《Java基础知识》动态代理（InvocationHandler）详解

/**
 * 创建用户接口
 */
public interface UserBean {
    String getUser();
}

import demo.knowledgepoints.section.inf.UserBean;

public class UserBeanImpl implements UserBean {

    private String user = null;

    //flag:0 无权限，1有权限。
    private String flag = null;

    public String getFlag() {
        return flag;
    }

    public void setFlag(String flag) {
        this.flag = flag;
    }

    public UserBeanImpl(String user,String flag)
    {
        this.user = user;
        this.flag = flag;
    }
    public String getUserName()
    {
        return user;
    }

    public String getUser()
    {
        System.out.println("this is getUser() method!");
        return user;
    }

    public void setUser(String user)
    {
        this.user = user;
        System.out.println("this is setUser() method!");
    }
}

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class UserBeanProxy implements InvocationHandler {

    private Object targetObject;

    public UserBeanProxy(Object targetObject)
    {
        this.targetObject = targetObject;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        UserBeanImpl userBean = (UserBeanImpl) targetObject;
        String flag = userBean.getFlag();
        Object result = null;

        //权限判断
        if("1".equals(flag) ){
            result = method.invoke(targetObject, args);
        }else{
            System.out.println("sorry , You don‘t have permission");
        }
        return result;
    }
}

import demo.knowledgepoints.section.inf.UserBean;

import java.lang.reflect.Proxy;

public class TestSection {
    public static void main(String[] args) {
        UserBeanImpl targetObject = new UserBeanImpl("蕾蕾","1");
        UserBeanProxy proxy = new UserBeanProxy(targetObject);
        //生成代理对象
        UserBean object = (UserBean) Proxy.newProxyInstance(targetObject.getClass().getClassLoader(),
                targetObject.getClass().getInterfaces(), proxy);

        String userName = object.getUser();
        System.out.println("userName: " + userName);
    }
}

UserBean object = (UserBean) Proxy.newProxyInstance(targetObject.getClass().getClassLoader(),
                targetObject.getClass().getInterfaces(), proxy);

public interface InvocationHandler {
    public Object invoke(Object proxy, Method method, Object[] args)
        throws Throwable;
}

public static Object newProxyInstance(ClassLoader loader,
                                      Class<?>[] interfaces,
                                      InvocationHandler h) throws IllegalArgumentException {
    //验证传入的InvocationHandler不能为空
    Objects.requireNonNull(h);
    //复制代理类实现的所有接口
    final Class<?>[] intfs = interfaces.clone();
    //获取安全管理器
    final SecurityManager sm = System.getSecurityManager();
    //进行一些权限检验
    if (sm != null) {
        checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
    }
    //该方法先从缓存获取代理类, 如果没有再去生成一个代理类
    Class<?> cl = getProxyClass0(loader, intfs);
    try {
        //进行一些权限检验
        if (sm != null) {
            checkNewProxyPermission(Reflection.getCallerClass(), cl);
        }
        //获取参数类型是InvocationHandler.class的代理类构造器
        final Constructor<?> cons = cl.getConstructor(constructorParams);
        final InvocationHandler ih = h;
        //如果代理类是不可访问的, 就使用特权将它的构造器设置为可访问
        if (!Modifier.isPublic(cl.getModifiers())) {
            AccessController.doPrivileged(new PrivilegedAction<Void>() {
                public Void run() {
                    cons.setAccessible(true);
                    return null;
                }
            });
        }
        //传入InvocationHandler实例去构造一个代理类的实例
        //所有代理类都继承自Proxy, 因此这里会调用Proxy的构造器将InvocationHandler引用传入
        return cons.newInstance(new Object[]{h});
    } catch (Exception e) {
        //统一用Exception捕获了所有异常
        throw new InternalError(e.toString(), e);
    }
}

private static Class<?> getProxyClass0(ClassLoader loader,
                                       Class<?>... interfaces) {
    //目标类实现的接口不能大于65535
    if (interfaces.length > 65535) {
        throw new IllegalArgumentException("interface limit exceeded");
    }
    //获取代理类使用了缓存机制
    return proxyClassCache.get(loader, interfaces);
}

//代理类生成工厂
private static final class ProxyClassFactory 
                implements BiFunction<ClassLoader, Class<?>[], Class<?>> {
    //代理类名称前缀
    private static final String proxyClassNamePrefix = "$Proxy";
    //用原子类来生成代理类的序号, 以此来确定唯一的代理类
    private static final AtomicLong nextUniqueNumber = new AtomicLong();
    @Override
    public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
        Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
        for (Class<?> intf : interfaces) {
            //这里遍历interfaces数组进行验证, 主要做三件事情
            //1.intf是否可以由指定的类加载进行加载
            //2.intf是否是一个接口
            //3.intf在数组中是否有重复
        }
        //生成代理类的包名
        String proxyPkg = null;
        //生成代理类的访问标志, 默认是public final的
        int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
        for (Class<?> intf : interfaces) {
            //获取接口的访问标志
            int flags = intf.getModifiers();
            //如果接口的访问标志不是public, 那么生成代理类的包名和接口包名相同
            if (!Modifier.isPublic(flags)) {
                //生成的代理类的访问标志设置为final
                accessFlags = Modifier.FINAL;
                //获取接口全限定名, 例如：java.util.Collection
                String name = intf.getName();
                int n = name.lastIndexOf(‘.‘);
                //剪裁后得到包名:java.util
                String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
                //生成的代理类的包名和接口包名是一样的
                if (proxyPkg == null) {
                    proxyPkg = pkg;
                } else if (!pkg.equals(proxyPkg)) {
                    //代理类如果实现不同包的接口, 并且接口都不是public的, 那么就会在这里报错
                    throw new IllegalArgumentException(
                        "non-public interfaces from different packages");
                }
            }
        }
        //如果接口访问标志都是public的话, 那生成的代理类都放到默认的包下：com.sun.proxy
        if (proxyPkg == null) {
            proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
        }
        //生成代理类的序号
        long num = nextUniqueNumber.getAndIncrement();
        //生成代理类的全限定名, 包名+前缀+序号, 例如：com.sun.proxy.$Proxy0
        String proxyName = proxyPkg + proxyClassNamePrefix + num;
        //这里是核心, 用ProxyGenerator来生成字节码, 该类放在sun.misc包下
        byte[] proxyClassFile = ProxyGenerator.generateProxyClass(proxyName,
                                  interfaces, accessFlags);
        try {
            //根据二进制文件生成相应的Class实例
            return defineClass0(loader, proxyName, proxyClassFile, 
                              0, proxyClassFile.length);
        } catch (ClassFormatError e) {
            throw new IllegalArgumentException(e.toString());
        }
    }
}