Java通過Executors提供四種線程池

http://cuisuqiang.iteye.com/blog/2019372

Java通過Executors提供四種線程池，分別為：
newCachedThreadPool創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 創建一個定長線程池，可控制線程最大并發數，超出的線程會在隊列中等待。
newScheduledThreadPool 創建一個定長線程池，支持定時及周期性任務執行。
newSingleThreadExecutor 創建一個單線程化的線程池，它只會用唯一的工作線程來執行任務，保證所有任務按照指定順序(FIFO, LIFO, 優先級)執行。

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(1) newCachedThreadPool
創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);??
????}??
???});??
??}??
?}??
}??

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線程池為無限大，當執行第二個任務時第一個任務已經完成，會復用執行第一個任務的線程，而不用每次新建線程。
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(2) newFixedThreadPool
創建一個定長線程池，可控制線程最大并發數，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();??
?????}??
????}??
???});??
??}??
?}??
}??

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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個數字。
定長線程池的大小最好根據系統資源進行設置。如Runtime.getRuntime().availableProcessors()

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(3)? newScheduledThreadPool
創建一個定長線程池，支持定時及周期性任務執行。延遲執行示例代碼如下：

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);??
?}??
}??

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表示延遲3秒執行。

定期執行示例代碼如下：

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);??
?}??
}??

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表示延遲1秒后每3秒執行一次。

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(4) newSingleThreadExecutor
創建一個單線程化的線程池，它只會用唯一的工作線程來執行任務，保證所有任務按照指定順序(FIFO, LIFO, 優先級)執行。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();??
?????}??
????}??
???});??
??}??
?}??
}??

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結果依次輸出，相當于順序執行各個任務。

你可以使用JDK自帶的監控工具來監控我們創建的線程數量，運行一個不終止的線程，創建指定量的線程，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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效果如下：

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選擇我們運行的程序：

監控運行狀態