JDK5中的一個亮點就是將Doug Lea的并發庫引入到Java標準庫中。Doug Lea確實是一個牛人，能教書，能出書，能編碼，不過這在國外還是比較普遍的，而國內的教授們就相差太遠了。
一般的服務器都需要線程池，比如Web、FTP等服務器，不過它們一般都自己實現了線程池，比如以前介紹過的Tomcat、Resin和Jetty等，現在有了JDK5，我們就沒有必要重復造車輪了，直接使用就可以，何況使用也很方便，性能也非常高。
Java代碼
package?concurrent; ??import?java.util.concurrent.ExecutorService; ??import?java.util.concurrent.Executors; ??public?class?TestThreadPool?{ ??public?static?void?main(String?args[])?throws?InterruptedException?{ ????ExecutorService?exec?=?Executors.newFixedThreadPool(2); ??for(int?index?=?0;?index?<?100;?index++)?{ ??Runnable?run?=?new?Runnable()?{ ??public?void?run()?{ ??long?time?=?(long)?(Math.random()?*?1000); ??System.out.println(“Sleeping?”?+?time?+?“ms”); ??try?{ ??Thread.sleep(time); ??}?catch?(InterruptedException?e)?{ ??} ??} ??}; ??exec.execute(run); ??} ????exec.shutdown(); ??} ??}?? package concurrent;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TestThreadPool {
public static void main(String args[]) throws InterruptedException {
// only two threads
ExecutorService exec = Executors.newFixedThreadPool(2);
for(int index = 0; index < 100; index++) {
Runnable run = new Runnable() {
public void run() {
long time = (long) (Math.random() * 1000);
System.out.println(“Sleeping ” + time + “ms”);
try {
Thread.sleep(time);
} catch (InterruptedException e) {
}
}
};
exec.execute(run);
}
// must shutdown
exec.shutdown();
}
}
上面是一個簡單的例子，使用了2個大小的線程池來處理100個線程。但有一個問題：在for循環的過程中，會等待線程池有空閑的線程，所以主線程會阻塞的。為了解決這個問題，一般啟動一個線程來做for循環，就是為了避免由于線程池滿了造成主線程阻塞。不過在這里我沒有這樣處理。[重要修正：經過測試，即使線程池大小小于實際線程數大小，線程池也不會阻塞的，這與Tomcat的線程池不同，它將Runnable實例放到一個“無限”的BlockingQueue中，所以就不用一個線程啟動for循環，Doug Lea果然厲害]
另外它使用了Executors的靜態函數生成一個固定的線程池，顧名思義，線程池的線程是不會釋放的，即使它是Idle。這就會產生性能問題，比如如果線程池的大小為200，當全部使用完畢后，所有的線程會繼續留在池中，相應的內存和線程切換（while(true)+sleep循環）都會增加。如果要避免這個問題，就必須直接使用ThreadPoolExecutor()來構造。可以像Tomcat的線程池一樣設置“最大線程數”、“最小線程數”和“空閑線程keepAlive的時間”。通過這些可以基本上替換Tomcat的線程池實現方案。
需要注意的是線程池必須使用shutdown來顯式關閉，否則主線程就無法退出。shutdown也不會阻塞主線程。
許多長時間運行的應用有時候需要定時運行任務完成一些諸如統計、優化等工作，比如在電信行業中處理用戶話單時，需要每隔1分鐘處理話單；網站每天凌晨統計用戶訪問量、用戶數；大型超時凌晨3點統計當天銷售額、以及最熱賣的商品；每周日進行數據庫備份；公司每個月的10號計算工資并進行轉帳等，這些都是定時任務。通過 java的并發庫concurrent可以輕松的完成這些任務，而且非常的簡單。
Java代碼
package?concurrent; ??import?static?java.util.concurrent.TimeUnit.SECONDS; ??import?java.util.Date; ??import?java.util.concurrent.Executors; ??import?java.util.concurrent.ScheduledExecutorService; ??import?java.util.concurrent.ScheduledFuture; ??public?class?TestScheduledThread?{ ??public?static?void?main(String[]?args)?{ ??final?ScheduledExecutorService?scheduler?=?Executors ??.newScheduledThreadPool(2); ??final?Runnable?beeper?=?new?Runnable()?{ ??int?count?=?0; ??public?void?run()?{ ??System.out.println(new?Date()?+?”?beep?”?+?(++count)); ??} ??}; ????final?ScheduledFuture?beeperHandle?=?scheduler.scheduleAtFixedRate( ??beeper,?1,?2,?SECONDS); ????final?ScheduledFuture?beeperHandle2?=?scheduler ??.scheduleWithFixedDelay(beeper,?2,?5,?SECONDS); ????scheduler.schedule(new?Runnable()?{ ??public?void?run()?{ ??beeperHandle.cancel(true); ??beeperHandle2.cancel(true); ??scheduler.shutdown(); ??} ??},?30,?SECONDS); ??} ??}?? package concurrent;
