解析JDK动态代理实现原理
JDK动态代理使用实例
代理模式的类图如上。关于静态代理的示例网上有很多，在这里就不讲了。
因为本篇讲述要点是JDK动态代理的实现原理，直接从JDK动态代理实例开始。
首先是Subject接口类。
package proxy.pattern;
public interface Subject {
void request() throws Exception;
}
接着是RealSubject类。
package proxy.pattern;
public class RealSubject implements Subject {
public void request() {
System.out.println("RealSubject execute request()");
}
}
下面是代理对象的InvocationHandler接口实现类。
package proxy.dynamic;
import proxy.pattern.RealSubject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class JdkProxySubject implements InvocationHandler {
private RealSubject realSubject;
public JdkProxySubject(RealSubject realSubject) {
this.realSubject = realSubject;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before");
Object result = null;
try {
result = method.invoke(realSubject, args);
} catch (Exception e) {
throw e;
} finally {
System.out.println("after");
}
return result;
}
}
InvocationHandler接口的定义如下。
/**
* {@code InvocationHandler} is the interface implemented by
* the <i>invocation handler</i> of a proxy instance.
*
* <p>Each proxy instance has an associated invocation handler.
* When a method is invoked on a proxy instance, the method
* invocation is encoded and dispatched to the {@code invoke}
* method of its invocation handler.
*/
//每一个代理对象都有一个相应的invocation handler。
//此接口应当由proxy instance的invocation handler实现
//当代理对象的方法被调用时，方法调用将会委派为invocation handler的invoke方法
//（这里有个疑问就是：方法委派是如何实现的？
//为什么调用代理对象的方法时，会调用invocation handler的invoke方法。
//答案在生成的代理类的源代码里，可以看最下面对代理类的源代码的分析）
public interface InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
}
客户端代码如下。
package proxy.dynamic;
import proxy.pattern.RealSubject;
import proxy.pattern.Subject;
import java.lang.reflect.Proxy;
public class Client {
public static void main(String[] args) throws Exception {
//通过设置参数，将生成的代理类的.class文件保存在本地
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
//通过调用Proxy.newProxyInstance生成代理对象
//方法参数为：1）classLoader 2）要代理的接口 3）代理对象的InvocationHandler
//(通过方法参数也可以看出来，JDK代理只能通过代理接口来来实现动态代理)
Subject subject = (Subject) Proxy.newProxyInstance(Client.class.getClassLoader(),
new Class[]{Subject.class}, new JdkProxySubject(new RealSubject()));
//调用代理对象的request方法。
//（根据InvocationHandler接口的定义，可以知道实际调用的是JdkProxySubject里的invoke方法）
subject.request();
}
}
运行结果如下。
before
RealSubject execute request()
after
JDK动态代理实现原理分析
生成代理对象的方法调用链为Proxy.newProxyInstance()-------->getProxyClass0()------->ProxyClassFactory.apply()-------->ProxyGenerator.generateProxyClass()。我们下面依次对各个阶段进行分析。
Proxy.newProxyInstance()
newProxyInstance方法的完整代码如下。
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*/
//
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
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;
}
});
}
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);
}
}
我们将错误判断等无用代码去掉后，代码如下。
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException {
final Class<?>[] intfs = interfaces.clone();
/*
* Look up or generate the designated proxy class.
*/
//根据提供的接口（在我们的例子中，即Subject接口），生成代理类
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
final Constructor<?> cons = cl.getConstructor(constructorParams);
//调用代理类的构造器，构造器参数为InvocationHandler的接口实现类（在我们的例子中，即JdkProxySubject）
return cons.newInstance(new Object[]{h});
}
下面，我们看看getProxyClass0方法，看看代理类是如何生成的。
getProxyClass0()
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// If the proxy class defined by the given loader implementing
// the given interfaces exists, this will simply return the cached copy;
// otherwise, it will create the proxy class via the ProxyClassFactory
//已经生成过代理类会放到cache里，没有生成过的话，会由ProxyClassFactory创建。
//因此，我们可以直接找到ProxyClassFactory，查看代理类是如何创建的。
return proxyClassCache.get(loader, interfaces);
}
因此，我们可以继续找到ProxyClassFactory，查看代理类是如何创建的。
ProxyClassFactory.apply()
ProxyClassFactory类完整的代码如下。
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
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.
*/
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;
/*
* Record the package of a non-public proxy interface so that the
* proxy class will be defined in the same package. Verify that
* all non-public proxy interfaces are in the same package.
*/
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 + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
}
我们将无用的代码去掉后，代码如下。
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
//第一步：确定包名，如果没有非public的代理接口，包名使用com.sun.proxy
String proxyPkg = null; // package to define proxy class in
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
if (proxyPkg == null) {
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
//第二步：确定类名。（在我们例子中，生成的代理类名为com.sun.proxy.$Proxy0）
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//第三步：根据类名、代理接口等信息，生成代理类。实际上可以理解为生成.class文件。
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
//第四步：根据生成的.class文件，返回一个代理类Class。此方法为native方法
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
}
}
ProxyGenerator.generateProxyClass()
ProxyGenerator.generateProxyClass生成.class文件的过程其实就是根据.class文件格式来一步步拼接处对应字节数组。
由于代码太长了，不贴了，可以看这个
ProxyGenerator.java。
生成的代理对象的.class文件
在创建代理对象前，可以通过下面语句，将生成的代理类的.class保存在本地。
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
生成的代理类的.class文件反编译后，是下面这个样子的。
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
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 proxy.pattern.Subject;
public final class $Proxy0 extends Proxy implements Subject {
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
public $Proxy0(InvocationHandler h) throws {
super(h);//通过这句，将invocationHandler实例传给了父类构造器。
}
public final boolean equals(Object var1) throws {
try {
return ((Boolean)super.h.invoke(this, m1, new Object[]{var1})).booleanValue();
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
//$Proxy0继承了Proxy，且将InvocationHandler h在构造时传给了Proxy。
//因此，super.h.invoke(this, m3, (Object[])null)其实就是调用的invoker handler的invoke方法。
//也正是像InvocationHandler定义中所说的，【当proxy instance的方法被调用时，方法调用将会委派为invocation handler的invoke方法】。
public final void request() throws Exception {
try {
super.h.invoke(this, m3, (Object[])null);
} catch (Exception | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final int hashCode() throws {
try {
return ((Integer)super.h.invoke(this, m0, (Object[])null)).intValue();
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
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("proxy.pattern.Subject").getMethod("request", new Class[0]);
m0 = Class.forName("java.lang.Object").getMethod("hashCode", new Class[0]);
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
Proxy中InvocationHandler的定义如下。
public class Proxy implements java.io.Serializable {
/**
* the invocation handler for this proxy instance.
* @serial
*/
protected InvocationHandler h;
/**
* Constructs a new {@code Proxy} instance from a subclass
* (typically, a dynamic proxy class) with the specified value
* for its invocation handler.
*
* @param h the invocation handler for this proxy instance
*
* @throws NullPointerException if the given invocation handler, {@code h},
* is {@code null}.
*/
protected Proxy(InvocationHandler h) {
Objects.requireNonNull(h);
this.h = h;
}
}
