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jdk动态代理及源码解析

介绍

Spring动态代理可以选择使用jdk动态代理, 或者cglib动态代理, jdk动态代理位于 java.lang.reflect 包下.

使用时涉及
接口: java.lang.reflect.InvocationHandler
动态代理类: java.lang.reflect.Proxy

注意: JDK 动态代理是基于接口的代理, 只能对实现接口的类生成代理, 不能对类进行代理

使用步骤

1.创建InvocationHandler接口的实现类, 并编写invoke方法的实现
2.创建被代理类的接口及实现类
3.使用动态代理类Proxy的静态方法生成代理类实例
4.使用实例调用方法

代码演示

按照步骤编写简易逻辑代码.

创建InvocationHandler接口的实现类

/**
* JDK 动态代理
* 基于接口的代理, 只能对实现接口的类生成代理, 不能对类进行代理
*
* @author liuzhihang
* @date 2018/5/17 10:36
*/
public class MyInvocationHandler implements InvocationHandler {

/**
* 目标对象
*/
private Object target;

public MyInvocationHandler(Object target) {
this.target = target;
}

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {

System.out.println("jdk 动态代理 before . . . ");

System.out.println("当前代理方法为:" + method);
Object invoke = method.invoke(target, args);

System.out.println("jdk 动态代理 after . . . ");
return invoke;
}
}

创建被代理类的接口及实现类

/**
* 被代理类的接口
* @author liuzhihang
* @date 2018/5/17 10:47
*/
public interface Subject {

/**
* 获取名字
* @return
*/
String getName();

/**
* 获取年龄
* @param name
* @return
*/
String getAge(String name);
}
/**
* 被代理类
*
* @author liuzhihang
* @date 2018/5/17 10:48
*/
public class SubjectImpl implements Subject {

@Override
public String getName() {
System.out.println("SubjectImpl的获取名字方法 . . .");
return "liuzhihang";
}

@Override
public String getAge(String name) {

System.out.println(name + "开始获取年龄 . . .");
return "25";
}
}

使用动态代理类Proxy的静态方法生成代理类实例

获取代理类实例有以下两种方式, 一种是通过Proxy.newProxyInstance(..)获取, 一种是通过 Proxy.getProxyClass(..) 方式获取
1.Proxy.newProxyInstance(..)

/**
* 当代理类实例调用方法时, 会自动跳转到代理类关联的 handler 对象, 通过 method.invoke(target, args) 进行调用
*
*
* @author liuzhihang
* @date 2018/5/17 10:49
*/
public class ProxyMainTest {

public static void main(String[] args) {

Subject subject = new SubjectImpl();

ClassLoader classLoader = subject.getClass().getClassLoader();
Class<?>[] interfaces = subject.getClass().getInterfaces();

MyInvocationHandler handler = new MyInvocationHandler(subject);
// 生成代理类实例
Subject proxyInstance = (Subject) Proxy.newProxyInstance(classLoader, interfaces, handler);

String name = proxyInstance.getName();
String instanceAge = proxyInstance.getAge("liuzhihang");
System.err.println(name + " " + instanceAge);

}
}

2.Proxy.getProxyClass(..)

/**
* 当代理类实例调用方法时, 会自动跳转到代理类关联的 handler 对象, 通过 method.invoke(target, args) 进行调用
* 此方式有异常抛出
*
* @author liuzhihang
* @date 2018/5/17 10:49
*/
public class ProxyMainTest {

public static void main(String[] args) {
try {

Subject subject = new SubjectImpl();

ClassLoader classLoader = subject.getClass().getClassLoader();
Class<?>[] interfaces = subject.getClass().getInterfaces();

MyInvocationHandler handler = new MyInvocationHandler(subject);

Class<?> proxyClass = Proxy.getProxyClass(classLoader, interfaces);
Constructor<?> constructor = proxyClass.getConstructor(InvocationHandler.class);

Subject subject1 = (Subject) constructor.newInstance(handler);
String name1 = subject1.getName();
String instanceAge1 = subject1.getAge("liuzhihang");
System.err.println(name1 + " " + instanceAge1);

} catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException | InstantiationException e) {
e.printStackTrace();
}

}

}

执行结果

D:\jdk1.8\bin\java.exe . . .
liuzhihang 25
jdk 动态代理 before . . .
当前代理方法为:public abstract java.lang.String com.liuzhihang.tool.proxy.jdk.Subject.getName()
SubjectImpl的获取名字方法 . . .
jdk 动态代理 after . . .
jdk 动态代理 before . . .
当前代理方法为:public abstract java.lang.String com.liuzhihang.tool.proxy.jdk.Subject.getAge(java.lang.String)
liuzhihang开始获取年龄 . . .
jdk 动态代理 after . . .

