檯打印的循環，我需要停止該程序。

任何信息對我都很有幫助。

Answer 1

造成的：java.lang.reflect.InvocationTargetException

這意味着你要調用的方法拋出異常。你需要看看它後面出現的異常並導致這個異常。它與你如何稱呼該方法無關。

我懷疑你得到一個的StackOverflowError

// calls the same method on the same proxy which will recurse until you get an error. 
return method.invoke(proxy, args); 

---

而是嘗試調用一個真正的對象上的方法做一些事情。

public class AnyInvocationHandler implements InvocationHandler { 
    final IAnyThing iat; 

    public AnyInvocationHandler(IAnyThing iat) { 
     this.iat = iat; 
    } 

    @Override 
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
     // call the same method on a real object. 
     return method.invoke(iat, args); 
    } 
} 

BTW你可以寫

Class interfaceClass = tsb.learning.reflection.IAnyThing.class; 
ClassLoader classLoader = interfaceClass.getClassLoader(); 
Class<?>[] interfaces = new Class<?>[] { interfaceClass }; 

Answer 2

裏面你AnyInvocationHandler，你可以在電話委託給你的AnyThing的實例：

public class AnyInvocationHandler implements InvocationHandler { 

    private AnyThing delegate = new AnyThing(); 

    @Override 
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 

     // to something useful here 
     [...] 
     // finally, invoke the method on implementation class. 
     return method.invoke(delegate, args); 
    } 
} 

Answer 3

在IAnyThing的唯一方法是doSomething()，所以我想在InvocationHandler你知道這個方法是什麼。把你的實現放在那裏。此外，在doSomething()旁邊，您還應該處理從java.lang.Object繼承的三種方法：

public static class AnyInvocationHandler implements InvocationHandler { 

    private static final Method doSomething; 

    static { 
     try { 
      doSomething = IAnyThing.class.getMethod("doSomething"); 
     } catch (NoSuchMethodException e) { 
      throw new ExceptionInInitializerError(e); 
     } 
    } 

    @Override 
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable { 
     if (method.getDeclaringClass() == Object.class) 
      return handleObjectMethod(proxy, method, args); 

     if (doSomething.equals(method)) { 
      doSomethingImpl(); 
      return null; 
     } 

     throw new AbstractMethodError(method.toString()); 
    } 

    private Object handleObjectMethod(Object proxy, Method method, Object[] args) { 
     switch (method.getName()) { 
      case "equals": 
       return proxy == args[0]; 
      case "hashCode": 
       return System.identityHashCode(proxy); 
      case "toString": 
       return proxy.getClass().getName() + "@" + Integer.toHexString(System.identityHashCode(proxy)); 
      default: 
       throw new AssertionError(); 
     } 
    } 

    private void doSomethingImpl() { 
     // implement.... 
    } 

}