Java 線程池詳解

系統啟動一個線程的成本是比較高的，因為它涉及到與操作系統的交互，使用線程池的好處是提高性能，當系統中包含大量並發的線程時，會導致系統性能劇烈下降，甚至導致JVM崩潰，而線程池的最大線程數參數可以控制系統中並發線程數不超過次數。

一、Executors 工廠類用來產生線程池，該工廠類包含以下幾個靜態工廠方法來創建對應的線程池。創建的線程池是一個ExecutorService對象，使用該對象的submit方法或者是execute方法執行相應的Runnable或者是Callable任務。線程池本身在不再需要的時候調用shutdown()方法停止線程池，調用該方法後，該線程池將不再允許任務添加進來，但是會直到已添加的所有任務執行完成後才死亡。

1、newCachedThreadPool()，創建一個具有緩存功能的線程池，提交到該線程池的任務（Runnable或Callable對象）創建的線程，如果執行完成，會被緩存到CachedThreadPool中，供後面需要執行的任務使用。

 import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class CacheThreadPool { static class Task implements Runnable { @Override public void run() { System.out.println(this + " " + Thread.currentThread().getName() + " AllStackTraces map size: " + Thread.currentThread().getAllStackTraces().size()); } } public static void main(String[] args) { ExecutorService cacheThreadPool = Executors.newCachedThreadPool(); //先添加三個任務到線程池for(int i = 0 ; i < 3; i++) { cacheThreadPool.execute(new Task()); } //等三個線程執行完成後，再次添加三個任務到線程池try { Thread.sleep(3000); } catch (InterruptedException e) { e.printStackTrace(); } for(int i = 0 ; i < 3; i++) { cacheThreadPool.execute(new Task()); } }}

執行結果如下：

 CacheThreadPool$Task@2d312eb9 pool-1-thread-1 AllStackTraces map size: 7CacheThreadPool$Task@59522b86 pool-1-thread-3 AllStackTraces map size: 7CacheThreadPool$Task@73dbb89f pool-1-thread-2 AllStackTraces map size: 7CacheThreadPool$Task@5795cedc pool-1-thread-3 AllStackTraces map size: 7CacheThreadPool$Task@256d5600 pool-1-thread-1 AllStackTraces map size: 7CacheThreadPool$Task@7d1c5894 pool-1-thread-2 AllStackTraces map size: 7

線程池中的線程對象進行了緩存，當有新任務執行時進行了復用。但是如果有特別多的並發時，緩存線程池還是會創建很多個線程對象。

2、newFixedThreadPool(int nThreads)創建一個指定線程個數，線程可複用的線程池。

 import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;public class FixedThreadPool { static class Task implements Runnable { @Override public void run() { System.out.println(this + " " + Thread.currentThread().getName() + " AllStackTraces map size: " + Thread.currentThread().getAllStackTraces().size()); } } public static void main(String[] args) { ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3); // 先添加三個任務到線程池for (int i = 0; i < 5; i++) { fixedThreadPool.execute(new Task()); } // 等三個線程執行完成後，再次添加三個任務到線程池try { Thread.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); } for (int i = 0; i < 3; i++) { fixedThreadPool.execute(new Task()); } }}

執行結果：

 FixedThreadPool$Task@7045c12d pool-1-thread-2 AllStackTraces map size: 7FixedThreadPool$Task@50fa0bef pool-1-thread-2 AllStackTraces map size: 7FixedThreadPool$Task@ccb1870 pool-1-thread-2 AllStackTraces map size: 7FixedThreadPool$Task@7392b4e3 pool-1-thread-1 AllStackTraces map size: 7FixedThreadPool$Task@5bdeff18 pool-1-thread-2 AllStackTraces map size: 7FixedThreadPool$Task@7d5554e1 pool-1-thread-1 AllStackTraces map size: 7FixedThreadPool$Task@24468092 pool-1-thread-3 AllStackTraces map size: 7FixedThreadPool$Task@fa7b978 pool-1-thread-2 AllStackTraces map size: 7

3、newSingleThreadExecutor(),創建一個只有單線程的線程池，相當於調用newFixedThreadPool(1)

4、newSheduledThreadPool(int corePoolSize),創建指定線程數的線程池，它可以在指定延遲後執行線程。也可以以某一周期重複執行某一線程，知道調用shutdown()關閉線程池。

示例如下：

 import java.util.concurrent.Executors;import java.util.concurrent.ScheduledExecutorService;import java.util.concurrent.TimeUnit;public class ScheduledThreadPool { static class Task implements Runnable { @Override public void run() { System.out.println("time " + System.currentTimeMillis() + " " + Thread.currentThread().getName() + " AllStackTraces map size: " + Thread.currentThread().getAllStackTraces().size()); } } public static void main(String[] args) { ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(3); scheduledExecutorService.schedule(new Task(), 3, TimeUnit.SECONDS); scheduledExecutorService.scheduleAtFixedRate(new Task(), 3, 5, TimeUnit.SECONDS); try { Thread.sleep(30 * 1000); } catch (InterruptedException e) { e.printStackTrace(); } scheduledExecutorService.shutdown(); }}

