Java編程讀寫(xiě)鎖詳解

更新時(shí)間：2018年08月06日 09:02:52 作者：屌絲碼農(nóng)

本篇文章給大家詳細(xì)分享了Java編程讀寫(xiě)鎖的相關(guān)原理以及知識(shí)點(diǎn)內(nèi)容，有興趣的朋友們可以參考下。

ReadWriteLock也是一個(gè)接口，提供了readLock和writeLock兩種鎖的操作機(jī)制，一個(gè)資源可以被多個(gè)線(xiàn)程同時(shí)讀，或者被一個(gè)線(xiàn)程寫(xiě)，但是不能同時(shí)存在讀和寫(xiě)線(xiàn)程。

基本規(guī)則：讀讀不互斥讀寫(xiě)互斥寫(xiě)寫(xiě)互斥

問(wèn)題：既然讀讀不互斥，為何還要加讀鎖

答：如果只是讀，是不需要加鎖的，加鎖本身就有性能上的損耗

如果讀可以不是最新數(shù)據(jù)，也不需要加鎖

如果讀必須是最新數(shù)據(jù)，必須加讀寫(xiě)鎖

讀寫(xiě)鎖相較于互斥鎖的優(yōu)點(diǎn)僅僅是允許讀讀的并發(fā)，除此之外并無(wú)其他。

結(jié)論：讀寫(xiě)鎖能夠保證讀取數(shù)據(jù)的嚴(yán)格實(shí)時(shí)性，如果不需要這種嚴(yán)格實(shí)時(shí)性，那么不需要加讀寫(xiě)鎖。

簡(jiǎn)單實(shí)現(xiàn)：

package readandwrite;
 
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.ReentrantReadWriteLock;
 
public class MyTest {
 private static ReentrantReadWriteLock rwl=new ReentrantReadWriteLock();
 private static double data=0;
 static class readClass implements Runnable{
  @Override
  public void run() {
   rwl.readLock().lock();
   System.out.println("讀數(shù)據(jù)："+data);
   rwl.readLock().unlock();
  }
 }
 
 static class writeClass implements Runnable{
  private double i;
  
  public writeClass(double i) {
   this.i = i;
  }
 
  @Override
  public void run() {
   rwl.writeLock().lock();
   data=i;
   System.out.println("寫(xiě)數(shù)據(jù)： "+data);
   rwl.writeLock().unlock();
  }
  
 }
  
 public static void main(String[] args) throws InterruptedException {
  ExecutorService pool=Executors.newCachedThreadPool();
  for(int i=0;i<10;i++){
   pool.submit(new readClass());
   pool.submit(new writeClass((double)new Random().nextDouble()));
   pool.submit(new writeClass((double)new Random().nextDouble()));
   Thread.sleep(1000);
  }
   
  pool.shutdown();
 }
 
 
}

之前我們提到的鎖都是排它鎖（同一時(shí)刻只允許一個(gè)線(xiàn)程進(jìn)行訪(fǎng)問(wèn)），而讀寫(xiě)鎖維護(hù)了一對(duì)鎖，一個(gè)讀鎖，一個(gè)寫(xiě)鎖。讀寫(xiě)鎖在同一時(shí)刻允許多個(gè)線(xiàn)程進(jìn)行讀操作，但是寫(xiě)線(xiàn)程訪(fǎng)問(wèn)過(guò)程中，所有的讀線(xiàn)程和其他寫(xiě)線(xiàn)程均被阻塞。如此，并發(fā)性有了很大的提升。這樣，在某些讀遠(yuǎn)遠(yuǎn)大于寫(xiě)的場(chǎng)景中，讀寫(xiě)鎖能夠提供比排它鎖更好的并發(fā)量和吞吐量。

