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This article introduces the relevant knowledge of "what are the states of threads in Java?". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

There are six types of thread states in Java.

1. NEW: a new thread object has been created, but the start () method has not been called yet.

two。 RUNNABLE: the two states of ready (ready) and running (running) in Java threads are generally called "running". After the thread object is created, other threads, such as the mainthread, call the object's start () method. The thread in this state is in the runnable thread pool, waiting to be selected by thread scheduling to obtain the right to use CPU, and is in the ready state (ready). A thread in the ready state becomes a running state (running) after obtaining a CPU time slice.

3. BLOCKED: indicates that the thread is blocking the lock.

4. WAITING: threads entering this state need to wait for other threads to take specific actions (notifications or interruptions).

5. Timeout wait (TIMED_WAITING): this state, unlike WAITING, can return on its own after a specified time.

6. TERMINATED: indicates that the thread has finished executing.

State diagram of a thread

1. Initial state

You can get a thread class by implementing the Runnable interface and inheriting Thread. When an instance of new comes out, the thread enters its initial state.

2.1. Ready state

The ready state just means that you are qualified to run, and if the scheduler does not pick you, you will always be ready. Call the start () method of the thread, which enters the ready state. The current thread sleep () method ends, other threads join () ends, waiting for the user to finish typing, one thread gets the object lock, and these threads will also enter the ready state. The current thread time slice is used up, the yield () method of the current thread is called, and the current thread enters the ready state. The thread in the lock pool gets the object lock and enters the ready state.

2.2. Running state

The state in which the thread is in when the thread scheduler selects a thread from the runnable pool as the current thread. This is also the only way for a thread to get into a running state.

3. Blocking state

Blocking state is the state of a thread blocking when it enters a method or block of code modified by the synchronized keyword (acquiring a lock).

4. wait for

Threads in this state are not allocated CPU execution time, they have to wait to be explicitly woken up, otherwise they will be waiting indefinitely.

5. Time-out waiting

Threads in this state are not allocated CPU execution time, but they do not have to wait indefinitely to be explicitly awakened by other threads, but they will wake up automatically after a certain amount of time.

6. Termination state

When the thread's run () method completes, or the main thread's main () method completes, we consider it terminated. This thread object may be alive, but it is no longer a separate thread of execution. Once a thread is terminated, it cannot be revived. Calling the start () method on a terminated thread throws a java.lang.IllegalThreadStateException exception.

7. Waiting queue

Before calling the wait (), notify () methods of obj, you must acquire the obj lock, that is, you must write it in the synchronized (obj) code snippet. Steps and diagrams related to waiting queues

Thread 1 acquires the lock of object An and is using object A.

Thread 1 calls the wait () method of object A.

Thread 1 releases the lock on object An and immediately enters the waiting queue.

The object in the lock pool competes for the lock of object A.

Thread 5 acquires the lock of object A, enters the synchronized block, and uses object A.

Thread 5 calls the notifyAll () method of object A to wake up all threads and enter the synchronization queue. If thread 5 calls the notify () method of object A, it wakes up a thread. I don't know who it will wake up, and the awakened thread enters the synchronization queue.

The synchronized where the notifyAll () method is located ends, and thread 5 releases the lock on object A.

The threads of the synchronous queue scramble for the object lock, but it is not known when thread 1 will be able to grab it.

Synchronization queue status

When the current thread wants to call the synchronization method of object A, it finds that the lock of object An is occupied by another thread, and the current thread enters the synchronization queue. In short, the synchronization queue is full of threads that want to compete for object locks.

When one thread 1 is awakened by another thread 2, thread 1 enters the synchronization queue to compete for the object lock. Synchronization queue is a concept that exists only in the context of synchronization, and an object corresponds to a synchronization queue.

Comparison of several methods

Thread.sleep (long millis), this method must be called by the current thread. The current thread enters the TIMED_WAITING state, but the object lock is not released. After millis, the thread automatically wakes up and enters the ready state. Purpose: the best way to give other threads a chance to execute.

Thread.yield (), which must be the current thread calling this method, the current thread abandons the acquired CPU time slice, but does not release the lock resources, and changes from the running state to the ready state, allowing OS to select the thread again. Purpose: let threads of the same priority take turns, but there is no guarantee that they will take turns. In practice, there is no guarantee that yield () will achieve the purpose of concession, because the concessionary thread may be selected again by the thread scheduler. Thread.yield () does not cause blocking. This method is similar to sleep (), except that the user cannot specify how long to pause.

T.join () / t.join (long millis), the join method of other thread t is called in the current thread, the current thread enters the WAITING/TIMED_WAITING state, and the current thread will not release the object lock that is already held. When thread t finishes execution or the millis time is up, the current thread enters the ready state.

Obj.wait (), the current thread calls the object's wait () method, and the current thread releases the object lock and enters the waiting queue. Rely on notify () / notifyAll () wake up or wait (long timeout) timeout time to wake up automatically.

Obj.notify () wakes up a single thread waiting on this object monitor, and the choice is arbitrary. NotifyAll () wakes up all threads waiting on this object monitor.

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