面试的时候,常常面试官会问spring 的aop底层原理,而aop的底层原理就是动态代理,每个JAVA程序员基本都能耳熟能详,对jdk动态代理和cglib动态代理的区别也了然于胸,jdk动态代理只能基于接口,而cglib并不需要。
那么jdk的动态代理为什么要基于接口呢?
下面我们先实现一个简单的jdk动态代理
接口类
public interface Person {
void say();
}
实现类
public class Man implements Person{
@Override
public void say() {
System.out.println("man");
}
}
代理类实现InvocationHandler接口,并重写invoke方法
public class JDKDynamicProxy implements InvocationHandler {
private Object target;
public JDKDynamicProxy(Object target) {
this.target = target;
}
public <T> T getProxy() {
return (T) Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("Do something before");
Object result = method.invoke(target, args);
System.out.println("Do something after");
return result;
}
}
调用一下
public class Test {
public static void main(String[] args) {
Person person = new JDKDynamicProxy(new Man()).getProxy();
person.say();
}
}
调用结果如下
Do something before
man
Do something after
说明动态代理生效了。
那么问题来了,jdk动态代理要求代理类实现InvocationHandler接口,重写invoke方法就可以实现动态代理,且构造函数注入Object即可,并无要求Object必须基于接口,而且重本质上说getProxy()获代理类(Object的原类),完全可以通过继承对象的方式。
因此再定义一个WonMan.class,不实现接口,在来测试一下
public class Woman {
public void say(){
System.out.println("woman");
}
}
调用接口
Woman woman = new JDKDynamicProxy(new Woman()).getProxy();
woman.say();
结果如下
Exception in thread "main" java.lang.ClassCastException: com.sun.proxy.$Proxy0 cannot be cast to com.ganhuojun.demo.proxy.Woman
at com.ganhuojun.demo.proxy.Test.main(Test.java:8)
说明,确实不支持没有接口的object动态代理。
接下来我们来分析下源码,探究下为什么只支持接口。因为动态代理的核心是获取代理类,我们下面方法开始看
Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
由于源码很多,只说简单说下核心部分,源码如下,红色部分是获取代理类
再看getProxyClass0方法,为了方便理解,将核心部分抽出来,如下
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// If the proxy class defined by the given loader implementing
// the given interfaces exists, this will simply return the cached copy;
// otherwise, it will create the proxy class via the ProxyClassFactory
return proxyClassCache.get(loader, interfaces);
}
proxyClassCache是WeakCache,再看get方法,一步一步往下看,最终会看到调用到ProxyClassFactory类的apply()方法,如下
此处发现红色部分是生成真正的代理类的方法,继续进去看一下,发现只需要将saveGeneratedFiles设置true,即可本地查看生成的代理类,如下
saveGeneratedFiles的值如下,
private static final boolean saveGeneratedFiles = (Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"));
此时我们将测试代码改成如下样子
System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
Person person = new JDKDynamicProxy(new Man()).getProxy();
person.say();
执行后发现本地多了一个代理类,如下图
打开看一下,源码如下,发现$Proxy0本身就继承了一个Proxy类,由于java不支持多继承,因此无法实现动态代理
public final class $Proxy0 extends Proxy implements Person {
private static Method m1;
private static Method m2;
private static Method m3;
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 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 void say() throws {
try {
super.h.invoke(this, m3, (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"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("com.ganhuojun.demo.proxy.Person").getMethod("say");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
结论:
生成的代理类继承了Proxy,由于java是单继承,所以只能实现接口,通过接口实现