Java動態代理Proxy.newProxyInstance原始碼到底干了什么？【2021-8-31】

動態代理

用一個簡單的例子來描述動態代理，你想租房子，一般的話，你需要四處找房子，很辛苦，你想在家躺著交了錢就行，所以你找了個代理（中介），代理去找好了房子，和房東商量，你過來看下房子簽合同交錢就好了，這就是代理的作用，（找中介需謹慎，中介不同于代碼，代碼不會騙人）
實際代碼中，當你有一個已有的方法，你在不希望修改它的前提下想要擴展它的功能，即可使用動態代理，典型案例就是Spring AOP，

例子代碼

/**
 * @author ddd
 * @create 2021-06-25    19:21
 * #desc 動態代理需要一個行為介面
 **/
public interface action {
    public void helloworld();
}

/**
 * @author ddd
 * @create 2021-06-25    
 * #desc 代理類 
 * **/
public class agent implements InvocationHandler {

    private  action action;

    public agent(action action){
        this.action=action;
    }
    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        System.out.println(" 辛苦找房！ " );
        Object getmethod=method.invoke(action,args);

        return getmethod;
    }
}

/**
 * @author ddd
 * @create 2021-06-25   
 * @desc 顧客
 **/
public class Custom implements action{

    @Override
    public void helloworld() {
        System.out.println(" 看房，簽合同，交錢！ ");
    }
}

/**
 * @author ddd
 * @create 2021-06-25   
 * @desc 運行類
 **/
public class Main {
    public static void main(String[] args) {
        System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
        action action=new Custom();
        InvocationHandler handlerProxy=new agent(action);
        /*
        這一步會在記憶體中生成動態代理類
         */
        action actionInstance = (My.DesignPattern.Agent.Jdk.action) Proxy.newProxyInstance(handlerProxy.getClass().getClassLoader(),
                action.getClass().getInterfaces(),
                handlerProxy);
        actionInstance.helloworld();

    }
}

Proxy.newProxyInstance()

大家都說動態代理很重要，用起來也很方便，被代理類實作一個行為介面，代理類實作InvocationHandler 介面，呼叫Proxy.newProxyInstance()即可生成一個代理類，那到底是怎么生成的代理類？我們進下原始碼Proxy.newProxyInstance()

public static Object newProxyInstance(ClassLoader loader,
                                          Class<?>[] interfaces,
                                          InvocationHandler h)
            throws IllegalArgumentException
    {
        /**
         * 傳入的代理類非空
         */
        Objects.requireNonNull(h);

        // 使用克隆的方式拿到介面,新知識，什么是克隆？
        final Class<?>[] intfs = interfaces.clone();
        // 系統安全性檢查
        final SecurityManager sm = System.getSecurityManager();
        if (sm != null) {
            // check  Proxy  Access 看名字就可以了解，檢查是否可以代理
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

        /*
         * Look up or generate the designated proxy class.
         *
         * 上面英文是原始碼注釋
         * 查找或者生成一個代理
         * 哎！有意思，查找，或者，生成，說明有兩種方式
         */
        Class<?> cl = getProxyClass0(loader, intfs);

        /*
         * Invoke its constructor with the designated invocation handler.
         *
         * 原始碼注釋：使用構造方法生成代理類
         */
        try {
            if (sm != null) {
                checkNewProxyPermission(Reflection.getCallerClass(), cl);
            }
            /**
             * 使用構造方法生成代理類
             */
            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;
                    }
                });
            }
            /**
             * 回傳new的物件
             */
            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);
        }
    }

getProxyClass0(loader, intfs)

上面有一行代碼的注釋是查找或者生成一個代理類，我們來這個方法的原始碼

    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
        // 又封裝一層，接著進入proxyClassCache.get
        return proxyClassCache.get(loader, interfaces);
    }

proxyClassCache.get(loader, interfaces)

    public V get(K key, P parameter) {
        Objects.requireNonNull(parameter);

        expungeStaleEntries();
        /**
         * 從快取中查找，咦，這就是查找一個代理類，但是我們還沒有生成過
         * 那在后續肯定有生成代理類放入快取的操作，
         * 利用快取的話再次訪問的速度就會很快
         *
         * key是什么，handlerProxy.getClass().getClassLoader()
         */
        Object cacheKey = CacheKey.valueOf(key, refQueue);

