读写锁操作

说到ReentrantReadWriteLock,首先要做的是与ReentrantLock划清界限.它和后者都是单独的实现,彼此之间没有继承或实现的关系.然后就是总结这个锁机制的特性了:

(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.

ReentrantReadWriteLock竞争条件

ReentrantReadWriteLock会使用两把锁来解决问题,一个读锁,一个写锁

线程进入读锁的前提条件:
没有其他线程的写锁,
没有写请求或者有写请求,但调用线程和持有锁的线程是同一个

线程进入写锁的前提条件:
没有其他线程的读锁
没有其他线程的写锁

 

import java.util.HashMap;import java.util.Map;import java.util.concurrent.locks.Lock;import java.util.concurrent.locks.ReentrantReadWriteLock;public class ReentrantReadWriteLockSample {public static void main(String[] args) {testReadLock();//testWriteLock();}public static void testReadLock() {final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();support.initCache();Runnable runnable = new Runnable() {public void run() {support.get("test");}};new Thread(runnable).start();new Thread(runnable).start();new Thread(new Runnable() {public void run() {support.put("test", "test");}}).start();}public static void testWriteLock() {final ReadWriteLockSampleSupport support = new ReadWriteLockSampleSupport();support.initCache();new Thread(new Runnable() {public void run() {support.put("key1", "value1");}}).start();new Thread(new Runnable() {public void run() {support.put("key2", "value2");}}).start();new Thread(new Runnable() {public void run() {support.get("key1");}}).start();}}class ReadWriteLockSampleSupport {private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();private final Lock readLock = lock.readLock();private final Lock writeLock = lock.writeLock();private volatile boolean completed;private Map<String,String> cache;public void initCache() {readLock.lock();if(!completed) {// Must release read lock before acquiring write lock                            //必须释放锁之前获得读写锁readLock.unlock(); // (1)writeLock.lock(); // (2)                            // Recheck state because another thread might have acquired                             //检查状态，因为另一个线程可能获得                            // write lock and changed state before we did.                            //写锁和状态改变之前，我们所做的。if(!completed) {cache = new HashMap<String,String>(32);completed = true;}// Downgrade by acquiring read lock before releasing write lock                            //获得读锁之前释放写锁readLock.lock(); // (3)writeLock.unlock(); // (4) Unlock write, still hold read}System.out.println("empty? " + cache.isEmpty());readLock.unlock();}public String get(String key) {readLock.lock();System.out.println(Thread.currentThread().getName() + " read.");startTheCountdown();try{return cache.get(key);}finally{readLock.unlock();}}public String put(String key, String value) {writeLock.lock();System.out.println(Thread.currentThread().getName() + " write.");startTheCountdown();try{return cache.put(key, value);}finally {writeLock.unlock();}}/** * A simple countdown,it will stop after about 5s.  */public void startTheCountdown() {long currentTime = System.currentTimeMillis();for(;;) {long diff = System.currentTimeMillis() - currentTime;if(diff > 5000) {break;}}}}

 

这个例子改造自JDK的API提供的示例,其中ReadWriteLockSampleSupport辅助类负责维护一个Map,当然前提是这个Map大部分的多线程下都是读取,只有很少的比例是多线程竞争修改Map的值.其中的initCache()简单的说明了特性(a),(b).在这个方法中如果把注释(1)和(2)处的代码调换位置,就会发现轻而易举的死锁了,当然是因为特性(1)的作用了.而注释(3),(4)处的代码位置则再次证明了特性 (a),并且有力的反映了特性(b)--WriteLock在cache初始化完毕之后,降级为ReadLock.另外get(),put()方法在线程获取锁之后会在方法中呆上近5s的时间.

ReentrantReadWriteLockSample中的两个静态测试方法则分别测试了ReadLock和WriteLock的排斥性. testReadLock()中,开启三个线程,前两者试图获取ReadLock而后者去获取WriteLock.执行结果可以看到:ReadWriteLockSampleSupport的get()方法中的打印结果在前两个线程中几乎同时显示,而put()中的打印结果则要等上近5s.这就说明了,ReadLock可以多线程持有并且排斥WriteLock的持有线程.testWriteLock()中,也开启三个线程.前两个是去获取WriteLock,最后一个获取ReadLock.执行的结果是三个打印结果都有近5s的间隔时间,这说明了WriteLock是独占的,比较独!

使用ReentrantReadWriteLock可以推广到大部分读,少量写的场景,因为读线程之间没有竞争,所以比起sychronzied,性能好很多.
如果需要较为精确的控制缓存,使用ReentrantReadWriteLock倒也不失为一个方案.

-----------------------------------------------------第二个例子-----------------------------------------------------------------------

 

读任务

package cn.itcast.heima.readwritelock;//可执行任务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      }    }  }}

写任务

package cn.itcast.heima.readwritelock;//可执行任务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    }  }}

读写操作的逻辑

package cn.itcast.heima.readwritelock;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("\t");             }      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<50000000; 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    }  }}

测试

package cn.itcast.heima.readwritelock;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 ReadTask(lockOperator, "读者A");    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);     }}