JDK动态代理源码学习

一、使用示例：

1、定义DTO：

public class User {

    private String userName;

    private String passWord;

    private String email;

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getPassWord() {
        return passWord;
    }

    public void setPassWord(String passWord) {
        this.passWord = passWord;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}

2、定义接口： 

public interface UserDao {

    void addUser(User user);
}

3、实现接口：  

public class UserDaoImpl implements UserDao {

    @Override
    public void addUser(User user) {
        System.out.println("connect to mySQL dataBase.......");
        System.out.println("添加用户信息成功...");
    }
}

4、实现被代理对象的增强类：

public class LogHandler implements InvocationHandler {

    private Object target;

    public LogHandler(Object target) {
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println("开始记录日志");
        method.invoke(target, args);
        System.out.println("日志记录完成");
        return null;
    }
}

5、调用测试： 

public static void main(String[] args) {
    UserDao dao = new UserDaoImpl();
    LogHandler handler = new LogHandler(dao);

    UserDao proxy = (UserDao) Proxy.newProxyInstance(dao.getClass().getClassLoader(), dao.getClass().getInterfaces(), handler);    
    proxy.addUser(user);
}

二、动态生成的代理文件查看： 

1、在调用动态代理的main方法中加上：

System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");

2、利用ProxyGenerator生成：  

public static void main(String[] args) {
        byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", UserDaoImpl.class.getInterfaces());
        String path = "E:\$Proxy0.class";
        try(FileOutputStream fos = new FileOutputStream(path)) {
            fos.write(classFile);
            fos.flush();
            System.out.println("代理类class文件写入成功");
        } catch (Exception e) {
            System.out.println("写文件错误");
        }
}

这样在运行代码的时候就会在项目的根目录（或者指定目录）下生成 com.sun.proxy.$ProxyX.class 了，反编译查看：

public final class $Proxy0 extends Proxy implements UserDao {
    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 void addUser(User var1) throws  {
        try {
            super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }
  
    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("com.ucar.test.service.UserDao").getMethod("addUser", Class.forName("com.ucar.test.dto.User"));
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

可以看到： 

1) 代理类$Proxy0从Proxy派生而来，并且实现了UserDao接口；

2) static块中生成了Object类的三个方法：toString、equals、hashCode，并且也生成了UserDao接口的addUser方法;

3) 调用端通过Proxy.newProxyInstance获取的对象实例也就是这个$Proxy0类型的对象，默认情况下这个类是不保存到磁盘上的，直接在内存中通过ClassLoader加载，因此在断点调试时是无法定位到这个类里面的；

4) 当调用UserDao接口的addUser方法时，其实是调用InvocationHandler.invoke方法，InvocationHandler的实现也就是我们前面实现的LogHandler类，这样就将调用转到LogHandler.invoke方法；

三、源码分析：

1、首先从Proxy.newProxyInstance开始分析：

public static Object newProxyInstance(ClassLoader loader, Class<?>[] interfaces, InvocationHandler h) throws IllegalArgumentException{
        // null检查，h为null就抛出NullPointerException
        Objects.requireNonNull(h);
        // 将接口类对象数组clone一份。
        final Class<?>[] intfs = interfaces.clone();
        
        //执行权限检查
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /*
         * Look up or generate the designated proxy class.
         */
         // 查找或者是生成一个特定的代理类对象
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            // 是static final 修饰的，源码： private static final Class<?>[] constructorParams ={ InvocationHandler.class };
            // 从代理类对象中查找参数为InvocationHandler的构造器
            final Constructor<?> cons = cl.getConstructor(constructorParams);
            final InvocationHandler ih = h;
            // 检测构造器是否是Public修饰，如果不是则强行转换为可以访问的。
            if (!Modifier.isPublic(cl.getModifiers())) {
                AccessController.doPrivileged(new PrivilegedAction<Void>() {
                    public Void run() {
                        cons.setAccessible(true);
                        return null;
                    }
                });
            }
            // 通过反射，将h作为参数，实例化代理类，返回代理类实例。
            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);
        }
}

这个方法主要工作： 

1) getProxyClass0(loader, intfs)方法生成了上面的代理类$Proxy0，生成时主要根据intfs接口获取接口并生成，因此这就是JDK动态代理的类必须要定义接口的愿意；

2) cons.newInstance(new Object[]{h})：利用反射技术实例化代理类，并返回实例化对象；

2、getProxyClass0方法：

/**
     * 生成一个代理类对象，
     * Generate a proxy class.  Must call the checkProxyAccess method
     * to perform permission checks before calling this.
     */
    private static Class<?> getProxyClass0(ClassLoader loader,
                                           Class<?>... interfaces) {
        // 接口类对象数组不能大于65535个，否则抛出异常
        if (interfaces.length > 65535) {
            throw new IllegalArgumentException("interface limit exceeded");
        }
        // 从代理类对象缓存中，根据类加载器和接口类对象数组查找代理类对象，
        // 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);
    }

在这个方法中，是直接从一个叫proxyClassCache缓存中读取的，来看一下这个缓存的声明：

private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());

