JDK动态代理步骤详解(源码分析)

动态代理步骤

1.创建一个实现接口invocationhandler的类，它必须实现invoke方法

2.创建被代理的类以及接口

3.通过proxy的静态方法

通过proxy的静态方法

proxyobject proxyobject = new proxyobject();
    invocationhandler invocationhandler = new dynamicproxy(proxyobject);
    classloader classloader = proxyobject.getclass().getclassloader();
    proxyobjectinterface proxy = (iroom) proxy.newproxyinstance(classloader,new class[]
    {proxyobjectinterface.class},invocationhandler);
    proxy.execute();

    public class dynamicproxy implements invocationhandler {
    private object object;

    public dynamicproxy(object object){
        this.object = object;
    }

    @override
    public object invoke(object proxy, method method, object[] args) throws throwable {
        object result = method.invoke(object,args);
        return result;
    }
}

创建一个代理 newproxyinstance

public static object newproxyinstance(classloader loader,
                                          class<?>[] interfaces,
                                          invocationhandler h)
        throws illegalargumentexception
    {
        //检验h不为空，h为空抛异常
        objects.requirenonnull(h);
        //接口的类对象拷贝一份
        final class<?>[] intfs = interfaces.clone();
        //进行一些安全性检查
        final securitymanager sm = system.getsecuritymanager();
        if (sm != null) {
            checkproxyaccess(reflection.getcallerclass(), loader, intfs);
        }
        /*
         * look up or generate the designated proxy class.
         *  查询（在缓存中已经有）或生成指定的代理类的class对象。
         */
        class<?> cl = getproxyclass0(loader, intfs);
        /*
         * invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checknewproxypermission(reflection.getcallerclass(), cl);
            }
            //得到代理类对象的构造函数，这个构造函数的参数由constructorparams指定
            //参数constructorparames为常量值：
            private static final class<?>[] constructorparams = { invocationhandler.class };
            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;
                    }
                });
            }
            //这里生成代理对象，传入的参数new object[]{h}后面讲
            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);
        }
    }

先对h进行判空处理。
这段代码核心就是通过getproxyclass0(loader, intfs)得到代理类的class对象，然后通过class对象得到构造方法，进而创建代理对象。下一步看getproxyclass0这个方法。从1可知，先接口得到接口类，当接口的数量超过65535，则报异常。

//此方法也是proxy类下的方法
    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
        //意思是：如果代理类被指定的类加载器loader定义了，并实现了给定的接口interfaces，
        //那么就返回缓存的代理类对象，否则使用proxyclassfactory创建代理类。
        return proxyclasscache.get(loader, interfaces);
    }

proxyclasscache 是一个弱引用的缓存
这里看到proxyclasscache，有cache便知道是缓存的意思，正好呼应了前面look up or generate the designated proxy class。查询（在缓存中已经有）或生成指定的代理类的class对象这段注释。

在进入get方法之前，我们看下 proxyclasscache是什么？高能预警，前方代码看起来可能有乱，但我们只需要关注重点即可。

private static final weakcache<classloader, class<?>[], class<?>>
        proxyclasscache = new weakcache<>(new keyfactory(), new proxyclassfactory());
//k代表key的类型，p代表参数的类型，v代表value的类型。
// weakcache<classloader, class<?>[], class<?>>  proxyclasscache  说明proxyclasscache存的值是class<?>对象，正是我们需要的代理类对象。
final class weakcache<k, p, v> {
    private final referencequeue<k> refqueue
        = new referencequeue<>();
    // the key type is object for supporting null key
    private final concurrentmap<object, concurrentmap<object, supplier<v>>> map
        = new concurrenthashmap<>();
    private final concurrentmap<supplier<v>, boolean> reversemap
        = new concurrenthashmap<>();
    private final bifunction<k, p, ?> subkeyfactory;
    private final bifunction<k, p, v> valuefactory;
    public weakcache(bifunction<k, p, ?> subkeyfactory,
                     bifunction<k, p, v> valuefactory) {
        this.subkeyfactory = objects.requirenonnull(subkeyfactory);
        this.valuefactory = objects.requirenonnull(valuefactory);
    }

其中map变量是实现缓存的核心变量，他是一个双重的map结构: (key, sub-key) -> value。其中key是传进来的classloader进行包装后的对象，sub-key是由weakcache构造函数传人的keyfactory()生成的。value就是产生代理类的对象，是由weakcache构造函数传人的proxyclassfactory()生成的。如下，回顾一下:

proxyclasscache是个weakcache类的对象，调用proxyclasscache.get(loader, interfaces)； 可以得到缓存的代理类或创建代理类（没有缓存的情况）。