import static java.util.concurrent.TimeUnit.SECONDS;
import java.util.Date;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
public class TestScheduledThread {
public static void main(String[] args) {
final ScheduledExecutorService scheduler = Executors
.newScheduledThreadPool(2);
final Runnable beeper = new Runnable() {
int count = 0;
public void run() {
System.out.println(new Date() + ” beep ” + (++count));
}
};
// 1秒鐘后運行，并每隔2秒運行一次
final ScheduledFuture beeperHandle = scheduler.scheduleAtFixedRate(
beeper, 1, 2, SECONDS);
// 2秒鐘后運行，并每次在上次任務運行完后等待5秒后重新運行
final ScheduledFuture beeperHandle2 = scheduler
.scheduleWithFixedDelay(beeper, 2, 5, SECONDS);
// 30秒后結束關閉任務，并且關閉Scheduler
scheduler.schedule(new Runnable() {
public void run() {
beeperHandle.cancel(true);
beeperHandle2.cancel(true);
scheduler.shutdown();
}
}, 30, SECONDS);
}
}
為了退出進程，上面的代碼中加入了關閉Scheduler的操作。而對于24小時運行的應用而言，是沒有必要關閉Scheduler的。
在實際應用中，有時候需要多個線程同時工作以完成同一件事情，而且在完成過程中，往往會等待其他線程都完成某一階段后再執行，等所有線程都到達某一個階段后再統一執行。
比如有幾個旅行團需要途經深圳、廣州、韶關、長沙最后到達武漢。旅行團中有自駕游的，有徒步的，有乘坐旅游大巴的；這些旅行團同時出發，并且每到一個目的地，都要等待其他旅行團到達此地后再同時出發，直到都到達終點站武漢。
這時候CyclicBarrier就可以派上用場。CyclicBarrier最重要的屬性就是參與者個數，另外最要方法是await()。當所有線程都調用了await()后，就表示這些線程都可以繼續執行，否則就會等待。
Java代碼
package?concurrent; ??import?java.text.SimpleDateFormat; ??import?java.util.Date; ??import?java.util.concurrent.BrokenBarrierException; ??import?java.util.concurrent.CyclicBarrier; ??import?java.util.concurrent.ExecutorService; ??import?java.util.concurrent.Executors; ??public?class?TestCyclicBarrier?{ ????private?static?int[]?timeWalk?=?{?5,?8,?15,?15,?10?}; ????private?static?int[]?timeSelf?=?{?1,?3,?4,?4,?5?}; ????private?static?int[]?timeBus?=?{?2,?4,?6,?6,?7?}; ????static?String?now()?{ ??SimpleDateFormat?sdf?=?new?SimpleDateFormat(“HH:mm:ss”); ??return?sdf.format(new?Date())?+?“:?“; ??} ????static?class?Tour?implements?Runnable?{ ??private?int[]?times; ??private?CyclicBarrier?barrier; ??private?String?tourName; ??public?Tour(CyclicBarrier?barrier,?String?tourName,?int[]?times)?{ ??this.times?=?times; ??this.tourName?=?tourName; ??this.barrier?=?barrier; ??} ??public?void?run()?{ ??try?{ ??Thread.sleep(times[0]?*?1000); ??System.out.println(now()?+?tourName?+?”?Reached?Shenzhen”); ??barrier.await(); ??Thread.sleep(times[1]?*?1000); ??System.out.println(now()?+?tourName?+?”?Reached?Guangzhou”); ??barrier.await(); ??Thread.sleep(times[2]?*?1000); ??System.out.println(now()?+?tourName?+?”?Reached?Shaoguan”); ??barrier.await(); ??Thread.sleep(times[3]?*?1000); ??System.out.println(now()?+?tourName?+?”?Reached?Changsha”); ??barrier.await(); ??Thread.sleep(times[4]?*?1000); ??System.out.println(now()?+?tourName?+?”?Reached?Wuhan”); ??barrier.await(); ??}?catch?(InterruptedException?e)?{ ??}?catch?(BrokenBarrierException?e)?{ ??} ??} ??} ????public?static?void?main(String[]?args)?{ ????CyclicBarrier?barrier?=?new?CyclicBarrier(3); ??ExecutorService?exec?=?Executors.newFixedThreadPool(3); ??exec.submit(new?Tour(barrier,?“WalkTour”,?timeWalk)); ??exec.submit(new?Tour(barrier,?“SelfTour”,?timeSelf)); ??exec.submit(new?Tour(barrier,?“BusTour”,?timeBus)); ??exec.shutdown(); ??} ??}?? package concurrent;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class TestCyclicBarrier {
