interface MyInterface {
    void doSomething();
}

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 call");
        Object result = method.invoke(target, args);
        System.out.println("After method call");
        return result;
    }
}

public class DynamicProxyExample {
    public static void main(String[] args) {
        MyInterface myObject = new MyInterface() {
            @Override
            public void doSomething() {
                System.out.println("Doing something");
            }
        };

        MyInvocationHandler handler = new MyInvocationHandler(myObject);
        MyInterface proxy = (MyInterface) Proxy.newProxyInstance(
                MyInterface.class.getClassLoader(),
                new Class<?>[]{MyInterface.class},
                handler
        );

        proxy.doSomething(); // This will call the proxy's doSomething method, which forwards to the handler's invoke method.
    }
}

public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h) throws IllegalArgumentException {
        //將接口clone，之后對(duì)此clone類進(jìn)行操作
        final Class<?>[] intfs = interfaces.clone();
        //進(jìn)行權(quán)限檢查
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }
        /*
         * Look up or generate the designated proxy class.
         * 根據(jù)傳入的類加載器和接口類數(shù)組，生成相應(yīng)的代理類
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }

            //獲取構(gòu)造
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doexposingd(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            //返回代理對(duì)象
            return cons.newInstance(new Object[]{h});
        } catch (IllegalAccessException|InstantiationException e) {
            throw new InternalError(e.toString(), e);
        } 
    }

    /**
     * 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");
        }

        // 如果由實(shí)現(xiàn)給定接口的給定加載器定義的代理類存在，將返回緩存的副本;
        // 否則，它將通過ProxyClassFactory創(chuàng)建代理類
        return proxyClassCache.get(loader, interfaces);
    }

/**
     * a cache of proxy classes
     * 動(dòng)態(tài)代理類的弱緩存容器
     * KeyFactory:根據(jù)接口的數(shù)量，映射一個(gè)最佳的key生成函數(shù),其中表示接口的類對(duì)象被弱引用；
     * 也就是key對(duì)象被弱引用繼承自WeakReference（key0、key1、key2、keyX），  保存接口密鑰(hash值)
     * ProxyClassFactory：生成動(dòng)態(tài)類的工廠
     */
    private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
        proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

  	// key 表示緩存鍵，parameter 表示參數(shù)。它的返回值類型為 V，表示緩存值。  
		public V get(K key, P parameter) {
        // 判斷parameter不能為空
        Objects.requireNonNull(parameter);

        expungeStaleEntries();

        Object cacheKey = CacheKey.valueOf(key, refQueue);
        // 該方法將 key 和 refQueue 作為參數(shù)調(diào)用 CacheKey.valueOf() 方法，生成一個(gè)緩存鍵 cacheKey，并從緩存中獲取與該緩存鍵相關(guān)的 valuesMap。
        // 如果 valuesMap 不存在，則創(chuàng)建一個(gè)新的 ConcurrentHashMap 對(duì)象，并將其加入到緩存中。valuesMap 是一個(gè) ConcurrentMap 對(duì)象，它用于存儲(chǔ)緩存鍵下的子鍵和值的提供者，其中子鍵由 subKeyFactory 參數(shù)生成。
        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;
            }
        }

        // 利用Key的工廠類根據(jù)key和parameter參數(shù)生成鍵
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        // 根據(jù)該鍵獲取到Supplier對(duì)象，表示結(jié)果的提供者。Supplier接口包含了一個(gè)get方法
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier可能是一個(gè)工廠類實(shí)例，或者是一個(gè)CacheValue<V>實(shí)例
                V value = supplier.get();
                if (value != null) {
                    return value;
                }
            }
            if (factory == null) {
                // 核心的創(chuàng)建代理類的原理在Factory類里實(shí)現(xiàn)
                // Factory類實(shí)際上也是Supplier接口的實(shí)現(xiàn)類，實(shí)現(xiàn)了get方法
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // 將上面的Factory類對(duì)象賦值給suppler，在下一次循環(huán)時(shí)，就可以調(diào)用Factory類的get方法來生成代理類
                    supplier = factory;
                }
            } else {
                if (valuesMap.replace(subKey, supplier, factory)) {
                    supplier = factory;
                } else {
                    // retry with current supplier
                    supplier = valuesMap.get(subKey);
                }
            }
        }
    }

