ReadWriteLock 原理

 

 

package com.study.lock.locks1;

import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.LockSupport;


public class JamesReadWriteLock {
    volatile AtomicInteger readCount = new AtomicInteger(0);
    AtomicInteger writeCount = new AtomicInteger(0);

    //独占锁 拥有者
    AtomicReference<Thread> owner = new AtomicReference<>();

    //等待队列
    public volatile LinkedBlockingQueue<WaitNode> waiters = new LinkedBlockingQueue<WaitNode>();
    class WaitNode{
        int type = 0;   //0 为想获取独占锁的线程，  1为想获取共享锁的线程
        Thread thread = null;
        int arg = 0;

        public WaitNode(Thread thread, int type, int arg){
            this.thread = thread;
            this.type = type;
            this.arg = arg;
        }
    }


    //获取独占锁
    public void lock() {
        int arg = 1;
        //尝试获取独占锁，若成功，退出方法，    若失败...
        if (!tryLock(arg)){
            //标记为独占锁
            WaitNode waitNode = new WaitNode(Thread.currentThread(), 0, arg);
            waiters.offer(waitNode);    //进入等待队列

            //循环尝试拿锁
            for(;;){
                //若队列头部是当前线程
                WaitNode head = waiters.peek();
                if (head!=null && head.thread == Thread.currentThread()){
                    if (!tryLock(arg)){      //再次尝试获取 独占锁
                        LockSupport.park();     //若失败，挂起线程
                    } else{     //若成功获取
                        waiters.poll();     //  将当前线程从队列头部移除
                        return;         //并退出方法
                    }
                }else{  //若不是队列头部元素
                    LockSupport.park();     //将当前线程挂起
                }
            }
        }
    }

    //释放独占锁
    public boolean unlock() {
        int arg = 1;

        //尝试释放独占锁 若失败返回true，若失败...
        if(tryUnlock(arg)){
            WaitNode next = waiters.peek(); //取出队列头部的元素
            if (next !=null){
                Thread th = next.thread;
                LockSupport.unpark(th);     //唤醒队列头部的线程
            }
            return true;                //返回true
        }
        return false;
    }

    //尝试获取独占锁
    public boolean tryLock(int acquires) {
        //如果read count ！=0 返回false
        if (readCount.get() !=0)
            return false;

        int wct = writeCount.get();     //拿到 独占锁 当前状态

        if (wct==0){
            if (writeCount.compareAndSet(wct, wct + acquires)){     //通过修改state来抢锁
                owner.set(Thread.currentThread());  //  抢到锁后，直接修改owner为当前线程
                return true;
            }
        }else if (owner.get() == Thread.currentThread()){
            writeCount.set(wct + acquires);     //修改count值
            return true;
        }

        return false;
    }

    //尝试释放独占锁
    public boolean tryUnlock(int releases) {
        //若当前线程没有 持有独占锁
        if(owner.get()!= Thread.currentThread()){
            throw new IllegalMonitorStateException();       //抛IllegalMonitorStateException
        }

        int wc= writeCount.get();
        int nextc = wc - releases;      //计算 独占锁剩余占用
        writeCount.set(nextc);      //不管是否完全释放，都更新count值

        if (nextc==0){  //是否完全释放
            owner.compareAndSet(Thread.currentThread(), null);
            return true;
        }else{
            return false;
        }

    }



    //获取共享锁
    public void lockShared() {
        int arg = 1;

        if (tryLockShared(arg) < 0){    //如果tryAcquireShare失败
            //将当前进程放入队列
            WaitNode node = new WaitNode(Thread.currentThread(), 1, arg);
            waiters.offer(node);  //加入队列

            for (;;){
                //若队列头部的元素是当前线程
                WaitNode head = waiters.peek();
                if (head!=null && head.thread == Thread.currentThread()){
                    if (tryLockShared(arg) >=0){    //尝试获取共享锁，  若成功
                        waiters.poll();      //将当前线程从队列中移除

                        WaitNode next = waiters.peek();
                        if (next!=null && next.type==1){    //如果下一个线程也是等待共享锁
                            LockSupport.unpark(next.thread);    //将其唤醒
                        }
                        return;     //退出方法
                    }else{                      //若尝试失败
                        LockSupport.park();     //挂起线程
                    }
                }else{  //若不是头部元素
                    LockSupport.park();
                }

            }
        }
    }

    //解锁共享锁
    public boolean unLockShared() {
        int arg = 1;

        if (tryUnLockShared(arg)){     //当read count变为0，才叫release share成功
            WaitNode next = waiters.peek();
            if (next!=null){
                LockSupport.unpark(next.thread);
            }
            return true;
        }
        return false;
    }

    //尝试获取共享锁
    public int tryLockShared(int acquires) {
        for (;;){
            if (writeCount.get()!=0 &&
                    owner.get() != Thread.currentThread())
                return -1;

            int rct = readCount.get();
            if (readCount.compareAndSet(rct, rct + acquires)){
                return 1;
            }
        }
    }

    //尝试解锁共享锁
    public boolean tryUnLockShared(int releases) {
        for(;;){
            int rc = readCount.get();
            int nextc = rc - releases;
            if (readCount.compareAndSet(rc, nextc)){
                return nextc==0;
            }
        }
    }



}