Java并发编程与技术内幕:聊聊锁的技术内幕（中）

我就是我 2021-09-11 00:34 398阅读 0赞

摘要：本文主要讲了读写锁。

一、读写锁ReadWriteLock

在上文中回顾了并发包中的可重入锁ReentrantLock，并且也分析了它的源码。从中我们知道它是一个单一锁（笔者自创概念），意思是在多人读、多人写、或同时有人读和写时。只能有一个人能拿到锁，执行代码。但是在很多场景。我们想控制它能多人同时读，但是又不让它多人写或同时读和写时。（想想这是不是和数据库的可重复读有点类型？），这时就可以使用读写锁：ReadWriteLock。

下面来看一个应用

package com.lin;
    
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.locks.ReadWriteLock;
    import java.util.concurrent.locks.ReentrantReadWriteLock;
    
    public class ReadWriteLockTest {
    	
       public  static  void main(String[] args) {
    	   //创建一个锁对象 ,非公平锁
           ReadWriteLock lock = new ReentrantReadWriteLock(false);  
           //创建一个线程池  
           ExecutorService pool = Executors.newCachedThreadPool(); 
    	   //设置一个账号，设置初始金额为10000
           Account account = new Account(lock,"123456",10000);
            
           //账号取钱10次，存钱10次，查询20次
           for(int i=1;i<=10;i++) {
        	   Operation operation1 = new Operation(account,"take");
        	   Operation operation2 = new Operation(account,"query");
        	   Operation operation3 = new Operation(account,"save");
        	   Operation operation4 = new Operation(account,"query");
        	   pool.execute(operation1);
        	   pool.execute(operation2);
        	   pool.execute(operation3);
        	   pool.execute(operation4);
           }
           pool.shutdown();
           while(!pool.isTerminated()){  
               //wait for all tasks to finish  
           }  
           System.out.println("账号"+ account.getAccoutNo() +",最后金额为："+account.getMoney());  
       }
    
    }
    
    class Operation implements Runnable{
    	
    	private Account account;//账号
    		
    	private String type;
    	
    	Operation(Account account,String type){
    		this.account = account;
    		this.type = type;
    	}
    	
    
    	public void run() {
    	    if ("take".equals(type)) { //每次取100元
    	    	 //获取写锁  
    	    	account.getLock().writeLock().lock();
    	    	account.setMoney(account.getMoney() -100);
    	    	System.out.println( "取走100元,账号"+ account.getAccoutNo()+" 还有"+account.getMoney()+"元");
    	    	account.getLock().writeLock().unlock();
    	    	
    	    }
    	    else if ("query".equals(type)) {
    	    	 //获取写锁  
    	    	account.getLock().readLock().lock();
    	    	System.out.println( "查询账号"+ account.getAccoutNo()+" 还有"+account.getMoney()+"元");
    	    	account.getLock().readLock().unlock();
    	    	
    	    }
    	    else if ("save".equals(type)) {
    	    	 //获取写锁  
    	    	account.getLock().writeLock().lock();
    	    	account.setMoney(account.getMoney() + 100);
    	    	System.out.println( "存入100元,账号"+ account.getAccoutNo()+" 还有"+account.getMoney()+"元");
    	    	account.getLock().writeLock().unlock();
    	    }
    	}
    	
    }
    
    class Account  {
    	
    	private int money;//账号上的钱
    	
    	private ReadWriteLock lock;//读写写
    	
    	private String accoutNo;//账号
    	
    	Account(ReadWriteLock lock,String accoutNo,int money) {
    		this.lock = lock;
    		this.accoutNo = accoutNo;
    		this.money = money;
    	}
    	
    	public int getMoney() {
    		return money;
    	}
    
    	public void setMoney(int money) {
    		this.money = money;
    	}
    
    	public ReadWriteLock getLock() {
    		return lock;
    	}
    
    	public void setLock(ReadWriteLock lock) {
    		this.lock = lock;
    	}
    
    	public String getAccoutNo() {
    		return accoutNo;
    	}
    
    	public void setAccoutNo(String accoutNo) {
    		this.accoutNo = accoutNo;
    	}	
    		
