[java] Spring - @Transactional - What happens in background?

I want to know what actually happens when you annotate a method with @Transactional? Of course, I know that Spring will wrap that method in a Transaction.

But, I have the following doubts:

1. I heard that Spring creates a proxy class? Can someone explain this in more depth. What actually resides in that proxy class? What happens to the actual class? And how can I see Spring's created proxied class
2. I also read in Spring docs that:

Note: Since this mechanism is based on proxies, only 'external' method calls coming in through the proxy will be intercepted. This means that 'self-invocation', i.e. a method within the target object calling some other method of the target object, won't lead to an actual transaction at runtime even if the invoked method is marked with @Transactional!

Source: http://static.springsource.org/spring/docs/2.0.x/reference/transaction.html

Why only external method calls will be under Transaction and not the self-invocation methods?

It may be late but I came across something which explains your concern related to proxy (only 'external' method calls coming in through the proxy will be intercepted) nicely.

For example, you have a class that looks like this

@Component("mySubordinate")
public class CoreBusinessSubordinate {

    public void doSomethingBig() {
        System.out.println("I did something small");
    }

    public void doSomethingSmall(int x){
        System.out.println("I also do something small but with an int");    
  }
}

and you have an aspect, that looks like this:

@Component
@Aspect
public class CrossCuttingConcern {

    @Before("execution(* com.intertech.CoreBusinessSubordinate.*(..))")
    public void doCrossCutStuff(){
        System.out.println("Doing the cross cutting concern now");
    }
}

When you execute it like this:

 @Service
public class CoreBusinessKickOff {

    @Autowired
    CoreBusinessSubordinate subordinate;

    // getter/setters

    public void kickOff() {
       System.out.println("I do something big");
       subordinate.doSomethingBig();
       subordinate.doSomethingSmall(4);
   }

}

Results of calling kickOff above given code above.

I do something big
Doing the cross cutting concern now
I did something small
Doing the cross cutting concern now
I also do something small but with an int

but when you change your code to

@Component("mySubordinate")
public class CoreBusinessSubordinate {

    public void doSomethingBig() {
        System.out.println("I did something small");
        doSomethingSmall(4);
    }

    public void doSomethingSmall(int x){
       System.out.println("I also do something small but with an int");    
   }
}


public void kickOff() {
  System.out.println("I do something big");
   subordinate.doSomethingBig();
   //subordinate.doSomethingSmall(4);
}

You see, the method internally calls another method so it won't be intercepted and the output would look like this:

I do something big
Doing the cross cutting concern now
I did something small
I also do something small but with an int

You can by-pass this by doing that

public void doSomethingBig() {
    System.out.println("I did something small");
    //doSomethingSmall(4);
    ((CoreBusinessSubordinate) AopContext.currentProxy()).doSomethingSmall(4);
}

Code snippets taken from: https://www.intertech.com/Blog/secrets-of-the-spring-aop-proxy/

When Spring loads your bean definitions, and has been configured to look for @Transactional annotations, it will create these proxy objects around your actual bean. These proxy objects are instances of classes that are auto-generated at runtime. The default behaviour of these proxy objects when a method is invoked is just to invoke the same method on the "target" bean (i.e. your bean).

However, the proxies can also be supplied with interceptors, and when present these interceptors will be invoked by the proxy before it invokes your target bean's method. For target beans annotated with @Transactional, Spring will create a TransactionInterceptor, and pass it to the generated proxy object. So when you call the method from client code, you're calling the method on the proxy object, which first invokes the TransactionInterceptor (which begins a transaction), which in turn invokes the method on your target bean. When the invocation finishes, the TransactionInterceptor commits/rolls back the transaction. It's transparent to the client code.

As for the "external method" thing, if your bean invokes one of its own methods, then it will not be doing so via the proxy. Remember, Spring wraps your bean in the proxy, your bean has no knowledge of it. Only calls from "outside" your bean go through the proxy.

Does that help?

As a visual person, I like to weigh in with a sequence diagram of the proxy pattern. If you don't know how to read the arrows, I read the first one like this: Client executes Proxy.method().

1. The client calls a method on the target from his perspective, and is silently intercepted by the proxy
2. If a before aspect is defined, the proxy will execute it
3. Then, the actual method (target) is executed
4. After-returning and after-throwing are optional aspects that are executed after the method returns and/or if the method throws an exception
5. After that, the proxy executes the after aspect (if defined)
6. Finally the proxy returns to the calling client

(I was allowed to post the photo on condition that I mentioned its origins. Author: Noel Vaes, website: www.noelvaes.eu)

The simplest answer is:

On whichever method you declare @Transactional the boundary of transaction starts and boundary ends when method completes.

If you are using JPA call then all commits are with in this transaction boundary.

Lets say you are saving entity1, entity2 and entity3. Now while saving entity3 an exception occur, then as enitiy1 and entity2 comes in same transaction so entity1 and entity2 will be rollback with entity3.

Transaction :

1. entity1.save
2. entity2.save
3. entity3.save

Any exception will result in rollback of all JPA transactions with DB.Internally JPA transaction are used by Spring.

All existing answers are correct, but I feel cannot give just this complex topic.

For a comprehensive, practical explanation you might want to have a look at this Spring @Transactional In-Depth guide, which tries its best to cover transaction management in ~4000 simple words, with a lot of code examples.