代理设计模式——JDK动态代理源码分析

1.代理类的生成机制

代理类通过Proxy类的newProxyInstance方法生成，那首先我们查看newProxyInstance的源码：

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.
     * 查找或生成指定的代理类
     */
    Class<?> cl = getProxyClass0(loader, intfs);

    /*
     * Invoke its constructor with the designated invocation handler.
     * 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);
        }

        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);
    }
}

通过方法中的注释我们可以知道getProxyClass0()来生成代理类的class对象，然后使用代理类的class对象进行实例化操作，实例化时将InvocationHandler对象作为参数传入newInstance方法中，方法中的其他代码主要是做了获取所有传入接口、获取安全管理器、权限校验等工作。这里我们主要关注class对象的生成过程，即getProxyClass0()方法。

/**
 * Generate a proxy class.  Must call the checkProxyAccess method
 * to perform permission checks before calling this.
 * 生成一个代理类。必须调用checkProxyAccess方法，在调用这个方法之前需要检查权限。
 */
private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) {
    //2^16-1=65535
    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
    // 如果实现给定接口的给定装载器定义的代理类存在，则返回缓存副本；
    // 否则通过 ProxyClassFactory创建代理类
    return proxyClassCache.get(loader, interfaces);
}

我们先看注释，ProxyClassFactory是生成代理类的关键。

private static final class ProxyClassFactory
    implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
    // prefix for all proxy class names
    // 所有代理类名称的前缀
    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序号的原子类
    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.
             * 验证类装入器是否将此接口的名称解析为相同的类对象。
             */
            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.
             * 验证类对象实际上表示一个接口。
             */
            if (!interfaceClass.isInterface()) {
                throw new IllegalArgumentException(
                    interfaceClass.getName() + " is not an interface");
            }
            /*
             * Verify that this interface is not a duplicate.
             * 验证此接口不是副本。
             */
            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.
         * 记录非公共代理接口的包，以便代理类将在同一个包中定义。验证所有非公共代理接口都在同一个包中。
         */
        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
            // 如果没有非公共代理接口，使用com.sun.proxy package 做前缀
            proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
        }

        /*
         * Choose a name for the proxy class to generate.
         * 选择要生成的代理类的名称。
         */
        long num = nextUniqueNumber.getAndIncrement();
        String proxyName = proxyPkg + proxyClassNamePrefix + num;

        /*
         * Generate the specified proxy class.
         * 生成指定的代理类。
         */
        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).
             * 这里的ClassFormatError表示(排除代理类生成代码中的bug)提供给
             * proxyclass创建的参数有其他一些无效方面(比如超出了虚拟机限制)。
             */
            throw new IllegalArgumentException(e.toString());
        }
    }
}

同样先查看注释，我们可以将该工厂类实现的功能进行梳理

• 1.提供代理类名称的前缀和用于标记唯一代理类的原子类序号
• 2.创建一个map，将所有的接口类传入，判断接口类是否可以由指定类进行加载、是否是一个接口、接口是否重复
• 3.生成类名
  • 3.1 验证所有非公有代理接口，进行权限判断、生成包名、截取包名、进行包名判断
  • 3.2 代理类生成的包路径位置
  • 3.3 整合成一个代理类的最终的命名规则 = 包名 + 前缀 + 序号
• 4.使用ProxyGenerator.generateProxyClass方法生成代理类

generateProxyClass方法真正进行class文件的代码生成。由于ProxyGenerator这个类在sun.misc这个包下，并没有直接提供源码可供参考，这里就直接概述一下class文件的生成过程

• 1.收集要生成的代理方法，组装成ProxyMethod对象。
  • 1.1 首先会为toString、hashCode、equals三个方法生成对象
  • 1.2 遍历所有接口，为每一个接口的每一个方法生成对象
• 2.收集所有生成类需要的字段信息和方法信息
• 3.组装成class文件

2.代理类生成机制总结

• 1.newProxyInstance方法中使用getProxyClass0()方法生成代理对象
• 2.在getProxyClass0()中创建ProxyClassFactory工厂用于生成代理对象
• 3.ProxyClassFactory工厂中调用ProxyGenerator.generateProxyClass方法生成代理类字节码文件，并返回代理类class对象
• 4.newProxyInstance方法中将该class对象通过newInstance(Object … initargs)传入InvocationHandler对象进行实例化。

3.代理类的具体内容

首先在本地生成代理类的class文件，然后通过jad工具进行反编译，在IDEA上可直接查看class文件的反编译内容

生成class文件：

public void proxyClassTest() throws IOException{
    IOrderSerivce orderService= (IOrderSerivce) new OrderServiceDynamicProxy(new OrderService()).getInstance();
    byte[] bytes= ProxyGenerator.generateProxyClass("$Proxy0",new Class[]{OrderService.class});
    FileOutputStream fos=new FileOutputStream(new File("$Proxy0.class"));
    fos.write(bytes);
    fos.close();
}

代理文件：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import proxy.datasource.Order;
import proxy.datasource.OrderService;

public final class $Proxy0 extends Proxy implements OrderService {
    private static Method m1;
    private static Method m8;
    private static Method m2;
    private static Method m3;
    private static Method m6;
    private static Method m5;
    private static Method m7;
    private static Method m9;
    private static Method m0;
    private static Method m4;

    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 notify() throws  {
        try {
            super.h.invoke(this, m8, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    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 createOrder(Order var1) throws  {
        try {
            return (Integer)super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var3) {
            throw var3;
        } catch (Throwable var4) {
            throw new UndeclaredThrowableException(var4);
        }
    }

    public final void wait(long var1) throws InterruptedException {
        try {
            super.h.invoke(this, m6, new Object[]{var1});
        } catch (RuntimeException | InterruptedException | Error var4) {
            throw var4;
        } catch (Throwable var5) {
            throw new UndeclaredThrowableException(var5);
        }
    }

    public final void wait(long var1, int var3) throws InterruptedException {
        try {
            super.h.invoke(this, m5, new Object[]{var1, var3});
        } catch (RuntimeException | InterruptedException | Error var5) {
            throw var5;
        } catch (Throwable var6) {
            throw new UndeclaredThrowableException(var6);
        }
    }

    public final Class getClass() throws  {
        try {
            return (Class)super.h.invoke(this, m7, (Object[])null);
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final void notifyAll() throws  {
        try {
            super.h.invoke(this, m9, (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);
        }
    }

    public final void wait() throws InterruptedException {
        try {
            super.h.invoke(this, m4, (Object[])null);
        } catch (RuntimeException | InterruptedException | 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"));
            m8 = Class.forName("proxy.datasource.OrderService").getMethod("notify");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m3 = Class.forName("proxy.datasource.OrderService").getMethod("createOrder", Class.forName("proxy.datasource.Order"));
            m6 = Class.forName("proxy.datasource.OrderService").getMethod("wait", Long.TYPE);
            m5 = Class.forName("proxy.datasource.OrderService").getMethod("wait", Long.TYPE, Integer.TYPE);
            m7 = Class.forName("proxy.datasource.OrderService").getMethod("getClass");
            m9 = Class.forName("proxy.datasource.OrderService").getMethod("notifyAll");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
            m4 = Class.forName("proxy.datasource.OrderService").getMethod("wait");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

可以看到代理类中的为每一个方法定义了一个公共的静态Method引用，并且在静态代码块中为每一个方法的引用进行赋值，赋值的方式如Class.forName(“java.lang.Object”).getMethod(“hashCode”);。然后定义了所有的方法，在方法的内部使用InvocationHandler对象的Invoke方法，将对应Method对象传入实现反射调用方法。