Java动态代理的原理

动态代理是使用非常广泛的技术，诸如大名鼎鼎的spring、mybatis等框架都大量使用动态代理。动态代理区别于静态代理的地方在于：静态代理需要为每一个被代理类都创建一个代理类，在编译期代理类就已经生成，而动态代理则是在程序运行期动态地生成代理类，所有被代理对象的执行逻辑都被转移到InvocationHandler对象的invoke方法，在invoke方法里通过反射来调用被代理的对象方法，当然也可以执行其它的逻辑。动态代理的一个显而易见的好处是：不用为每个被代理类都手动创建一个代理类。

按照惯例，先谈使用再讲原理。举个例子先。

场景：七夕临近，公司交友社团本着消灭单身狗的美好愿景，为闷骚的程序猿（媛）们开通了月老服务，任何有心仪目标但是又羞于开口的单身程序狗都可以委托“月老”来约心中的男神女神出来。

Suitor(追求者)接口

public interface Suitor {

    void sayHi(Beauty beauty);

}

Programmer(程序猿)

public class Programmer implements Suitor {

    @Override
    public void sayHi(Beauty beauty) {
        System.out.println("Hello, " + beauty.getName() + "! Would you like to have dinner with me tonight?");
    }

}

Beauty(MM)

public class Beauty {

    private String name;

    public Beauty(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

}

处理邀请MM动作的InvocationHandler

public class SuitorInvocationHandler implements InvocationHandler {

    Suitor suitor;

    public SuitorInvocationHandler(Suitor suitor) {
        this.suitor = suitor;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        return method.invoke(suitor, args);
    }

}

测试，创建代理对象并对MM发出邀请

public class DynamicProxyTest {

    @Test
    public void test() {
        Beauty beauty = new Beauty("MM");
        InvocationHandler handler = new SuitorInvocationHandler(new Programmer());
        Suitor suitor = (Suitor) Proxy.newProxyInstance(Suitor.class.getClassLoader(), new Class<?>[]{Suitor.class}, handler);
        suitor.sayHi(beauty);
    }

}

结果

Hello, MM! Would you like to have dinner with me tonight?

可见，代理对象成功地激发了被代理对象想执行的业务逻辑。

废话不多说，看看代理对象到底是如何生成的，业务逻辑又是如何转嫁到InvocationHandler上的。

Proxy.newProxyInstance中调用生成代理类的逻辑 Class<?> cl = getProxyClass0(loader, intfs);

@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();
        if (sm != null) {
            checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
        }

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

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

            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;
                    }
                });
            }
            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);
        }
    }

最终生成代理类的逻辑在Proxy.ProxyClassFactory.apply方法内

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) {
                }
                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
            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);
            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());
            }
        }

真正生成类字节码文件的逻辑在byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName, interfaces, accessFlags)。上面主要做的是接口校验、生成类名和确定代理类的访问修饰符等工作。那我们接着看看代理类具体是如何生成的吧。

private byte[] generateClassFile() {
        // 将Object类中的方法添加到代理类中
        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;
        // 获取接口中的所有方法并添加到代理类中
        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();

        List var12;
        // 验证具有相同方法签名的方法返回类型是否相同
        while(var11.hasNext()) {
            var12 = (List)var11.next();
            checkReturnTypes(var12);
        }

        Iterator var15;
        try {
            // 添加代理类的构造方法
            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();
                   // 代理类的字段都是Method类型，存储的待invoke的接口方法和从Object继承的方法
                    // 字段用private static修饰，10是ACC_PRIVATE和ACC_STATIC相与的结果
                    this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
                    // 生成代理类方法
                    this.methods.add(var16.generateMethod());
                }
            }
            // 生成静态代码块初始化私有静态字段
            this.methods.add(this.generateStaticInitializer());
        } catch (IOException var10) {
            throw new InternalError("unexpected I/O Exception", var10);
        }
        // 如果方法数量超过65535个则抛异常
        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);
            }
        }
    }

那么生成的代理类到底长什么样子呢？通过以下代码，将内存中的代理类字节数组写入文件。

    byte[] classFile = ProxyGenerator.generateProxyClass("$Proxy0", Programmer.class.getInterfaces());
        FileOutputStream fos = new FileOutputStream(new File("programmer.class"));
        try {
            fos.write(classFile);
            fos.flush();
            System.out.println("successfully generated proxy class file");
        } catch (Exception e) {
            System.out.println("failed to generate proxy class file");
        }

生成的类文件：

public final class $Proxy0 extends Proxy implements Suitor {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    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});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final void sayHi(Beauty var1) throws  {
        try {
            super.h.invoke(this, m3, new Object[]{var1});
        } 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 int hashCode() throws  {
        try {
            return (Integer)super.h.invoke(this, m0, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
            m3 = Class.forName("com.wxy.popcorn.test.proxy.dynamic.Suitor").getMethod("sayHi", Class.forName("com.wxy.popcorn.test.proxy.dynamic.Beauty"));
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

不难看出，代理类的私有静态字段维护了所有接口方法和从Object继承的方法。这些方法的调用最终都转移到InvocationHandler.invoke。

That's all！