多线程八 Lock

r囧r小猫 2021-10-29 09:42 368阅读 0赞

> 前面我们可以使用synchronized关键字来实现线程之间的同步互斥,**lock接口同样也是在JDK1.5中提出**,同样是解决线程安全性问题的另一种解决方案,而且它更强大,更灵活本片博客介绍对其展开介绍;  
>

### Lock接口有如下几个实现类: ###

*  **ReentrantLock--JDK实现的锁**
 *  **ReentrantReadWritrLock.ReadLock**
 *  **ReentrantReadWriteLock.WriteLock**

### 打个例子 ###

public class Demo01 { private int i=0; Lock Mylock = new ReentrantLock(); public int add() throws InterruptedException { try{ Mylock.lock(); i++; return i; } finally { Mylock.unlock(); } }

如上代码 i++ 被 Mylock.lock()和 Mylock.unlock()围住,显示的获取和释放锁,具有同步性...

> 其中,**ReentrantLock**一个可重入的互斥锁 Lock，它**具有与使用 synchronized 方法和语句所访问的隐式监视器锁相同的一些基本行为和语义，但功能更强大。**

> Lock 与 Synchronized 相比较,显而易见的就是,**Lock需要显示的去获取锁,释放锁,比较繁琐**,但是繁琐带来了更大好处,让代码更灵活...**Lock是对Synchronized的封装...**

比如:

1.  在控制锁的获取和释放,以及何时何地获取释放
2.  使用Lock,可以很方便的实现锁的公平性**ReentrantLock(boolean fair)**
3.  强大的API
    
     *  非阻塞获取锁:**tryLock()**
     *  可中断式获取线程**acquireSharedInterruptibly(int arg)**
     *  超时获取线程**tryAcquireSharedNanos(int arg , long nanosTimeout)**

--------------------

### 一 . Conditon&Reentrantlock ###

#### 1. Condition实现正确的通知/等待 ####

*  **注意点,一定要在condition.wait()方法调用之前,使用lock.lock()方法获取对象同步锁**,否则抛出异常

public void waitMethod(){ try{ lock.lock(); System.out.println(Thread.currentThread().getName()+"等待了..."+System.currentTimeMillis()); try { condition.await(); } catch (InterruptedException e) { e.printStackTrace(); } }finally { lock.unlock(); } } public void signalMethod(){ try{ lock.lock(); System.out.println(Thread.currentThread().getName()+"被唤醒了..."+System.currentTimeMillis()); condition.signal(); }finally { lock.unlock(); } } public static void main(String[] args) { demo1 demo1 = new demo1(); new Thread(()->{ demo1.waitMethod(); }).start(); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } demo1.signalMethod(); } }

运行结果:

Thread-0等待了...1549450097987
    main被唤醒了...1549450099988

#### 2 使用多个Condition实现,通知部分线程 ####

> 接下来就是重头戏了**Condition控制通知指定的线程醒来**

运行结果:

Thread-0等待了...1549450097987
    main被唤醒了...1549450099988

### 2 使用多个Condition实现,通知部分线程 ###

> 接下来就是重头戏了**Condition控制通知指定的线程醒来**

/* * 使用多个Condition, 唤醒指定的线程 * */ public class demo2 { private Lock lock = new ReentrantLock(); private Condition conditionA = lock.newCondition(); private Condition conditionB = lock.newCondition(); public void waitA(){ try{ lock.lock(); System.out.println(Thread.currentThread().getName()+"等待了..."+System.currentTimeMillis()); try { conditionA.await(); System.out.println(Thread.currentThread().getName()+"await之后的代码..."+System.currentTimeMillis()); } catch (InterruptedException e) { e.printStackTrace(); } }finally { lock.unlock(); } } public void waitB(){ try{ lock.lock(); System.out.println(Thread.currentThread().getName()+"等待了..."+System.currentTimeMillis()); try { conditionB.await(); System.out.println(Thread.currentThread().getName()+"await之后的代码..."+System.currentTimeMillis()); } catch (InterruptedException e) { e.printStackTrace(); } }finally { lock.unlock(); } } public void signalAALL(){ try{ lock.lock(); conditionA.signalAll(); System.out.println(Thread.currentThread().getName()+" 执行唤醒操作. ."+System.currentTimeMillis()); }finally { lock.unlock(); } } public void signalBBLL(){ try{ lock.lock(); System.out.println(Thread.currentThread().getName()+" 被唤醒了.."+System.currentTimeMillis()); conditionB.signalAll(); }finally { lock.unlock(); } } public static void main(String[] args) { demo2 demo2 = new demo2(); ExecutorService executorService = Executors.newCachedThreadPool(); executorService.execute(new Runnable() { @Override public void run() { demo2.waitA(); // demo2.waitB(); } }); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } demo2.signalAALL(); System.out.println("mian 线程结束..."); } }

