《实战Java高并发程序设计》学习记录5之ReentrantLo

ReentrantLock

• 可重入
  来看个demo：

package someTest;

import java.util.concurrent.locks.ReentrantLock;

public class ReentrantLockDemo implements Runnable{
    public static ReentrantLock lock=new ReentrantLock();
    public static int i=0;
    @Override
    public void run() {
        for(int j=0;j<100000;j++) {
            lock.lock();
            try {
                i++;
            }finally {
                lock.unlock();
            }
        }
    }
    public static void main(String[] args) throws InterruptedException {
        ReentrantLockDemo demo=new ReentrantLockDemo();
        Thread t1=new Thread(demo);
        Thread t2=new Thread(demo);
        t1.start();
        t2.start();
        t1.join();
        t2.join();
        System.out.println(i);
    }
}

输出结果：

200000

需要注意的问题：

一个线程执行了几次lock.lock();代码，对应的，在finally块当中就要执行几次lock.unlock();代码，否则别的线程就进不来了

• 可中断
  来看个demo，这可是个面试题

package someTest;

import java.util.concurrent.locks.ReentrantLock;

public class ReentrantLockInterruptDemo implements Runnable{
    public static ReentrantLock lock1=new ReentrantLock();
    public static ReentrantLock lock2=new ReentrantLock();
    int lock;
    public ReentrantLockInterruptDemo(int lock) {
        this.lock=lock;
    }
    @Override
    public void run() {
        try {
            if(lock==1) {
                lock1.lockInterruptibly();
                try {
                    Thread.sleep(500);
                }catch (InterruptedException e) {
                    e.printStackTrace();
                }
                lock2.lockInterruptibly();
            }else {
                lock2.lockInterruptibly();
                try {
                    Thread.sleep(500);
                }catch (InterruptedException e) {
                    e.printStackTrace();
                }
                lock1.lockInterruptibly();
            }
        }catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            if(lock1.isHeldByCurrentThread()) {
                lock1.unlock();
            }
            if(lock2.isHeldByCurrentThread()) {
                lock2.unlock();
            }
            System.out.println(Thread.currentThread().getId()+":线程退出");
        }
    }
    public static void main(String[] args) throws InterruptedException {
        ReentrantLockInterruptDemo r1=new ReentrantLockInterruptDemo(1);
        ReentrantLockInterruptDemo r2=new ReentrantLockInterruptDemo(2);
        Thread t1=new Thread(r1);
        Thread t2=new Thread(r2);
        t1.start();
        t2.start();Thread.sleep(1000);
        DeadlockChecker.check();
    }
    
}

辅助程序：DeadlockChecker.java,一个守护线程，看是否有死锁线程

package someTest;

import java.lang.management.ManagementFactory;
import java.lang.management.ThreadInfo;
import java.lang.management.ThreadMXBean;
    
/** 检查死锁  */
public class DeadlockChecker {
    private final static ThreadMXBean mbean = ManagementFactory.getThreadMXBean();
    
    final static Runnable deadLockChecker = new Runnable() {
        @Override
        public void run() {
            while(true){
                long[] deadLockedThreadIds = mbean.findDeadlockedThreads();
                if(deadLockedThreadIds != null){
                    ThreadInfo[] threadInfos = mbean.getThreadInfo(deadLockedThreadIds);
                    for(Thread t : Thread.getAllStackTraces().keySet()){
                        for(ThreadInfo ti : threadInfos){
                            if(ti.getThreadId() == t.getId()){
                                t.interrupt();
                            }
                        }
                    }
                }
                try {
                    Thread.sleep(5000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
            
        }
    };
    
    public static void check() {
        Thread t = new Thread(deadLockChecker);
        t.setDaemon(true);
        t.start();
    }
}

程序输出：

java.lang.InterruptedException
10:线程退出
11:线程退出
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:898)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
    at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
    at someTest.ReentrantLockInterruptDemo.run(ReentrantLockInterruptDemo.java:22)
    at java.lang.Thread.run(Thread.java:748)
java.lang.InterruptedException
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireInterruptibly(AbstractQueuedSynchronizer.java:898)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1222)
    at java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:335)
    at someTest.ReentrantLockInterruptDemo.run(ReentrantLockInterruptDemo.java:30)
    at java.lang.Thread.run(Thread.java:748)

• 可定时
  来看个demo：

package someTest;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantLock;

public class TimedLock implements Runnable{
    public static ReentrantLock lock=new ReentrantLock();

    @Override
    public void run() {
        try {
            if(lock.tryLock(5, TimeUnit.SECONDS)) {
                Thread.sleep(6000);
            }else {
                System.out.println("get lock failed");
            }
        }catch (InterruptedException e) {
            e.printStackTrace();
        }finally {
            if(lock.isHeldByCurrentThread())
                lock.unlock();
        }
    }
    public static void main(String[] args) {
        TimedLock ins=new TimedLock();
        Thread t1=new Thread(ins);
        Thread t2=new Thread(ins);
        t1.start();
        t2.start();
    }
    
}

输出结果：

get lock failed

• 公平锁
  这个看看就好，一般公平锁的性能不是很好，先到先得，需要处理一个排队的问题

public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

补充

• jps：查看当前运行的java线程
• jstack:查看线程栈上的信息