// 徒步需要的時間: Shenzhen, Guangzhou, Shaoguan, Changsha, Wuhan
private static int[] timeWalk = { 5, 8, 15, 15, 10 };
// 自駕游
private static int[] timeSelf = { 1, 3, 4, 4, 5 };
// 旅游大巴
private static int[] timeBus = { 2, 4, 6, 6, 7 };static String now() {
SimpleDateFormat sdf = new SimpleDateFormat(“HH:mm:ss”);
return sdf.format(new Date()) + “: “;
}static class Tour implements Runnable {
private int[] times;
private CyclicBarrier barrier;
private String tourName;
public Tour(CyclicBarrier barrier, String tourName, int[] times) {
this.times = times;
this.tourName = tourName;
this.barrier = barrier;
}
public void run() {
try {
Thread.sleep(times[0] * 1000);
System.out.println(now() + tourName + ” Reached Shenzhen”);
barrier.await();
Thread.sleep(times[1] * 1000);
System.out.println(now() + tourName + ” Reached Guangzhou”);
barrier.await();
Thread.sleep(times[2] * 1000);
System.out.println(now() + tourName + ” Reached Shaoguan”);
barrier.await();
Thread.sleep(times[3] * 1000);
System.out.println(now() + tourName + ” Reached Changsha”);
barrier.await();
Thread.sleep(times[4] * 1000);
System.out.println(now() + tourName + ” Reached Wuhan”);
barrier.await();
} catch (InterruptedException e) {
} catch (BrokenBarrierException e) {
}
}
}public static void main(String[] args) {
// 三個旅行團
CyclicBarrier barrier = new CyclicBarrier(3);
ExecutorService exec = Executors.newFixedThreadPool(3);
exec.submit(new Tour(barrier, “WalkTour”, timeWalk));
exec.submit(new Tour(barrier, “SelfTour”, timeSelf));
exec.submit(new Tour(barrier, “BusTour”, timeBus));
exec.shutdown();
}
}
運行結果：
00:02:25: SelfTour Reached Shenzhen
00:02:25: BusTour Reached Shenzhen
00:02:27: WalkTour Reached Shenzhen
00:02:30: SelfTour Reached Guangzhou
00:02:31: BusTour Reached Guangzhou
00:02:35: WalkTour Reached Guangzhou
00:02:39: SelfTour Reached Shaoguan
00:02:41: BusTour Reached Shaoguan
并發庫中的BlockingQueue是一個比較好玩的類，顧名思義，就是阻塞隊列。該類主要提供了兩個方法put()和take()，前者將一個對象放到隊列中，如果隊列已經滿了，就等待直到有空閑節點；后者從head取一個對象，如果沒有對象，就等待直到有可取的對象。
下面的例子比較簡單，一個讀線程，用于將要處理的文件對象添加到阻塞隊列中，另外四個寫線程用于取出文件對象，為了模擬寫操作耗時長的特點，特讓線程睡眠一段隨機長度的時間。另外，該Demo也使用到了線程池和原子整型（AtomicInteger），AtomicInteger可以在并發情況下達到原子化更新，避免使用了synchronized，而且性能非常高。由于阻塞隊列的put和take操作會阻塞，為了使線程退出，特在隊列中添加了一個“標識”，算法中也叫“哨兵”，當發現這個哨兵后，寫線程就退出。
當然線程池也要顯式退出了。
Java代碼
