透过字节码分析Java动态代理机制。

Posted 2022-03-02 linlf03

一、创建动态代理代码

1、创建接口

public interface Subject 
    void request();

　　

2、创建接口实现类

public class RealSubject  implements  Subject

    @Override
    public void request() 
        System.out.println("From real subject");
    

　　

3、创建DynamicSubject类

public class DynamicSubject implements InvocationHandler 

    private  Object sub;

    public  DynamicSubject(Object obj)
        this.sub = obj;
    

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable 
        System.out.println("before calling:" + method);
        method.invoke(this.sub, args);
        System.out.println("after calling:" + method);
        return null;
    

　　

4、创建测试类

public class Client 
    public static void main(String[] args) 

        System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");

        RealSubject rs = new RealSubject();
        InvocationHandler ds = new DynamicSubject(rs);

        Class<?> cls = rs.getClass();
        Subject subject = (Subject) Proxy.newProxyInstance(cls.getClassLoader(), cls.getInterfaces(), ds);
        subject.request();

        System.out.println(subject.getClass());
        System.out.println(subject.getClass().getSuperclass());
    

　　输出结果：

before calling:public abstract void com.example.jvm.bytecode.Subject.request()
From real subject
after calling:public abstract void com.example.jvm.bytecode.Subject.request()
class com.sun.proxy.$Proxy0
class java.lang.reflect.Proxy

　　

查看源码，动态代理类在ProxyGenerator类的generateProxyClass生成。

   public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) 
        ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
        final byte[] var4 = var3.generateClassFile();
        if(saveGeneratedFiles) 
            AccessController.doPrivileged(new PrivilegedAction<Void>() 
                public Void run() 
                    try 
                        int var1 = var0.lastIndexOf(46);
                        Path var2;
                        if(var1 > 0) 
                            Path var3 = Paths.get(var0.substring(0, var1).replace(‘.‘, File.separatorChar), new String[0]);
                            Files.createDirectories(var3, new FileAttribute[0]);
                            var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
                         else 
                            var2 = Paths.get(var0 + ".class", new String[0]);
                        

                        Files.write(var2, var4, new OpenOption[0]);
                        return null;
                     catch (IOException var4x) 
                        throw new InternalError("I/O exception saving generated file: " + var4x);
                    
                
            );
        

        return var4;
    

　　可以发现saveGeneratedFiles为true时，可以将代理类保存到磁盘上。

因此在测试类中sun.misc.ProxyGenerator.saveGeneratedFiles属性设置为true

System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles","true");


最终生成的.class 文件位置如下图