How to make the main thread wait for the sub thread to execute before executing

Summarize the three methods ：

（1）join Method , Direct each thread to join Just go in .

（2）countdownlatch, Internally, a counting operation is maintained , adopt cas The algorithm guarantees the atomicity of the operation , When the thread has finished executing , call countDown() Method , Subtract one from the count , Main thread call countdownlatch Object's await() Method , Suspends the current thread , until countdownlatch The count of objects is 0, Main thread execution .

（3） Something like a barrier , When the sub thread is finished , call CyclicBarrier Object's await() Method , Until a specified number of threads reach the barrier , Will continue to execute the main thread . Compare each child thread to && A condition of a symbol , Then only when all the conditions are met , To execute the main thread .