JDK中的Proxy动态代理原理剖析
主要API类是：
```
Proxy.newProxyInstance
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
返回一个指定接口的代理类实例，该接口可以将方法调用指派到指定的调用处理程序。此方法相当于：
Proxy.getProxyClass(loader, interfaces).
getConstructor(new Class[] { InvocationHandler.class }).
newInstance(new Object[] { handler });
```
```
Proxy.newProxyInstance 抛出 IllegalArgumentException，原因与 Proxy.getProxyClass 相同。
参数：
loader - 定义代理类的类加载器
interfaces - 代理类要实现的接口列表
h - 指派方法调用的调用处理程序
返回：
一个带有代理类的指定调用处理程序的代理实例，它由指定的类加载器定义，并实现指定的接口
抛出：
IllegalArgumentException - 如果违反传递到 getProxyClass 的参数上的任何限制
NullPointerException - 如果 interfaces 数组参数或其任何元素为 null，或如果调用处理程序 h 为 null
```
先声明一个接口
```
package com.czq.proxy;
public interface IPackageManager {
String getPackageInfo() ;
}
```
实现该接口
```
package com.czq.proxy;
public class PackageManagerImpl implements IPackageManager {
@Override
public String getPackageInfo() {
String s = "com.czq.proxy";
// System.out.println(s);
return s;
}
@Override
public String toString() {
return "PackageManagerImpl";
}
}
```
实现InvocationHandler 接口，关键之处在这
```
package com.czq.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class PackageManagerWoker implements InvocationHandler {
private Object mTarget = null;
public PackageManagerWoker(Object target) {
super();
this.mTarget = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System. out.println("1" );
System. out.println("method:" +method);
if (args != null) {
for (int i = 0; i < args.length; i++) {
System. out.println("args[" + i + "]:" + args[i]);
}
}
Object result = method.invoke( mTarget, args);
System. out.println("2" );
return result;
}
```
测试：
```
package com.czq.proxy;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
import java.lang.reflect.Proxy;
public class Test {
public static void main(String[] args) {
// 从源码中得知，设置这个值，可以把生成的代理类，输出出来。
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
IPackageManager pkgManger = new PackageManagerImpl();
// System.out.println("pkgManger.toString:"+pkgManger.toString());
PackageManagerWoker woker = new PackageManagerWoker(pkgManger);
IPackageManager pm = (IPackageManager) Proxy.newProxyInstance(pkgManger.getClass().getClassLoader(), pkgManger
.getClass().getInterfaces(), woker);
// System.out.println("pm.getName:" +pm.getClass().getName())
System. out.println("pm.toString:" +pm.toString());
System.out.println(pm.getPackageInfo());
}
}
```
输出结果如下：
```
1
method:public java.lang.String java.lang.Object.toString()
2
pm.toString:PackageManagerImpl
1
method:public abstract java.lang.String com.czq.proxy.IPackageManager.getPackageInfo()
2
com.czq.proxy
```
得出结论：
pm.getPackageInfo()方法会走到PackageManagerWoker的invoke方法。
思考问题：
PackageManagerWoker不继承IPackageManager。不能强转成IPackageManager。
也就是pm对象不是PackageManagerWoker对象。
那pm 是哪个对象，是什么类呢？为什么还能强转成IPackageManager
打印pm的className
```
System.out.println("pm.getName:" +pm.getClass().getName())
```
得出的结果：
```
pm.getName:com.sun.proxy.$Proxy0
```
也就是pm对象是com.sun.proxy.$Proxy0这个类new出的对象。这个类是刚刚Proxy.newProxyInstance自动生成的class
那这个class里面写的是什么呢？
查看源码：Proxy.java
```
/**
* 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
/*
* 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)) {
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);
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());
}
}
}
```
通过ProxyGenerator 生成了这个class。
查看ProxyGenerator源码：
```
/**
* Generate a proxy class given a name and a list of proxy interfaces.