说明weakcache中有get这个方法。先看下weakcache类的定义（这里先只给出变量的定义和构造函数），继续看它的get（）；

//k和p就是weakcache定义中的泛型，key是类加载器，parameter是接口类数组
public v get(k key, p parameter) {
        //检查parameter不为空
        objects.requirenonnull(parameter);
         //清除无效的缓存
        expungestaleentries();
        // cachekey就是(key, sub-key) -> value里的一级key，
        object cachekey = cachekey.valueof(key, refqueue);
        // lazily install the 2nd level valuesmap for the particular cachekey
        //根据一级key得到 concurrentmap<object, supplier<v>>对象。如果之前不存在，则新建一个concurrentmap<object, supplier<v>>和cachekey（一级key）一起放到map中。
        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
        //这部分就是调用生成sub-key的代码，上面我们已经看过怎么生成的了
        object subkey = objects.requirenonnull(subkeyfactory.apply(key, parameter));
        //通过sub-key得到supplier
        supplier<v> supplier = valuesmap.get(subkey);
        //supplier实际上就是这个factory
        factory factory = null;

        while (true) {
            //如果缓存里有supplier ，那就直接通过get方法，得到代理类对象，返回，就结束了，一会儿分析get方法。
            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
            //下面的所有代码目的就是：如果缓存中没有supplier，则创建一个factory对象，把factory对象在多线程的环境下安全的赋给supplier。
            //因为是在while（true）中，赋值成功后又回到上面去调get方法，返回才结束。
            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);
                }
            }
        }
    }

所以接下来我们看factory类中的get方法。接下来看supplier的get()

public synchronized v get() { // serialize access
            // re-check
            supplier<v> supplier = valuesmap.get(subkey);
            //重新检查得到的supplier是不是当前对象
            if (supplier != this) {
                // something changed while we were waiting:
                // might be that we were replaced by a cachevalue
                // or were removed because of failure ->
                // return null to signal weakcache.get() to retry
                // the loop
                return null;
            }
            // else still us (supplier == this)
            // create new value
            v value = null;
            try {
                 //代理类就是在这个位置调用valuefactory生成的
                 //valuefactory就是我们传入的 new proxyclassfactory()
                //一会我们分析proxyclassfactory()的apply方法
                value = objects.requirenonnull(valuefactory.apply(key, parameter));
            } finally {
                if (value == null) { // remove us on failure
                    valuesmap.remove(subkey, this);
                }
            }
            // the only path to reach here is with non-null value
            assert value != null;

            // wrap value with cachevalue (weakreference)
            //把value包装成弱引用
            cachevalue<v> cachevalue = new cachevalue<>(value);

            // put into reversemap
            // reversemap是用来实现缓存的有效性
            reversemap.put(cachevalue, boolean.true);

            // try replacing us with cachevalue (this should always succeed)
            if (!valuesmap.replace(subkey, this, cachevalue)) {
                throw new assertionerror("should not reach here");
            }

            // successfully replaced us with new cachevalue -> return the value
            // wrapped by it
            return value;
        }
    }

拨云见日，来到proxyclassfactory的apply方法，代理类就是在这里生成的。

首先看proxyclasscache的定义weakcache<classloader, class<?>[], class<?>>，泛型里面第一个表示加载器k，第二个表示接口类p，第三个则是生成的代理类v。而v的生成则是通过proxyclassfactory生成的。调用其apply（）；

//这里的bifunction<t, u, r>是个函数式接口，可以理解为用t，u两种类型做参数，得到r类型的返回值
private static final class proxyclassfactory
        implements bifunction<classloader, class<?>[], class<?>>
    {
        // prefix for all proxy class names
        //所有代理类名字的前缀
        private static final string proxyclassnameprefix = "$proxy";
        // next number to use for generation of unique proxy class names
        //用于生成代理类名字的计数器
        private static final atomiclong nextuniquenumber = new atomiclong();
        @override
        public class<?> apply(classloader loader, class<?>[] interfaces) {
            map<class<?>, boolean> interfaceset = new identityhashmap<>(interfaces.length);
            //验证代理接口，可不看
            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());
                }
            }
            //生成的代理类的包名 
            string proxypkg = null;     // package to define proxy class in
            //代理类访问控制符: public ,final
            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.
             */
            //验证所有非公共的接口在同一个包内；公共的就无需处理
            //生成包名和类名的逻辑，包名默认是com.sun.proxy，
            // 类名默认是$proxy 加上一个自增的整数值
            //如果被代理类是 non-public proxy interface ，则用和被代理类接口一样的包名
            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();
            //代理类的完全限定名，如com.sun.proxy.$proxy0.calss
            string proxyname = proxypkg + proxyclassnameprefix + num;

            /*
             * generate the specified proxy class.
             */
            //核心部分，生成代理类的字节码
            byte[] proxyclassfile = proxygenerator.generateproxyclass(
                proxyname, interfaces, accessflags);
            try {
                //把代理类加载到jvm中，至此动态代理过程基本结束了
                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());
            }
        }
    }