package?concurrent; ??import?java.io.File; ??import?java.io.FileFilter; ??import?java.util.concurrent.BlockingQueue; ??import?java.util.concurrent.ExecutorService; ??import?java.util.concurrent.Executors; ??import?java.util.concurrent.LinkedBlockingQueue; ??import?java.util.concurrent.atomic.AtomicInteger; ????public?class?TestBlockingQueue?{ ??static?long?randomTime()?{ ??return?(long)?(Math.random()?*?1000); ??} ????public?static?void?main(String[]?args)?{ ????final?BlockingQueue?queue?=?new?LinkedBlockingQueue(100); ????final?ExecutorService?exec?=?Executors.newFixedThreadPool(5); ??final?File?root?=?new?File(“F://JavaLib”); ????final?File?exitFile?=?new?File(“”); ????final?AtomicInteger?rc?=?new?AtomicInteger(); ????final?AtomicInteger?wc?=?new?AtomicInteger(); ????Runnable?read?=?new?Runnable()?{ ??public?void?run()?{ ??scanFile(root); ??scanFile(exitFile); ??} ????public?void?scanFile(File?file)?{ ??if?(file.isDirectory())?{ ??File[]?files?=?file.listFiles(new?FileFilter()?{ ??public?boolean?accept(File?pathname)?{ ??return?pathname.isDirectory() ??||?pathname.getPath().endsWith(“.java”); ??} ??}); ??for?(File?one?:?files) ??scanFile(one); ??}?else?{ ??try?{ ??int?index?=?rc.incrementAndGet(); ??System.out.println(“Read0:?”?+?index?+?”?“ ??+?file.getPath()); ??queue.put(file); ??}?catch?(InterruptedException?e)?{ ??} ??} ??} ??}; ??exec.submit(read); ????for?(int?index?=?0;?index?<?4;?index++)?{ ????final?int?NO?=?index; ??Runnable?write?=?new?Runnable()?{ ??String?threadName?=?“Write”?+?NO; ??public?void?run()?{ ??while?(true)?{ ??try?{ ??Thread.sleep(randomTime()); ??int?index?=?wc.incrementAndGet(); ??File?file?=?queue.take(); ????if?(file?==?exitFile)?{ ????queue.put(exitFile); ??break; ??} ??System.out.println(threadName?+?“:?”?+?index?+?”?“ ??+?file.getPath()); ??}?catch?(InterruptedException?e)?{ ??} ??} ??} ??}; ??exec.submit(write); ??} ??exec.shutdown(); ??} ??}?? package concurrent;
import java.io.File;
import java.io.FileFilter;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.atomic.AtomicInteger;public class TestBlockingQueue {
static long randomTime() {
return (long) (Math.random() * 1000);
}public static void main(String[] args) {
// 能容納100個文件
final BlockingQueue queue = new LinkedBlockingQueue(100);
// 線程池
final ExecutorService exec = Executors.newFixedThreadPool(5);
final File root = new File(“F://JavaLib”);
// 完成標志
final File exitFile = new File(“”);
// 讀個數
final AtomicInteger rc = new AtomicInteger();
// 寫個數
final AtomicInteger wc = new AtomicInteger();
// 讀線程
Runnable read = new Runnable() {
public void run() {
scanFile(root);
scanFile(exitFile);
}public void scanFile(File file) {
if (file.isDirectory()) {
File[] files = file.listFiles(new FileFilter() {
public boolean accept(File pathname) {
return pathname.isDirectory()
|| pathname.getPath().endsWith(“.java”);
}
});
for (File one : files)
scanFile(one);
} else {
try {
int index = rc.incrementAndGet();
System.out.println(“Read0: ” + index + ” “
+ file.getPath());
queue.put(file);
} catch (InterruptedException e) {
}
}
}
};
exec.submit(read);
// 四個寫線程
for (int index = 0; index < 4; index++) {
// write thread
final int NO = index;
Runnable write = new Runnable() {
String threadName = “Write” + NO;
public void run() {
while (true) {
try {
Thread.sleep(randomTime());
int index = wc.incrementAndGet();
