Be aware that FindBugs can only detect some problematic situations (related or not to concurrency code). For example, if you delete the unlock() call in the run() method of the Task class and repeat the analysis, FindBugs won't alert you that
you will get the lock in the task but you will never be able to free it.

Use the tools of the static code analysis as a form of assistance to improve the quality of your code, but do not expect it to detect all the bugs.