ReentrantReadWriteLock 的使用

ReentrantReadWriteLock 是一个锁对象,他支持读数据锁和写数据锁;

适合的使用场景

1. 多线程环境
2. 并且共同访问同一个资源数据
3. 要求可以共享读数据,同时读
4. 不能同时写数据

注意事项

• 读操作可以多线程同时一起读数据
• 但在写的时候,多个线程只能有一个线程在写,并且写的同时不允许去读数据

使用方式

   public static void main(String[] args) {
        //创建并发访问的账户
        MyCount myCount = new MyCount("95599200901215522", 10000);
        //创建一个锁对象
        ReadWriteLock lock = new ReentrantReadWriteLock(false);
        //创建一个线程池
        ExecutorService pool = Executors.newFixedThreadPool(2);
//        ExecutorService pool = Executors.newCachedThreadPool();
        //创建一些并发访问用户，一个信用卡，存的存，取的取，好热闹啊
        User u1 = new User("张三", myCount, -4000, lock, false);
        User u2 = new User("张三他爹", myCount, 6000, lock, false);
        User u3 = new User("张三他弟", myCount, -8000, lock, false);
        User u4 = new User("张三", myCount, 800, lock, false);
        User u5 = new User("张三他爹", myCount, 0, lock, true);
        //在线程池中执行各个用户的操作,2个线程池大家在用
        // u1 和 u2 两个抢一个执行lock
        pool.execute(u1);
        pool.execute(u2);
        pool.execute(u3);
        pool.execute(u4);
        pool.execute(u5);
        //关闭线程池
        pool.shutdown();
    }
}


class User implements Runnable {
    private String name;                 //用户名
    private MyCount myCount;         //所要操作的账户
    private int iocash;                 //操作的金额，当然有正负之分了
    private ReadWriteLock myLock;                 //执行操作所需的锁对象
    private boolean ischeck;         //是否查询

    User(String name, MyCount myCount, int iocash, ReadWriteLock myLock, boolean ischeck) {
        this.name = name;
        this.myCount = myCount;
        this.iocash = iocash;
        this.myLock = myLock;
        this.ischeck = ischeck;
    }

    public void run() {
        if (ischeck) {
            //获取读锁
            myLock.readLock().lock();
            System.out.println("读：" + name + "正在查询" + myCount + "账户，当前金额为" + myCount.getCash());
//            释放读锁
            myLock.readLock().unlock();
        } else {
            //获取写锁: 同一时间只能有一个线程执行操作
            myLock.writeLock().lock();
            //执行现金业务
            System.out.println("写：" + name + "正在操作" + myCount + "账户，金额为" + iocash + "，当前金额为" + myCount.getCash());
            myCount.setCash(myCount.getCash() + iocash);
            System.out.println("写：" + name + "操作" + myCount + "账户成功，金额为" + iocash + "，当前金额为" + myCount.getCash());
            //释放写锁
            myLock.writeLock().unlock();
        }
    }
}


class MyCount {
    private String oid;         //账号
    private int cash;             //账户余额

    MyCount(String oid, int cash) {
        this.oid = oid;
        this.cash = cash;
    }

    public String getOid() {
        return oid;
    }

    public void setOid(String oid) {
        this.oid = oid;
    }

    public int getCash() {
        return cash;
    }

    public void setCash(int cash) {
        this.cash = cash;
    }

    @Override
    public String toString() {
        return "MyCount{" +
                "oid='" + oid + '\'' +
                ", cash=" + cash +
                '}';
    }
}

打印结果

写：张三他爹正在操作MyCount{oid='95599200901215522', cash=10000}账户，金额为6000，当前金额为10000
写：张三他爹操作MyCount{oid='95599200901215522', cash=16000}账户成功，金额为6000，当前金额为16000
写：张三正在操作MyCount{oid='95599200901215522', cash=16000}账户，金额为-4000，当前金额为16000
写：张三操作MyCount{oid='95599200901215522', cash=12000}账户成功，金额为-4000，当前金额为12000
写：张三他弟正在操作MyCount{oid='95599200901215522', cash=12000}账户，金额为-8000，当前金额为12000
写：张三他弟操作MyCount{oid='95599200901215522', cash=4000}账户成功，金额为-8000，当前金额为4000
写：张三正在操作MyCount{oid='95599200901215522', cash=4000}账户，金额为800，当前金额为4000
写：张三操作MyCount{oid='95599200901215522', cash=4800}账户成功，金额为800，当前金额为4800
读：张三他爹正在查询MyCount{oid='95599200901215522', cash=4800}账户，当前金额为4800

实际项目使用

  public void cluster() {
        mClusterTaskLock.writeLock().lock();//加锁避免并发修改数据
        try {
            // Attempt to cancel the in-flight request.
            mClusterTask.cancel(true);
            mClusterTask = new ClusterTask();

            if (Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
                mClusterTask.execute(mMap.getMapStatus().zoom);
            } else {
                mClusterTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR, mMap.getMapStatus().zoom);
            }
        } finally {
            mClusterTaskLock.writeLock().unlock();//用完释放
        }
    }

读和写一起使用

    private Set<? extends Cluster<T>> getClustersInternal(int discreteZoom) {
        Set<? extends Cluster<T>> results;
        mCacheLock.readLock().lock();
        results = mCache.get(discreteZoom);
        mCacheLock.readLock().unlock();

        if (results == null) {
            mCacheLock.writeLock().lock();
            results = mCache.get(discreteZoom);
            if (results == null) {
                results = mAlgorithm.getClusters(discreteZoom);
                mCache.put(discreteZoom, results);
            }
            mCacheLock.writeLock().unlock();
        }
        return results;
    }