    }

输出结果：

![20160717142507833][]

在上面的例子中，设置了一个账号。金额为10000，然后开了10条线程每次取100，10条线程每次存100，20条线程一直查。从结果中我们可以看到是正确的。

二、源码分析

1、ReadWriteLock

public interface ReadWriteLock {
    
        Lock readLock();//返回读锁
    
        Lock writeLock();//返回写锁
    }

ReadWriteLock就只是一个接口类，真正实现 类在ReentrantReadWriteLock

2、ReentrantReadWriteLock

（1）包含变量

public class ReentrantReadWriteLock
            implements ReadWriteLock, java.io.Serializable {
        private static final long serialVersionUID = -6992448646407690164L;
        /**读锁*/
        private final ReentrantReadWriteLock.ReadLock readerLock;
        /** 写锁 */
        private final ReentrantReadWriteLock.WriteLock writerLock;
        /** 内部类，在ReentrantLock也有它 */
        final Sync sync;

看了一个它的变量还是比较简单的，其中Sync类在ReentrantLock类中笔者已介绍过。

（2）构造函数

public ReentrantReadWriteLock() {
            this(false);
        }
    
        /**
         * 设置读写锁
         */
        public ReentrantReadWriteLock(boolean fair) {
            sync = fair ? new FairSync() : new NonfairSync();//是否公平
            readerLock = new ReadLock(this);
            writerLock = new WriteLock(this);
        }

看了看FairSync的源码其实和上节中讲的基本一样。这里就不再展开。

（3）ReadLock

看了下读锁，其实很简单，发现里面封装的都 是调用Sync类的方法，看来它才是重点。在ReadLock类的lock方法中，我们看到了sync.acquireShared(1);这里就可以认为是一个共享锁

public static class ReadLock implements Lock, java.io.Serializable {
            private static final long serialVersionUID = -5992448646407690164L;
            private final Sync sync; //和上面的一样，是同一个类

protected ReadLock(ReentrantReadWriteLock lock) {
                sync = lock.sync;
            }
    
            public void lock() {
                sync.acquireShared(1); //共享锁
            }
    
    
            public void lockInterruptibly() throws InterruptedException {
                sync.acquireSharedInterruptibly(1);//响应中断，跳出阻塞
            }
    
    
            public boolean tryLock() {
                return sync.tryReadLock();//取得锁才返回true
            }
    
            public boolean tryLock(long timeout, TimeUnit unit)
                    throws InterruptedException {
                return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
            }
            public void unlock() {
                sync.releaseShared(1);//释放锁
            }

再进来看看

public final void acquireShared(int arg) {
            if (tryAcquireShared(arg) < 0)
                doAcquireShared(arg);
        }

再进去就看不了，这里的方法其实就是认为取得一个共享锁。

（4）WriteLock

public static class WriteLock implements Lock, java.io.Serializable {
            private static final long serialVersionUID = -4992448646407690164L;
            private final Sync sync;
    
            protected WriteLock(ReentrantReadWriteLock lock) {
                sync = lock.sync;
            }
    
            public void lock() {
                sync.acquire(1); //表明只是取得一个锁，但不是独占
            }
    
            public void lockInterruptibly() throws InterruptedException {
                sync.acquireInterruptibly(1);
            }

在WriteLock锁中的lock方法和ReadLock是有所不同的。WriteLock它是一个独占锁。也就是有线程拿到后，其它线程就得阻塞等待了。

public final void acquire(int arg) {
            if (!tryAcquire(arg) &&
                acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
                selfInterrupt();
        }

[20160717142507833]: /images/20210811/7ae16566ff404fc383de95bfcadb167d.png