JDK动态代理
带着疑问看本文：
- JDK动态代理为什么只能代理接口，不能代理类？
- JDK动态代理如何生成？生成的代理类存放在哪里？
本文不介绍代理模式相关知识，只探讨JDK动态代理的技术问题。
JDK动态代理使用示例
public interface Person {
void doSomething();
}
public class DefaultPerson implements Person {
@Override
public void doSomething() {
System.out.println("do something");
}
}
public class PersonHandler implements InvocationHandler {
private Object target;
public PersonHandler(Object target) {
this.target = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before ...");
Object ret = null;
try {
ret = method.invoke(target, args);
} catch (Exception e) {
//todo handle invoke exception
System.out.println("after throws ...");
return ret;
}
System.out.println("after return ...");
return ret;
}
}
DefaultPerson target = new DefaultPerson();
PersonHandler handler = new PersonHandler(target);
Person person = (Person)Proxy.newProxyInstance(Person.class.getClassLoader(),
new Class[]{Person.class},
handler);
person.doSomething();
通过执行我们看到，生成的代理类对doSomething()方法进行了拦截，执行了PersonHandler中的invoke方法逻辑，从而实现了对doSomething()方法的增强。
再执行如下方法：
System.out.println(person.hashCode());
System.out.println(person.toString());
我们观察到，代理类不仅对我们指定的接口中的方法进行了拦截，对其他方法也进行了拦截。事实上，代理类中所有的方法均被委托给了PersonHandler中的invoke方法。也就是说，代理类的所有方法的执行逻辑都是invoke方法，再由invoke中的method.invoke(target, args)调用目标类上的原方法。
代理类的类型
执行如下代码：
System.out.println(person instanceof DefaultPersonService);
System.out.println(DefaultPerson.class.isAssignableFrom(person.getClass()));
System.out.println(Person.class.isAssignableFrom(person.getClass()));
System.out.println(Proxy.class.isAssignableFrom(person.getClass()));
观察输出结果，思考如下问题：
代理类 与 Person 、DefaultPerson、Proxy 之间是什么关系?
我们再来观察下生成的类是什么样子：
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
package com.sun.proxy;
import com.zzvcom.lspring.beans.Person;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy0 extends Proxy implements Person {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m0;
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final void doSomething() throws {
try {
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
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);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("com.zzvcom.lspring.beans.Person").getMethod("doSomething");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
我们可以看到：
- 代理类是final的，不能再被继承
- 代理类是我们指定接口的实现类
- 代理类继承了Proxy类，是Proxy的子类
- 代理类有一个构造方法，其参数是InvocationHandler
- 代理类把
equals，toString、hashCode以及指定接口中的doSomething方法全都委托给InvocationHandler执行了
此外，通过观察每个方法中都有super.h.invoke(this, m1, ..)调用，可以看到，invoke的第一个参数this指的是当前代理类的实例，第二个参数m指定的原始对象或Object中声明的方法，因为他们都是通过反射从原始对象中获取的。代理类对equals，toString、hashCode这些放Object中声明的方法进行了重写，那么Object中的其他方法呢?如wait(),notify(),getClass(),clone(),finilize()等。。。
wait(),notify(),getClass()是final方法，无法被重写,对其增强也就无从谈起。
clone()和finilize()呢？
代理类的生成过程
查看Proxy.newProxyInstance(..)的源码：
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);
}
}
- 该方法通过getProxyClass0(loader, intfs)生成代理类并使用loader类加载器加载该类形成Class文件
- 通过cl.getConstructor(constructorParams)拿到类的构造方法，注意该构造方法以InvocationHandler为参数。
- 通过cons.newInstance(new Object[]{h})执行构造方法，创建类实例。
再看getProxyClass0函数的源码：
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
return proxyClassCache.get(loader, interfaces);
}
直接使用proxyClassCache获取Class文件。proxyClassCache是一个WeakCache,其Key是ClassLoader，Value是Class对象。具体的代理类生成逻辑在ProxyClassFactory中：
@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());
}
}
- 尝试加载所有的接口类，如果有接口无法加载，则直接报错
- 对所有接口进行访问性验证，如果有接口来自其他包且不是public则直接报错
- 使用ProxyGenerator.generateProxyClass(..)生成类的二进制字节码
- 调用native方法defineClass0(..)生成Class文件
从生成过程中我们可以看到，生成的Class类是弱引用，也就意味着当JVM空间不足时，这些代理类会被垃圾回收器回收，再次使用时需要重新生成。
CGLib动态代理相关文章：
https://objectcomputing.com/resources/publications/sett/november-2005-create-proxies-dynamically-using-cglib-library
https://www.ffutop.com/posts/2018-07-10-cglib-enhancer/
https://dzone.com/articles/cglib-missing-manual
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