Re enter the read-write lock ReentrantReadWriteLock Feature summary

• Reentrant ： Because it's a re entrant lock , And in the same thread , Therefore, the lock can be obtained repeatedly , Ordinary Lock Even if it is the same thread, it cannot get repeatedly , It's not reentrant ; You have to release as many times as you re-enter .
• Mutex and lock up degradation ： Reading is not mutually exclusive , Reading and writing & Writing mutually exclusive ;;; Rules are not applicable in the same thread , But in the same thread, such as this example, the case of lock degradation is OK Of , But if you read the lock first and then add a write lock, it is a lock upgrade , Deadlock will occur
• Fairness ：ReentrantReadWriteLock Declare as true Fair lock , namely “ Thread waiting for lock sorting ” And “ The thread finally gets the lock sort ” It's consistent , Otherwise seize
• Solutions in distributed environment  redission Distributed environment readWriteLock

public class App {
    private static final ReadWriteLock readWriteLock = new ReentrantReadWriteLock(true);

    public static void main(String[] args) throws Exception {
        Thread t = new Thread(() -> {
            readWriteLock.writeLock().lock();
            System.out.println("writeLock1");
            readWriteLock.writeLock().lock();// Because it's a re entrant lock , And in the same thread , Therefore, the lock can be obtained repeatedly , Ordinary Lock Even if it is the same thread, it cannot get repeatedly ;
            System.out.println("writeLock1");
            readWriteLock.readLock().lock(); // Different threads ： Reading is not mutually exclusive , Reading and writing & Writing mutually exclusive ;;; Rules are not applicable in the same thread , But in the same thread, such as this example, the case of lock degradation is OK Of , But if you read the lock first and then add a write lock, it is a lock upgrade , Deadlock will occur 
            System.out.println("readLock1");
            try {
                Thread.sleep(5000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            readWriteLock.writeLock().unlock();
            readWriteLock.writeLock().unlock();// The front is locked twice , So it needs to be released twice 
            readWriteLock.readLock().unlock();
        });
        t.start();
        Thread.sleep(1000);
        // start-up N Threads ： Test fair lock 、 Not fair lock 
        for (int i = 0; i < 100; i++) {
            int finalI = i;
            new Thread(() -> {
                System.out.println(" Thread waiting for lock sorting "+finalI);
                // Because the write lock has been t Thread acquisition ,t The write lock obtained by the thread is sorted by the read lock here , So it is blocked here .5s after t Thread release lock , Because of the ReentrantReadWriteLock Declare as true Fair lock , namely “ Thread waiting for lock sorting ” And “ The thread finally gets the lock sort ” It's consistent 
                readWriteLock.readLock().lock();
                System.out.println(" The thread finally gets the lock sort "+ finalI);
            }).start();
        }
    }
}