說說動態代理中碰到的一個小問題

JDK內置 Proxy類和 InvocationHandler接口來提供動態代理的實現。在實現鏈接池的時候動態代理就能夠派上用場了。經過代理接管close方法, connectoin關閉的時候就不須要真正關閉，而只是放回鏈接池，具體實現原理能夠參考紅薯關於鏈接池的文章。

我要寫的呢是關於在測試使用動態代理時碰到的一個問題，先看個人代碼:

首先是接口 IFunction

public interface IFunction {
    void doSomething () throws IllegalStateException;
}

接口實現 FunctionImpl

public class FunctionImpl implements IFunction {
    @Override
    public void doSomething() throws IllegalStateException {
        // 方法什麼也不作, 只拋異常
        throw new IllegalStateException();
    }
}

攔截 IFunctioin 的動態代理類 FunctionHandler

public class FunctionHandler  implements InvocationHandler{

    private IFunction fun;

    public FunctionHandler(IFunction function) {
        this.fun = function;
    }

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

最後是簡單的調用

public class Client {

    public static void main(String[] args) {
        IFunction fun = (IFunction) Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(),
                new Class[]{IFunction.class}, new FunctionHandler(new FunctionImpl()));
        try {
            fun.doSomething();
        } catch (IllegalStateException e) {
            e.printStackTrace();
        }
    }
}

如此，一個簡單的動態代理完成了, 一眼瞥上去沒什麼問題, 惋惜一運行就拋異常了

Exception in thread "main" java.lang.reflect.UndeclaredThrowableException
	at com.sun.proxy.$Proxy0.doSomething(Unknown Source)
	at designpattern.proxy.Client.main(Client.java:18)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: java.lang.reflect.InvocationTargetException
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at designpattern.proxy.FunctionHandler.invoke(FunctionHandler.java:21)
	... 7 more
Caused by: java.lang.IllegalStateException
	at designpattern.proxy.FunctionImpl.doSomething(FunctionImpl.java:9)
	... 12 more

很奇怪啊，接口只聲明瞭IllegalStateException, 結果卻拋出了 InvocationTargetException 及 UndeclaredThrowableException.

接下來咱們看看這兩個不速之客是如何產生的。

首先這兩個異常確定是接口實現類拋出 IllegalStateException 引發的, 因而能夠定位到 java.lang.reflect.Method 的 invoke(Object obj, Object... args), 該方法文檔已經說明: 當代理的方法拋出異常時 invoke(Object obj, Object... args) 方法會拋出 InvocationTargetException 異常, 也就是說個人 IllegalStateException 此時會被包裝成 InvocationTargetException。

好，如今已經知道 InvocationTargetException 是由於Method反射機制包裝產生的。

接下來再看 UndeclaredThrowableException 如何產生。

在 InvocationHandler 聲明的方法 invoke(Object proxy, Method method, Object[] args) 的文檔中有這麼一句話

Throwable the exception to throw from the method invocation on the proxy instance. The exception's type must be assignable either to any of the exception types declared in the throws clause of the interface method or to the unchecked exception types java.lang.RuntimeException or code java.lang.Error. If a checked exception is thrown by this method that is not assignable to any of the exception types declared in the throws clause of the interface method, then an UndeclaredThrowableException containing the exception that was thrown by this method will be thrown by the method invocation on the proxy instance.

這裏也就是說被代理接口的方法在執行的時候拋出的受檢異常必須是接口定義中聲明的異常, 若是拋出的受檢異常未被接口聲明, 那麼此時這個異常就會被包裝成UndeclaredThrowableException。

那麼也就清楚了，以前已經看到Method.invoke()時拋出了異常InvocationTargetException 剛好不在 接口聲明的異常範圍內, 所以動態代理執行的時候會拋出異常 UndeclaredThrowableException。

對於這個問題能夠改良下 FunctionHandler 的代碼就可解決

public class FunctionHandler  implements InvocationHandler{

    private IFunction fun;

    public FunctionHandler(IFunction function) {
        this.fun = function;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        try {
            return method.invoke(fun, args);
        } catch (InvocationTargetException e) {
            throw e.getCause();
        }
    }
}

這樣再次運行測試獲得的結果以下

java.lang.IllegalStateException
	at designpattern.proxy.FunctionImpl.doSomething(FunctionImpl.java:9)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at designpattern.proxy.FunctionHandler.invoke(FunctionHandler.java:23)
	at com.sun.proxy.$Proxy0.doSomething(Unknown Source)
	at designpattern.proxy.Client.main(Client.java:18)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:606)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

兩個不速之客已經消失了, "老闆不再用擔憂個人異常", 哈哈。

我的博客地址: http://hellofalcon.com/post/p-jdkdynamicproxy/