看一下WeakCache类的大概结构：

final class WeakCache<K, P, V> {

    private final ReferenceQueue<K> refQueue= new ReferenceQueue<>();
    // the key type is Object for supporting null key
    // key的类型为Object，支持null key,这里的null key并不是真的可以使用null最为key,而是一个new Objdec()对象实例。ConcurrentHashMap,不允许键或值null，而HashMap可以。ConcurrentHashMap是线程安全的，HashMap不是。
    private final ConcurrentMap<Supplier<V>, Boolean> reverseMap= new ConcurrentHashMap<>();
    
    private final BiFunction<K, P, ?> subKeyFactory;
    private final BiFunction<K, P, V> valueFactory;

    //构造方法
    public WeakCache(BiFunction<K, P, ?> subKeyFactory,
                     BiFunction<K, P, V> valueFactory) {
        this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
        this.valueFactory = Objects.requireNonNull(valueFactory);
    }

    
    public V get(K key, P parameter) {
       //下面会详细介绍这个方法
    }

  ......
}

3、WeakCache.get方法：

    // key是类加载器，parameter为接口类对象数组
    public V get(K key, P parameter) {
        // 接口类对象数组null检查。
        Objects.requireNonNull(parameter);
        
        // 删除过时的条目
        expungeStaleEntries();
        // 生成缓存key对象实例，如果key = null，cacheKey = new Object();
        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
        // 从缓存map中读取指定cacheKey的缓存数据valuesMap
        ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
        
        if (valuesMap == null) {
            //如果valuesMap为null,则新增
            //putIfAbsent方法解释：如果值存在则返回值，并且不对原来的值做任何更改，如果不存在则新增，并返回null
            //Absent的意思是缺席，不在
            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
        // 获取subKey，这里用到了上面提到的Proxy的静态内部类KeyFactory:subKeyFactory.apply(ket,parameter)
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        // 从valuesMap中获取supplier
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue<V> instance
                // 4,从工厂中获取代理类对象
                V value = supplier.get();
                if (value != null) {
                    // 5，返回
                    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) {
                //1,实例化工厂
                factory = new Factory(key, parameter, subKey, valuesMap);
            }

            if (supplier == null) {
                //2，保存到valuesMap中
                supplier = valuesMap.putIfAbsent(subKey, factory);
                if (supplier == null) {
                    // successfully installed Factory
                    // 3，赋值
                    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);
                }
            }
        }
    }

其中supplier就是factory； 

4、Factory.get方法：

private final class Factory implements Supplier<V> {

        private final K key;
        private final P parameter;
        private final Object subKey;
        private final ConcurrentMap<Object, Supplier<V>> valuesMap;

        Factory(K key, P parameter, Object subKey,
                ConcurrentMap<Object, Supplier<V>> valuesMap) {
            this.key = key;
            this.parameter = parameter;
            this.subKey = subKey;
            this.valuesMap = valuesMap;
        }

        @Override
        public synchronized V get() { // serialize access
            // re-check
            // 检查
            Supplier<V> 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 {
                // valueFactory就是WeakCache的valueFactory属性，因为Factory是WeakCache的内部类，所以可以直接访问WeakCache的valueFactory属性
                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<V> cacheValue = new CacheValue<>(value);

            // try replacing us with CacheValue (this should always succeed)
            if (valuesMap.replace(subKey, this, cacheValue)) {
                // put also in reverseMap
                reverseMap.put(cacheValue, Boolean.TRUE);
            } else {
                throw new AssertionError("Should not reach here");
            }

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

这里最关键的代码：valueFactory.apply(key, parameter)；valueFactory就是Proxy的静态内部类ProxyClassFactory； 

5、ProxyClassFactory.apply方法：

     /**
     * 一个利用给定的类加载器和接口类数组生成，定义并返回代理类对象的工厂方法
     * A factory function that generates, defines and returns the proxy class given
     * the ClassLoader and array of interfaces.
     */
    private static final class ProxyClassFactory
        implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    {
        // prefix for all proxy class names
        // 所有代理类对象的前缀
        private static final String proxyClassNamePrefix = "$Proxy";

        // next number to use for generation of unique proxy class names
        // 用于生成唯一代理类名称的下一个数字
        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        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 {
                    // 加载接口类，获得接口类的类对象，第二个参数为false表示不进行实例化
                    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());
                }
            }
            // package to define proxy class in
            // 代理类的包名
            String proxyPkg = null;     
            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 = com.sun.proxy package
                proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
            }

            /*
             * 生成代理类的类名
             * Choose a name for the proxy class to generate.
             */
            long num = nextUniqueNumber.getAndIncrement();
            String proxyName = proxyPkg + proxyClassNamePrefix + num;

            /*
             * Generate the specified proxy class.
             */
            //生成代理类class文件
            byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags);
            try {
                // 返回代理类对象
                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());
            }
        }
    }
  }

这里的ProxyGenerator.generateProxyClass生成了代理类的字节码文件; 

