详见：http://blog.yemou.net/article/query/info/tytfjhfascvhzxcyt124
在java多线程中，为了提高效率有些共享资源允许同时进行多个读的操作，但只允许一个写的操作，比如一个文件，只要其内容不变可以让多个线程同时读，不必做排他的锁定，排他的锁定只有在写的时候需要，以保证别的线程不会看到数据不完整的文件。
下面是个关于多线程读写锁的例子，我稍微做了下修改，蛮容易理解的，来至于http://www.highya.com/redirect.php?fid=113&tid=7180&goto=nextoldset。
这里模拟了这样一个场景: 在ReadWriteLockOperator对象里设置一个共享资源 shareResources 。
有3个读者(A, B, C)一直连续的从 shareResources 获取信息, 然后输出到控制台 ;有一个作者每隔60秒往shareResources 加入信息, 加信息的过程相对耗时, 在这段时间, 任何读者都不能访问 shareResources。
写了4个类来验证这种情况，只在windows下做了测试。
ReadTask.java 读任务
WriteTask.java 写任务
ReadWriteLockLogic.java 读写操作的逻辑
ReadWriteLockTest.java 带有main方法的测试类
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public class ReadTask extends Thread { //logic bean private ReadWriteLockLogic readWriteLockOperator; //读者 private String reader; public ReadTask(ReadWriteLockLogic readWriteLockOperator, String reader) { this.readWriteLockOperator = readWriteLockOperator; this.reader = reader; } private ReadTask(){} // 执行任务 public void run() { if(this.readWriteLockOperator != null){ try { while(!isInterrupted()){ Thread.sleep(200); System.out.println(reader + " read:" + Thread.currentThread().toString() + " : " + this.readWriteLockOperator.read()); } } catch (Exception e) { // TODO: handle exception } } }} |
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public class WriteTask extends Thread{ //logic bean private ReadWriteLockLogic readWriteLockOperator; //作者 private String writer; public WriteTask(ReadWriteLockLogic readWriteLockOperator, String writer) { this.readWriteLockOperator = readWriteLockOperator; this.writer = writer; } private WriteTask(){} // 一个很耗时的写任务 public void run() { try { while(!isInterrupted()){ Thread.sleep(100); this.readWriteLockOperator.write(this.writer, "hehehhe"); } } catch (Exception e) { // TODO: handle exception } }} |
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import java.util.ArrayList;import java.util.List;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReadWriteLock;import java.util.concurrent.locks.ReentrantReadWriteLock;//读写操作的逻辑public class ReadWriteLockLogic { // 初始化一个 ReadWriteLock private ReadWriteLock lock = new ReentrantReadWriteLock(); //共享资源 private List<String> shareResources = new ArrayList<String>(0); //读 public String read() { // 得到 readLock 并锁定 Lock readLock = lock.readLock(); readLock.lock(); try { // 读相对省时，做空循环 大约0.5second for(int i=0 ;i<2500000; i++){ System.out.print(""); } // 做读的工作 StringBuffer buffer = new StringBuffer(); for (String shareResource : shareResources) { buffer.append(shareResource); buffer.append(" "); } return buffer.toString(); } finally { readLock.unlock();//一定要保证锁的释放 } } //写 public void write(String writer, String content) { // 得到 writeLock 并锁定 Lock writeLock = lock.writeLock(); writeLock.lock(); try { System.out.println(writer + " write ===" + Thread.currentThread().toString()); // 写比较费时，所以做空循环 大约13second for(int i=0 ;i<10000000; i++){ System.out.print(""); System.out.print(""); } // 做写的工作 int count = shareResources.size(); for (int i=count; i < count + 1; i++) { shareResources.add(content + "_" + i); } } finally { writeLock.unlock();//一定要保证锁的释放 } }} |
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import java.util.concurrent.ExecutionException;import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class ReadWriteLockTest { public static void main(String[] args) throws InterruptedException, ExecutionException { //1 创建一个具有排程功能的线程池 ScheduledExecutorService service = Executors.newScheduledThreadPool(5); //2 读写锁的logic bean ReadWriteLockLogic lockOperator = new ReadWriteLockLogic(); //3 生成一个可执行任务(该任务执行完毕可以返回结果 或者 抛出异常；而Runnable接口的run方法则不行) Runnable writeTask1 = new WriteTask(lockOperator, "作者A"); //4 延时0秒后每2秒重复执行writeTask1; service.scheduleAtFixedRate(writeTask1, 0, 60, TimeUnit.SECONDS); //5 创建3个读任务 Runnable readTask1 = new WriteTask(lockOperator, "作者B"); Runnable readTask2 = new ReadTask(lockOperator, "读者B"); Runnable readTask3 = new ReadTask(lockOperator, "读者C"); //6 延时0秒后每秒执行一次task1; service.scheduleAtFixedRate(readTask1, 1, 1, TimeUnit.SECONDS); service.scheduleAtFixedRate(readTask2, 2, 1, TimeUnit.SECONDS); service.scheduleAtFixedRate(readTask3, 3, 1, TimeUnit.SECONDS); }} |
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作者A write ===Thread[pool-1-thread-1,5,main]
作者B write ===Thread[pool-1-thread-4,5,main]
读者C read:Thread[pool-1-thread-3,5,main] : hehehhe_0 hehehhe_1
读者B read:Thread[pool-1-thread-2,5,main] : hehehhe_0 hehehhe_1
作者A write ===Thread[pool-1-thread-1,5,main]
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通过观察控制台，可以看到作者a出现后，大约5秒作者b才会出现，而又过了5秒后，读者c和读者b同时会出现，接着5秒后，作者a又出现了。这说明了，读锁之间没有排斥，可以多线程持有并且排斥WriteLock的持有线程。而WriteLock是全部排斥的，是独占的,比较独!
下面是附赠的读写锁的小知识，来至http://www.txdnet.cn/essay/view.jsp?tid=1288670091703&cid=2
(a).重入方面其内部的WriteLock可以获取ReadLock,但是反过来ReadLock想要获得WriteLock则永远都不要想.
(b).WriteLock可以降级为ReadLock,顺序是:先获得WriteLock再获得ReadLock,然后释放WriteLock,这时候线程将保持Readlock的持有.反过来ReadLock想要升级为WriteLock则不可能,为什么?参看(a),呵呵.
(c).ReadLock可以被多个线程持有并且在作用时排斥任何的WriteLock,而WriteLock则是完全的互斥.这一特性最为重要,因为对于高读取频率而相对较低写入的数据结构,使用此类锁同步机制则可以提高并发量.
(d).不管是ReadLock还是WriteLock都支持Interrupt,语义与ReentrantLock一致.
(e).WriteLock支持Condition并且与ReentrantLock语义一致,而ReadLock则不能使用Condition,否则抛出UnsupportedOperationException异常.