Process finished with exit code 0

结论: 代理实例在每次调用方法是都会通过代理类进行调用

相关源码解析

完整注释可自己查看相关源码, 源码过程应当DeBug多走走.
1.调用 Proxy.newProxyInstance 方法

 /**
* 返回指定接口的代理类实例,该接口将方法调用分派给指定的调用处理程序
*/
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException {

// 非空校验
Objects.requireNonNull(h);

final Class<?>[] intfs = interfaces.clone();

// 获取系统安全接口
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
// 校验权限
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}

/*
* 从缓存中获取代理类 或者 生成新的代理类
*/
Class<?> cl = getProxyClass0(loader, intfs);

/*
* 通过反射获取构造函数对象并生成代理类实例
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
// 获取构造
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
// 验证代理类的修饰符
if (!Modifier.isPublic(cl.getModifiers())) {
// 修改访问权限
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
// 将此对象的可访问标志设置为指定的布尔值, true表示反射对象在使用时应禁止Java语言访问检查, false表示反射对象应强制执行Java语言访问检查
cons.setAccessible(true);
return null;
}
});
}
//生成实例, 并将参数传入构造
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);
}
}

可以看出获取代理类是在 Class<?> cl = getProxyClass0(loader, intfs); 处, 继续相关逻辑
2.获取代理类相关逻辑

/**
* 生成代理类, 之前必须进行权限检查
*/
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}

//如果由实现给定接口的给定加载器定义的代理类存在,则它将简单地返回缓存副本; 否则,它将通过Proxy Class Factory创建代理类
return proxyClassCache.get(loader, interfaces);
}

3.proxyClassCache.get(loader, interfaces);
java.lang.reflect.WeakCache#get(..) 介绍

 /**
* 通过缓存查找值, 如果缓存中没有给定的(key,sub Key)对的条目或条目已被清除,则它总是评估{Key sub Key Factory}函数并可选择评估{Factory value}函数
*/
public V get(K key, P parameter) {
// 非空校验
Objects.requireNonNull(parameter);
// 判断移除队列
expungeStaleEntries();
// 缓存key
Object cacheKey = CacheKey.valueOf(key, refQueue);

// 延迟加载使用二级map
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
ConcurrentMap<Object, Supplier<V>> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}

// 创建子key 并根据key 检索supplier
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
// 根据key获取supplier
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;

while (true) {
if (supplier != null) {
// supplier 可能为 Factory 或者 CacheValue<V> 的实例, 从缓存中获取到则直接返回
V value = supplier.get();
if (value != null) {
return value;
}
}

// factory不存在则创建
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}

// supplier 为null
if (supplier == null) {
// 从valuesMap获取supplier
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// successfully installed Factory
supplier = factory;
}
// else retry with winning supplier
} else {
if (valuesMap.replace(subKey, supplier, factory)) {
// successfully replaced
// cleared CacheEntry / unsuccessful Factory
// with our Factory
supplier = factory;
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}

可以发现重点在 Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter)); 获取 subKey 的过程中.
4.subKeyFactory.apply(key, parameter)
Debug发现在此处调用的是 java.lang.reflect.Proxy.ProxyClassFactory 静态内部类,
此处根据接口的数量生成二级缓存

 /**
* 一个工厂函数, 用于生成, 定义并返回给定ClassLoader和接口数组的代理类
*/
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// 所有代理类的前缀
private static final String proxyClassNamePrefix = "$Proxy";

// next number to use for generation of unique proxy class names
private static final AtomicLong nextUniqueNumber = new AtomicLong();

@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {

// 在IdentityHashMap中, 当且仅当两个key严格相等(key1==key2)时,IdentityHashMap才认为两个key相等
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
// 循环接口数组
for (Class<?> intf : interfaces) {
/*
* 验证类加载器是否将此接口的名称解析为同一个Class对象
*/
Class<?> interfaceClass = null;
try {
// 获取接口的 class
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* 验证interfaceClass是否为接口
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* 验证接口是否重复
*/
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;

/*
* 验证所有非公开代理接口是否在同一个包中
*/
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) {
// 如果没有非公开的代理接口,使用 com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}

/*
* 为要生成的代理类选择一个名称
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;

/*
* 生成代理类
*/
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());
}
}
}

5.生辰给代理类
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);
可以在测试类中添加以下内容打印出代理类:

System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

代理类内容如下:

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

package com.sun.proxy;

import com.liuzhihang.tool.proxy.jdk.Subject;
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 Subject {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m4;
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 String getName() throws {
try {
return (String)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 String getAge(String var1) throws {
try {
return (String)super.h.invoke(this, m4, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}

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.liuzhihang.tool.proxy.jdk.Subject").getMethod("getName");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("com.liuzhihang.tool.proxy.jdk.Subject").getMethod("getAge", Class.forName("java.lang.String"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}

可以看出生成的$Proxy0类继承Proxy动态代理类并实现了Subject被代理接口, 实现所有方法
通过 super.h.invoke(this, m1, new Object[]{var1}) 内部调用了 InvocationHandler.invoke(…)方法, 通过反射调用代理实例的方法

文章作者: liuzhihang
文章链接: https://liuzhihang.com/2018/05/17/jdk-dynamic-proxy-and-source-code-analysis.html
版权声明: 本博客所有文章除特别声明外,均采用 CC BY-NC-SA 4.0 许可协议。转载请注明来自 Notes

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