Can i change a proxy to point at a differnt target at runtime ?

[sudheerk84]

I have a use case as follows

Entity ent = new Entity();

Entity has methods - setAAA(), setBBB(), ....


Service A - calls getEntity() - which returns entity object.

When service A calls getEntity().setAAA() - i want to intercept this method and return a new entity object.


So i am think of creating a proxy inside getEntity() - and return a proxy.

When i intercept setAAA() - method i want to craete new Entity object and point make teh proxy to pint at this new Entity().

Is this possible ?

I am planning to use Proxyfactory to create spring proxies using API.

In short - inside a proxy at runtime i want to cganhe its target object - ? Is this possible ?


Are there better ways to do this ?

[Grzegorz Grzybek]

Hello

Thanks to many implementations of TargetSource, you can do the following:

    ProxyFactory pf = new ProxyFactory();
    pf.setExposeProxy(true);
    pf.addInterface(MyService.class);
    pf.setTargetSource(new HotSwappableTargetSource(new MyServiceImpl()));
    pf.addAdvice(new MethodInterceptor()
    {
      @Override
      public Object invoke(MethodInvocation invocation) throws Throwable
      {
        ((HotSwappableTargetSource)((Advised)AopContext.currentProxy()).getTargetSource()).swap(new MyServiceImpl());
        return "test";
      }
    });
    MyService service = (MyService)pf.getProxy();

I hope this is what you need.

regards
Grzegorz Grzybek

[sudheerk84]

Amazing !!! . That was exactly what i was looking for.

Can i use this in my production. Is the HotSwappableTargetSource very expensive to use ?

Also one more confiormation - hope the previous target becomes eligible for garbage collection.

[Grzegorz Grzybek]

Hi

First - HotSwappableTargetSource is very lightweight - just a reference to the target.

Second - swap() returns previous target, so it will be GCed when there's no reference left.

Also - when you want to replace the target during particular method interception, you have to be careful if the target must be the original target while intercepting other methods. You can always write your own TargetSource, which will remember the first target.

regards
Grzegorz Grzybek

[sudheerk84]

Great. I have now implemented my own target source and everything seems fine.

One question i had in your example is ..

Is there any specific reason to create newAdvice() or new Advisor(). - [new MethodInterceptor() in your example]

Can i use this advisor/advice as a singleton and inject in various usecases . Is it thread safe enough ?

[sudheerk84]

i am getting an issue with one particular usecase

When i change the target at runtime , for that particular call , it doesnt change the target .

i have written my own TargetSource which which just replaces the old target with the new one.

When ever a setter method is called on this proxy , i just call the method on the targetSource to change the target.

But now if i perform any setting of a value , this value is set on the old target object and not on the new object.

Doesnt the change target work even for the current call ?

[I am using before advice where i am changing the target]

[sudheerk84]

One thing I figured out that since i was using before advice i could not control the current calling thread which has already stored the target.

So i tried implementing MethodInterceptor. Still for the current method invocation it uses the old target.


One way i see is to use a around advice and i explicitly call the method using the new target by reflection.

But is there a better way out ?

[Grzegorz Grzybek]

Hi

Can you put your code on the Github? I could see and try to help...

regards
Grzegorz Grzybek

[sudheerk84]

Thanks a lot for having a look.

I have not used github before , i will paste the code here - i have created a small use case for this

This is the output of teh program

Get called from AccountDAO
creating proxy now
accountcom.test.entity.account.Account@e45076
Before advice called

com.test.entity.account.Account@fd68b1 :old a value100
com.test.entity.account.Account@10382a9:new a value0


That is the setter value has been set on the old objectrather than teh new one.

