Java Concurrent

Java Concurrent

ExecutorService

• ExecutorService exec = Executors.newCachedThreadPool(); // create a cached pool
• ExecutorService exec = Executors.newFixedThreadPool(4); // fixed sized thread pool
• ExecutorService exec = Executors.newSingleThreadExecutor(); // single thread's pool

ThreadPoolExecutor

ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(3, 6, 2, TimeUnit.HOURS, queue);

• 3, corePoolSize
• 6, maximunPoolSize
• 2, keep alive time (idle threads will be gc after such a long time)
• TimeUnit.HOURS, time units
• queue, LinkedBlockingQueue<Runnable>, or new ArrayBlockingQueue<Runnable>(8)

当新任务在方法 execute(java.lang.Runnable) 中提交时，如果运行的线程少于 corePoolSize，则创建新线程来处理请求（即使存在空闲线程）。如果运行的线程多于 corePoolSize 而少于 maximumPoolSize，则仅当队列（queue）满时才创建新线程。如果设置的 corePoolSize 和 maximumPoolSize 相同，则创建了固定大小的线程池。如果将 maximumPoolSize 设置为基本的无界值（如 Integer.MAX_VALUE），则允许池适应任意数量的并发任务。

# AtomicInteger

• AtomicInteger ac = new AtomicInteger();
• int index = ac.incrementAndGet();

CountDownLatch

• countDown(); // decrease the count by 1
• await(); // suspend the current thread until the count becomes 0
• CountDownLatch c1 = new CountDownLatch(2); // like the condition in OS

Runnable VS Callable

Runnable和Callable的区别是，
(1)Callable规定的方法是call(),Runnable规定的方法是run().
(2)Callable的任务执行后可返回值，而Runnable的任务是不能返回值得
(3)call方法可以抛出异常，run方法不可以
(4)运行Callable任务可以拿到一个Future对象，表示异步计算的结果。它提供了检查计算是否完成的方法，以等待计算的完成，并检索计算的结果。通过Future对象可以了解任务执行情况，可取消任务的执行，还可获取执行结果。

Semaphore

• Semaphore s = new Semaphore(4); // allow at most 4 threads concurrently access a piece of code
• s.acquire() ; // try to get the qualification to access a piece of code, wait if the current semaphore is 0
• s.release(); // release the lock/semaphore