上篇主要说明了jdk动态代理的基本使用，这篇文章主要说说jdk底层是怎么实现的
这里源码版本是1.8.0-b132
首先来看看它是如何生成代理类的：
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, interfaces);
}
/*
*查找或生成指定的代理类
*/
Class<?> cl = getProxyClass0(loader, interfaces);
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//使用带有InvocationHandler 的构造函数生成代理类的实例
return cons.newInstance(new Object[]{h});
} catch (IllegalAccessException|InstantiationException e) {
throw new InternalError(e.toString(), e);
} catch (InvocationTargetException e) {
Throwable t = e.getCause();
if (t instanceof RuntimeException) {
throw (RuntimeException) t;
} else {
throw new InternalError(t.toString(), t);
}
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString(), e);
}
}
接下里我们看看是怎么生成代理类的 也就是
Class<?> cl = getProxyClass0(loader, interfaces); 这个getProxyClass0方法
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// 如果已经存在相应的代理类，则直接返回，否则才会通过ProxyClassFactory
// 来创建代理
return proxyClassCache.get(loader, interfaces);
}
接下来我们看看proxyClassCache是怎么实现的
源代码里面有
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
具体的缓存逻辑这里我们暂时不关心
这里主要关注ProxyClassFactory的实现
ProxyClassFactory是Proxy的一个静态内部类，实现了WeakCache的内部接口BiFunction的apply方法，主要作用是 根据给定的接口数组以及类加载器 生成代理类
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// 代理类前缀
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
private static final AtomicLong nextUniqueNumber = new AtomicLong();
//返回该代理类的class实例
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
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");
}
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null; // package to define proxy class in
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
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");
}
}
}
if (proxyPkg == null) {
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* 生成代理类的字节码
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//根据字节码生成Class实例
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
throw new IllegalArgumentException(e.toString());
}
}
}
默认情况 生成的代理没保存在本地
我们可以通过 在添加以下代码 保存动态代理类生成到本地
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
接下来我们来分析下生成的代理类
package com.sun.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import proxyAop.UserService;
public final class $Proxy0 extends Proxy
implements UserService
{
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
public $Proxy0(InvocationHandler paramInvocationHandler)
throws
{
super(paramInvocationHandler);
}
public final boolean equals(Object paramObject)
throws
{
try
{
return ((Boolean)this.h.invoke(this, m1, new Object[] { paramObject })).booleanValue();
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final String toString()
throws
{
try
{
return ((String)this.h.invoke(this, m2, null));
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final void add()
throws
{
try
{
this.h.invoke(this, m3, null);
return;
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
public final int hashCode()
throws
{
try
{
return ((Integer)this.h.invoke(this, m0, null)).intValue();
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
throw new UndeclaredThrowableException(localThrowable);
}
}
static
{
try
{
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
m2 = Class.forName("java.lang.Object").getMethod("toString", new Class[0]);
m3 = Class.forName("proxyAop.UserService").getMethod("add", new Class[0]);
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
return;
}
catch (NoSuchMethodException localNoSuchMethodException)
{
throw new NoSuchMethodError(localNoSuchMethodException.getMessage());
}
catch (ClassNotFoundException localClassNotFoundException)
{
throw new NoClassDefFoundError(localClassNotFoundException.getMessage());
}
}
}
可以看到：
该代理类继承了Proxy类，实现了所代理的接口，由于java是单继承，这里已经继承了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，只支持对接口的代理。
并提供了一个使用InvocationHandler作为参数的构造方法。
通过静态代码块来初始化接口中的Method对象，以及Object类的equals、hashCode、toString方法。
当代理类调用相关方法的时候 会执行生成该代理类所实例化的InvocationHandler的对象的 invoke方法。