Good example of livelock?

Question

I understand what livelock is, but I was wondering if anyone had a good code-based example of it? And by code-based, I do not mean \"two people trying to get p

Answer 1

As there is no answer marked as accepted answer, I have attempted to create live lock example;

Original program was written by me in Apr 2012 to learn various concept of multithreading. This time I have modified it to create deadlock, race condition, livelock etc.

So let's understand the problem statement first;

Cookie Maker Problem

There are some ingredient containers: ChocoPowederContainer, WheatPowderContainer. CookieMaker takes some amount of powder from ingredient containers to bake a Cookie. If a cookie maker finds a container empty it checks for another container to save time. And waits until Filler fills the required container. There is a Filler who checks container on regular interval and fills some quantity if a container needs it.

Please check the complete code on github;

Let me explain you implementation in brief.

• I start Filler as daemon thread. So it'll keep filling containers on regular interval. To fill a container first it locks the container -> check if it needs some powder -> fills it -> signal all makers who are waiting for it -> unlock container.
• I create CookieMaker and set that it can bake up to 8 cookies in parallel. And I start 8 threads to bake cookies.
• Each maker thread creates 2 callable sub-thread to take powder from containers.
• sub-thread takes a lock on a container and check if it has enough powder. If not, wait for some time. Once Filler fills the container, it takes the powder, and unlock the container.
• Now it completes other activities like: making mixture and baking etc.

Let's have a look in the code:

CookieMaker.java

private Integer getMaterial(final Ingredient ingredient) throws Exception{
        :
        container.lock();
        while (!container.getIngredient(quantity)) {
            container.empty.await(1000, TimeUnit.MILLISECONDS);
            //Thread.sleep(500); //For deadlock
        }
        container.unlock();
        :
}

IngredientContainer.java

public boolean getIngredient(int n) throws Exception {
    :
    lock();
    if (quantityHeld >= n) {
        TimeUnit.SECONDS.sleep(2);
        quantityHeld -= n;
        unlock();
        return true;
    }
    unlock();
    return false;
}

Everything runs fine until Filler is filling the containers. But if I forget to start the filler, or filler goes on unexpected leave, sub-threads keep changing their states to allow other maker to go and check the container.

I have also create a daemon ThreadTracer which keeps watch on thread states and deadlocks. This the output from console;

2016-09-12 21:31:45.065 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:RUNNABLE, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:TIMED_WAITING]
2016-09-12 21:31:45.065 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:TIMED_WAITING, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:TIMED_WAITING]
WheatPowder Container has 0 only.
2016-09-12 21:31:45.082 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:TIMED_WAITING, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:RUNNABLE]
2016-09-12 21:31:45.082 :: [Maker_0:WAITING, Maker_1:WAITING, Maker_2:WAITING, Maker_3:WAITING, Maker_4:WAITING, Maker_5:WAITING, Maker_6:WAITING, Maker_7:WAITING, pool-7-thread-1:TIMED_WAITING, pool-7-thread-2:TIMED_WAITING, pool-8-thread-1:TIMED_WAITING, pool-8-thread-2:TIMED_WAITING, pool-6-thread-1:TIMED_WAITING, pool-6-thread-2:TIMED_WAITING, pool-5-thread-1:TIMED_WAITING, pool-5-thread-2:TIMED_WAITING, pool-1-thread-1:TIMED_WAITING, pool-3-thread-1:TIMED_WAITING, pool-2-thread-1:TIMED_WAITING, pool-1-thread-2:TIMED_WAITING, pool-4-thread-1:TIMED_WAITING, pool-4-thread-2:TIMED_WAITING, pool-3-thread-2:TIMED_WAITING, pool-2-thread-2:TIMED_WAITING]

You'll notice that sub-threads and changing their states and waiting.