对象的方法中一旦加入synchronized修饰,则任何时刻只能有一个线程访问synchronized修饰的方法。假设有个数据对象拥有写方法与读 方法,多线程环境中要想保证数据的安全,需对该对象的读写方法都要加入 synchronized同步块。这样任何线程在写入时,其它线程无法读取与改变 数据;如果有线程在读取时,其他线程也无法读取或写入。这种方式在写入操作远大于读操作时,问题不大,而当读取远远大于写入时,会造成性能瓶颈,因为此种 情况下读取操作是可以同时进行的,而加锁操作限制了数据的并发读取。
ReadWriteLock解决了这个问题,当写操作时,其他线程无法读取或写入数据,而当读操作时,其它线程无法写入数据,但却可以读取数据 。
package demo.readwritelock; import java.text.SimpleDateFormat; import java.util.Date; import java.util.concurrent.locks.Lock; import java.util.concurrent.locks.ReadWriteLock; import java.util.concurrent.locks.ReentrantReadWriteLock; public class ReadWriteLockDemo { static SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"); public static void main(String[] args) throws InterruptedException { Data data = new Data(); Worker worker1 = new Worker(data, true); Worker worker2 = new Worker(data, false); worker1.start(); worker2.start(); } static class Worker extends Thread{ Data data; boolean read; public Worker(Data data, boolean read){ this.data = data; this.read = read; } public void run(){ if(read){ data.get(); } else { data.set(); } } } static class Data{ ReadWriteLock lock = new ReentrantReadWriteLock(); Lock read = lock.readLock(); Lock write = lock.writeLock(); public void set(){ write.lock(); System.out.println(Thread.currentThread().hashCode()+"set:begin"+sdf.format(new Date())); try{ Thread.sleep(5000); }catch(Exception e){ }finally{ System.out.println(Thread.currentThread().hashCode()+"set:end"+sdf.format(new Date())); write.unlock(); } } public int get(){ read.lock(); System.out.println(Thread.currentThread().hashCode()+"get:begin"+sdf.format(new Date())); try{ Thread.sleep(5000); }catch(Exception e){ }finally{ System.out.println(Thread.currentThread().hashCode()+"get:end"+sdf.format(new Date())); read.unlock(); } return 1; } } }
运行结果:
1043636732get:begin2016-01-26 10:26:42 1043636732get:end2016-01-26 10:26:47 1125883825set:begin2016-01-26 10:26:47 1125883825set:end2016-01-26 10:26:52
上面是当读数据的时候,写数据是不被允许的。下面展示可以同时读取同一个对象的数据的情况实例:
public static void main(String[] args) { Data data = new Data(); Worker worker1 = new Worker(data, true); Worker worker2 = new Worker(data, true); worker1.start(); worker2.start(); }
运行结果如下:
1981440623set:begin2016-01-26 10:38:35 332711452set:begin2016-01-26 10:38:35 332711452set:end2016-01-26 10:38:40 1981440623set:end2016-01-26 10:38:40