How to use @Async correctly in Spring

Question

I have a question about Spring's @Async annotation and how to correctly use it. Let's say I have these methods:

@Async
public void test(String param1) {
    test2(param1, null);
}

@Async
public void test2(String param1, String param2) {
    test3(param1, param2, null);
}

@Async
public void test3(String param1, String param2, String param3) {
    // do some heavy work
}

Do I need to have @Async on all three methods for it to be called asynchronously or is it enough only to have it on test3 that will actually do the work?

Answer 1

You do need it only at one method. Just because, after a new thread is started with the first @Async-method, is is asynchronous to the invoking method.

But what this mean for you, is highly depending on what your example should illustrate:

1) So in your case, an @Async for test1(String param1) is enough when you ever invoke test2 and test3 through test1.

@Async
public void test1(String param1) {
    test2(param1, null);
}

private void test2(String param1, String param2) {
    test3(param1, param2, null);
}

private void test3(String param1, String param2, String param3) {
    // do something
}

note that the methods 2 and 3 are private

---

2) But if your example is for illustrating a method overloading pattern for default parameters (Method chaining), then it is more complicate. Then you would need to have the @Async annotation at the method that do the real stuff. Because you just want to execute the real execution in an async-way, but not one async invocation for each chaining step.

public void test(String param1) {
    test(param1, null);
}


public void test(String param1, String param2) {
    //this invocation runs async only when real AspectJ is used
    test(param1, param2, null);
}

@Async
public void test(String param1, String param2, String param3) {
    // do something
}

note that all methods are named just test (method chaining pattern)

The problem with this is, that Spring (without real AspectJ load- or compile-time weaving) will not run a method async if it is invoked via this! (see https://stackoverflow.com/a/22561903/280244)