[Concurrent programming] LockSupport class

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1. Official website document: https://www.apiref.com/java11-zh/java.base/java/util/concurrent/locks/LockSupport.html

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1. Preliminary knowledge

1.1 Why use LockSupport?

 3 ways to make threads wait for awakening:

1. Use the wait() method in Object to make the thread wait, and use the notify method in Object to wake up the thread
2. Use the await() method of Condition in the JUC package to make the thread wait, and use the signal() method to wake up the thread
3. The LockSupport class can block the current thread and wake up the specified blocked thread

1.2  Wait() and notify() in the Object class implement thread waiting and awakening

1. The wait and notify methods must be used in pairs in a synchronized block or synchronized method.  Both wait and notify methods remove the synchronization code block and see the abnormal situation
   in the running effect: Exception in thread “A” Exception in thread “B”
   java.lang.IllegalMonitorStateException
2. Wait first and then notify (if you notify first and then wait, another thread will always be waiting)
3. Synchronized is a keyword belonging to the JVM level. monitorenter (the bottom layer is done through the monitor object, in fact, wait/notify and other methods also rely on the monitor object to call wait/notify and other methods only in a synchronized block or method)