Hang occurring when Thread.getAllStackTraces() is called

Question

I'm running the below code to display the running threads of my program

public void printTasks() {
    System.out.println("get threads start");
    for (Thread thread : Thread.getAllStackTraces().keySet()) {
        //This block filters the non-java threads running
        if (!thread.getThreadGroup().getName().equals("system")) {
            System.out.println("hello");
        }
    }
    System.out.println("get threads end");
}

the problem is that sometimes the the code hangs just after print "get threads start", I suspicious the hang happens in this line "Thread.getAllStackTraces()"

Note: My program performs a set of tasks using threads, due to this, it creates about ~70 threads and the hangs is intermittent, only 1 of every 6 or 7 times that I call this method the issue shows up

my questions are:

• Is this a known issue?
• Is there a way to prevent this behavior?
• Is there a safer method to list the running threads?

Edit: I'm using java 1.8, the issue happens in Linux OEL and Windows Server, in both cases it is intermittent, the software is run as standalone desktop application

Thank you in advance

Answer 1

I just found out what the problem was (at least this is my hypothesis).

The code used to retrieve the running threads was experiencing a race condition.

The threads that my program was creating are constantly changing, some start others end in very short periods of time (1 second or less)

the function Thread.getAllStackTraces() returns a HashMap of thread and stack trace (line a) and then in the next line (line b) I was trying to get the thread's group name

 for (Thread thread : Thread.getAllStackTraces().keySet()) { <--- a
     if (!thread.getThreadGroup().getName().equals("system")) { <--- b

but the thread lasted so little that it disappeared before the second line was reached and thus I ended up trying to get a value from the map using an invalid key (this was the race condition)

Note : in case that you never had experienced this before, when you try to read the value of a HashMap that doesn't exist you might end up waiting forever for the result

Solution

Ensure the thread still exists before trying to read its attributes

public void printTasks() {
    System.out.println("get threads start");
    for (Thread thread : Thread.getAllStackTraces().keySet()) {
        //This block filters the non-java threads running
        if (Thread.getAllStackTraces().containsKey(thread) && //<--new line here
            thread.getThreadGroup().getName().equals("system")) {
            System.out.println("hello");
        }
    }
    System.out.println("get threads end");
}

A second approach would be to try to get an snapshot of the content of getAllStackTraces() to read immutable objects

A better solution is welcome, hope this helps someone