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Lock的使用 1.使用ReentrantLock类 * 在java多线程中,可以使用synchronized关键字来实现线程之间同步互斥,但在JDK1.5中新增加了ReentrantLock类 也能达到同样的效果,并且再扩展功能上也更加强大,比如有嗅探锁定,多路分支通知等功能,而且在使用上也比 synchronized更加的灵活。 *使用ReentrantLock实现同步举个栗子:
public class MyService { private Lock lock=new ReentrantLock(); public void testMethod(){ lock.lock(); for(int i=0;i<5;i++){ System.out.println("ThreadName="+Thread.currentThread().getName()+(""+(i+1))); } lock.unlock(); }}
public class MyThread extends Thread { private MyService myService; public MyThread(MyService myService){ super(); this.myService=myService; } @Override public void run() { myService.testMethod(); }}
public class Run { public static void main(String[] args) { MyService service = new MyService(); MyThread a1 = new MyThread(service); MyThread a2 = new MyThread(service); MyThread a3 = new MyThread(service); MyThread a4 = new MyThread(service); MyThread a5 = new MyThread(service); a1.start(); a2.start(); a3.start(); a4.start(); a5.start(); }}很简单,使用lock加锁,unlock解锁 * 使用Condition实现等待/通知 关键字synchronized与wait()和notify()/notifyAll()方法相结合可以实现等待/通知模式,类ReentrantLock也 可以实现同样的功能,但需要借助于Condition对象。Condition类是在JDK5中出现的技术,使用它有更好的灵活性, 比如可以实现多路通知功能,也就是在一个Lock对象里面可以创建多个Condition(即对象监视器)实例,线程对象可以 注册在指定的Condition中,从而可以有选择性地进行线程通知,在调度线程上更加灵活。 注意:在调用condition.await()方法前,必须用lock.lock()代码获取同步监视器。 * 使用Condition实现等待/通知
举个栗子
public class MyService { private Lock lock = new ReentrantLock(); public Condition condition = lock.newCondition(); public void await() { try { lock.lock(); System.out.println(" await时间为" + System.currentTimeMillis()); condition.await(); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } public void signal() { try { lock.lock(); System.out.println("signal时间为 " + System.currentTimeMillis()); condition.signal(); } finally { lock.unlock(); } }}
public class ThreadA extends Thread { private MyService service; public ThreadA(MyService service) { super(); this.service = service; } @Override public void run() { service.await(); }}
public class Run { public static void main(String[] args) throws InterruptedException { MyService service = new MyService(); ThreadA a = new ThreadA(service); a.start(); Thread.sleep(3000); service.signal(); }}
*使用多个Condition实现通知部分线程
public class MyService { private Lock lock = new ReentrantLock(); public Condition conditionA = lock.newCondition(); public Condition conditionB = lock.newCondition(); public void awaitA() { try { lock.lock(); System.out.println("begin awaitA时间为" + System.currentTimeMillis() + "ThreadName=" + Thread.currentThread().getName()); conditionA.await(); System.out.println(" end awaitA时间为" + System.currentTimeMillis() + " ThreadName=" + Thread.currentThread().getName()); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } public void awaitB() { try { lock.lock(); System.out.println("begin awaitB时间为 " + System.currentTimeMillis() + " ThreadName=" + Thread.currentThread().getName()); conditionB.await(); System.out.println(" end awaitB时间为 " + System.currentTimeMillis() + " ThreadName=" + Thread.currentThread().getName()); } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } public void signalAll_A() { try { lock.lock(); System.out.println(" signalAll_A时间为" + System.currentTimeMillis() + " ThreadName=" + Thread.currentThread().getName()); conditionA.signalAll(); } finally { lock.unlock(); } } public void signalAll_B() { try { lock.lock(); System.out.println(" signalAll_B时间为" + System.currentTimeMillis() + " ThreadName=" + Thread.currentThread().getName()); conditionB.signalAll(); } finally { lock.unlock(); } }}测试后,只有线程A被唤醒了。使用ReentrantLock对象可以唤醒指定种类的线程,这是控制部分线程行为的方便方式。
*公平锁与非公平锁
公平与非公平锁:锁Lock分为 "公平锁”和"非公平锁",公平锁表示线程获取锁的顺序是按照线程加锁的顺序来分配的, 即先来先得的FIFO先进先出顺序。而非公平锁就是一种获取锁的抢占机制,是随机获得锁的,和公平锁不一样的就是先来的 不一定先得到锁,这个方式可能造成某些线程一直拿不到锁,结果也就是不公平的了。 lock =new ReentrantLock(true);//公平锁 lock =new ReentrantLock(false);//非公平锁转载地址:http://jmjqi.baihongyu.com/