        public synchronized V get() {               
            V value = null;
            try {
                //valueFactory實(shí)際上是一個(gè)ProxyClassFactory實(shí)例，調(diào)用的是ProxyClassFactory.apply方法
                value = Objects.requireNonNull(valueFactory.apply(key, parameter));
            } finally {
                if (value == null) { 
                    valuesMap.remove(subKey, this);
                }
            }
            // wrap value with CacheValue (WeakReference)
            CacheValue<V> 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");
            }
            return value;
        }
    }

        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) {
                }
                /*
                 * Verify that the Class object actually represents an interface.
                 */
                if (!interfaceClass.isInterface()) {
                    throw new IllegalArgumentException(
                        interfaceClass.getName() + " is not an interface");
                }
            }

            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);
            return defineClass0(loader, proxyName,
                                    proxyClassFile, 0, proxyClassFile.length);

    }

    private byte[] generateClassFile() {
        //生成hashCode方法
        this.addProxyMethod(hashCodeMethod, Object.class);
        //生成equals方法
        this.addProxyMethod(equalsMethod, Object.class);
        //生成toString方法
        this.addProxyMethod(toStringMethod, Object.class);
        Class[] var1 = this.interfaces;
        int var2 = var1.length;

        int var3;
        Class var4;
        for(var3 = 0; var3 < var2; ++var3) {
            var4 = var1[var3];
            Method[] var5 = var4.getMethods();
            int var6 = var5.length;

            for(int var7 = 0; var7 < var6; ++var7) {
                Method var8 = var5[var7];
                this.addProxyMethod(var8, var4);
            }
        }

        Iterator var11 = this.proxyMethods.values().iterator();

        List var12;
        while(var11.hasNext()) {
            var12 = (List)var11.next();
            checkReturnTypes(var12);
        }

        Iterator var15;
        try {
            this.methods.add(this.generateConstructor());
            var11 = this.proxyMethods.values().iterator();

            while(var11.hasNext()) {
                var12 = (List)var11.next();
                var15 = var12.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
                    this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
                    this.methods.add(var16.generateMethod());
                }
            }

            this.methods.add(this.generateStaticInitializer());
        } catch (IOException var10) {
            throw new InternalError("unexpected I/O Exception", var10);
        }

        if (this.methods.size() > 65535) {
            throw new IllegalArgumentException("method limit exceeded");
        } else if (this.fields.size() > 65535) {
            throw new IllegalArgumentException("field limit exceeded");
        } else {
            this.cp.getClass(dotToSlash(this.className));
            this.cp.getClass("java/lang/reflect/Proxy");
            var1 = this.interfaces;
            var2 = var1.length;

            for(var3 = 0; var3 < var2; ++var3) {
                var4 = var1[var3];
                this.cp.getClass(dotToSlash(var4.getName()));
            }

            this.cp.setReadOnly();
            ByteArrayOutputStream var13 = new ByteArrayOutputStream();
            DataOutputStream var14 = new DataOutputStream(var13);

            try {
                var14.writeInt(-889275714);
                var14.writeShort(0);
                var14.writeShort(49);
                this.cp.write(var14);
                var14.writeShort(this.accessFlags);
                var14.writeShort(this.cp.getClass(dotToSlash(this.className)));
                var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
                var14.writeShort(this.interfaces.length);
                Class[] var17 = this.interfaces;
                int var18 = var17.length;

                for(int var19 = 0; var19 < var18; ++var19) {
                    Class var22 = var17[var19];
                    var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
                }

                var14.writeShort(this.fields.size());
                var15 = this.fields.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
                    var20.write(var14);
                }

                var14.writeShort(this.methods.size());
                var15 = this.methods.iterator();

                while(var15.hasNext()) {
                    ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
                    var21.write(var14);
                }

                var14.writeShort(0);
                return var13.toByteArray();
            } catch (IOException var9) {
                throw new InternalError("unexpected I/O Exception", var9);
            }
        }
    }