Java Concurrency
在多线程环境下，为了保证共享数据的原子和内存可见性，需要进行锁操作。在JAVA中提供了内置锁和显示锁。本文使用用例结合，来介绍以下锁的用法：
内置锁(synchronized)
- 内置锁用来锁定代码块，在进入代码的时候获取锁定，在退出（或者异常退出）释放锁定。内置锁是互斥的，意味中同一时刻只能有一个线程获取该锁，其它线程只能等待或者阻塞直到锁的释放。如下面代码中，假如线程1执行addOne操作，当线程2调用getOne时，就需要等待线程1执行完成并释放锁。
public class ProductPool {
private Integer product = new Integer(0);
public synchronized Integer getProduct() {
return product;
}
public synchronized void addOne() {
this.product = this.product + 1;
LOG.info("produce value: {}", this.product);
}
public synchronized Integer getOne() {
Integer old = new Integer(this.product);
this.product = this.product - 1;
return old;
}
}
- 内置锁是可以重入的。当线程A获取锁执行某操作，如果在当前线程A内，某个步骤也需要获取该锁，该步骤是可以获取到锁的。如下例子，当ChildClass的对象执行doPrint时已经获取到了锁，内部继续调用super.doPrint，如果不能重入就会发生死锁。在同一线程内，锁可以重入。
public class SynchronizedDeakLock {
private static final Logger LOG = LoggerFactory.getLogger(SynchronizedLock.class);
public class BaseClass {
public synchronized void doPrint() {
LOG.info("base class print");
}
}
public class ChildClass extends BaseClass {
@Override
public synchronized void doPrint() {
LOG.info("child class do print");
super.doPrint();
}
}
@Test
public void testDeadLock() {
ChildClass childClass = new ChildClass();
childClass.doPrint();
}
}
- 内置锁使用非常简单，在需要同步的方法、代码块上加入synchronized就行了，不需要显示的获取和释放锁。且内置锁是JVM内置的，它可以执行部分优化，比如在线程封闭锁对象（该对象使用了锁，但是却不是共享对象，只在某一个线程使用）的锁消除，改变锁的粒度来消除内置锁的同步等。
- 在某些情况下，我们希望获取锁但又不想一直等待，所以我们指定获取到锁的最大时间，如果获取不到就超时。内置锁对这种细粒度的控制是不支持的，JAVA提供了一种新的锁机制：显示锁。下章，我们就对该话题进行讨论。
ReentrantLock
ReentrantLock是JAVA 5提供的细粒度的锁，作为内置锁在某些场景的补充。比如：支持线程获取锁的时间设置，支持获取锁线程对interrupt事件响应。但是在使用时必须显示的获取锁，然后在finally中释放。如果不释放，相当于在程序中放置了个定时炸弹，后期很难发现。它实现了Lock的以下API(部分例子为了达到测试效果没有unlock, 实际使用中绝对不能这样)：
1 . void lock() 获取锁，一致等待直到获取。下面的例子中，在主线程中获取锁且不释放， 子线程调用lock方法来获取锁。可以看到，子线程一致处于RUNNABLE状态，即使它被interrupt。
@Test
public void testLockWithInterrupt() throws InterruptedException {
final Lock lock = new ReentrantLock();
lock.lock();
Thread childThread = new Thread(() -> {
lock.lock();
}, "t1 thread");
childThread.start();
childThread.interrupt();
LOG.info("the child thread state: {}", childThread.getState().name());
assertFalse(childThread.isInterrupted());
}
2 . void lockInterruptibly() throws InterruptedException; 获取锁直到线程被interrupt, 线程抛出InterruptedException。下面的例子中，主线程获取锁且不释放，子线程调用lockInterruptibly方法来获取锁。首先在子线程获取不到锁的时候，会处于一直等待状态；当主线程中调用子线程interrupt时，子线程会抛出InterruptedException。
@Test(expected = InterruptedException.class)
public void testLockInterruptibly() throws Exception {
final Lock lock = new ReentrantLock();
lock.lock();
Thread.sleep(1000);
Thread mainThread = Thread.currentThread();
Thread t1=new Thread(new Runnable(){
@Override
public void run() {
try {
lock.lockInterruptibly();
} catch (InterruptedException e) {
LOG.error(" thread interrupted: {}", e);
mainThread.interrupt();
}
}
}, "t1 thread");
t1.start();
Thread.sleep(1000);
t1.interrupt();
Thread.sleep(1000000);
}
3 . boolean tryLock() 获取锁，如果获取不到则立即返回false。
@Test
public void testTryLock() throws InterruptedException {
CountDownLatch countDownLatch = new CountDownLatch(1);
ReentrantLock reentrantLock = new ReentrantLock();
Runnable runnable = () -> {
reentrantLock.lock();
try {
Thread.sleep(2 * 1000l);
countDownLatch.countDown();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
