Question

How could I “pause” a method in between an execution, and trigger it externally to continue the execution of this method?

The Thread is a servlet, and the executeAndWait method is started by a button click.
Now I want to do polling on a SessionScoped variable.
If this variable changes its state (eg to true), the polling/waiting method should continue.

I first though about Observer pattern. But with this I can just trigger the execution of methods. But I need to sleep/wait and trigger the continue of a method.

How else could this be done apart from polling to a “global” variable.?

class MyThread {

    @Inject
    MyBean bean;

    doExecuteAndWait() {
        //this waits for the var to turn into "true" state
        while(!bean.isVarValid()) {
            Thread.sleep(1000);
        }
        //continue execution if somevar is set to true externally
    }
}


class MySignal {
    @Inject
    MyBean bean;

    //this triggers the continuation of the method from class above
    toggleVar() {
        bean.setVarValid(true);
    }   
}

@SessionScoped
class MyBean() {
    varValid = false;

    isVarValid() {
        return varValid;
    }

    setVarValid(Boolean status) {
        this.varValid = status;
    }
}

Answer 1

You could use a CountDownLatch.

Initialize it to 1, call await() in MyThread, and then call the countDown() method from MySignal when it should be released.

This way you don’t need to keep polling the value of a variable.