一、背景
代理模式，作为常用的设计模式之一，在实际项目中或多或少都会被使用到。当前一些主流的项目框架中，也有不少代理模式的身影。
代理模式中，代理类与主体类实现同样的接口，代理类持有实体类的引用，并接受客户端对代理类中实体引用的外部注入，并代理实体类的功能。
注：描述中的这种外部注入形式有个专有技术名词：依赖注入
代理模式通用的类图为：
按照代理类的产生方式，是在运行期之前就静态的存在还是在运行期动态产生，可以将代理模式分为静态代理和动态代理。
二、静态代理
在真正理解动态代理之前，有必要先简单回顾下静态代理的一般过程。
直接看一个具体的实例。
1，定义接口：
package com.corn.proxy.pstatic;
public interface Subject {
String action();
}
复制代码2，定义主体类：
package com.corn.proxy.pstatic;
public class RealSubject implements Subject {
@Override
public String action() { System.out.println("action in RealSubject"); return "action done"; } } 复制代码3，定义静态代理类：
package com.corn.proxy.pstatic;
public class ProxySubject implements Subject{
private Subject realSubject;
public ProxySubject(Subject realSubject) {
this.realSubject = realSubject;
}
@Override
public String action() { // 主体action前执行 System.out.println("do sth before RealSubject action"); String result = this.realSubject.action(); // 主体action后执行 System.out.println("do sth after RealSubject action"); return result; } } 复制代码4,客户端注入实体并访问：
package com.corn.proxy.pstatic;
public class Client {
public static void main(String[] args) {
Subject realSubject = new RealSubject();
ProxySubject proxySubject = new ProxySubject(realSubject);
proxySubject.action();
}
}
复制代码run，输出结果：
do sth before RealSubject action action in RealSubject do sth after RealSubject action 复制代码上述代码过程亦对应了代理模式类图结构。
三、动态代理
本质上，动态代理也是遵循上述通用的代理模式类图关系，与静态代理相比，其动态主要体现在： 1，具体代理类（ProxySubject）的生成是在运行期动态产生的，而非编译期就已经静态存在；
2，具体代理类（ProxySubject）与被代理类的代理关系（ProxySubject持有RealSubject的引用），是想办法动态注入进入的；
3，具体代理类（ProxySubject）对被代理类的功能的代理是在动态生成的代理类内部，想办法去动态的调用被代理类的对应方法的。
无论是具体代理类的动态产生，还是与被代理类的关系建立，以及对被代理类方法的代理调用，这中间，都用到了两个关键的中间媒介，即Proxy和InvocationHandler。
Proxy类，其中提供了动态生成代理类的静态方法，并持有实现了InvocationHandler接口的引用。同时，所有生成的代理类也都是Proxy类的子类。
InvocationHandler接口，只包含一个抽象出来的方法名：invoke，使得实现InvocationHandler接口的类去具体实现，在实现中通过持有被代理类实体（RealSubject），并通过反射，去调用对应的实体方法。
因此，动态代理总体上的执行流程为：
当客户端通过Proxy的静态方法生成动态代理类后，调用动态代理类对应的接口方法时，内部会调用其内部持有的InvocationHandler接口的实例对象的invoke方法，并得以调用到实际被代理实体的相应方法。
将总体的类之间关系如果用类图表示，与通用的代理模式类图稍有区别。
相应代码实现代码过程如下：
1，定义接口Subject：
package com.corn.proxy.pdynamic;
public interface Subject {
String action();
}
复制代码2，定义主体类：
package com.corn.proxy.pdynamic;
public class RealSubject implements Subject{
@Override
public String action() { System.out.println("action in RealSubject"); return "action done"; } } 复制代码3，定义实现了InvocationHandler接口的类：
package com.corn.proxy.pdynamic;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class ProxyInvocationHandler implements InvocationHandler {
protected Subject subject;
public ProxyInvocationHandler(Subject subject) {
this.subject = subject;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("do something before in ProxyInvocationHandler"); return method.invoke(subject, args); } } 复制代码4，客户端注入实体并访问：
package com.corn.proxy.pdynamic;
import java.lang.reflect.Proxy;
public class Client {
public static void main(String[] args) {
Subject realSubject = new RealSubject();
ProxyInvocationHandler proxyInvocationHandler = new ProxyInvocationHandler(realSubject);
Subject proxyRealSubject = (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(),