*
* @param name the class name of the proxy class
* @param interfaces proxy interfaces
* @param accessFlags access flags of the proxy class
*/
public static byte[] generateProxyClass(final String name,
Class<?>[] interfaces,
int accessFlags)
{
ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
final byte[] classFile = gen.generateClassFile();
if (saveGeneratedFiles) {
java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<Void>() {
public Void run() {
try {
int i = name.lastIndexOf('.');
Path path;
if (i > 0) {
Path dir = Paths.get(name.substring(0, i).replace('.', File.separatorChar));
Files.createDirectories(dir);
path = dir.resolve(name.substring(i+1, name.length()) + ".class");
} else {
path = Paths.get(name + ".class");
}
Files.write(path, classFile);
return null;
} catch (IOException e) {
throw new InternalError(
"I/O exception saving generated file: " + e);
}
}
});
}
return classFile;
}
```
发现 saveGeneratedFiles 为true报错生成的class的源码。
这个saveGeneratedFiles 怎么赋值呢？
```
/** debugging flag for saving generated class files */
private final static boolean saveGeneratedFiles =
java.security.AccessController.doPrivileged(
new GetBooleanAction(
"sun.misc.ProxyGenerator.saveGeneratedFiles")).booleanValue();
```
也就是把sun.misc.ProxyGenerator.saveGeneratedFiles 改成true就可以输出结果了。
```
// 从源码中得知，设置这个值，可以把生成的代理类，输出出来。
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
```
注意，需要再工程根目录下，增加 com/sun/proxy目录，否则会报错如下：
```
Exception in thread "main" java.lang.InternalError: I/O exception saving generated file: java.io.FileNotFoundException : com\sun\proxy\$Proxy0.class (系统找不到指定的路径。)
at sun.misc.ProxyGenerator$1.run(ProxyGenerator.java:336 )
at sun.misc.ProxyGenerator$1.run(ProxyGenerator.java:327 )
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.ProxyGenerator.generateProxyClass(ProxyGenerator.java:326)
at java.lang.reflect.Proxy$ProxyClassFactory.apply(Proxy.java:672)
at java.lang.reflect.Proxy$ProxyClassFactory.apply(Proxy.java:592)
at java.lang.reflect.WeakCache$Factory.get(WeakCache.java:244)
at java.lang.reflect.WeakCache.get(WeakCache.java:141 )
at java.lang.reflect.Proxy.getProxyClass0(Proxy.java:455 )
at java.lang.reflect.Proxy.newProxyInstance(Proxy.java:738)
at com.czq.proxy.Test.main( Test.java:18)
```
把proxy0输出的结果如下：
反编译看看proxy0是内容是啥，有什么秘密
```
package com.sun.proxy;
import com.czq.proxy.IPackageManager;
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 IPackageManager
{
private static Method m3; // 生成对应的方法对象
private static Method m1;
private static Method m0;
private static Method m2;
// proxy0 继承Proxy，实现IPackageManager 接口，需要传入 InvocationHandler，初始化对应的h对象。
// 我们的h对象就是PackageManagerWoker，所以我们会调用到PackageManagerWoker的 invoke方法。
// 所以是proxy0，调用InvocationHandler的 invoke 方法，传入对应的方法。InvocationHandler 放射调用对应的tagret中的方法。
public $Proxy0(InvocationHandler paramInvocationHandler)
throws
{
super(paramInvocationHandler);
}
public final String getPackageInfo()
throws
{
try
{
return (String)this.h.invoke(this, m3, null);
}
catch (RuntimeException localRuntimeException)
{
throw localRuntimeException;
}
catch (Throwable localThrowable)
{
}
throw new UndeclaredThrowableException(localThrowable);
}
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 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);
}
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);
}
static
{
try
{
// 把各个方法，对应到成员变量上
m3 = Class.forName("com.czq.proxy.IPackageManager").getMethod("getPackageInfo", new Class[0]);
m1 = Class.forName("java.lang.Object").getMethod("equals", new Class[] { Class.forName("java.lang.Object") });
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());
}
}
```
结论如下：
1. proxy0 继承Proxy，实现IPackageManager 接口，需要传入 InvocationHandler，初始化对应的h对象。
2. 我们的h对象就是PackageManagerWoker，所以我们会调用到PackageManagerWoker的 invoke方法。
3. 所以是proxy0，调用InvocationHandler的 invoke 方法，传入对应的方法。InvocationHandler 放射调用对应的tagret中的方法。套了2层
整体类图如下
