jdk动态代理

使用代理

接口定义

public interface StudentService {
    List<StudentService> queryStudent(int id);
}

实现接口

public class StudentServiceImpl implements StudentService {
    @Override
    public List<StudentService> queryStudent(int id) {
        System.out.println("----queryStudent----");
        return new ArrayList<>();
    }
}

创建代理

public static void main( String[] args )
    {
        System.getProperties().put("jdk.proxy.ProxyGenerator.saveGeneratedFiles", "true");
        StudentService studentService = new StudentServiceImpl();
        StudentService proxyStudent = (StudentService) Proxy.newProxyInstance(studentService.getClass().getClassLoader(), studentService.getClass().getInterfaces(), new InvocationHandler() {
            @Override
            public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
                System.out.println("方法前置处理");
                Object obj = method.invoke(studentService, args);
                System.out.println("方法后置处理");
                return obj;

            }
        });
        proxyStudent.queryStudent(1);
    }

实现原理

生成代理类的字节码文件，在本项目目录下
System.getProperties().put(“jdk.proxy.ProxyGenerator.saveGeneratedFiles”, “true”);

内容如下：

public final class $Proxy0 extends Proxy implements StudentService {
    private static final Method m0;
    private static final Method m1;
    private static final Method m2;
    private static final Method m3;

    public $Proxy0(InvocationHandler var1) {
        super(var1);
    }

    public final int hashCode() {
        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 boolean equals(Object var1) {
        try {
            return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    public final String toString() {
        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 List queryStudent(int var1) {
        try {
            return (List)super.h.invoke(this, m3, new Object[]{var1});
        } catch (RuntimeException | Error var2) {
            throw var2;
        } catch (Throwable var3) {
            throw new UndeclaredThrowableException(var3);
        }
    }

    static {
        try {
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
            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.hoitung.fyz.service.StudentService").getMethod("queryStudent", Integer.TYPE);
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }

    private static MethodHandles.Lookup proxyClassLookup(MethodHandles.Lookup var0) throws IllegalAccessException {
        if (var0.lookupClass() == Proxy.class && var0.hasFullPrivilegeAccess()) {
            return MethodHandles.lookup();
        } else {
            throw new IllegalAccessException(var0.toString());
        }
    }
}

总结

1. Proxy.newProxyInstance方法用Proxy(h)构造器创建Proxy的子类，第3个参数InvocationHandler赋值给成员变量h。代理对象实现第2个接口中的所有方法，同时接口中的每个方法都会生成一个内部方法m1，m2, m3…，每个方法都通过反射映射到所在的类和方法。
2. 代理对象调用实现接口的方法时都是调用h的invoke方法。