6、ProxyGenerator.generateProxyClass方法：

public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
    ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
   // 真正用来生成代理类字节码文件的方法在这里
    final byte[] var4 = var3.generateClassFile();
   // 保存代理类的字节码文件
    if(saveGeneratedFiles) {
        AccessController.doPrivileged(new PrivilegedAction() {
            public Void run() {
                try {
                    int var1 = var0.lastIndexOf(46);
                    Path var2;
                    if(var1 > 0) {
                        Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar), 
                                                                               new String[0]);
                        Files.createDirectories(var3, new FileAttribute[0]);
                        var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                    } else {
                        var2 = Paths.get(var0 + ".class", new String[0]);
                    }

                    Files.write(var2, var4, new OpenOption[0]);
                    return null;
                } catch (IOException var4x) {
                    throw new InternalError("I/O exception saving generated file: " + var4x);
                }
            }
        });
    }

    return var4;
}

其中generateClassFile是真正用于生成代理类字节码文件的方法；saveGeneratedFiles判断是否要将代理类字节码文件保存到本地磁盘文件，其定义为：

private static final boolean saveGeneratedFiles = (Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"));

 可以看到saveGeneratedFiles是从系统环境变量中读取的”sun.misc.ProxyGenerator.saveGeneratedFiles”变量并转换成布尔类型；

7、generateClassFile方法：

private byte[] generateClassFile() {
    //下面一系列的addProxyMethod方法是将接口中的方法和Object中的方法添加到代理方法中(proxyMethod)
    this.addProxyMethod(hashCodeMethod, Object.class);
    this.addProxyMethod(equalsMethod, Object.class);
    this.addProxyMethod(toStringMethod, Object.class);
    Class[] var1 = this.interfaces;
    int var2 = var1.length;

    int var3;
    Class var4;
   //获得接口中所有方法并添加到代理方法中
    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();
    //验证具有相同方法签名的方法的返回类型是否一致
    List var12;
    while(var11.hasNext()) {
        var12 = (List)var11.next();
        checkReturnTypes(var12);
    }

    //后面一系列的步骤用于写代理类Class文件
    Iterator var15;
    try {
         //生成代理类的构造函数
        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();
                //将代理类字段声明为Method，并且字段修饰符为 private static.
               //因为 10 是 ACC_PRIVATE和ACC_STATIC的与运算 故代理类的字段都是 private static Method ***
                this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, 
                               "Ljava/lang/reflect/Method;", 10));
               //生成代理类的方法
                this.methods.add(var16.generateMethod());
            }
        }
       //为代理类生成静态代码块对某些字段进行初始化
        this.methods.add(this.generateStaticInitializer());
    } catch (IOException var10) {
        throw new InternalError("unexpected I/O Exception", var10);
    }

    if(this.methods.size() > 'uffff') { //代理类中的方法数量超过65535就抛异常
        throw new IllegalArgumentException("method limit exceeded");
    } else if(this.fields.size() > 'uffff') {// 代理类中字段数量超过65535也抛异常
        throw new IllegalArgumentException("field limit exceeded");
    } else {
        // 后面是对文件进行处理的过程
        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);
        }
    }
}

其中addProxyMethod实现将接口与Object中一些方法添加到代理类中； 

8、addProxyMethod： 

private void addProxyMethod(Method var1, Class<?> var2) {
    String var3 = var1.getName();//获得方法名称
    Class[] var4 = var1.getParameterTypes();//获得方法参数类型
    Class var5 = var1.getReturnType();//获得方法返回类型
    Class[] var6 = var1.getExceptionTypes();//异常类型
    String var7 = var3 + getParameterDescriptors(var4);//获得方法签名
    Object var8 = (List)this.proxyMethods.get(var7);//根据方法前面获得proxyMethod的value
    if(var8 != null) {//处理多个代理接口中方法重复的情况
        Iterator var9 = ((List)var8).iterator();

        while(var9.hasNext()) {
            ProxyGenerator.ProxyMethod var10 = (ProxyGenerator.ProxyMethod)var9.next();
            if(var5 == var10.returnType) {
                ArrayList var11 = new ArrayList();
                collectCompatibleTypes(var6, var10.exceptionTypes, var11);
                collectCompatibleTypes(var10.exceptionTypes, var6, var11);
                var10.exceptionTypes = new Class[var11.size()];
                var10.exceptionTypes = (Class[])var11.toArray(var10.exceptionTypes);
                return;
            }
        }
    } else {
        var8 = new ArrayList(3);
        this.proxyMethods.put(var7, var8);
    }

    ((List)var8).add(new ProxyGenerator.ProxyMethod(var3, var4, var5, var6, var2, null));
}

JDK动态代理的原理是根据定义好的规则，用传入的接口创建一个新类，JDK的动态代理只能代理接口中的方法，是针对接口生成代理类。这就是为什么采用动态代理时为什么只能用接口引用指向代理，而不能用传入的类引用执行动态类。

四、参考资料：

https://www.jianshu.com/p/471c80a7e831

https://blog.csdn.net/u012834750/article/details/82499648

https://www.cnblogs.com/zhangchengzi/p/9713807.html