Behavioral analysis of queue jumping under different lock modes in Java.

1. ReentrantReadWriteLock can set fair lock mode and unfair lock mode.

// 公平锁模式
ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock(true);
//非公平锁模式 默认情况
ReentrantReadWriteLock reentrantReadWriteLock = new ReentrantReadWriteLock(false);

Before obtaining a fair lock, check the readerShouldBlock() method. Before obtaining a write lock, check the writerShouldBlock() method first, and then decide whether to queue or jump in the queue.

2. In unfair lock mode, writerShouldBlock() and readerShouldBlock() implement

final boolean writerShouldBlock() {
    return false; // writers can always barge
}
final boolean readerShouldBlock() {
    return apparentlyFirstQueuedIsExclusive();
}

Unfair lock can jump the queue when acquiring the write lock. Use policy decisions when reading locks.

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