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简介
jdk中独占锁的实现除了使用关键字synchronized外,还可以使用ReentrantLock。虽然在性能上ReentrantLock和synchronized没有什么区别，但ReentrantLock相比synchronized而言功能更加丰富，使用起来更为灵活，也更适合复杂的并发场景。
1. 加锁解锁
synchronized加锁解锁的过程是隐式的,用户不用手动操作,优点是操作简单，但显得不够灵活。一般并发场景使用synchronized的就够了；ReentrantLock 需要手动加锁和解锁,**且解锁的操作尽量要放在finally代码块中**,保证线程正确释放锁。
ReentrantLock lock = new ReentrantLock();
try {
lock.lock();
} catch (Exception e) {
log.info("业务异常", e);
} finally {
lock.unlock();
}
2. 公平锁 & 非公平锁
非公平锁
由 cpu 自动分配线程的执行权限, 不受程序控制, 可能一个线程独占锁很长时间另一个线程获取不到锁
public static void main(String[] args) throws InterruptedException {
ReentrantLock lock = new ReentrantLock();
ExecutorService executorService = Executors.newCachedThreadPool();
executorService.submit(() -> {
for (int i = 0; i < 10; i++) {
lock.lock();
log.info("ThreadA :{}", DateFormatUtils.format(new Date(), "yyyyMMdd HH:mm:ss"));
lock.unlock();
}
});
executorService.submit(() -> {
for (int i = 0; i < 10; i++) {
lock.lock();
log.info("ThreadB :{}", DateFormatUtils.format(new Date(), "yyyyMMdd HH:mm:ss"));
lock.unlock();
}
});
TimeUnit.SECONDS.sleep(20);
executorService.shutdown();
}
12:21:37.535 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.545 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.545 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:21:37
12:21:37.546 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.547 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.548 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
12:21:37.548 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:21:37
公平锁
公平锁可以照顾到每个线程,根据日志我们可以看到两个线程交替执行
ReentrantLock lock = new ReentrantLock(true);
12:24:19.476 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.485 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.486 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.487 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.487 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.487 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.487 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.488 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.488 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.488 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.488 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.489 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.489 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.489 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.490 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.490 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.490 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.490 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
12:24:19.490 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadA :20190130 12:24:19
12:24:19.490 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample2 - ThreadB :20190130 12:24:19
3. 可以中断
synchronized 不能响应中断,该代码执行后,控制台不能够输出"线程B"
@Slf4j
public class ReentrantLockExample4 {
public static void main(String[] args) throws InterruptedException {
Object lock = new Object();
ExecutorService executorService = Executors.newCachedThreadPool();
executorService.submit(() -> {
synchronized (lock) {
while (true){
}
}
});
TimeUnit.SECONDS.sleep(1);
ExecutorService executorService2 = Executors.newCachedThreadPool();
executorService2.submit(() -> {
synchronized (lock) {
log.info("线程Ｂ");
}
});
//发送中断信号
executorService.shutdownNow();
}
}
ReentrantLock
根据输出结果我们可以看到,ReentrantLock响应了中断事件
@Slf4j
public class ReentrantLockExample3 {
public static void main(String[] args) throws InterruptedException {
ReentrantLock lock = new ReentrantLock(true);
ExecutorService executorService = Executors.newCachedThreadPool();
executorService.submit(() -> {
try {
lock.lockInterruptibly();
while (true) {
}
} catch (InterruptedException e) {
log.info("线程中断", e);
} finally {
lock.unlock();
}
});
//发送中断信号
executorService.shutdownNow();
}
}
输出结果
12:41:39.523 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample3 - 线程中断
java.lang.InterruptedException: null
at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireInterruptibly(AbstractQueuedSynchronizer.java:1261)
at java.base/java.util.concurrent.locks.ReentrantLock.lockInterruptibly(ReentrantLock.java:317)
at cn.wyj.learn.juc.ReentrantLockExample3.lambda$main$0(ReentrantLockExample3.java:18)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
at java.base/java.lang.Thread.run(Thread.java:834)
4. 获取锁时限时等待
基于这个特性我可以使用 ReentrantLock 编程避免死锁,但是要注意活锁的问题
@Slf4j
public class ReentrantLockExample5 {
public static void main(String[] args) throws InterruptedException {
ReentrantLock lock = new ReentrantLock();
ExecutorService executorService = Executors.newCachedThreadPool();
executorService.submit(() -> {
lock.lock();
try {
TimeUnit.SECONDS.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
log.info("线程A执行完成............");
lock.unlock();
});
executorService.submit(() -> {
try {
while (true) {
boolean b = lock.tryLock(2, TimeUnit.SECONDS);
if (!b) {
log.info("还没获取到锁过会再来吧............");
continue;
}
log.info("线程B执行任务完成");
lock.unlock();
break;
}
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
}
:48:50.511 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:52.515 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:54.517 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:56.518 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 还没获取到锁过会再来吧............
12:48:58.508 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 线程A执行完成............
12:48:58.508 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample5 - 线程B执行任务完成
5. Condition实现等待通知机制
基于 ReentrantLock Condition 可以实现阻塞队列等功能
@Slf4j
public class ReentrantLockExample6 {
public static void main(String[] args) throws InterruptedException {
ReentrantLock lock = new ReentrantLock();
Condition condition = lock.newCondition();
ExecutorService executorService = Executors.newCachedThreadPool();
executorService.submit(() -> {
lock.lock();
log.info("等待线程B执行完成");
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
log.info("线程A执行完成............");
lock.unlock();
});
executorService.submit(() -> {
lock.lock();
condition.signal();
log.info("线程B执行任务完成");
lock.unlock();
});
}
}
12:52:47.556 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample6 - 等待线程B执行完成
12:52:47.565 [pool-1-thread-2] INFO cn.wyj.learn.juc.ReentrantLockExample6 - 线程B执行任务完成
12:52:47.565 [pool-1-thread-1] INFO cn.wyj.learn.juc.ReentrantLockExample6 - 线程A执行完成............