然后调用getmethod（）,将equals(),hashcode(),tostring()等方法添加进去。然后遍历所有接口的方法，添加到代理类中。最后将这些方法进行排序。

private static list<method> getmethods(class<?>[] interfaces) {
        list<method> result = new arraylist<method>();
        try {
            result.add(object.class.getmethod("equals", object.class));
            result.add(object.class.getmethod("hashcode", emptyarray.class));
            result.add(object.class.getmethod("tostring", emptyarray.class));
        } catch (nosuchmethodexception e) {
            throw new assertionerror();
        }

        getmethodsrecursive(interfaces, result);
        return result;
    }
private static void getmethodsrecursive(class<?>[] interfaces, list<method> methods) {
        for (class<?> i : interfaces) {
            getmethodsrecursive(i.getinterfaces(), methods);
            collections.addall(methods, i.getdeclaredmethods());
        }
    }

最后输出相关proxy class

package com.zhb.jdk.proxy;
import java.io.fileoutputstream;
import java.io.ioexception;
import java.lang.reflect.proxy;

import com.zhb.jdk.dynamicproxy.helloworldimpl;

import sun.misc.proxygenerator;

/**
 * @author zhb
 * @date 2018年8月31日下午11:35:07
 * @todo todo
 */
public class dynamicproxytest {

    public static void main(string[] args) {

        iuserservice target = new userserviceimpl();
        myinvocationhandler handler = new myinvocationhandler(target);
        //第一个参数是指定代理类的类加载器（我们传入当前测试类的类加载器）
        //第二个参数是代理类需要实现的接口（我们传入被代理类实现的接口，这样生成的代理类和被代理类就实现了相同的接口）
        //第三个参数是invocation handler，用来处理方法的调用。这里传入我们自己实现的handler
        iuserservice proxyobject = (iuserservice) proxy.newproxyinstance(dynamicproxytest.class.getclassloader(),
                target.getclass().getinterfaces(), handler);
        proxyobject.add("陈粒");

        string path = "d:/$proxy0.class";
        byte[] classfile = proxygenerator.generateproxyclass("$proxy0", helloworldimpl.class.getinterfaces());
        fileoutputstream out = null;

        try {
            out = new fileoutputstream(path);
            out.write(classfile);
            out.flush();
        } catch (exception e) {
            e.printstacktrace();
        } finally {
            try {
                out.close();
            } catch (ioexception e) {
                e.printstacktrace();
            }
        }

    }
}
// decompiled by jad v1.5.8e2. copyright 2001 pavel kouznetsov.
// jad home page: http://kpdus.tripod.com/jad.html
// decompiler options: packimports(3) fieldsfirst ansi space

import com.zhb.jdk.proxy.iuserservice;
import java.lang.reflect.*;

public final class $proxy0 extends proxy
    implements iuserservice
{

    private static method m1;
    private static method m2;
    private static method m3;
    private static method m0;
    //代理类的构造函数，其参数正是是invocationhandler实例，
    //proxy.newinstance方法就是通过通过这个构造函数来创建代理实例的
    public $proxy0(invocationhandler invocationhandler)
    {
        super(invocationhandler);
    }
     // object类中的三个方法，equals，tostring， hashcode
    public final boolean equals(object obj)
    {
        try
        {
            return ((boolean)super.h.invoke(this, m1, new object[] {
                obj
            })).booleanvalue();
        }
        catch (error ) { }
        catch (throwable throwable)
        {
            throw new undeclaredthrowableexception(throwable);
        }
    }

    public final string tostring()
    {
        try
        {
            return (string)super.h.invoke(this, m2, null);
        }
        catch (error ) { }
        catch (throwable throwable)
        {
            throw new undeclaredthrowableexception(throwable);
        }
    }
    //接口代理方法
    public final void add(string s)
    {
        try
        {
            // invocation handler的 invoke方法在这里被调用
            super.h.invoke(this, m3, new object[] {
                s
            });
            return;
        }
        catch (error ) { }
        catch (throwable throwable)
        {
            throw new undeclaredthrowableexception(throwable);
        }
    }

    public final int hashcode()
    {
        try
        {
            // 在这里调用了invoke方法。
            return ((integer)super.h.invoke(this, m0, null)).intvalue();
        }
        catch (error ) { }
        catch (throwable throwable)
        {
            throw new undeclaredthrowableexception(throwable);
        }
    }

    // 静态代码块对变量进行一些初始化工作
    static 
    {
        try
        {
            m1 = class.forname("java.lang.object").getmethod("equals", new class[] {
                class.forname("java.lang.object")
            });
            m2 = class.forname("java.lang.object").getmethod("tostring", new class[0]);
            m3 = class.forname("com.zhb.jdk.proxy.iuserservice").getmethod("add", new class[] {
                class.forname("java.lang.string")
            });
            m0 = class.forname("java.lang.object").getmethod("hashcode", new class[0]);
        }
        catch (nosuchmethodexception nosuchmethodexception)
        {
            throw new nosuchmethoderror(nosuchmethodexception.getmessage());
        }
        catch (classnotfoundexception classnotfoundexception)
        {
            throw new noclassdeffounderror(classnotfoundexception.getmessage());
        }
    }
}

以上就是jdk动态代理步骤详解(源码分析)的详细内容，更多关于jdk动态代理的资料请关注其它相关文章！