jdk动态代理实现原理
写在前面:
大神和diao炸天的亲请绕道..
关于代理模式的概念这里省去,大家可以放鸟尽情搜..
关于为什么叫动态代理,个人理解是代理的类是在运行时动态生成的,大家也可以参考网上的理解..
文笔很差,所以文字较少,都在代码和注释中..
=======一点不华丽的分割线-------------------------
开门见山,lets go..
java中可以通过jdk提供的 Proxy.newProxyInstance静态方法来创建动态代理对象,下面先来看看这个方法的实现
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h)
throws IllegalArgumentException {
//InvocationHandler不能为空,因为对代理对象的所有方法调用实际上都会委托到InvocationHandler的invoke方法,
//这个我们后面通过查看产生的代理类的源代码便会一目了然
if (h == null) {
throw new NullPointerException();
}
//这个是核心的地方,通过提供的ClassLoader和interface列表来产生代理类,具体的实现可以参考getProxyClass这个方法的实现,
//真正的工作是由sun.misc.ProxyGenerator这个类来完成的,可以google查看具体的逻辑.在我们的程序中通过设置
//System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true")可以查看产生的类文件
Class cl = getProxyClass(loader, interfaces);
//因为代理类继承了Proxy类.而Proxy中定义了构造函数protected Proxy(InvocationHandler h),所以可以反射得到Constructer实例
//创建代理对象
try {
Constructor cons = cl.getConstructor(constructorParams);
return (Object) cons.newInstance(new Object[] { h });
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString());
} catch (IllegalAccessException e) {
throw new InternalError(e.toString());
} catch (InstantiationException e) {
throw new InternalError(e.toString());
} catch (InvocationTargetException e) {
throw new InternalError(e.toString());
}
}
下面通过个例子来说明下:
先来定义一个接口,jdk的动态代理基于接口来创建代理类,不能基于类的原因是java不支持多重继承,而代理类都会继承Proxy类(个人理解).
/**
* Subject
*
* @author Kevin Fan
* @since 2013-9-13 下午2:43:33
*/
public interface Subject {
void pub(String key, String content);
String sub(String key);
}
再来一个具体的实现,在代理模式中可以叫它的实例可以叫target,这个是真正执行操作的对象
/**
* SimpleSubject
*
* @author Kevin Fan
* @since 2013-9-13 下午2:45:03
*/
public class SimpleSubject implements Subject {
private Map<String, String> msg = new ConcurrentHashMap<String, String>();
public void pub(String key, String content) {
System.out.println("pub msg: key is " + key + ", content is " + content);
msg.put(key, content);
}
public String sub(String key) {
if (msg.containsKey(key)) {
String ret = msg.get(key);
System.out.println("sub msg: key is " + key + ", result is " + ret);
return ret;
}
return null;
}
}
好,接下来我们来写个动态代理工厂,根据 不同的target来创建动态代理对象
/**
* SubjectProxyFactory
*
* @author Kevin Fan
* @since 2013-9-13 下午2:47:24
*/
public class SubjectProxyFactory {
//TODO: cache
public static Subject getSubject(final Subject realSubject) {
return (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(), new Class[] { Subject.class },
new InvocationHandler() {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("\naction before method invocation....");
Object retVal = method.invoke(realSubject, args);
System.out.println("action after method invocation....\n");
return retVal;
}
});
}
}
可以看到这是一个简单的实现,只是在真实对象执行前后各打一句信息..
接下来用一个 main函数来把这些结合起来
/**
* Demo
*
* @author Kevin Fan
* @since 2013-9-13 下午2:50:28
*/
public class Demo {
public static void main(String[] args) {
//设置此系统属性,以查看代理类文件
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
//创建真实对象
Subject subj = new SimpleSubject();
subj.pub("name", "kevin.fan");
subj.sub("name");
//创建代理对象
Subject proxy = SubjectProxyFactory.getSubject(subj);
proxy.pub("hobby", "r&b music");
proxy.sub("name");
}
}
ok,小手抖一下,走你,看下执行结果
pub msg: key is name, content is kevin.fan sub msg: key is name, result is kevin.fan action before method invocation.... pub msg: key is hobby, content is r&b music action after method invocation.... action before method invocation.... sub msg: key is name, result is kevin.fan action after method invocation....
可以看到在调用代理对象的方法时,添加的额外动作已经生效,接下来我们看下生成的代理类的代码..
import com.aliyun.demo.kevin.coder.lang.proxy.Subject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
//这里很清楚了,代理类继承了Proxy类,并且实现了Proxy.newProxyInstance这个方法中传入的接口
public final class $Proxy0 extends Proxy
implements Subject
{
//这些方法在下面的static init block中进行初始化
private static Method m4;
private static Method m1;
private static Method m3;
private static Method m0;
private static Method m2;
static
{
try
{
m4 = Class.forName("com.aliyun.demo.kevin.coder.lang.proxy.Subject").getMethod("sub", new Class[] { Class.forName("java.lang.String") });
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
m3 = Class.forName("com.aliyun.demo.kevin.coder.lang.proxy.Subject").getMethod("pub", new Class[] { Class.forName("java.lang.String"), Class.forName("java.lang.String") });
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
return;
}
catch (NoSuchMethodException localNoSuchMethodException)
{
throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
}
catch (ClassNotFoundException localClassNotFoundException)
{
throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
}
}
//构造函数,接收一个 InvocationHandler作为参数,这就是为什么Proxy.newProxyInstance方法里可以
//通过InvocationHandler实例作为参数来反射获取Constructer实例
public $Proxy0 paramInvocationHandler)
throws
{
super(paramInvocationHandler);
}
//下面通过这个来看下代理对象中方法是怎样调用的
public final String sub(String paramString)
throws
{
try
{
//全部是通过调用InvocationHandler的invoke方法,传入对应的方法和参数
return (String)this.h.invoke(this, m4, new Object[] { paramString });
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final boolean equals(Object paramObject)
throws
{
try
{
return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final void pub(String paramString1, String paramString2)
throws
{
try
{
this.h.invoke(this, m3, new Object[] { paramString1, paramString2 });
return;
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final int hashCode()
throws
{
try
{
return ((Integer)this.h.invoke(this, m0, null)).intValue();
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final String toString()
throws
{
try
{
return (String)this.h.invoke(this, m2, null);
}
catch (Error|RuntimeException localError)
{
throw localError;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
}
-------------END
by Kevin Fan