JDK动态代理源码大概解析

接口和实现类，和测试

public interface UserService {

    String getName();
}

public class UserServiceImpl implements UserService{
    @Override
    public String getName() {
        return "hello";
    }
}

UserService userService=(UserService)Proxy.newProxyInstance(UserServiceImpl.class.getClassLoader(), new Class[]{UserService.class}, new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                UserServiceImpl userService1=new UserServiceImpl();
                return method.invoke(userService1,args)+" hanson";
            }
        });
        System.out.println(userService.getName());

输出结果

hello hanson

2，说下为设么会有两个类，一个接口，一个实现。相信大家都知道。这个代理的实现的前提是有接口。如果这个不清楚，可以去查阅下jd动态代理实现的前提。当然本文看完后也就知道了为设么必须要有接口了。明白最终怎么实现的增强。

3。接下来就看下整个生成代理类的方法调用链路。

@CallerSensitive
    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();
        //校验就不细看了不是本次的主题
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        
        //得到代理类 class信息
        Class cl = getProxyClass0(loader, intfs);

        
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            //通过参数信息获取构造器
            final Constructor cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
        
            if (!Modifier.isPublic(cl.getModifiers())) {
                //设置访问权限
                AccessController.doPrivileged(new PrivilegedAction() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            //返回生成的代理实例
            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);
        }
    }

上述过程大概的执行主流程。进入Class cl = getProxyClass0(loader, intfs);这个方法进入后

得到 

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);
    }

跟进到  proxyClassCache.get(loader, interfaces);通过代理类缓存得到 代理类。继续跟进

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
        //缓存中获取首次是不存在的 初始化一个map valu为一个函数，下面的逻辑会用到
        ConcurrentMap> valuesMap = map.get(cacheKey);
        if (valuesMap == null) {
            ConcurrentMap> oldValuesMap
                = map.putIfAbsent(cacheKey,
                                  valuesMap = new ConcurrentHashMap<>());
            if (oldValuesMap != null) {
                valuesMap = oldValuesMap;
            }
        }

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

        while (true) {
            //函数不为空就取出相应的类
            if (supplier != null) {
                // supplier might be a Factory or a Cachevalue 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) {
                //工厂为空就实例化
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // successfully installed Factory
                    //因为上一步已经将工厂实例化了，所以下次循环可以直接调用获取。
                    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);
                }
            }
        }
    }

下一步继续看 重要的信息 安装工厂类的过程

@Override
        public synchronized V get() { // serialize access
            // re-check
            Supplier supplier = valuesMap.get(subKey);
            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 {
                //重要入口，此处完成类的生成
                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)
            Cachevalue cachevalue = new Cachevalue<>(value);

            // put into reverseMap
            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;
        }
    }

下一步走进入  上步中提到的重要入口方法

valueFactory.apply(key, parameter)，继续进入看

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

            Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);

            for (Class intf : interfaces) {
                
                Class interfaceClass = null;
                try {
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                //保证传入的接口可被整行加载,同时必须时接口,同时必须有可访问权限
                if (interfaceClass != intf) {
                    throw new IllegalArgumentException(
                        intf + " is not visible from class loader");
                }
                
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
                
                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;

            
            //获取包路径信息,同时代理的接口必须在相同的包（非public）
            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 + ".";
            }

            
            //正常情况下生成固定路径 $Proxy0,1,2 原子递增具体生成
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            
            //根据信息生成字节流文件信息.
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
                proxyName, interfaces, accessFlags);
            try {
                //生成具体类信息
                return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);
            } catch (ClassFormatError e) {
                
                throw new IllegalArgumentException(e.toString());
            }
        }
    }

接着进入 上文提到的 

byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
    proxyName, interfaces, accessFlags);此处操作字节信息生成字节流文件.

public static byte[] generateProxyClass(final String var0, Class[] var1, int var2) {
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        //生成字节信息
        final byte[] var4 = var3.generateClassFile();
        if (saveGeneratedFiles) {
            AccessController.doPrivileged(new PrivilegedAction() {
                public Void run() {
                    try {
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if (var1 > 0) {
                            //路径转换根据不同系统window或unix
                            Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
                            Files.createDirectories(var3);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                        } else {
                            var2 = Paths.get(var0 + ".class");
                        }
                        //写入文件
                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                    } catch (IOException var4x) {
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    }
                }
            });
        }

        return var4;
    }

最后就是进入上面的字节信息生成逻辑

final byte[] var4 = var3.generateClassFile();

此方法进行字节操作生成字节流信息.到此完成了代理过程。

有问题欢迎各位老铁留言指出共同提高。