lock.tryLock()方法的使用

package concurrent;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class TestTryLock {

    private List<Object> list = new ArrayList<Object>();
    private Lock         lock = new ReentrantLock();

    public static void main(String[] args) {
        final TestTryLock test = new TestTryLock();
        new Thread("第一个线程  ") {

            @Override
            public void run() {
                test.doSomething(Thread.currentThread());
            }
        }.start();

        new Thread("第二个线程  ") {

            @Override
            public void run() {
                test.doSomething(Thread.currentThread());
            }
        }.start();
    }

    public void doSomething(Thread thread) {
        if (lock.tryLock()) {
            try {
                System.out.println(thread.getName() + "得到了锁.");
                for (int i = 0; i < 10; i++) {
                    list.add(i);
                }
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                System.out.println(thread.getName() + "释放了锁.");
                lock.unlock();
            }
        } else {
            System.out.println(thread.getName() + "获取锁失败.");
        }
    }
}

以上代码运行结果如下：

第一个线程  得到了锁.
第一个线程  释放了锁.
第二个线程  得到了锁.
第二个线程  释放了锁.

package concurrent;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class TestTryLock {

    private List<Object> list = new ArrayList<Object>();
    private Lock         lock = new ReentrantLock();

    public static void main(String[] args) {
        final TestTryLock test = new TestTryLock();
        new Thread("第一个线程  ") {

            @Override
            public void run() {
                test.doSomething(Thread.currentThread());
            }
        }.start();

        new Thread("第二个线程  ") {

            @Override
            public void run() {
                test.doSomething(Thread.currentThread());
            }
        }.start();
    }

    public void doSomething(Thread thread) {
        if (lock.tryLock()) {
            try {
                System.out.println(thread.getName() + "得到了锁.");
                for (int i = 0; i < 10; i++) {
                    list.add(i);
                    Thread.sleep(10);
                }
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                System.out.println(thread.getName() + "释放了锁.");
                lock.unlock();
            }
        } else {
            System.out.println(thread.getName() + "获取锁失败.");
        }
    }
}

运行结果如下：

第二个线程  得到了锁.
第一个线程  获取锁失败.
第二个线程  释放了锁.

问题如下：
我知道lock()方法去获取锁，当获取不到锁的时候，会一直等待。直到获取到锁。
tryLock()方法获取锁的时候，制作一次试探，如果获取锁失败，就不会一直等待的。如果是这样的话，如我Demo所示的这样，在业务逻辑中使用tryLock很容易造成程序不可控。比较疑惑这个tryLock的使用方法。。求大神解释。。谢谢~~

java jdk spring-mvc

阅读 25.5k

4 个回答

得票最新

magooxx

4924

发布于
2016-05-31

✓ 已被采纳

这个最好把Lock的四个锁法都比较一下（容我copy些东西）：

void lock();

If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until the lock has been acquired.

在等待获取锁的过程中休眠并禁止一切线程调度

void lockInterruptibly() throws InterruptedException;

If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happens:
The lock is acquired by the current thread; or Some other thread interrupts the current thread, and interruption of lock acquisition is supported.

在等待获取锁的过程中可被中断

boolean tryLock();

Acquires the lock if it is available and returns immediately with the value true. If the lock is not available then this method will return immediately with the value false.

获取到锁并返回true；获取不到并返回false

*boolean tryLock(long time, TimeUnit unit) throws InterruptedException;

If the lock is available this method returns immediately with the value true. If the lock is not available then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happens:The lock is acquired by the current thread; or Some other thread interrupts the current thread, and interruption of lock acquisition is supported; or The specified waiting time elapses.

在指定时间内等待获取锁；过程中可被中断

假如线程A和线程B使用同一个锁LOCK，此时线程A首先获取到锁LOCK.lock()，并且始终持有不释放。如果此时B要去获取锁，有四种方式：

LOCK.lock(): 此方式会始终处于等待中，即使调用B.interrupt()也不能中断，除非线程A调用LOCK.unlock()释放锁。
LOCK.lockInterruptibly(): 此方式会等待，但当调用B.interrupt()会被中断等待，并抛出InterruptedException异常，否则会与lock()一样始终处于等待中，直到线程A释放锁。
LOCK.tryLock(): 该处不会等待，获取不到锁并直接返回false，去执行下面的逻辑。
LOCK.tryLock(10, TimeUnit.SECONDS)：该处会在10秒时间内处于等待中，但当调用B.interrupt()会被中断等待，并抛出InterruptedException。10秒时间内如果线程A释放锁，会获取到锁并返回true，否则10秒过后会获取不到锁并返回false，去执行下面的逻辑。