Spring's dynamic proxy but was actually of type 'com.sun.proxy. $ Proxy14 Exception

Today in writing Springof introducing the agent when a reported error:

Exception in thread "main" org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'inter1' is expected to be of type 'com.dengchengchao.springtest.intertest.Inter1Impl' but was actually of type 'com.sun.proxy.$Proxy14'

Probably means that type conversion error.

Source code is as follows:

ApplicationContext  ctx = new AnnotationConfigApplicationContext(Conf.class);
Inter1 inter1 = ctx.getBean("inter1", Inter1Impl.class);

inter1.say1();

Inter2 inter2=(Inter2) inter1;
inter2.say2();

Later, googlea little way into the agent found the CGLIBline.

We all know the JDKonly agent interface, for a class of non-agent interfaces, it should be used CGLIB.

Because CGLIBa proxy class through inheritance to achieve, and JDKis achieved by interface.

But I am here Inter1clearly is an interface. Later, double-check code, in fact, found that the use of Javaa proxy is also OK, as long as the change the following line of code:

Inter1 inter1 = ctx.getBean("inter1", Inter1.class);

In other words, needs to be converted into type should be Inter1.classand can not be specific class Inter1Impl.

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Why Javaproxy only supports interface agent , here we come Grilled deep look:

First, we define an interface:

public interface People {
    void eat();
}

Then define an implementation class:

public class Student implements People{

    @Override
    public void eat() {
        System.out.println("用手吃");
    }
}

We then define a proxy class:

public class StudentInvokeHandler implements InvocationHandler {

    private Object target;

    public StudentInvokeHandler(Object target) {
        this.target = target;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable    {

        System.out.println("饭前洗手");
        Object retVal = method.invoke(target, args);
        System.out.println("饭后吃水果");
        return retVal;
    }
}

Next, through a proxy to callStudent

public static void main(String[] args) {
    //初始化Student
    Student student = new Student();
    //初始化Student代理类
    StudentInvokeHandler studentInvokeHandler = new StudentInvokeHandler(student);
    //通过代理获取代理独享
    People studentProxy = (People) Proxy.newProxyInstance(StudentInvokeHandler.class.getClassLoader(), new Class[]{People.class}, studentInvokeHandler);
    //通过代理对象调用eat方法
    studentProxy.eat();
}

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Can be seen, Javathe agency is very simple, but the bottom is how to achieve it?

Referring to elaborate JDK dynamic proxy implementation principle , we mainset about the JVMproperty

public static void main(String[] args) {
    //将生成的代理类文件保存
    System.getProperties().put("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
    Student student = new Student();
    StudentInvokeHandler studentInvokeHandler = new StudentInvokeHandler(student);
    People studentProxy = (People) Proxy.newProxyInstance(StudentInvokeHandler.class.getClassLoader(), new Class[]{People.class}, studentInvokeHandler);
    studentProxy.eat();
}

After the run, the project can be found in the root directory of the com/sun/proxy/$Proxy0.classfile, which is the proxy Studentobjects generated .classfiles:

public final class $Proxy0 extends Proxy implements People {
    private static Method m1;
    private static Method m3;
    private static Method m2;
    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 void eat() throws  {
        try {
            super.h.invoke(this, m3, (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 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"));
            m3 = Class.forName("com.dengchengchao.springtest.proxy.People").getMethod("eat");
            m2 = Class.forName("java.lang.Object").getMethod("toString");
            m0 = Class.forName("java.lang.Object").getMethod("hashCode");
        } catch (NoSuchMethodException var2) {
            throw new NoSuchMethodError(var2.getMessage());
        } catch (ClassNotFoundException var3) {
            throw new NoClassDefFoundError(var3.getMessage());
        }
    }
}

Through the above file we can find:

• Generated proxy class inherits Proxyrealized Peopleinterfaces

  This is why JDKthe proxy can only proxy interface, not proxy the specific class, because Javanot multiple inheritance, it can only implement the interface

• Because the interface is achieved, thus generating a proxy object forproxy

  proxy instanceof People  //true
  proxy instanceof Student //false

This is the beginning of the root cause of the problems we encountered, proxyjust to achieve the Peopleinterface is not inherited from Studentclass, so you can not proxyobject to a Studenttype, it was reported fault.

Understand the problem, the underlying future use as JDKa proxy class, it will not make another mistake.

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