realSubject.getClass().getInterfaces(), proxyInvocationHandler);
proxyRealSubject.action();
}
}
复制代码run，输出结果：
do something before in ProxyInvocationHandler action in RealSubject 复制代码四、实现原理
从上述动态代理执行流程和类图分析中已经可以看出，动态代理的关键，在于通过InvocationHandler和Proxy媒介，在运行时动态生成动态代理类，生成的动态代理类依然实现了Subject接口，并在调用方法时回调了InvocationHandler实现类的invoke方法，InvocationHandler实现类的invoke方法通过反射，回调了被代理实体的对应方法。
看起来有点绕。
在生成动态代理过程中，Java工程和Android项目中有点区别，先从源码角度看下Java工程的具体实现过程。
Android studio中不能直接新建Java工程，但可以在Android项目中通过建立Java Library的方式，建立Java库。默认情况下，Android Studio关联的JDK为AS内置的，是看不到源码的，为此，需要将其改成自己下载的JDK路径，此路中包含了源码src目录。 具体修改方式为：
1，查看本地JDK目录
/usr/libexec/java_home -V
复制代码输出
Matching Java Virtual Machines (1):
1.8.0_162, x86_64: "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home /Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home 复制代码2，Android Studio替换JDK设置 File >> Other Setting >> Default Project Structure，在JDK Location中将默认设置
/Applications/Android Studio.app/Contents/jre/jdk/Contents/Home
复制代码修改为：
/Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home
复制代码完成设置后，可以直接看到JDK源码。
ProxyInvocationHandler接口只有一个方法：
public interface InvocationHandler {
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable;
}
复制代码下面重点看下Proxy类newProxyInstance过程。
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); } } 复制代码其中调用的getProxyClass0对应的具体实现为：
/**
* Generate a proxy class. Must call the checkProxyAccess method
* to perform permission checks before calling this.
*/
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); } 复制代码于是，我们跟到对应的ProxyClassFactory：
/**
* A factory function that generates, defines and returns the proxy class given * the ClassLoader and array of interfaces. */ 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()); } } } 复制代码其中，关键的过程在于：
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
return defineClass0(loader, proxyName, proxyClassFile, 0, proxyClassFile.length); } catch (ClassFormatError e) { ... } 复制代码通过ProxyGenerator类中的generateProxyClass方法生成对应的动态代理类二进制代码，并通过 ClassLoader加载后，通过反射，生成对应的类实例。
ProxyGenerator类是直接集成在rt.jar中的，包名为sun.misc，为扩展类。Android项目中因JDK版本问题，默认是不集成的，那Android中此处是如何实现的呢？
同样的，在Android项目中直接使用动态代理，跟踪源码调用过程。
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
// Android-removed: SecurityManager calls
/*
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 { // Android-removed: SecurityManager / permission checks. /* if (sm != null) { checkNewProxyPermission(Reflection.getCallerClass(), cl); } */ final Constructor<?> cons = cl.getConstructor(constructorParams); final InvocationHandler ih = h; if (!Modifier.isPublic(cl.getModifiers())) { // BEGIN Android-changed: Excluded AccessController.doPrivileged call. /* AccessController.doPrivileged(new PrivilegedAction<Void>() { public Void run() { cons.setAccessible(true); return null; } }); */ cons.setAccessible(true); // END Android-removed: Excluded AccessController.doPrivileged call. } 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); } } 复制代码很明显，Android JDK中newProxyInstance方法中部分实现做了修改。但主要执行路径依然不变。
/**
* Generate a proxy class. Must call the checkProxyAccess method
* to perform permission checks before calling this.
