读写锁ReentrantReadWriteLock 的实现原理

package com.ams.thread.lesson8;

import cn.hutool.core.thread.ThreadUtil;
import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/**
 * 关注微信公众号"AI码师"获取项目源码及2021面试题一套
 *
 * @author: AI码师
 * Date: 2021/12/30 6:08 上午
 * Description:
 */
@Slf4j
public class Example20 {
    private static ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
    private static Lock readLock = lock.readLock();
    private static Lock writeLock = lock.writeLock();

    public static void main(String[] args) {
        new Thread(new Thread1("读线程1")).start();
        new Thread(new Thread1("读线程2")).start();
        new Thread(new Thread1("读线程3")).start();
        new Thread(new Thread1("读线程4")).start();
        new Thread(new Thread2("写线程1")).start();
        new Thread(new Thread2("写线程2")).start();
    }


    /**
     * 读锁线程
     */
    static class Thread1 implements Runnable {
        private String name;
        public Thread1(String name) {
            this.name = name;
        }
        @Override
        public void run() {
            readLock.lock();
            int i=0;
            while (i++<10){
                log.info("线程 {} 在运行",name);
                ThreadUtil.sleep(1000);
            }
            readLock.unlock();
        }
    }
    /**
     * 写锁线程
     */
    static class Thread2 implements Runnable {
        private String name;
        public Thread2(String name) {
            this.name = name;
        }
        @Override
        public void run() {
            writeLock.lock();
            int i=0;
            while (i++<5){
                log.info("线程 {} 在运行",name);
                ThreadUtil.sleep(1000);
            }
            writeLock.unlock();
        }
    }
}

 protected final boolean tryAcquire(int acquires) {
            Thread current = Thread.currentThread();
            int c = getState();
            int w = exclusiveCount(c);
            if (c != 0) {
                // (Note: if c != 0 and w == 0 then shared count != 0)
                if (w == 0 || current != getExclusiveOwnerThread())
                    return false;
                if (w + exclusiveCount(acquires) > MAX_COUNT)
                    throw new Error("Maximum lock count exceeded");
                // Reentrant acquire
                setState(c + acquires);
                return true;
            }
            if (writerShouldBlock() ||
                !compareAndSetState(c, c + acquires))
                return false;
            setExclusiveOwnerThread(current);
            return true;
        }

        protected final boolean tryRelease(int releases) {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int nextc = getState() - releases;
            boolean free = exclusiveCount(nextc) == 0;
            if (free)
                setExclusiveOwnerThread(null);
            setState(nextc);
            return free;
        }

 protected final int tryAcquireShared(int unused) {
            Thread current = Thread.currentThread();
            int c = getState();
            if (exclusiveCount(c) != 0 &&
                getExclusiveOwnerThread() != current)
                return -1;
            int r = sharedCount(c);
            if (!readerShouldBlock() &&
                r < MAX_COUNT &&
                compareAndSetState(c, c + SHARED_UNIT)) {
                if (r == 0) {
                    firstReader = current;
                    firstReaderHoldCount = 1;
                } else if (firstReader == current) {
                    firstReaderHoldCount++;
                } else {
                    HoldCounter rh = cachedHoldCounter;
                    if (rh == null || rh.tid != getThreadId(current))
                        cachedHoldCounter = rh = readHolds.get();
                    else if (rh.count == 0)
                        readHolds.set(rh);
                    rh.count++;
                }
                return 1;
            }
            return fullTryAcquireShared(current);
        }

  protected final boolean tryReleaseShared(int unused) {
            Thread current = Thread.currentThread();
            if (firstReader == current) {
                // assert firstReaderHoldCount > 0;
                if (firstReaderHoldCount == 1)
                    firstReader = null;
                else
                    firstReaderHoldCount--;
            } else {
                HoldCounter rh = cachedHoldCounter;
                if (rh == null || rh.tid != getThreadId(current))
                    rh = readHolds.get();
                int count = rh.count;
                if (count <= 1) {
                    readHolds.remove();
                    if (count <= 0)
                        throw unmatchedUnlockException();
                }
                --rh.count;
            }
            for (;;) {
                int c = getState();
                int nextc = c - SHARED_UNIT;
                if (compareAndSetState(c, nextc))
                    // Releasing the read lock has no effect on readers,
                    // but it may allow waiting writers to proceed if
                    // both read and write locks are now free.
                    return nextc == 0;
            }
        }