File file = queue.take();
// 隊列已經無對象
if (file == exitFile) {
// 再次添加”標志”，以讓其他線程正常退出
queue.put(exitFile);
break;
}
System.out.println(threadName + “: ” + index + ” “
+ file.getPath());
} catch (InterruptedException e) {
}
}
}
};
exec.submit(write);
}
exec.shutdown();
}
}
從名字可以看出，CountDownLatch是一個倒數計數的鎖，當倒數到0時觸發事件，也就是開鎖，其他人就可以進入了。在一些應用場合中，需要等待某個條件達到要求后才能做后面的事情；同時當線程都完成后也會觸發事件，以便進行后面的操作。
CountDownLatch最重要的方法是countDown()和await()，前者主要是倒數一次，后者是等待倒數到0，如果沒有到達0，就只有阻塞等待了。
一個CountDouwnLatch實例是不能重復使用的，也就是說它是一次性的，鎖一經被打開就不能再關閉使用了，如果想重復使用，請考慮使用CyclicBarrier。
下面的例子簡單的說明了CountDownLatch的使用方法，模擬了100米賽跑，10名選手已經準備就緒，只等裁判一聲令下。當所有人都到達終點時，比賽結束。
同樣，線程池需要顯式shutdown。
Java代碼
package?concurrent; ????import?java.util.concurrent.CountDownLatch; ??import?java.util.concurrent.ExecutorService; ??import?java.util.concurrent.Executors; ????public?class?TestCountDownLatch?{ ??public?static?void?main(String[]?args)?throws?InterruptedException?{ ????final?CountDownLatch?begin?=?new?CountDownLatch(1); ????final?CountDownLatch?end?=?new?CountDownLatch(10); ????final?ExecutorService?exec?=?Executors.newFixedThreadPool(10); ??for(int?index?=?0;?index?<?10;?index++)?{ ??final?int?NO?=?index?+?1; ??Runnable?run?=?new?Runnable(){ ??public?void?run()?{ ??try?{ ??begin.await(); ??Thread.sleep((long)?(Math.random()?*?10000)); ??System.out.println(“No.”?+?NO?+?”?arrived”); ??}?catch?(InterruptedException?e)?{ ??}?finally?{ ??end.countDown(); ??} ??} ??}; ??exec.submit(run); ??} ??System.out.println(“Game?Start”); ??begin.countDown(); ??end.await(); ??System.out.println(“Game?Over”); ??exec.shutdown(); ??} ??}?? package concurrent;import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class TestCountDownLatch {
public static void main(String[] args) throws InterruptedException {
// 開始的倒數鎖
final CountDownLatch begin = new CountDownLatch(1);
// 結束的倒數鎖
final CountDownLatch end = new CountDownLatch(10);
// 十名選手
final ExecutorService exec = Executors.newFixedThreadPool(10);
for(int index = 0; index < 10; index++) {
final int NO = index + 1;
Runnable run = new Runnable(){
public void run() {
try {
begin.await();
Thread.sleep((long) (Math.random() * 10000));
System.out.println(“No.” + NO + ” arrived”);
} catch (InterruptedException e) {
} finally {
end.countDown();
}
}
};
exec.submit(run);
}
System.out.println(“Game Start”);
begin.countDown();
end.await();
System.out.println(“Game Over”);
exec.shutdown();
}
}
運行結果:
Game Start
No.4 arrived
No.1 arrived
No.7 arrived
No.9 arrived
No.3 arrived
No.2 arrived
No.8 arrived
No.10 arrived
No.6 arrived
No.5 arrived
Game Over
有時候在實際應用中，某些操作很耗時，但又不是不可或缺的步驟。比如用網頁瀏覽器瀏覽新聞時，最重要的是要顯示文字內容，至于與新聞相匹配的圖片就沒有那么重要的，所以此時首先保證文字信息先顯示，而圖片信息會后顯示，但又不能不顯示，由于下載圖片是一個耗時的操作，所以必須一開始就得下載。
Java的并發庫的Future類就可以滿足這個要求。Future的重要方法包括get()和cancel()，get()獲取數據對象，如果數據沒有加載，就會阻塞直到取到數據，而 cancel()是取消數據加載。另外一個get(timeout)操作，表示如果在timeout時間內沒有取到就失敗返回，而不再阻塞。
下面的Demo簡單的說明了Future的使用方法：一個非常耗時的操作必須一開始啟動，但又不能一直等待；其他重要的事情又必須做，等完成后，就可以做不重要的事情。
Java代碼