*/
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); } 复制代码同样的，ProxyClassFactory类。
/**
* A factory function that generates, defines and returns the proxy class given * the ClassLoader and array of interfaces. */ 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 the default package. proxyPkg = ""; } { // Android-changed: Generate the proxy directly instead of calling // through to ProxyGenerator. List<Method> methods = getMethods(interfaces); Collections.sort(methods, ORDER_BY_SIGNATURE_AND_SUBTYPE); validateReturnTypes(methods); List<Class<?>[]> exceptions = deduplicateAndGetExceptions(methods); Method[] methodsArray = methods.toArray(new Method[methods.size()]); Class<?>[][] exceptionsArray = exceptions.toArray(new Class<?>[exceptions.size()][]); /* * Choose a name for the proxy class to generate. */ long num = nextUniqueNumber.getAndIncrement(); String proxyName = proxyPkg + proxyClassNamePrefix + num; return generateProxy(proxyName, interfaces, loader, methodsArray, exceptionsArray); } } } 复制代码其中关键的，我们发现
// Android-changed: Generate the proxy directly instead of calling
// through to ProxyGenerator.
...
return generateProxy(proxyName, interfaces, loader, methodsArray, exceptionsArray); 复制代码跟踪进去看下具体的执行过程
@FastNative
private static native Class<?> generateProxy(String name, Class<?>[] interfaces,
ClassLoader loader, Method[] methods,
Class<?>[][] exceptions);
// END Android-changed: How proxies are generated.
复制代码原来，Android JDK中对generateProxy进行了处理，直接使用的是本地的方法。
那么，生成动态代理时，如果遇到线程安全问题呢？
ProxyClassFactory代码中，有如下处理过程：
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(); ... /* * 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) { ... } } 复制代码显然，通过线程安全的文件命名方式，预防了针对同样的接口使用多线程情况下，使用动态代理可能出现的线程安全问题。
那么，生成的动态代理到底长什么样子？我们可以直接调用系统的ProxyGenerator.generateProxyClass试一下。
package com.corn.proxy.pdynamic;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Proxy;
import sun.misc.ProxyGenerator;
public class Client {
public static void main(String[] args) {
Subject realSubject = new RealSubject();
ProxyInvocationHandler proxyInvocationHandler = new ProxyInvocationHandler(realSubject);
Subject proxyRealSubject = (Subject) Proxy.newProxyInstance(realSubject.getClass().getClassLoader(),
realSubject.getClass().getInterfaces(), proxyInvocationHandler);
proxyRealSubject.action();
String proxyName = "ProxySubject"; byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, RealSubject.class.getInterfaces()); try (FileOutputStream fos = new FileOutputStream("/Users/corn/T/ProxySubject.class")){ fos.write(proxyClassFile); fos.flush(); } catch (FileNotFoundException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } } } 复制代码run，对应目录下生成ProxySubject.class文件，通过工具可以看到对应的字节码内容。
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
import com.corn.proxy.pdynamic.Subject;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class ProxySubject extends Proxy implements Subject {
private static Method m1;
private static Method m2;
private static Method m0;
private static Method m3;
public ProxySubject(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 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); } } public final String action() throws { try { return (String)super.h.invoke(this, m3, (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")); m2 = Class.forName("java.lang.Object").getMethod("toString"); m0 = Class.forName("java.lang.Object").getMethod("hashCode"); m3 = Class.forName("com.corn.proxy.pdynamic.Subject").getMethod("action"); } catch (NoSuchMethodException var2) { throw new NoSuchMethodError(var2.getMessage()); } catch (ClassNotFoundException var3) { throw new NoClassDefFoundError(var3.getMessage()); } } } 复制代码果然，在生成的动态代理中，实现了Subject接口，并在对应的action方法中，调用了InvocationHandler实现类的实例的invoke方法。
五、结语
动态代理作为代理模式中的一种，避免了在运行期之前直接定义静态代理类，对于需要大量的代理类的情形下，是非常有用的。同时，我们也应该看到，在整体流程上，其实动态代理与静态代理总体上还是一样的。动态代理无论在代理类的创建过程中，还是对代理方法的调用，过程中都用到了反射，在一定程度上性能上有所损耗，实际使用中需要适量权衡。
动态代理模式，作为设计模式中相对比较难理解的一种，主要在于其过程经过了层层封装，最后只是通过Proxy类和InvocationHandler对外直接暴露了使用接口，对使用方直接屏蔽了具体的细节。但对于我们理解这种模式本身来说，了解并适当掌握其中的过程，还是有所受益的。
作者：HappyCorn
链接：https://juejin.im/post/5d047101f265da1bb2772e09
来源：掘金
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