一個(gè)關(guān)于讀寫(xiě)鎖的Demo：

分析：設(shè)計(jì)一個(gè)模擬隊(duì)列，擁有一個(gè)data成員變量用于存儲(chǔ)數(shù)據(jù)和存取兩種操作。

import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class ReadWriteLockDemo
{

  public static void main(String[] args)
  {
    DefQueue queue = new DefQueue();
    for (int i = 1; i < 10; i++)
    {
      //啟動(dòng)線(xiàn)程進(jìn)行讀操作
      new Thread(new Runnable()
      {
        @Override
        public void run()
        {
          while (true)
          {
            queue.get();
          }
        }

      }).start();

      //啟動(dòng)線(xiàn)程進(jìn)行寫(xiě)操作
      new Thread(new Runnable()
      {
        @Override
        public void run()
        {
          while(true)
          {
            queue.put(new Random().nextInt(10000));
          }
        }
      }).start();
    }
  }

}

class DefQueue
{
  private int data;
  ReadWriteLock rwLock = new ReentrantReadWriteLock();

  public void get()
  {
    rwLock.readLock().lock();//加讀鎖
    try
    {
      System.out.println(Thread.currentThread().getName() + "be ready to get data");
      Thread.sleep((long) (Math.random() * 1000));

      System.out.println(Thread.currentThread().getName() + "get the data:  " + data);

    } catch (InterruptedException e)
    {
      e.printStackTrace();
    } finally
    {
      rwLock.readLock().unlock();//釋放讀鎖
    }
  }

  public void put(int data)
  {
    rwLock.writeLock().lock();//加寫(xiě)鎖

    try
    {
      System.out.println(Thread.currentThread().getName() + " be ready to write data");

      Thread.sleep((long) (Math.random() * 1000));

      this.data = data;

      System.out.println(Thread.currentThread().getName() + " has wrote the data: "+data);
    } catch (InterruptedException e)
    {
      e.printStackTrace();
    } finally
    {
      rwLock.writeLock().unlock();//釋放寫(xiě)鎖
    }

  }
}

程序部分運(yùn)行結(jié)果：

Thread-0be ready to get data
Thread-0get the data:  0
Thread-1 be ready to write data
Thread-1 has wrote the data: 1156
Thread-2be ready to get data
Thread-2get the data:  1156
Thread-3 be ready to write data
Thread-3 has wrote the data: 9784
Thread-3 be ready to write data
Thread-3 has wrote the data: 4370
Thread-3 be ready to write data
Thread-3 has wrote the data: 1533
Thread-4be ready to get data
Thread-4get the data:  1533
Thread-5 be ready to write data
Thread-5 has wrote the data: 2345
Thread-6be ready to get data
Thread-6get the data:  2345
Thread-9 be ready to write data
Thread-9 has wrote the data: 9463
Thread-9 be ready to write data
Thread-9 has wrote the data: 9301
Thread-9 be ready to write data
Thread-9 has wrote the data: 549
Thread-9 be ready to write data
Thread-9 has wrote the data: 4673
Thread-9 be ready to write data

我們可以看到打印語(yǔ)句結(jié)果很正常。

下面我們?cè)賮?lái)實(shí)現(xiàn)一個(gè)模擬緩沖區(qū)的小Demo：

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;

/*
 * @author vayne
 * 
 * 多線(xiàn)程實(shí)現(xiàn)緩存的小demo
 */
class Cachend
{
  volatile Map<String, String> cachmap = new HashMap<String, String>();//加volatile關(guān)鍵字保證可見(jiàn)性。

  ReadWriteLock rwLock = new ReentrantReadWriteLock();//這個(gè)讀寫(xiě)鎖要定義在方法外面，使得每一個(gè)線(xiàn)程用的是同一個(gè)讀寫(xiě)鎖。
  public String getS(String key)           //如果定義在方法內(nèi)部，就是跟方法棧有關(guān)的讀寫(xiě)鎖。這樣可能不是同一個(gè)鎖。
  {
    rwLock.readLock().lock();
    String value = null;
    try
    {
      value = cachmap.get(key);

      if (cachmap.get(key) == null)//這里要重新獲得key對(duì)應(yīng)的value值
      {
        rwLock.readLock().unlock();
        rwLock.writeLock().lock();
        try
        {
          if (cachmap.get(key) == null)//這里也是
          {
            value = "" + Thread.currentThread().getName();

            cachmap.put(key, value);