package?concurrent; ????import?java.util.concurrent.Callable; ??import?java.util.concurrent.ExecutionException; ??import?java.util.concurrent.ExecutorService; ??import?java.util.concurrent.Executors; ??import?java.util.concurrent.Future; ????public?class?TestFutureTask?{ ??public?static?void?main(String[]?args)throws?InterruptedException, ??ExecutionException?{ ??final?ExecutorService?exec?=?Executors.newFixedThreadPool(5); ??Callable?call?=?new?Callable()?{ ??public?String?call()?throws?Exception?{ ??Thread.sleep(1000?*?5); ??return?“Other?less?important?but?longtime?things.”; ??} ??}; ??Future?task?=?exec.submit(call); ????Thread.sleep(1000?*?3); ??System.out.println(“Let’s?do?important?things.”); ????String?obj?=?task.get(); ??System.out.println(obj); ????exec.shutdown(); ??} ??}?? package concurrent;import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;public class TestFutureTask {
public static void main(String[] args)throws InterruptedException,
ExecutionException {
final ExecutorService exec = Executors.newFixedThreadPool(5);
Callable call = new Callable() {
public String call() throws Exception {
Thread.sleep(1000 * 5);
return “Other less important but longtime things.”;
}
};
Future task = exec.submit(call);
// 重要的事情
Thread.sleep(1000 * 3);
System.out.println(“Let’s do important things.”);
// 其他不重要的事情
String obj = task.get();
System.out.println(obj);
// 關閉線程池
exec.shutdown();
}
}
運行結果：
Let’s do important things.
Other less important but longtime things.
考慮以下場景：瀏覽網頁時，瀏覽器了5個線程下載網頁中的圖片文件，由于圖片大小、網站訪問速度等諸多因素的影響，完成圖片下載的時間就會有很大的不同。如果先下載完成的圖片就會被先顯示到界面上，反之，后下載的圖片就后顯示。
Java的并發庫的CompletionService可以滿足這種場景要求。該接口有兩個重要方法：submit()和take()。submit用于提交一個runnable或者callable，一般會提交給一個線程池處理；而take就是取出已經執行完畢runnable或者callable實例的Future對象，如果沒有滿足要求的，就等待了。 CompletionService還有一個對應的方法poll，該方法與take類似，只是不會等待，如果沒有滿足要求，就返回null對象。
Java代碼
package?concurrent; ????import?java.util.concurrent.Callable; ??import?java.util.concurrent.CompletionService; ??import?java.util.concurrent.ExecutionException; ??import?java.util.concurrent.ExecutorCompletionService; ??import?java.util.concurrent.ExecutorService; ??import?java.util.concurrent.Executors; ??import?java.util.concurrent.Future; ????public?class?TestCompletionService?{ ??public?static?void?main(String[]?args)?throws?InterruptedException, ??ExecutionException?{ ??ExecutorService?exec?=?Executors.newFixedThreadPool(10); ??CompletionService?serv?= ??new?ExecutorCompletionService(exec); ????for?(int?index?=?0;?index?<?5;?index++)?{ ??final?int?NO?=?index; ??Callable?downImg?=?new?Callable()?{ ??public?String?call()?throws?Exception?{ ??Thread.sleep((long)?(Math.random()?*?10000)); ??return?“Downloaded?Image?”?+?NO; ??} ??}; ??serv.submit(downImg); ??} ????Thread.sleep(1000?*?2); ??System.out.println(“Show?web?content”); ??for?(int?index?=?0;?index?<?5;?index++)?{ ??Future?task?=?serv.take(); ??String?img?=?task.get(); ??System.out.println(img); ??} ??System.out.println(“End”); ????exec.shutdown(); ??} ??}?? package concurrent;import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;public class TestCompletionService {