This is my spring xml

    <bean id="accountDao" class="com.test.dao.account.AccountDAO">
		 <property name="advisor" ref="settersAdvisor" /> 
	</bean>

     <bean id="swapableAroundAdvice" class="com.test.framework.SwappableMethodInvoker"/>

	<bean id="settersAdvisor"
		class="org.springframework.aop.support.RegexpMethodPointcutAdvisor">
		<property name="advice">
			<!-- <ref local="swapableBeforeAdvice" /> -->
			<ref local="swapableAroundAdvice" /> 
		</property>
		<property name="patterns">
			<list>
				<value>com.test.entity.account.*.set.*</value>
				<value>com.test.entity.account.*.increment.*</value>
			</list>
		</property>
	</bean>

This is my account class

[code]

public class Account implements IAccount {

private boolean writable;

private int a;

private String b;

public Account(int a, String b) {
super();
this.a = a;
this.b = b;
}

public int getA() {
return a;
}

public void setA(int a) {
this.a = a;
}

public String getB() {
return b;
}

public void setB(String b) {
this.b = b;
}

public boolean isWritable() {
return writable;
}

public void setWritable(boolean writable) {
this.writable = writable;
}

public void incrementa() {
this.a = a + 1;
// return this.a;
}

@Override
public void asset() {
// TODO Auto-generated method stub

}

public Object clone() {
try {
return super.clone();
} catch (CloneNotSupportedException e) {
return null;
}
}


[code]


This is my account dao

public class AccountDAO {

    private Advisor advisor;

    public IAccount get(long id) {
        System.out.println("Get called from AccountDAO");
        IAccount a = new Account(0, "suji");
        ProxyFactory pf = new ProxyFactory();
        pf.setExposeProxy(true);
        pf.addInterface(IAccount.class);
        // pf.setTargetSource(new SwappableTargetSource(a));
        pf.setTargetSource(new HotSwappableTargetSource(a));
        pf.addAdvisor(advisor);
        System.out.println("creating proxy now");
        return (IAccount) pf.getProxy();
    }

    public void setAdvisor(Advisor advisor) {
        this.advisor = advisor;
    }

This is my swappable method invoker

public class SwappableMethodInvoker implements MethodInterceptor {

    @Override
    public Object invoke(MethodInvocation invocation) throws Throwable {
        // TODO Auto-generated method stub
        System.out.println("Before advice called");

        Account clonedobj = (Account) ((Account) invocation.getThis()).clone();

        ((HotSwappableTargetSource) ((Advised) AopContext.currentProxy())
                .getTargetSource()).swap(clonedobj);

      
  /* 

If i uncomment these lines everything works fine
return invocation.getMethod().invoke(clonedobj,
         invocation.getArguments());  */

        return invocation.proceed();

    }
}

An this is my main program

    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext(
                "classpath:META-INF/spring/aop-poc-module-context.xml");
        AccountDAO accountDao = (AccountDAO) ctx.getBean("accountDao");
        IAccount account = accountDao.get(1);
        System.out.println("account" + account);
        IAccount accountOld = (Account) ((HotSwappableTargetSource) ((Advised) account)
                .getTargetSource()).getTarget();
        account.setA(100);
        IAccount accountnew = (Account) ((HotSwappableTargetSource) ((Advised) account)
                .getTargetSource()).getTarget();
        System.out.println(accountOld + "  :old a value" + accountOld.getA());
        System.out.println(accountnew + ":new a value" + accountnew.getA());
    }

[Grzegorz Grzybek]

Hi

When you're in your interceptor the invocation object is already constructed and keeps a reference to the target obtained from target source. So changing the target in target source has no effect.

See org.springframework.aop.framework.JdkDynamicAopPro xy.invoke(Object, Method, Object[]) - the target is acquired from target source, ReflectiveMethodInvocation is constructed and eventually its "proceed()" method invoked. So using:

return invocation.getMethod().invoke(clonedobj,
         invocation.getArguments());

is the only option in your case. Of course this is not equivalent of calling "invocation.proceed" (see org.springframework.aop.framework.ReflectiveMethod Invocation.proceed() where there are other interceptors invoked).

TargetSource should be used to manipulate the target from the code external to proxy invocation.

Hope this helps.

regards
Grzegorz Grzybek