java动态代理机制原理

Proxy的public static Object newProxyInstance(ClassLoader loader,Class<?>[] interfaces,InvocationHandler h)，

首先通过getProxyClass 创建一个新的Class 对象，//（Class cl = getProxyClass(loader, interfaces);）

getProxyClass实现所有的包含所有的interfaces接口，并且名字为String proxyName = proxyPkg + proxyClassNamePrefix + num;

其中proxyClassNamePrefix为"$Proxy"，proxyPkg 为包名，num并发控制数字

然后 Constructor cons = cl.getConstructor(constructorParams);，其中Class [] constructorParams ={ InvocationHandler.class }

return (Object) cons.newInstance(new Object[] { h });

可知：proxyName 的Class对象包含有InvocationHandler.class数组的构造器，

最后 return (Object) cons.newInstance(new Object[] { h })，即用实现InvocationHandler接口的对象作为参数实例化proxyName Class。

测试：建立一个方法打印出Class的内容：

//此类修改于http://blog.csdn.net/rokii/archive/2009/04/03/4046098.aspx，原来的方法不能正确的打印出类的内容

public static void printClassDefinition(Class clz) {

String clzModifier = Modifier.toString(clz.getModifiers())+" ";

String superClz = clz.getSuperclass().getName();

if (superClz != null && !superClz.equals("")) {

superClz = "extends " + superClz;

}

Class[] interfaces = clz.getInterfaces();

String inters = "";

for (int i = 0; i < interfaces.length; i++) {

if (i == 0) {

inters += "implements ";

}

inters += interfaces[i].getName();

}

System.out.println(clzModifier + clz.getName() + " " + superClz + " "

+ inters);

System.out.println("{");

System.out.println("//Fields:");

Field[] fields = clz.getDeclaredFields();

for (int i = 0; i < fields.length; i++) {

String modifier = Modifier.toString(fields[i].getModifiers())+" ";

String fieldName = fields[i].getName();

String fieldType = fields[i].getType().getName();

System.out.println(" " + modifier + fieldType + " " + fieldName

+ ";");

}

System.out.println("//Constructs:");

Constructor[] constructs=clz.getConstructors();

for(int i=0;i<constructs.length;i++){

Constructor construct=constructs[i];

System.out.println(" "+construct.toString()+";");

}

System.out.println("//Methods:");

Method[] methods = clz.getDeclaredMethods();

for (int i = 0; i < methods.length; i++) {

Method method = methods[i];

String modifier = Modifier.toString(method.getModifiers())+" ";

String methodName = method.getName();

Class returnClz = method.getReturnType();

String retrunType = returnClz.getName();

Class[] clzs = method.getParameterTypes();

String paraList = "(";

for (int j = 0; j < clzs.length; j++) {

paraList += clzs[j].getName();

if (j != clzs.length - 1) {

paraList += ", ";

}

}

paraList += ")";

clzs = method.getExceptionTypes();

String exceptions = "";

for (int j = 0; j < clzs.length; j++) {

if (j == 0) {

exceptions += "throws ";

}

exceptions += clzs[j].getName();

if (j != clzs.length - 1) {

exceptions += ", ";

}

}

exceptions += ";";

String methodPrototype = modifier + retrunType + " " + methodName

+ paraList + exceptions;

System.out.println(" " + methodPrototype);

}

System.out.println("}");

}

某个代理类返回对象用printClassDefinition打印如下：

public final $Proxy0 extends java.lang.reflect.Proxy implements BusinessProcessor

{

//Fields:

private static java.lang.reflect.Method m1;

private static java.lang.reflect.Method m0;

private static java.lang.reflect.Method m3;

private static java.lang.reflect.Method m4;

private static java.lang.reflect.Method m2;

//Constructs:

public $Proxy0(java.lang.reflect.InvocationHandler);

//Methods:

public final void processBusiness(int);

public final void testMethod();

public final int hashCode();

public final boolean equals(java.lang.Object);

public final java.lang.String toString();

}

可知，$Proxy0的运作原理如下：

用实现java.lang.reflect.InvocationHandler接口的对象初始化。

Fields指向的是实现java.lang.reflect.InvocationHandler接口的对象的调用方法，

Fields和Methods是一一对应的

当$Proxy0对象调用方法时，比如调用public final void processBusiness(int);

在processBusiness的内部，会把int参数传递给某个对应的Field，

然后通过Field，调用实现java.lang.reflect.InvocationHandler接口对象的方法