摘要：用户在调用被动态代理过的对象的方法时，调用的请求会被自动发给代理对象实现了接口的方法，由方法来实现对请求的统一处理。动态代理实现切面代替该语句块一定会在前面执行。
用户在调用被动态代理过的对象的方法时，调用的请求会被自动发给代理对象（实现了InvocationHandler接口）的invoke()方法，由invoke()方法来实现对请求的统一处理。
源码分析
Java.lang.reflect.Proxy
public class Proxy implements java.io.Serializable {
private static final Class[] constructorParams = { InvocationHandler.class };
private static final WeakCache[], Class>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
......
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class[] intfs = interfaces.clone(); //浅克隆
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
//生成代理类的class
Class cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//getConstructor(InvocationHandler.class) 获取代理类的构造器
final Constructor cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//代理类实例化返回
return cons.newInstance(new Object[]{h});
}
......
}
private static Class getProxyClass0(ClassLoader loader, Class... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// key为WeakReference，若缓存中存在相同key，则返回ProxyClass，若不存在，则用ProxyClassFactory创建ProxyClass。
return proxyClassCache.get(loader, interfaces); //java.lang.reflect.WeakCache 这个内核分析？
}
......
}
private static final class ProxyClassFactory implements BiFunction[], Class> {
// 所有代理类ProxyClass名字的前缀
private static final String proxyClassNamePrefix = "$Proxy";
// 用计数器生成代理类ProxyClass的名字
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class apply(ClassLoader loader, Class[] interfaces) {
Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//对每一个interface Class进行验证
for (Class intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
* 用IdentityHashMap来验证interface Class的唯一性，因为IdentityHashMap的key比较是==，即比较内存地址。
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null;
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
// 对于非公共接口，ProxyClass的包名设置为接口的包名
for (Class intf : interfaces) {
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) {
accessFlags = Modifier.FINAL;
String name = intf.getName();
int n = name.lastIndexOf(".");
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
if (proxyPkg == null) {
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) {
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
// 对于公共接口，ProxyClass的包名为com.sun.proxy
if (proxyPkg == null) {
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
long num = nextUniqueNumber.getAndIncrement();
//ProxyClass的完全限定名
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//由c++写的ProxyGenerator生成代理类的二进制字节码
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
// 根据二进制字节码返回相应的ProxyClass
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString());
}
}
}
$Proxy.class
以下是反编译$Proxy0.class：
public final class $Proxy0 extends Proxy implements MyInterface1{
private static Method m1;
private static Method m3;
private static Method m0;
private static Method m2;
public $Proxy0(InvocationHandler paramInvocationHandler) {
super(paramInvocationHandler);
}
public final boolean equals(Object paramObject) {
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 sayHello(String paramString) {
try {
this.h.invoke(this, m3, new Object[] { paramString });
return;
}
catch (Error|RuntimeException localError) {
throw localError;
}
catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
public final int hashCode() {
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() {
try {
return (String)this.h.invoke(this, m2, null);
}
catch (Error|RuntimeException localError) {
throw localError;
}
catch (Throwable localThrowable) {
throw new UndeclaredThrowableException(localThrowable);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
m3 = Class.forName("com.mikan.proxy.MyInterface1").getMethod("sayHello", new Class[] { 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());
}
}
}
$Proxy0.class的总结：
对于要代理没有接口的类，则必须继承该没有接口的类，从而覆盖其方法，但由于java不能多继承，这里代理类已经继承了Proxy类了，不能再继承其他的类，所以JDK的动态代理不支持对实现类的代理，只支持接口的代理。
构造方法的入参为InvocationHandler，这说明该代理类是实现了InvocationHandler接口的类的代理类，而InvocationHandler实现类再代理真正的实体类。
静态代码块会初始化真正实体类的Method对象，以及Object类的equals、hashCode、toString的Method对象。
总结：
Proxy 会从内部类WeakCache中根据ClassLoader 和interfaces获取ProxyClass（若缓存中存在相同key，则返回ProxyClass，若不存在，则用ProxyClassFactory创建ProxyClass（由ProxyGenerator生成二进制码，再将写成*.class文件））。
AOP JDK动态代理实现
切面InvocationHandler：
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
// BeforeAdvice
Object retVal = null;
try {
retVal = method.invoke(target, args); // AroundAdvice 代替该语句
// AfterReturningAdvice
}
catch (Throwable e) {
// AfterThrowingAdvice
}
finally {
//finally块一定会在return前面执行。
// AfterAdvice
}
return retVal;
}
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