public static void main(String[] args) throws InterruptedException,
ExecutionException {
ExecutorService exec = Executors.newFixedThreadPool(10);
CompletionService serv =
new ExecutorCompletionService(exec);for (int index = 0; index < 5; index++) {
final int NO = index;
Callable downImg = new Callable() {
public String call() throws Exception {
Thread.sleep((long) (Math.random() * 10000));
return “Downloaded Image ” + NO;
}
};
serv.submit(downImg);
}Thread.sleep(1000 * 2);
System.out.println(“Show web content”);
for (int index = 0; index < 5; index++) {
Future task = serv.take();
String img = task.get();
System.out.println(img);
}
System.out.println(“End”);
// 關閉線程池
exec.shutdown();
}
}
運行結果:
Show web content
Downloaded Image 1
Downloaded Image 2
Downloaded Image 4
Downloaded Image 0
Downloaded Image 3
End
操作系統的信號量是個很重要的概念，在進程控制方面都有應用。Java并發庫的Semaphore可以很輕松完成信號量控制，Semaphore可以控制某個資源可被同時訪問的個數，acquire()獲取一個許可，如果沒有就等待，而release()釋放一個許可。比如在Windows下可以設置共享文件的最大客戶端訪問個數。
Semaphore維護了當前訪問的個數，提供同步機制，控制同時訪問的個數。在數據結構中鏈表可以保存“無限”的節點，用Semaphore可以實現有限大小的鏈表。另外重入鎖ReentrantLock也可以實現該功能，但實現上要負責些，代碼也要復雜些。
下面的Demo中申明了一個只有5個許可的Semaphore，而有20個線程要訪問這個資源，通過acquire()和release()獲取和釋放訪問許可。
Java代碼
package?concurrent; ????import?java.util.concurrent.ExecutorService; ??import?java.util.concurrent.Executors; ??import?java.util.concurrent.Semaphore; ????public?class?TestSemaphore?{ ??public?static?void?main(String[]?args)?{ ????ExecutorService?exec?=?Executors.newCachedThreadPool(); ????final?Semaphore?semp?=?new?Semaphore(5); ????for?(int?index?=?0;?index?<?20;?index++)?{ ??final?int?NO?=?index; ??Runnable?run?=?new?Runnable()?{ ??public?void?run()?{ ??try?{ ????semp.acquire(); ??System.out.println(“Accessing:?”?+?NO); ??Thread.sleep((long)?(Math.random()?*?10000)); ????semp.release(); ??}?catch?(InterruptedException?e)?{ ??} ??} ??}; ??exec.execute(run); ??} ????exec.shutdown(); ??} ??}?? package concurrent;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;public class TestSemaphore {
public static void main(String[] args) {
// 線程池
ExecutorService exec = Executors.newCachedThreadPool();
// 只能5個線程同時訪問
final Semaphore semp = new Semaphore(5);
// 模擬20個客戶端訪問
for (int index = 0; index < 20; index++) {
final int NO = index;
Runnable run = new Runnable() {
public void run() {
try {
// 獲取許可
semp.acquire();
System.out.println(“Accessing: ” + NO);
Thread.sleep((long) (Math.random() * 10000));
// 訪問完后，釋放
semp.release();
} catch (InterruptedException e) {
}
}
};
exec.execute(run);
}
// 退出線程池
exec.shutdown();
}
}
運行結果：
Accessing: 0
Accessing: 1
Accessing: 2
Accessing: 3
Accessing: 4
Accessing: 5
Accessing: 6
Accessing: 7
Accessing: 8
Accessing: 9
Accessing: 10
Accessing: 11
Accessing: 12
Accessing: 13
Accessing: 14
Accessing: 15
Accessing: 16
Accessing: 17
Accessing: 18
Accessing: 19
轉載于:https://www.cnblogs.com/liaomin416100569/archive/2010/04/21/9331750.html
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