结果:

pool-1-thread-1等待了...1549459953814
    main 执行唤醒操作. .1549459955805
    mian 线程结束...
    pool-1-thread-1await之后的代码...1549459955805

*  通过结果可以看到,ReentrantLock对象,**可以唤醒指定种类的线程**,使用个Condition让线程等待,就用哪个Condition去把它唤醒

--------------------

### 公平锁与非公平锁 ###

> **Lock锁分为公平锁和非公平锁,所谓公平锁,就是表示线程获取锁的顺序,是按照线程加锁的顺序来实现的,也就是FIFO的顺序先进先出的顺序,而非公平锁描述的则是一种锁的随机抢占机制,还可能会导致一些线程根本抢不着锁而被饿死,结果就是不公平了**

*  ReentrantLock支持公平锁

ReentrantLock()  
创建一个 ReentrantLock 的实例。

<table> 
 <thead> 
  <tr> 
   <th>构造方法名</th> 
   <th>简介</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>ReentrantLock(boolean fair)</td> 
   <td>创建一个具有给定公平策略的 ReentrantLock。</td> 
  </tr> 
 </tbody> 
</table>

### ReentrantLock几组常用API ###

第一组:  
方法名 | 作用  
\---|---  
int **getHoldCount()** | **查询当前线程**保存此锁的个数,(执行lock.lock()的次数),即重入的次数  
int **getQueueLength()** | 返回**正在等待获取此锁(调用该方法的锁)的线程的个数,比如有五条线程已经准备就绪了,其中一条线程首先执行lock.await()方法,紧接着调用该方法,获取到的返回值为4,说明有四条线程正在等待此lock  
int** getWaitQueueLength(Condition condition)\*\* | **返回正在等待和此锁相关的condition的线程数**

第二组:  
方法名 | 作用  
\---|---  
boolean **hasQueuedThread(Thread thread)** | **判断当前线程**是否正在等到获取到此锁  
boolean **hasQueuedThreads()** | **在所有的线程中查询,是否有线程正在等待此所的锁定**  
boolean **hasWaiters(Condition condition)** | 判断是否有线程**正在等待**和此condition相关的条件

第三组

<table> 
 <thead> 
  <tr> 
   <th>方法名</th> 
   <th>作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>boolean <strong>isFair()</strong></td> 
   <td>判断此锁是不是公平的</td> 
  </tr> 
  <tr> 
   <td>boolean <strong>isHeldByCurrentThread()</strong></td> 
   <td>判断<strong>当前线程是否拿到了锁</strong></td> 
  </tr> 
  <tr> 
   <td>boolean <strong>isLocked()</strong></td> 
   <td>判断此锁<strong>是否由任意线程保持</strong></td> 
  </tr> 
 </tbody> 
</table>

第四组

### lock()与lockInterruptibly() ###

> **假如出现下面几步,我们这两种加锁方法,会有什么反应?**,第一: 线程A启动,使用lock()加锁,第二步: CPU的执行权被线程B抢到,且线程B使用interrupted()方法给线程A打上中断的标记..第三步: 线程A继续执行

*  使用lock()加锁,假如我们没有使用isInterrupted()判断的话,代码会按照原来的顺序依次全部执行,没有异常,线程AB正常结束
 *  使用lockInterruptibly()加锁,线程A被中断后,会调用lockInterruptibly()报异常,进入catch代码块

### tryLock()与Lock() ###

> **boolean tryLock()与void lock() 前者是有返回值的**,两者都能去获取锁,而tryLock()**直意尝试获取锁,有就拿,没有算了**,它多了一步判断,它在调用时,会去判断此锁是否被其他线程锁定,如果没有,则获取,并返回ture

if(lock.tryLock()){ //do something 1 ... }else{ //do something 2... }

**可以看到,使用tryLock(),不至于当前线程被阻塞住**

<table> 
 <thead> 
  <tr> 
   <th>方法名</th> 
   <th>作用</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>boolean <strong>tryLock(Long timeout,TimeUnit unit)</strong></td> 
   <td>如果在给定的<strong>等待时间</strong>内,锁没有被其他线程拿到,并且当前线程也没有被中断,获取锁</td> 
  </tr> 
 </tbody> 
</table>