運行結果如下：

 time 1458921795240 pool-1-thread-1 AllStackTraces map size: 6time 1458921795241 pool-1-thread-2 AllStackTraces map size: 6time 1458921800240 pool-1-thread-1 AllStackTraces map size: 7time 1458921805240 pool-1-thread-1 AllStackTraces map size: 7time 1458921810240 pool-1-thread-1 AllStackTraces map size: 7time 1458921815240 pool-1-thread-1 AllStackTraces map size: 7time 1458921820240 pool-1-thread-1 AllStackTraces map size: 7

由運行時間可看出，任務是按照5秒的周期執行的。

5、newSingleThreadScheduledExecutor()創建一個只有一個線程的線程池，同調用newScheduledThreadPool(1)。

二、ForkJoinPool和ForkJoinTask

ForkJoinPool是ExecutorService的實現類，支持將一個任務劃分為多個小任務並行計算，在把多個小任務的計算結果合併成總的計算結果。它有兩個構造函數

ForkJoinPool(int parallelism)創建一個包含parallelism個並行線程的ForkJoinPool。

ForkJoinPool(),以Runtime.availableProcessors()方法返回值作為parallelism參數來創建ForkJoinPool。

ForkJoinTask 代表一個可以並行，合併的任務。它是實現了Future<T>接口的抽像類，它有兩個抽象子類，代表無返回值任務的RecuriveAction和有返回值的RecursiveTask。可根據具體需求繼承這兩個抽像類實現自己的對象，然後調用ForkJoinPool的submit 方法執行。

RecuriveAction 示例如下，實現並行輸出0-300的數字。

 import java.util.concurrent.ForkJoinPool;import java.util.concurrent.RecursiveAction;import java.util.concurrent.TimeUnit;public class ActionForkJoinTask { static class PrintTask extends RecursiveAction { private static final int THRESHOLD = 50; private int start; private int end; public PrintTask(int start, int end) { this.start = start; this.end = end; } @Override protected void compute() { if (end - start < THRESHOLD) { for(int i = start; i < end; i++) { System.out.println(Thread.currentThread().getName() + " " + i); } } else { int middle = (start + end) / 2; PrintTask left = new PrintTask(start, middle); PrintTask right = new PrintTask(middle, end); left.fork(); right.fork(); } } } public static void main(String[] args) { ForkJoinPool pool = new ForkJoinPool(); pool.submit(new PrintTask(0, 300)); try { pool.awaitTermination(2, TimeUnit.SECONDS); } catch (InterruptedException e) { e.printStackTrace(); } pool.shutdown(); }}

在拆分小任務後，調用任務的fork（）方法，加入到ForkJoinPool中並行執行。

RecursiveTask示例，實現並行計算100個整數求和。拆分為每20個數求和後獲取結果，在最後合併為最後的結果。

 import java.util.Random;import java.util.concurrent.ExecutionException;import java.util.concurrent.ForkJoinPool;import java.util.concurrent.Future;import java.util.concurrent.RecursiveTask;public class TaskForkJoinTask { static class CalTask extends RecursiveTask<Integer> { private static final int THRESHOLD = 20; private int arr[]; private int start; private int end; public CalTask(int[] arr, int start, int end) { this.arr = arr; this.start = start; this.end = end; } @Override protected Integer compute() { int sum = 0; if (end - start < THRESHOLD) { for (int i = start; i < end; i++) { sum += arr[i]; } System.out.println(Thread.currentThread().getName() + " sum:" + sum); return sum; } else { int middle = (start + end) / 2; CalTask left = new CalTask(arr, start, middle); CalTask right = new CalTask(arr, middle, end); left.fork(); right.fork(); return left.join() + right.join(); } } } public static void main(String[] args) { int arr[] = new int[100]; Random random = new Random(); int total = 0; for (int i = 0; i < arr.length; i++) { int tmp = random.nextInt(20); total += (arr[i] = tmp); } System.out.println("total " + total); ForkJoinPool pool = new ForkJoinPool(4); Future<Integer> future = pool.submit(new CalTask(arr, 0, arr.length)); try { System.out.println("cal result: " + future.get()); } catch (InterruptedException e) { e.printStackTrace(); } catch (ExecutionException e) { e.printStackTrace(); } pool.shutdown(); }}

執行結果如下：

 total 912ForkJoinPool-1-worker-2 sum:82ForkJoinPool-1-worker-2 sum:123ForkJoinPool-1-worker-2 sum:144ForkJoinPool-1-worker-3 sum:119ForkJoinPool-1-worker-2 sum:106ForkJoinPool-1-worker-2 sum:128ForkJoinPool-1-worker-2 sum:121ForkJoinPool-1-worker-3 sum:89cal result: 912

子任務執行完後，調用任務的join（）方法獲取子任務執行結果，再相加獲得最後的結果。