        // lazily install the 2nd level valuesMap for the particular cacheKey
        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
        Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
        /**
         * Supplier生產者介面，函式型介面，有必要去了解一下
         * 這里用于生產代理類
         */
        Supplier<V> supplier = valuesMap.get(subKey);
        Factory factory = null;

        while (true) {
            if (supplier != null) {
                // supplier might be a Factory or a CacheValue<V> instance
                /**
                 * 非空即可獲得代理類
                 * 第一次進入valuesMap是空的，map.get()獲取不到
                 * 所以回圈條件是while（ture）
                 * 后面會將一個Factory賦值給supplier，然后通過factory.get()來獲取
                 */
                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
                    /**
                     * 如果為空，就賦值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);
                }
            }
        }
    }

Factory:factory.get()

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
            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 {
                 /**
                 * 關鍵代碼只在這一句，valueFactory.apply(key, parameter)非空即可
                 *  再去看valueFactory.apply(key, parameter)
                 */
                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);

            // 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;
        }
    }

Proxy:ProxyClassFactory:apply()

private static final class ProxyClassFactory
            implements BiFunction<ClassLoader, Class<?>[], Class<?>>
    {
        // prefix for all proxy class names
        private static final String proxyClassNamePrefix = "$Proxy";

        // next number to use for generation of unique proxy class names
        private static final AtomicLong nextUniqueNumber = new AtomicLong();

        @Override
        public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
            /**
             * 獲取所有介面資訊并便利
             */
            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 {
                    /**
                     * Class.forName作用是要求JVM查找并加載指定的類,相當于將介面實體化
                     * ClassLoader loader是我們一開始傳入的自己類的加載器
                     */
                    interfaceClass = Class.forName(intf.getName(), false, loader);
                } catch (ClassNotFoundException e) {
                }
                /**
                 * 判斷當前類的實體化是否與我們創建的類是同一個加載器生成的
                 */
                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());
                }
            }

            String proxyPkg = null;     // package to define proxy class in
                                        // 代理類放在哪個路徑下
            // 介面的權限
            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) {
                /**
                 * 獲取介面的權限
                 */
                int flags = intf.getModifiers();
                /**
                 * 如果是public，則放在同一路徑下
                 */
                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);
            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());
            }
        }
    }

生成的代理類

以上程序就生成了代理類，那最終我們生成的代理類在哪呢？
在開頭的例子中，我們在Main類中加了這么一行代碼，它可以將我們生成的代理類保存到本地

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

運行之后就可以看到

代理類的helloworld()

我們來看代理類的helloworld方法

public final class $Proxy0 extends Proxy implements action{
	---
	/*
	而我們一開始在最初的main方法中，
	 InvocationHandler handlerProxy=new agent(action);
     action actionInstance = (My.DesignPattern.Agent.Jdk.action)Proxy.newProxyInstance(handlerProxy.getClass().getClassLoader(),
                action.getClass().getInterfaces(),
                handlerProxy);
	又通過$Proxy0構造方法傳了new agent(action)，是InvocationHandler 的實作類
	所以Proxy 類中的InvocationHandl就是我們寫的代理類
	*/
	public $Proxy0(InvocationHandler var1) throws  {
        super(var1);
    }
public final void helloworld() throws  {
        try {
         /**
         * 呼叫了父類的h變數的invoke，即agent類中的invoke
         */
            super.h.invoke(this, m3, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }
    ---
    }

總結

1、動態代理基于反射，可以在程式運行時得到一個類的全部資訊，
2、反射基于JVM的類加載機制和JVM記憶體模型中類，介面模板的存放，
3、動態代理通過以上技術在運行時動態生成一個代理物件，
4、為什么說是動態，因為agent傳入的引數是action介面，可以在運行時傳入任何實作了action介面的類來完成代理，使用更加靈活，
5、此文僅個人理解，并且因為反射涉及JVM知識，內容過多，未進一步深入，