[Java并发编程（一）] 线程池 FixedThreadPool vs CachedThreadPool ...
摘要
介绍 Java 并发包里的几个主要 ExecutorService 。
正文
CachedThreadPool
CachedThreadPool 是通过 java.util.concurrent.Executors 创建的 ThreadPoolExecutor 实例。这个实例会根据需要，在线程可用时，重用之前构造好的池中线程。这个线程池在执行 大量短生命周期的异步任务时（many short-lived asynchronous task），可以显著提高程序性能。调用 execute 时，可以重用之前已构造的可用线程，如果不存在可用线程，那么会重新创建一个新的线程并将其加入到线程池中。如果线程超过 60 秒还未被使用，就会被中止并从缓存中移除。因此，线程池在长时间空闲后不会消耗任何资源。
注意队列实例是：new SynchronousQueue
/**
* Creates a thread pool that creates new threads as needed, but
* will reuse previously constructed threads when they are
* available. These pools will typically improve the performance
* of programs that execute many short-lived asynchronous tasks.
* Calls to <tt>execute</tt> will reuse previously constructed
* threads if available. If no existing thread is available, a new
* thread will be created and added to the pool. Threads that have
* not been used for sixty seconds are terminated and removed from
* the cache. Thus, a pool that remains idle for long enough will
* not consume any resources. Note that pools with similar
* properties but different details (for example, timeout parameters)
* may be created using {@link ThreadPoolExecutor} constructors.
*
* @return the newly created thread pool
*/
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
FixedThreadPool
FixedThreadPool 是通过 java.util.concurrent.Executors 创建的 ThreadPoolExecutor 实例。这个实例会复用 固定数量的线程 处理一个 共享的无边界队列 。任何时间点，最多有 nThreads 个线程会处于活动状态执行任务。如果当所有线程都是活动时，有多的任务被提交过来，那么它会一致在队列中等待直到有线程可用。如果任何线程在执行过程中因为错误而中止，新的线程会替代它的位置来执行后续的任务。所有线程都会一致存于线程池中，直到显式的执行 ExecutorService.shutdown() 关闭。
注意队列实例是：new LinkedBlockingQueue
/**
* Creates a thread pool that reuses a fixed number of threads
* operating off a shared unbounded queue. At any point, at most
* <tt>nThreads</tt> threads will be active processing tasks.
* If additional tasks are submitted when all threads are active,
* they will wait in the queue until a thread is available.
* If any thread terminates due to a failure during execution
* prior to shutdown, a new one will take its place if needed to
* execute subsequent tasks. The threads in the pool will exist
* until it is explicitly {@link ExecutorService#shutdown shutdown}.
*
* @param nThreads the number of threads in the pool
* @return the newly created thread pool
* @throws IllegalArgumentException if {@code nThreads <= 0}
*/
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
SingleThreadPool
SingleThreadPool 是通过 java.util.concurrent.Executors 创建的 ThreadPoolExecutor 实例。这个实例只会使用单个工作线程来执行一个无边界的队列。（注意，如果单个线程在执行过程中因为某些错误中止，新的线程会替代它执行后续线程）。它可以保证认为是按顺序执行的，任何时候都不会有多于一个的任务处于活动状态。和 newFixedThreadPool(1) 的区别在于，如果线程遇到错误中止，它是无法使用替代线程的。
/**
* Creates an Executor that uses a single worker thread operating
* off an unbounded queue. (Note however that if this single
* thread terminates due to a failure during execution prior to
* shutdown, a new one will take its place if needed to execute
* subsequent tasks.) Tasks are guaranteed to execute
* sequentially, and no more than one task will be active at any
* given time. Unlike the otherwise equivalent
* <tt>newFixedThreadPool(1)</tt> the returned executor is
* guaranteed not to be reconfigurable to use additional threads.
