java 1.8 动态代理源码分析

Posted 2020-09-22 寰殇丶天使

JDK8动态代理源码分析

动态代理的基本使用就不详细介绍了：

例子：

class proxyed implements pro{
    @Override
    public void text() {
        System.err.println("本方法");
    }
}

interface pro {
    void text();
}

public class JavaProxy implements InvocationHandler {
        private Object source;
        public JavaProxy(Object source) {
            super();
            this.source = source;
        }
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            System.out.println("before");
            Object invoke = method.invoke(source, args);
            System.out.println("after");
            return invoke;
        }
        public Object getProxy(){
            return Proxy.newProxyInstance(getClass().getClassLoader(), source.getClass().getInterfaces(), this);
        }
        public static void main(String[] args) throws IllegalAccessException, InvocationTargetException, InstantiationException, NoSuchMethodException {
            //第一种，自己写
            //1.设置saveGeneratedFiles值为true则生成 class字节码文件方便分析
            System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
            //2.获取动态代理类
            Class proxyClazz = Proxy.getProxyClass(pro.class.getClassLoader(),pro.class);
            //3.获得代理类的构造函数，并传入参数类型InvocationHandler.class
            Constructor constructor = proxyClazz.getConstructor(InvocationHandler.class);
            //4.通过构造函数来创建动态代理对象，将自定义的InvocationHandler实例传入
            pro iHello = (pro) constructor.newInstance(new JavaProxy(new proxyed()));
            //5.通过代理对象调用目标方法
            iHello.text();
            //第二种，调用JDK提供的方法，实现了2~4步
            Proxy.newProxyInstance(JavaProxy.class.getClassLoader(),proxyed.class.getInterfaces(),new JavaProxy(new proxyed()));
        }
}

 

 

入口：newProxyInstance

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException {
        //Objects.requireNonNull 判空方法，之后所有的单纯的判断null并抛异常，都是此方法
        Objects.requireNonNull(h);
        //clone 类实现的所有接口
        final Class<?>[] intfs = interfaces.clone();
        //获取当前系统安全接口
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            //Reflection.getCallerClass返回调用该方法的方法的调用类;loader：接口的类加载器
            //进行包访问权限、类加载器权限等检查
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /*
         * Look up or generate the designated proxy class.
         *  查找或生成代理类
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         * 使用指定的调用处理程序调用它的构造函数
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            //获取构造
            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;
                    }
                });
            }
            //返回 代理对象
            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);
        }
    }

 

　　从上面的分析中可以看出，newProxyInstance帮我们执行了生成代理类----获取构造器----生成代理对象这三步；

　　我们重点分析生成代理类

getProxyClass0

　　/**
     * a cache of proxy classes：动态代理类的弱缓存容器
     * KeyFactory:根据接口的数量，映射一个最佳的key生成函数,其中表示接口的类对象被弱引用；也就是key对象被弱引用继承自WeakReference（key0、key1、key2、keyX），保存接口密钥(hash值)
     * ProxyClassFactory：生成动态类的工厂
     * 注意，两个都实现了BiFunction<ClassLoader, Class<?>[], Object>接口
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

    /**
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     * 生成代理类，调用前必须进行 checkProxyAccess权限检查，所以newProxyInstance进行了权限检查
     */
    private static Class<?> getProxyClass0(ClassLoader loader, Class<?>... interfaces) {
        //实现接口的最大数量<65535;谁写的类能实现这么多接口
        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
        // 如果缓存中有，就直接返回，否则会生成
        return proxyClassCache.get(loader, interfaces);
    }

proxyClassCache.get

public V get(K key, P parameter) {
        //key：类加载器；parameter：接口数组
        Objects.requireNonNull(parameter);
        //清除已经被GC回收的弱引用
        expungeStaleEntries();

        //CacheKey弱引用类，refQueue已经被回收的弱引用队列；构建一个CacheKey
        Object cacheKey = CacheKey.valueOf(key, refQueue);
        
        //map一级缓存，获取valuesMap二级缓存
        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;
            }
        }

        // subKeyFactory类型是KeyFactory，apply返回表示接口的key
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        //Factory 实现了supplier，我们实际是获取缓存中的Factory，调用其get方法
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;
        
        //下面用到了 CAS+重试 实现的多线程安全的 非阻塞算法
        while (true) {
            if (supplier != null) {
                // 只需要知道，最终会调用get方法，此supplier可能是缓存中取出来的，也可能是Factory新new出来的
                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);
                }
            }
        }
    }

supplier.get

　　这个方法中会调用ProxyClassFactory的apply方法，就不过多介绍

ProxyClassFactory.apply

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.
                 * 类加载器和接口名解析出的是同一个
                 */
            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.
                 * 确保是一个接口
                 */
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException( interfaceClass.getName() + " is not an interface");
            }
                /*
                 * Verify that this interface is not a duplicate.
                 * 确保接口没重复
                 */
            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;
            /*
             * 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
             */
        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.
             * proxyClassNamePrefix = $Proxy
             * nextUniqueNumber 是一个原子类，确保多线程安全，防止类名重复，类似于：$Proxy0，$Proxy1......
             */
        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;
            /*
             * Generate the specified proxy class.
             * 生成类字节码的方法:重点
             */
        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());
        }
    }

