JDK 动态代理运行原理

Posted 2022-08-22 叶长风

JDK 动态代理运行原理

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• 1. 程序演示
• 1. 源码讲解
• 1. 总结

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这几天有空研究了下JDk的动态代理，JDK的动态代理类都在java.lang.reflect包下，写了一些小程序来演示了相关类的使用，同时做了一些与CGLIb的对比，以后有空再讲述下lombok中相关注解的使用。

1. 程序演示

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接口：HelloWorld:

public interface HelloWorld 

    void sayHello();

对应的实现类为：
HelloWorldImpl:

public class HelloWorldImpl implements HelloWorld 
    @Override
    public void sayHello() 
        System.out.println("Hello world");
    

接口与对应的实现的逻辑是比较简单的，在这只是讲述JDK动态代理的原理，业务逻辑也无需要复杂的业务逻辑。

代理类MyInvocationHandler:

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

/**
 * Created by xiaxuan on 17/11/7.
 */
public class MyInvocationHandler implements InvocationHandler 

    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("before method invoke : " + method.getName());
        return method.invoke(target, args);
    

测试类TestProxy:

import java.lang.reflect.Proxy;

/**
 * Created by xiaxuan on 17/11/7.
 */
public class TestProxy 

    public static void main(String[] args) 
        HelloWorld hw = (HelloWorld) Proxy.newProxyInstance(HelloWorld.class.getClassLoader(), new Class[] HelloWorld.class, new MyInvocationHandler(new HelloWorldImpl()));
        hw.sayHello();

    

运行结果为:

使用上其实还是挺简单的，以下是关键的动态代理的源码讲解。

2. 源码讲解

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我们先进入Proxy.newProxyInstance()中查看，如下：

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);

        ......
        
    

我省略了后面的代码，上面的代码中关键的一行为

getProxyClass0(loader, intfs);

转到对应的方法为：

    /**
     * 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);
    

从proxyClassCache中取出class，进入到get方法中，如下：

    public V get(K key, P parameter) 
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
        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;
            
        

        // create subKey and retrieve the possible Supplier<V> stored by that
        // subKey from valuesMap
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) 
            if (supplier != null) 
                // supplier might be a Factory or a CacheValue<V> instance
                V value = supplier.get();
                if (value != null) 
                    return value;
                
            
            // else no supplier in cache
            // or a supplier that returned null (could be a cleared CacheValue
            // or a Factory that wasn't successful in installing the CacheValue)

            // lazily construct a Factory
            if (factory == null) 
                factory = new Factory(key, parameter, subKey, valuesMap);
            

            if (supplier == null) 
                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);
                
            
        
    

这里面关键的代码是以下两行:

Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);

这里真正生成代理类的源码为 subKeyFactory.apply(key, parameter)
但 Supplier<V> supplier = valuesMap.get(subKey),
这行代码中的Supplier对象并不是在运行到这的时候就能取到，而是在使用当前Supplier对象的时候才会实例化出来，这个是java8中的一个延迟加载的新特性。

进到方法subKeyFactory.apply(key, parameter)中，查看代码:

 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());
            
        
    

其中这行代码

byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);

用来生成新的字节码替代运行。

我们可以用这行代码生成我们的代理类查看一下，新的方法如下：

public class TestProxy 

    public static void main(String[] args) throws IOException 
        HelloWorld hw = (HelloWorld) Proxy.newProxyInstance(HelloWorld.class.getClassLoader(), new Class[] HelloWorld.class, new MyInvocationHandler(new HelloWorldImpl()));
        hw.sayHello();
        createProxyClassFile();

    

    //还原我们的代理类
    public static void createProxyClassFile() throws IOException 
        byte[] bytes = ProxyGenerator.generateProxyClass("$Proxy0", new Class[]  HelloWorld.class );
        FileOutputStream out = new FileOutputStream("$Proxy0.class");
        out.write(bytes);
        out.close();
    


 ```

 使用intellij运行程序，生成的class保存在当前项目的根目录下，可以直接打开$Proxy.class文件，内容如下：

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

import cn.com.proxyDemo.HelloWorld;
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 HelloWorld 
    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)).booleanValue();
         catch (RuntimeException | Error var3) 
            throw var3;
         catch (Throwable var4) 
            throw new UndeclaredThrowableException(var4);
        
    

    public final void sayHello() 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)).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"));
            m3 = Class.forName("cn.com.proxyDemo.HelloWorld").getMethod("sayHello", new Class[0]);
            m2 = Class.forName("java.lang.Object").getMethod("toString", 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());
        
    

这个就是最终的代理类，继承Proxy并且实现了我们自己定义的HelloWorld。

在我们调用sayHello()方法的时候，实际上调用的是代理类中下下面一行代码：

 public final void sayHello() throws  
        try 
            super.h.invoke(this, m3, (Object[])null);
         catch (RuntimeException | Error var2) 
            throw var2;
         catch (Throwable var3) 
            throw new UndeclaredThrowableException(var3);
        
    

而这行代码
super.h.invoke(this, m3, (Object[])null);
就是对应调用到我们写的
MyInvocationHandler中的

@Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable 
        System.out.println("before method invoke : " + method.getName());
        return method.invoke(target, args);
    

最终就起到一个动态代理的作用。

总结

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动态代理，在第一次生成的对应的代理对象后，将其存在缓存中，然后再次调用的时候就直接从缓存中取出代理对象，然后调用对应的代理方法实现需要的效果。

在此就需要提下JDK这种动态代理和CGLIB这种的区别了， CGLIB一般是在编译阶段对生成的class进行替换，在实际运行的时候不需要再去生成字节码替换调用了，而JDK动态代理的话，在运行阶段生成代理类进行调用一般来说会稍微慢一些。

以后有空讲讲lombok中的@DATA注解的用法和原理，就是使用ASM在源码编译阶段生成class进行替换，相对于JDK动态代理来说速度要快许多。