### await()和awaitUninterrupted() ###

> 当线程A await() 线程B给线程A打上中断的标记

*  1.中断 2,抛出中断异常 3. 进入catch块

> 当线程A awaitUninterrupted() 线程B给线程A打上中断的标记

*  1.中断

### boolean awaitUtil(Date deadLine); ###

> 出现一下几种情况,线程被唤醒

*  等待的时间结束
 *  在等待.被中断的过程被提前唤醒
 *  被中断

--------------------

### 读写锁(排它锁,共享锁) ###

> 在一个不需要操作实例变量的方法中,完全可以使用读写锁来提升该方法的代码运行速度

*  读操作相关的锁是**共享锁**,写操作相关的锁为**排他锁**

ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

*  读读共享 lock.readLock().lock();
 *  写写互斥 lock.writeLock().lock();
 *  读写互斥

**获取读写锁**

public void read() { lock.readLock().lock(); System.out.println("获取到了读锁" + Thread.currentThread().getName() + " " + System.currentTimeMillis()); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.readLock().unlock(); } } public void write(){ try { lock.writeLock().lock(); System.out.println(""+Thread.currentThread().getName()+" " +System.currentTimeMillis()); Thread.sleep(2000); }catch (Exception e){ e.printStackTrace(); }finally { lock.writeLock().unlock(); } }

**创建四条线程测试**

获取到了读锁Thread-0 1548851348805 获取到了读锁Thread-1 1548851348805 Thread-2 1548851350806 Thread-3 1548851352806

### JDK8新增StampedLock对ReentrantReadWriteLock进行增强 ###

stamped:贴上邮票的; 盖上邮戳的,拿到锁之后会返回给我们一个票据,根据这个Stamp的值判断是在读的时候发生了写,还是在写的时候发生了读操作

**解决的问题:**  
在高并发的情况下,读操作的次数远远大于写操作,,因为读写互斥,写操作可能就会出现饥饿的情况,一直抢占不到cpu的资源

解决方法:

1.  当然可以使用公平的ReadWriteLock,但是依然有性能问题
2.  StampedLock的乐观锁实现了读写共享提升了!

**StampedLock里面有两种锁**  
**乐观锁:**

> 读锁并不会阻塞写锁

public long tryOptimisticRead() {...}

悲观锁:

> 读写互斥,和ReentrantReadWriteLock实现相同的功能

API:

独占的获取写锁,若锁被其他线程获取到,则阻塞,注意它是相对于ReentrantReadWriteLock来讲,它是有返回值的,返回值的作用:

*  释放锁(unlock的需要参数)
 *  进行锁的转换

/** * Exclusively acquires the lock, blocking if necessary * until available. * * @return a stamp that can be used to unlock or convert mode */ public long writeLock() { long s, next; // bypass acquireWrite in fully unlocked case only return ((((s = state) & ABITS) == 0L && U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ? next : acquireWrite(false, 0L)); }

//一旦写锁可用立即获取,返回值可以用作释放锁或被锁的转换使用, 0表示没有获取到锁 /** * Exclusively acquires the lock if it is immediately available. * * @return a stamp that can be used to unlock or convert mode, * or zero if the lock is not available */ public long tryWriteLock() { long s, next; return ((((s = state) & ABITS) == 0L && U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ? next : 0L); } //延迟获取锁 /** * Exclusively acquires the lock if it is available within the * given time and the current thread has not been interrupted. * Behavior under timeout and interruption matches that specified * for method {@link Lock#tryLock(long,TimeUnit)}. * * @param time the maximum time to wait for the lock * @param unit the time unit of the {@code time} argument * @return a stamp that can be used to unlock or convert mode, * or zero if the lock is not available * @throws InterruptedException if the current thread is interrupted * before acquiring the lock */ public long tryWriteLock(long time, TimeUnit unit) ... // 非独占的获取读锁 /** * Non-exclusively acquires the lock, blocking if necessary * until available. * * @return a stamp that can be used to unlock or convert mode */ public long readLock() { long s = state, next; // bypass acquireRead on common uncontended case return ((whead == wtail && (s & ABITS) < RFULL && U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ? next : acquireRead(false, 0L)); } // 一旦锁可用,立即非独占的获取读锁 /** * Non-exclusively acquires the lock if it is immediately available. * * @return a stamp that can be used to unlock or convert mode, * or zero if the lock is not available */ public long tryReadLock() { for (;;) { long s, m, next; if ((m = (s = state) & ABITS) == WBIT) return 0L; else if (m < RFULL) { if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) return next; } else if ((next = tryIncReaderOverflow(s)) != 0L) return next; } }

乐观锁!!!它获取到的锁,**读写锁非互斥**  
返回一个标记,这个标记过一会用去 校验, 如果锁是排它锁,返回零

/** * Returns a stamp that can later be validated, or zero * if exclusively locked. * * @return a stamp, or zero if exclusively locked */ public long tryOptimisticRead() { long s; return (((s = state) & WBIT) == 0L) ? (s & SBITS) : 0L; }