 

ProxyGenerator.generateProxyClass

public static byte[] generateProxyClass(final String name, Class<?>[] interfaces, int accessFlags) {
        ProxyGenerator gen = new ProxyGenerator(name, interfaces, accessFlags);
        //真正生成字节码的方法
        final byte[] classFile = gen.generateClassFile();
        //如果saveGeneratedFiles为true 则生成字节码文件，所以在开始我们要设置这个参数
        //当然，也可以通过返回的bytes自己输出
        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;
    }

 

最终方法

　　

private byte[] generateClassFile() {
        /* ============================================================
         * Step 1: Assemble ProxyMethod objects for all methods to generate proxy dispatching code for.
         * 步骤1：为所有方法生成代理调度代码，将代理方法对象集合起来。
         */
        //增加 hashcode、equals、toString方法
        addProxyMethod(hashCodeMethod, Object.class);
        addProxyMethod(equalsMethod, Object.class);
        addProxyMethod(toStringMethod, Object.class);
        //增加接口方法
        for (Class<?> intf : interfaces) {
            for (Method m : intf.getMethods()) {
                addProxyMethod(m, intf);
            }
        }

        /*
         * 验证方法签名相同的一组方法，返回值类型是否相同；意思就是重写方法要方法签名和返回值一样
         */
        for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
            checkReturnTypes(sigmethods);
        }

        /* ============================================================
         * Step 2: Assemble FieldInfo and MethodInfo structs for all of fields and methods in the class we are generating.
         * 为类中的方法生成字段信息和方法信息
         */
        try {
            //增加构造方法
            methods.add(generateConstructor());
            for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
                for (ProxyMethod pm : sigmethods) {
                    // add static field for method‘s Method object
                    fields.add(new FieldInfo(pm.methodFieldName,
                            "Ljava/lang/reflect/Method;",
                            ACC_PRIVATE | ACC_STATIC));
                    // generate code for proxy method and add it
                    methods.add(pm.generateMethod());
                }
            }
            //增加静态初始化信息
            methods.add(generateStaticInitializer());
        } catch (IOException e) {
            throw new InternalError("unexpected I/O Exception", e);
        }

        if (methods.size() > 65535) {
            throw new IllegalArgumentException("method limit exceeded");
        }
        if (fields.size() > 65535) {
            throw new IllegalArgumentException("field limit exceeded");
        }

        /* ============================================================
         * Step 3: Write the final class file.
         * 步骤3：编写最终类文件
         */
        /*
         * Make sure that constant pool indexes are reserved for the following items before starting to write the final class file.
         * 在开始编写最终类文件之前，确保为下面的项目保留常量池索引。
         */
        cp.getClass(dotToSlash(className));
        cp.getClass(superclassName);
        for (Class<?> intf: interfaces) {
            cp.getClass(dotToSlash(intf.getName()));
        }

        /*
         * Disallow new constant pool additions beyond this point, since we are about to write the final constant pool table.
         * 设置只读，在这之前不允许在常量池中增加信息，因为要写常量池表
         */
        cp.setReadOnly();

        ByteArrayOutputStream bout = new ByteArrayOutputStream();
        DataOutputStream dout = new DataOutputStream(bout);

        try {
            // u4 magic;
            dout.writeInt(0xCAFEBABE);
            // u2 次要版本;
            dout.writeShort(CLASSFILE_MINOR_VERSION);
            // u2 主版本
            dout.writeShort(CLASSFILE_MAJOR_VERSION);

            cp.write(dout);             // (write constant pool)

            // u2 访问标识;
            dout.writeShort(accessFlags);
            // u2 本类名;
            dout.writeShort(cp.getClass(dotToSlash(className)));
            // u2 父类名;
            dout.writeShort(cp.getClass(superclassName));
            // u2 接口;
            dout.writeShort(interfaces.length);
            // u2 interfaces[interfaces_count];
            for (Class<?> intf : interfaces) {
                dout.writeShort(cp.getClass(
                        dotToSlash(intf.getName())));
            }
            // u2 字段;
            dout.writeShort(fields.size());
            // field_info fields[fields_count];
            for (FieldInfo f : fields) {
                f.write(dout);
            }
            // u2 方法;
            dout.writeShort(methods.size());
            // method_info methods[methods_count];
            for (MethodInfo m : methods) {
                m.write(dout);
            }
            // u2 类文件属性：对于代理类来说没有类文件属性;
            dout.writeShort(0); // (no ClassFile attributes for proxy classes)

        } catch (IOException e) {
            throw new InternalError("unexpected I/O Exception", e);
        }

        return bout.toByteArray();
    }

生成的字节码反编译

　　

final class $Proxy0 extends Proxy implements pro {
        //fields    
        private static Method m1;
        private static Method m2;
        private static Method m3;
        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 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 void text() throws  {
            try {
                //实际就是调用代理类的invoke方法 
                super.h.invoke(this, m3, (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 {
                //这里每个方法对象 和类的实际方法绑定
                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("spring.commons.api.study.CreateModel.pro").getMethod("text", 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());
            }
        }
    }