            System.out.println(Thread.currentThread().getName() + " put the value ::::" + value);
          }
        } finally
        {
          rwLock.readLock().lock();  //將鎖降級(jí)，這里跟下一句的順序不能反。
          rwLock.writeLock().unlock();//關(guān)于這里的順序問(wèn)題，下面我會(huì)提到。
        }
      }

    } finally
    {
      rwLock.readLock().unlock();
    }

    return cachmap.get(key);
  }
}

public class CachendDemo
{
  public static void main(String[] args)
  {
    Cachend ca = new Cachend();
    for (int i = 0; i < 4; i++)
    {
      new Thread(new Runnable()
      {
        @Override
        public void run()
        {
          System.out.println(Thread.currentThread().getName()+" "+ca.getS("demo1"));
          System.out.println(Thread.currentThread().getName()+" "+ca.cachmap.entrySet());
        }
      }).start();
    }
  }
}

運(yùn)行結(jié)果：

Thread-0 put the value ::::Thread-0
Thread-0 Thread-0
Thread-0 [demo1=Thread-0]
Thread-2 Thread-0
Thread-2 [demo1=Thread-0]
Thread-3 Thread-0
Thread-3 [demo1=Thread-0]
Thread-1 Thread-0
Thread-1 [demo1=Thread-0]

上面我給出了一些注釋?zhuān)鋵?shí)這個(gè)代碼是很不好寫(xiě)的，考慮的東西很多。下面我來(lái)講一下上面的代碼中提到的順序問(wèn)題。

對(duì)于讀寫(xiě)鎖我們應(yīng)該了解下面的一些性質(zhì)（這些性質(zhì)是由源代碼得出來(lái)的，因?yàn)樵创a的設(shè)計(jì)，所以才有下列性質(zhì)）：

如果存在讀鎖，則寫(xiě)鎖不能被獲取，原因在于：讀寫(xiě)鎖要確保寫(xiě)鎖的操作對(duì)讀鎖可見(jiàn)。，如果允許讀鎖在已被獲取的情況下對(duì)寫(xiě)鎖的獲取，那么正在運(yùn)行的其他讀線(xiàn)程就無(wú)法感知到當(dāng)前寫(xiě)線(xiàn)程的操作。因此，只有等待其他讀線(xiàn)程都釋放了讀鎖，寫(xiě)鎖才能被當(dāng)前線(xiàn)程獲取，而寫(xiě)鎖一旦被獲取，則其他讀寫(xiě)線(xiàn)程的后續(xù)訪(fǎng)問(wèn)將會(huì)被阻塞。
鎖降級(jí)：指的是寫(xiě)鎖降級(jí)成為讀鎖。具體操作是獲取到寫(xiě)鎖之后，在釋放寫(xiě)鎖之前，要先再次獲取讀鎖。這也就是上面我寫(xiě)注釋提醒大家注意的地方。為什么要這樣處理呢，答案就是為了保證數(shù)據(jù)可見(jiàn)性。如果當(dāng)前線(xiàn)程不獲取讀鎖而是直接釋放寫(xiě)鎖，假設(shè)此刻另一個(gè)線(xiàn)程（記作T）獲取了寫(xiě)鎖并修改了數(shù)據(jù)，那么當(dāng)前線(xiàn)程無(wú)法感知線(xiàn)程T的數(shù)據(jù)更新。如果當(dāng)前線(xiàn)程獲取讀鎖，即遵循鎖降級(jí)的步驟，則線(xiàn)程T將會(huì)被阻塞，知道當(dāng)前線(xiàn)程使用數(shù)據(jù)并釋放讀鎖之后，T才能獲取寫(xiě)鎖進(jìn)行數(shù)據(jù)更新。

