java通過Executors提供四種線程池，分別為：newCachedThreadPool創(chuàng)建一個可緩存線程池，如果線程池長度超過處理需要，可靈活回收空閑線程，若無可回收，則新建線程。newFixedThreadPool 創(chuàng)建一個定長線程池，可控制線程最大并發(fā)數(shù)，超出的線程會在隊列中等待。newScheduledThreadPool 創(chuàng)建一個定長線程池，支持定時及周期性任務執(zhí)行。newSingleThreadExecutor 創(chuàng)建一個單線程化的線程池，它只會用唯一的工作線程來執(zhí)行任務，保證所有任務按照指定順序(FIFO, LIFO, 優(yōu)先級)執(zhí)行。
(1) newCachedThreadPool創(chuàng)建一個可緩存線程池，如果線程池長度超過處理需要，可靈活回收空閑線程，若無可回收，則新建線程。示例代碼如下：
package test; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ThreadPoolExecutorTest { public static void main(String[] args) { ExecutorService cachedThreadPool = Executors.newCachedThreadPool(); for (int i = 0; i < 10; i++) { final int index = i; try { Thread.sleep(index * 1000); } catch (InterruptedException e) { e.PRintStackTrace(); } cachedThreadPool.execute(new Runnable() { public void run() { System.out.println(index); } }); } } }
線程池為無限大，當執(zhí)行第二個任務時第一個任務已經(jīng)完成，會復用執(zhí)行第一個任務的線程，而不用每次新建線程。 (2) newFixedThreadPool創(chuàng)建一個定長線程池，可控制線程最大并發(fā)數(shù)，超出的線程會在隊列中等待。示例代碼如下：
package test; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ThreadPoolExecutorTest { public static void main(String[] args) { ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3); for (int i = 0; i < 10; i++) { final int index = i; fixedThreadPool.execute(new Runnable() { public void run() { try { System.out.println(index); Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } } }); } } }因為線程池大小為3，每個任務輸出index后sleep 2秒，所以每兩秒打印3個數(shù)字。定長線程池的大小最好根據(jù)系統(tǒng)資源進行設置。如Runtime.getRuntime().availableProcessors()
(3) newScheduledThreadPool創(chuàng)建一個定長線程池，支持定時及周期性任務執(zhí)行。延遲執(zhí)行示例代碼如下：
package test; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; public class ThreadPoolExecutorTest { public static void main(String[] args) { ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5); scheduledThreadPool.schedule(new Runnable() { public void run() { System.out.println("delay 3 seconds"); } }, 3, TimeUnit.SECONDS); } }表示延遲3秒執(zhí)行。
定期執(zhí)行示例代碼如下：
package test; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; public class ThreadPoolExecutorTest { public static void main(String[] args) { ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(5); scheduledThreadPool.scheduleAtFixedRate(new Runnable() { public void run() { System.out.println("delay 1 seconds, and excute every 3 seconds"); } }, 1, 3, TimeUnit.SECONDS); } }表示延遲1秒后每3秒執(zhí)行一次。
(4) newSingleThreadExecutor創(chuàng)建一個單線程化的線程池，它只會用唯一的工作線程來執(zhí)行任務，保證所有任務按照指定順序(FIFO, LIFO, 優(yōu)先級)執(zhí)行。示例代碼如下：
package test; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ThreadPoolExecutorTest { public static void main(String[] args) { ExecutorService singleThreadExecutor = Executors.newSingleThreadExecutor(); for (int i = 0; i < 10; i++) { final int index = i; singleThreadExecutor.execute(new Runnable() { public void run() { try { System.out.println(index); Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } } }); } } }結果依次輸出，相當于順序執(zhí)行各個任務。
你可以使用JDK自帶的監(jiān)控工具來監(jiān)控我們創(chuàng)建的線程數(shù)量，運行一個不終止的線程，創(chuàng)建指定量的線程，來觀察：工具目錄：C:/Program Files/Java/jdk1.6.0_06/bin/jconsole.exe運行程序做稍微修改：
package test; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class ThreadPoolExecutorTest { public static void main(String[] args) { ExecutorService singleThreadExecutor = Executors.newCachedThreadPool(); for (int i = 0; i < 100; i++) { final int index = i; singleThreadExecutor.execute(new Runnable() { public void run() { try { while(true) { System.out.println(index); Thread.sleep(10 * 1000); } } catch (InterruptedException e) { e.printStackTrace(); } } }); try { Thread.sleep(500); } catch (InterruptedException e) { e.printStackTrace(); } } } }
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