reentrantLock.unlock();
}
};
Runnable interruptRunnable = () -> {
boolean result = reentrantLock.tryLock();
if (result) {
LOG.info("lock success");
reentrantLock.unlock();
} else {
LOG.info("lock failed");
}
};
new Thread(runnable).start();
new Thread(interruptRunnable).start();
countDownLatch.await();
}
4 . boolean tryLock(long time, TimeUnit unit) throws InterruptedException 在指定的时间内获取锁，且返回结果。
@Test
public void testTryLockWithTime() throws InterruptedException, ExecutionException {
final Lock lock = new ReentrantLock();
lock.lock();
CompletableFuture completableFuture = CompletableFuture.supplyAsync(() -> tryLock(lock));
assertFalse(completableFuture.get().booleanValue());
}
private boolean tryLock(Lock lock) {
try {
boolean result = lock.tryLock(100, TimeUnit.MILLISECONDS);
LOG.info("lock result: {}", result);
return result;
} catch (InterruptedException e) {
LOG.error("interrupted: {}", e);
}
return false;
}
Semaphore
信号量常常用来控制对某一资源的访问数量。例如，下面的测试中我们设置信号量的permits为5，当其中5个现在获取且没释放，其它访问线程是获取不到permit的。
@Test
public void testSemaphore() throws InterruptedException {
Semaphore semaphore = new Semaphore(5);
CountDownLatch countDownLatch = new CountDownLatch(2000);
Executor executor = Executors.newFixedThreadPool(10);
Runnable runnable = () -> {
boolean isAcquired = semaphore.tryAcquire();
if (isAcquired) {
try {
LOG.info("semaphore is acquired");
TimeUnit.MICROSECONDS.sleep(2);
} catch (InterruptedException ex) {
LOG.error("error: {}", ex);
} finally {
semaphore.release();
}
} else {
LOG.info("semaphore is not acquired");
}
countDownLatch.countDown();
};
IntStream.range(1, 2001).forEach(i ->
executor.execute(runnable)
);
countDownLatch.await();
}
线程池(Thread pool)
线程池中的任务相对独立，才能使它的性能达到最优。在线程池中，如果出现相互依赖的线程，这可能导致线程死锁。比如：我们开启一个只有1个线程的线程池，调用A任务时，A开始了B任务。然后A任务依赖B任务的完成。在实际执行中，A使用了线程池中的线程，B任务不能获取线程执行，导致A任务不停的处于等待，而B任务也在等待A释放线程。
@Test
public void testThreadPoolThreadDependency() {
ExecutorService executor = Executors.newSingleThreadExecutor();
Callable stringRunnable = () -> {
return "test";
};
Callable runnable = () -> {
Future result = executor.submit(stringRunnable);
try {
return result.get();
} catch (InterruptedException e) {
return null;
} catch (ExecutionException e) {
return null;
}
};
try {
LOG.info(executor.submit(runnable).get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
运行上面测试，会发现处于一直等待的情况，查看thread dump：
"pool-1-thread-1" #11 prio=5 os_prio=31 tid=0x00007fd6d606f000 nid=0x5703 waiting on condition [0x0000000122af2000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x0000000795f453d8> (a java.util.concurrent.FutureTask)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.FutureTask.awaitDone(FutureTask.java:429)
at java.util.concurrent.FutureTask.get(FutureTask.java:191)
at com.eyesee.concurrency.threadpool.ThreadPoolExecutorTest.lambda$testThreadPoolThreadDependency$1(ThreadPoolExecutorTest.java:25)
at com.eyesee.concurrency.threadpool.ThreadPoolExecutorTest$$Lambda$2/183264084.call(Unknown Source)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
at java.lang.Thread.run(Thread.java:745)
Locked ownable synchronizers:
- <0x0000000795f22fd0> (a java.util.concurrent.ThreadPoolExecutor$Worker)
源代码详见：https://github.com/jessepys/javaconcurrency