第二條對(duì)應(yīng)我們上面的程序就是，如果我們添加了“demo1”對(duì)應(yīng)的value值，然后釋放了寫(xiě)鎖，此時(shí)在當(dāng)前線(xiàn)程S還未獲得讀鎖時(shí)，另一個(gè)線(xiàn)程T又獲得了寫(xiě)鎖，那么就會(huì)將S的操作給覆蓋（如果取到的值已經(jīng)緩存在S中，那么T的操作就無(wú)法被S感知了，到最后依然會(huì)返回S操作的值）。

再來(lái)看一個(gè)DEMO：

讀寫(xiě)鎖,分為讀鎖和寫(xiě)鎖,多個(gè)讀鎖不互斥,讀鎖和寫(xiě)鎖互斥,寫(xiě)鎖與寫(xiě)鎖互斥,這是JVM自己控制的,你只要上好相應(yīng)的鎖即可,如果你的代碼只讀數(shù)據(jù),可以很多人同時(shí)讀,但不能同時(shí)寫(xiě),那就上讀鎖;如果你的代碼修改數(shù)據(jù),只能有一個(gè)人在寫(xiě),且不能同時(shí)讀取,那就上寫(xiě)鎖.總之,讀的時(shí)候上讀鎖,寫(xiě)的時(shí)候上寫(xiě)鎖!

看如下程序: 新建6個(gè)線(xiàn)程,3個(gè)線(xiàn)程用來(lái)讀,3個(gè)線(xiàn)程用來(lái)寫(xiě),

package javaplay.thread.test;
 
import java.util.Random;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
 
public class ReadWriteLockTest {
  public static void main(String[] args) {
    final Queue3 q3 = new Queue3();
    for (int i = 0; i < 3; i++) {
      new Thread() {
        public void run() {
          while (true) {
            q3.get();
          }
        }
      }.start();
      new Thread() {
        public void run() {
          while (true) {
            q3.put(new Random().nextInt(10000));
          }
        }
      }.start();
    }
  }
}
 
class Queue3 {
  private Object data = null;// 共享數(shù)據(jù)，只能有一個(gè)線(xiàn)程能寫(xiě)該數(shù)據(jù)，但可以有多個(gè)線(xiàn)程同時(shí)讀該數(shù)據(jù)。
  // 讀寫(xiě)鎖
  ReadWriteLock rwl = new ReentrantReadWriteLock();
 
  // 相當(dāng)于讀操作
  public void get() {
    rwl.readLock().lock();
    try {
      System.out.println(Thread.currentThread().getName() + " be ready to read data!");
      Thread.sleep((long) (Math.random() * 1000));
      System.out.println(Thread.currentThread().getName() + "have read data :" + data);
    } catch (InterruptedException e) {
      e.printStackTrace();
    } finally {
      rwl.readLock().unlock();
    }
  }
 
  // 相當(dāng)于寫(xiě)操作
  public void put(Object data) {
    rwl.writeLock().lock();
    try {
      System.out.println(Thread.currentThread().getName() + " be ready to write data!");
      Thread.sleep((long) (Math.random() * 1000));
      this.data = data;
      System.out.println(Thread.currentThread().getName() + " have write data: " + data);
    } catch (InterruptedException e) {
      e.printStackTrace();
    } finally {
      rwl.writeLock().unlock();
    }
  }
}

讀寫(xiě)鎖功能很強(qiáng)大！這樣可以實(shí)現(xiàn)正常的邏輯,如果我們把讀寫(xiě)鎖相關(guān)的代碼注釋,發(fā)現(xiàn)程序正準(zhǔn)備寫(xiě)的時(shí)候,就有線(xiàn)程讀了,發(fā)現(xiàn)準(zhǔn)備讀的時(shí)候,有線(xiàn)程去寫(xiě),這樣不符合我們的邏輯；通過(guò)Java5的新特新可以很輕松的解決這樣的問(wèn)題；

查看Java API ReentrantReadWriteLock 上面有經(jīng)典(緩存)的用法，下面是doc里面的偽代碼,，它演示的是一個(gè)實(shí)體的緩存，不是緩存系統(tǒng)，相當(dāng)于緩存代理，注意volatile的運(yùn)用:

package javaplay.thread.test;
 
import java.util.concurrent.locks.ReentrantReadWriteLock;
 
