Difference Between BLOCKED, WAITING, And TIMED_WAITING? - explained through real-life examples

BLOCKED, WAITING, and TIMED_WAITING are important thread states, but often confusing to many of us. One must have a proper understanding of both in order to analyze thread dumps. Using real-life examples, this article breaks down each state into simpler terms.

Any confusing concepts can be easily understood through examples rather than formal definitions given in Java doc. If they are real-life examples, it can be even more relatable. I would like to share some real-life examples which might help to understand these thread states.

任何困扰的概念可以被容易的理解通过例子，而非java文档中的正式定义。如果是真实生活中的例子，它也许会更有联系。

 

Fig: Transitive Graph 传递图 generated by http://fastthread.io/ showing which threads are blocking what threads

BLOCKED

Java doc formally defines BLOCKED state as: “A thread that is blocked waiting for a monitor lock is in this state.”

一个线程被阻塞是指线程正等待一个监控锁的状态。

Real-life example: Today you are going for a job interview. This is your dream job, which you have been targeting for last few years. You woke up early in the morning, got ready, put on your best outfit, looking sharp in front of the mirror. Now you step out to your garage and realize that your wife has already taken the car. In this scenario, you only have one car, so what will happen? In real life, a fight may happen :-). Here you are BLOCKED because your wife has already taken the car. You won't be able to go to the interview.

This is the BLOCKED state. Explaining it in technical terms, you are the thread T1 and your wife is the thread T2 and lock is the car. T1 is BLOCKED on the lock (i.e. the car), because T2 has already acquired this lock.

你是线程T1,你的妻子是线程T2，而车是锁。T1被阻塞与锁（即车），由于T2已经获取了这个锁。

Titbit精选: A Thread will enter in to BLOCKED state when it’s waiting for a monitor lock to enter a synchronized block/method or reenter a synchronized block/method after calling Object#wait() method.

 

WAITING

Java doc formally defines WAITING state as: “A thread that is waiting indefinitely for another thread to perform a particular action is in this state.”

Real-life example: Let’s say few minutes later your wife comes back home with the car. Now you realize that the interview time is approaching, and there is a long distance to drive to get there. So, you put all the power on the gas pedal in the car. You drive at 100 mph when the allowed speed limit is only 60 mph. Your luck, a traffic cop sees you driving over the speed limit, and he pulls you over to the curb. Now you are entering into the WAITING state, my friend. You stop driving the car and sit idly in the car until the cop investigates you, and then lets you go. Basically, until he lets you go, you are stuck in the WAITING state.

Explaining it in technical terms, you are thread T1 and the cop is thread T2. You released your lock (i.e. you stopped driving the car), and went into the WAITING state. Until the cop (i.e. T2) lets you go, you will be stuck in this WAITING state.

你是T1,警察是T2.你释放你的锁（你停下正在驾驶的车），你进入等待状态。直到警察（T2）让你走，否则你将一直陷入等待状态。

Titbit: A Thread will enter in to WAITING state when it’s calling one of the following methods:

 

1. Object#wait() with no timeout

2. Thread#join() with no timeout

3. LockSupport#park()

 

Thread that has called Object.wait() on an object is in WAITING state until another thread to call Object.notify() or Object.notifyAll() on that object. A thread that has called Thread.join() is in WAITING state for a specified thread to terminate.

 

TIMED_WAITING

Java doc formally defines TIMED_WAITING state as: “A thread that is waiting for another thread to perform an action for up to a specified waiting time is in this state.”

Real-life example:  Despite all the drama, you did extremely well in the interview, impressed everyone and got this high paying job. (Congratulations!) You come back home and tell to your neighbor about this new job and how excited you are about it. Your friend says that he is also working in the same office building. He suggests that the two of you should drive together. You think it’s a great idea. So on the first day of work, you go to his house. You stop your car in front of his house. You wait for 10 minutes, but your neighbor still doesn’t come out. You go ahead and start driving to work, as you don’t want to be delayed on your first day.  Now this is TIMED_WAITING.

Explaining it in technical terms, you are thread T1 and your neighbor is thread T2. You release lock(i.e. stop driving the car) and wait up to 10 minutes. If your neighbor, T2, doesn’t come out in 10 minutes, you start driving the car again.

你是T1，你的邻居是T2.你释放锁（停止驾驶）并且等待10分钟。如果你的邻居T2在10分钟之内没出现，你开始再一次驾驶车。

Titbit: A Thread will enter in to TIMED_WAITING state when it’s calling one of the following methods:

 

1. Thread#sleep()

2. Object#wait() with timeout

3. Thread#join() with timeout

4. LockSupport#parkNanos()

5. LockSupport#parkUntil()

Conclusion

When someone is analyzing thread dumps, understanding these different thread states are critical. How many threads are in RUNNABLE, BLOCKED, WAITING, and TIMED_WATING states? Which threads are blocked? Who is blocking them? What object used for locking? These are some of the important metrics度量 to be analyzed in thread dumps. These kinds of detailed thread dump analyses can easily be done through an online tool such as: http://fastthread.io/