*
* @return the newly created single-threaded Executor
*/
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
程序演示
-
LiftOff
public class LiftOff implements Runnable { protected int countDown = 10; // Default private static int taskCount = 0; private final int id = taskCount++; public LiftOff() {} public LiftOff(int countDown) { this.countDown = countDown; } public String status() { return "Thread ID: [" + String.format("%3d", Thread.currentThread().getId()) + "] #" + id + "(" + (countDown > 0 ? countDown : "LiftOff!") + ") "; } @Override public void run() { while (countDown-- > 0) { System.out.println(status()); Thread.yield(); } } } -
CachedThreadPoolCase
public class CachedThreadPoolCase { public static void main(String[] args) throws InterruptedException { ExecutorService exec = Executors.newCachedThreadPool(); for(int i = 0; i < 5; i++) { exec.execute(new LiftOff()); Thread.sleep(10); } exec.shutdown(); } }当 sleep 间隔为 5 milliseconds 时，共创建了 3 个线程，并交替执行。
当 sleep 间隔为 10 milliseconds 时，共创建了 2 个线程，交替执行。
-
FixedThreadPoolCase
public class FixedThreadPoolCase { public static void main(String[] args) throws InterruptedException { ExecutorService exec = Executors.newFixedThreadPool(3); for (int i = 0; i < 5; i++) { exec.execute(new LiftOff()); Thread.sleep(10); } exec.shutdown(); } }无论 sleep 间隔时间是多少，总共都创建 3 个线程，并交替执行。
-
SingleThreadCase
public class SingleThreadPoolCase { public static void main(String[] args) throws InterruptedException { ExecutorService exec = Executors.newSingleThreadExecutor(); for (int i = 0; i < 10; i++) { exec.execute(new LiftOff()); Thread.sleep(5); } exec.shutdown(); } }无论 sleep 间隔时间是多少，总共都只创建 1 个线程。
FixedThreadPool 与 CachedThreadPool 特性对比
| 特性 | FixedThreadPool | CachedThreadPool |
|---|---|---|
| 重用 | FixedThreadPool 与 CacheThreadPool差不多，也是能 reuse 就用，但不能随时建新的线程 | 缓存型池子，先查看池中有没有以前建立的线程，如果有，就 reuse ；如果没有，就建一个新的线程加入池中 |
| 池大小 | 可指定 nThreads，固定数量 | 可增长，最大值 Integer.MAX_VALUE |
| 队列大小 | 无限制 | 无限制 |
| 超时 | 无 IDLE | 默认 60 秒 IDLE |
| 使用场景 | 所以 FixedThreadPool 多数针对一些很稳定很固定的正规并发线程，多用于服务器 | 大量短生命周期的异步任务 |
| 结束 | 不会自动销毁 | 注意，放入 CachedThreadPool 的线程不必担心其结束，超过 TIMEOUT 不活动，其会自动被终止。 |
最佳实践
FixedThreadPool 和 CachedThreadPool 两者对高负载的应用都不是特别友好。
CachedThreadPool 要比 FixedThreadPool 危险很多。
如果应用要求高负载、低延迟，最好不要选择以上两种线程池：
- 任务队列的无边界：会导致内存溢出以及高延迟
- 长时间运行会导致
CachedThreadPool在线程创建上失控
因为两者都不是特别友好，所以推荐使用 ThreadPoolExecutor ，它提供了很多参数可以进行细粒度的控制。
-
将任务队列设置成有边界的队列
-
使用合适的
RejectionHandler- 自定义的RejectionHandler或 JDK 提供的默认 handler 。 -
如果在任务完成前后需要执行某些操作，可以重载
beforeExecute(Thread, Runnable) afterExecute(Runnable, Throwable) -
重载 ThreadFactory ，如果有线程定制化的需求
-
在运行时动态控制线程池的大小（Dynamic Thread Pool）
参考
iteye: Java 并发包中的几种 ExecutorService
stackoverflow: FixedThreadPool vs CachedThreadPool: the lesser of two evils
blogjava: 深入浅出多线程（4）对CachedThreadPool OutOfMemoryError问题的一些想法