/*
 * Sample usages. Here is a code sketch showing how to perform lock downgrading after updating a cache 
 * (exception handling is particularly tricky when handling multiple locks in a non-nested fashion):
 */
class CachedData {
  Object data;
  volatile boolean cacheValid;
  final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();
 
  void processCachedData() {
    rwl.readLock().lock();
    if (!cacheValid) {
      // Must release read lock before acquiring write lock
      rwl.readLock().unlock();
      rwl.writeLock().lock();
      try {
        // Recheck state because another thread might have
        // acquired write lock and changed state before we did.
        if (!cacheValid) {
          data = ...
          cacheValid = true;
        }
        // Downgrade by acquiring read lock before releasing write lock
        rwl.readLock().lock();
      } finally {
        rwl.writeLock().unlock(); // Unlock write, still hold read
      }
    }
 
    try {
      use(data);
    } finally {
      rwl.readLock().unlock();
    }
  }
}

假設(shè)現(xiàn)在多個(gè)線(xiàn)程來(lái)讀了，那第一個(gè)線(xiàn)程讀到的數(shù)據(jù)是空的，那它就要寫(xiě)就要填充數(shù)據(jù)，那么第二個(gè)第三個(gè)就應(yīng)該互斥等著，一進(jìn)來(lái)是來(lái)讀數(shù)據(jù)的所以上讀鎖，進(jìn)來(lái)后發(fā)現(xiàn)數(shù)據(jù)是空的，就先把讀鎖釋放再重新獲取寫(xiě)鎖，就開(kāi)始寫(xiě)數(shù)據(jù)，數(shù)據(jù)寫(xiě)完了，就把寫(xiě)鎖釋放，把讀鎖重新掛上，持有讀鎖時(shí)不能同時(shí)獲取寫(xiě)鎖，但擁有寫(xiě)鎖時(shí)可同時(shí)再獲取讀鎖，自己線(xiàn)程掛的寫(xiě)鎖可同時(shí)掛讀鎖的，這就是降級(jí)，就是除了讀鎖和寫(xiě)鎖外，還有讀寫(xiě)鎖也叫更新鎖，就是自己即可以讀又可以寫(xiě)的鎖，也就是在自己擁有寫(xiě)鎖還沒(méi)釋放寫(xiě)鎖時(shí)就獲取了讀鎖就降級(jí)為讀寫(xiě)鎖/更新鎖，但是不能在持有讀鎖時(shí)再獲取寫(xiě)鎖；

基于上面的例子,我們可以實(shí)現(xiàn)一個(gè)緩存系統(tǒng)：

package javaplay.thread.test;
 
import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.locks.ReadWriteLock;
import java.util.concurrent.locks.ReentrantReadWriteLock;
 
public class CacheDemo {
  private Map<String, Object> cache = new HashMap<>();
 
  public static void main(String[] args) {
 
  }
 
  // 可做到多個(gè)線(xiàn)程并必的讀 讀和寫(xiě)又互斥 系統(tǒng)性能很高
  // 這就是讀寫(xiě)鎖的價(jià)值
  private ReadWriteLock rwl = new ReentrantReadWriteLock();
 
  public Object getData(String key) {
    rwl.readLock().lock();
    Object value = null;
    try {
      value = cache.get(key);
      if (value == null) {// 避免首次多次查詢(xún)要加synchronized
        rwl.readLock().unlock();
        rwl.writeLock().lock();
        try {
          if (value == null) // 就算第二個(gè)第三個(gè)線(xiàn)程進(jìn)來(lái)時(shí)也不用再寫(xiě)了 跟偽代碼相同原理
            value = "aaa";// 實(shí)際去query db
        } finally {
          rwl.writeLock().unlock();
        }
        rwl.readLock().lock();
      }
    } finally {
      rwl.readLock().unlock();
    }
    return value;
  }
}
錯(cuò)誤之處：沒(méi)有把不存在的值put；要用get(key)來(lái)判空

感謝大家對(duì)腳本之家的支持。

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