thread livelock doubt

Question

On site http://download-llnw.oracle.com/javase/tutorial/essential/concurrency/starvelive.html there is a concept of livelock where first is said that here the threads are not blocked and after is said that thread are blocked... "They're still blocking each other" so I don't understand well... if they are blocked it seems like deadlock...

Can anyone submit me a simple Java code of a livelock situation better if it is able to simulate the Alphonse and Gaston example reported on Java's site?

Answer 1

With a deadlock, there are two or more threads: each thread holds one lock, and needs to acquire another lock. Each thread is sleeping on the lock the other thread holds. Because they are asleep, they are 'dead' -- not executing any code, and they never will.

With a livelock, two or more threads are alive and executing code, but they cannot continue past conditions in their code because they are too busy responding to events from other threads in the livelock.

My Java is seriously rusty at this point, so let's go with a hopefully-simple description:

Two threads, A and B, need to acquire locks 1 and 2 in order to perform some operation (eating a sandwich):

Thread A tries this:

Acquire lock 1
if try_lock(2) == success
  eat the sandwich
else
  drop 1
  sleep a bit
  go back to start


Thread B tries this:

Acquire lock 2
if try_lock(1) == success
  eat the sandwich
else
  drop 2
  sleep a bit
  go back to start

If these two threads always alternate instructions, neither one will ever eat the sandwich: they will both spend all their time acquiring, testing, and dropping locks, but they never get to make forward progress.