**校验**,如果锁还没被任何线程获取,获取被持有当前stamp的线程获取返回true , 如果 stamp为0,返回false

/** * Returns true if the lock has not been exclusively acquired * since issuance of the given stamp. Always returns false if the * stamp is zero. Always returns true if the stamp represents a * currently held lock. Invoking this method with a value not * obtained from {@link #tryOptimisticRead} or a locking method * for this lock has no defined effect or result. * * @param stamp a stamp * @return {@code true} if the lock has not been exclusively acquired * since issuance of the given stamp; else false */ public boolean validate(long stamp) { U.loadFence(); return (stamp & SBITS) == (state & SBITS); }

**锁的释放**

/** * If the lock state matches the given stamp, releases the * exclusive lock. * * @param stamp a stamp returned by a write-lock operation * @throws IllegalMonitorStateException if the stamp does * not match the current state of this lock */ public void unlockWrite(long stamp) { WNode h; if (state != stamp || (stamp & WBIT) == 0L) throw new IllegalMonitorStateException(); state = (stamp += WBIT) == 0L ? ORIGIN : stamp; if ((h = whead) != null && h.status != 0) release(h); } //释放任意匹配成功的锁 /** * If the lock state matches the given stamp, releases the * corresponding mode of the lock. * * @param stamp a stamp returned by a lock operation * @throws IllegalMonitorStateException if the stamp does * not match the current state of this lock */ public void unlock(long stamp) {

**读,写 锁之间的转换**

public long tryConvertToWriteLock(long stamp) { ..} public long tryConvertToWriteLock(long stamp) { ..} //释放读锁 public void unlockRead(long stamp) {..} //释放写锁 public void unlockWrite(long stamp) {..}

#### 简单使用后 ####

/* * StampedLock的简单使用 * */ public class StampedLock01 { private int balance; StampedLock stamptedLock = new StampedLock(); //悲观读 public void read(){ long s = stamptedLock.readLock(); try{ try { System.out.println("拿到读锁"+Thread.currentThread().getName()+System.currentTimeMillis()); Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } }finally { stamptedLock.unlockRead(s); } } // 乐观锁 public void OptimismRead(){ //获取乐观锁,拿到 标记 long stm = stamptedLock.tryOptimisticRead(); try { System.out.println("一开始读取到的balance=="+balance); //方便测试 睡一会 Thread.sleep(200); // 在读的时候,可能会出现现写操作-- 判断 if(!stamptedLock.validate(stm)){ //1 锁空闲可用 2 拥有当前stm的线程获取到锁,返回true 其他返回false //重新读取 long l = stamptedLock.readLock(); System.out.println("乐观锁中发现了在读时,发现写操作,重新读结果为: "+balance); // 更新标记, 用于锁的释放 stm=l; } }catch (Exception e){ e.printStackTrace(); }finally { //释放锁 stamptedLock.unlockRead(stm); } } // 带条件的读写锁 public void MyConditionReadWriteLock(int v){ //判断balance是否符合更新的条件 long stm = stamptedLock.readLock(); try{ // 为什么不会if 而用while while(stm>0){ //转换成写锁; long s1 = stamptedLock.tryConvertToWriteLock(stm); if (s1!=0){// 成功转换 balance+=v; System.out.println("进行写操作: balance=="+balance+99); stm=s1; break; }else{ //没有转换成功 //释放读锁 stamptedLock.unlockRead(stm); // 获取写 long s2 = stamptedLock.writeLock(); stm=s2; System.out.println("手动获取写锁..."); } } }finally { stamptedLock.unlock(stm); } } //独占的写 public void write(int v){ long stamp = stamptedLock.writeLock(); try{ balance+=v; System.out.println("进行写的操作...结果: "+balance+" "+Thread.currentThread().getName()+" "+"write中的标记为:"+stamp); Thread.sleep(2000); }catch (Exception e){ e.printStackTrace(); } finally { stamptedLock.unlockWrite(stamp); } } public static void main(String[] args) { StampedLock01 stampedLock01 = new StampedLock01(); //测试普通的read // 测试成功 异步执行 //测试独占的写 成功 /* new Thread(()->{ // 乐观读 stampedLock01.OptimismRead(); }).start(); new Thread(()->{ stampedLock01.write(3); }).start(); */ new Thread(()->{ stampedLock01.MyConditionReadWriteLock(1); }).start(); } }

--------------------

> 最后提一下 > 如何选择**大名鼎鼎的AQS最有名的实现类ReentrantLock**和**synchronized**这个JVM提供的锁,一开始synchronized是一个笨重的重量级锁,但是jdk1.5之后,进行了偏向锁,轻量级锁的优化,使它的性能和ReentrantLock擦不多了,于是没有特殊的要求,官方推荐使用synchronized  
> 什么情况下使用ReentrantLock呢?

*  使用它特有的公平锁
 *  使用它的Condition类,分组唤醒指定的线程
 *  ## 提供了能够中断正在等待锁的线程的机制,lock.lockInterrupted() ##

转载于:https://www.cnblogs.com/ZhuChangwu/p/11150337.html