執行緒池之ThreadPoolExecutor使用
阿新 • • 發佈：2019-06-24
ThreadPoolExecutor提供了四個構造方法：
我們以最後一個構造方法（引數最多的那個），對其引數進行解釋：
public ThreadPoolExecutor(int corePoolSize, // 1 int maximumPoolSize, // 2 long keepAliveTime, // 3 TimeUnit unit, // 4 BlockingQueue<Runnable> workQueue, // 5 ThreadFactory threadFactory, // 6 RejectedExecutionHandler handler ) { //7 if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0) throw new IllegalArgumentException(); if (workQueue == null || threadFactory == null || handler == null) throw new NullPointerException(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }
| 序號 | 名稱 | 型別 | 含義 |
|---|---|---|---|
| 1 | corePoolSize | int | 核心執行緒池大小 |
| 2 | maximumPoolSize | int | 最大執行緒池大小 |
| 3 | keepAliveTime | long | 執行緒最大空閒時間 |
| 4 | unit | TimeUnit | 時間單位 |
| 5 | workQueue | BlockingQueue<Runnable> | 執行緒等待佇列 |
| 6 | threadFactory | ThreadFactory | 執行緒建立工廠 |
| 7 | handler | RejectedExecutionHandler | 拒絕策略 |
如果對這些引數作用有疑惑的請看 ThreadPoolExecutor概述
知道了各個引數的作用後，我們開始構造符合我們期待的執行緒池。首先看JDK給我們預定義的幾種執行緒池：
一、預定義執行緒池
- FixedThreadPool
public static ExecutorService newFixedThreadPool(int nThreads) {
return new ThreadPoolExecutor(nThreads, nThreads,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>());
}
- corePoolSize與maximumPoolSize相等，即其執行緒全為核心執行緒，是一個固定大小的執行緒池，是其優勢；
- keepAliveTime = 0 該引數預設對核心執行緒無效，而FixedThreadPool全部為核心執行緒；
- workQueue 為LinkedBlockingQueue（無界阻塞佇列），佇列最大值為Integer.MAX_VALUE。如果任務提交速度持續大餘任務處理速度，會造成佇列大量阻塞。因為佇列很大，很有可能在拒絕策略前，記憶體溢位。是其劣勢；
- FixedThreadPool的任務執行是無序的；
適用場景：可用於Web服務瞬時削峰，但需注意長時間持續高峰情況造成的佇列阻塞。
- CachedThreadPool
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
- corePoolSize = 0，maximumPoolSize = Integer.MAX_VALUE，即執行緒數量幾乎無限制；
- keepAliveTime = 60s，執行緒空閒60s後自動結束。
- workQueue 為 SynchronousQueue 同步佇列，這個佇列類似於一個接力棒，入隊出隊必須同時傳遞，因為CachedThreadPool執行緒建立無限制，不會有佇列等待，所以使用SynchronousQueue；
適用場景：快速處理大量耗時較短的任務，如Netty的NIO接受請求時，可使用CachedThreadPool。
- SingleThreadExecutor
public static ExecutorService newSingleThreadExecutor() {
return new FinalizableDelegatedExecutorService
(new ThreadPoolExecutor(1, 1,
0L, TimeUnit.MILLISECONDS,
new LinkedBlockingQueue<Runnable>()));
}
咋一瞅，不就是newFixedThreadPool(1)嗎？定眼一看，這裡多了一層FinalizableDelegatedExecutorService包裝，這一層有什麼用呢，寫個dome來解釋一下：
public static void main(String[] args) {
ExecutorService fixedExecutorService = Executors.newFixedThreadPool(1);
ThreadPoolExecutor threadPoolExecutor = (ThreadPoolExecutor) fixedExecutorService;
System.out.println(threadPoolExecutor.getMaximumPoolSize());
threadPoolExecutor.setCorePoolSize(8);
ExecutorService singleExecutorService = Executors.newSingleThreadExecutor();
// 執行時異常 java.lang.ClassCastException
// ThreadPoolExecutor threadPoolExecutor2 = (ThreadPoolExecutor) singleExecutorService;
}
對比可以看出，FixedThreadPool可以向下轉型為ThreadPoolExecutor，並對其執行緒池進行配置，而SingleThreadExecutor被包裝後，無法成功向下轉型。因此，SingleThreadExecutor被定以後，無法修改，做到了真正的Single。
- ScheduledThreadPool
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
return new ScheduledThreadPoolExecutor(corePoolSize);
}
newScheduledThreadPool呼叫的是ScheduledThreadPoolExecutor的構造方法，而ScheduledThreadPoolExecutor繼承了ThreadPoolExecutor，構造是還是呼叫了其父類的構造方法。
public ScheduledThreadPoolExecutor(int corePoolSize) {
super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
new DelayedWorkQueue());
}
對於ScheduledThreadPool本文不做描述，其特性請關注後續篇章。
二、自定義執行緒池
以下是自定義執行緒池，使用了有界佇列，自定義ThreadFactory和拒絕策略的demo：
public class ThreadTest {
public static void main(String[] args) throws InterruptedException, IOException {
int corePoolSize = 2;
int maximumPoolSize = 4;
long keepAliveTime = 10;
TimeUnit unit = TimeUnit.SECONDS;
BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(2);
ThreadFactory threadFactory = new NameTreadFactory();
RejectedExecutionHandler handler = new MyIgnorePolicy();
ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, unit,
workQueue, threadFactory, handler);
executor.prestartAllCoreThreads(); // 預啟動所有核心執行緒
for (int i = 1; i <= 10; i++) {
MyTask task = new MyTask(String.valueOf(i));
executor.execute(task);
}
System.in.read(); //阻塞主執行緒
}
static class NameTreadFactory implements ThreadFactory {
private final AtomicInteger mThreadNum = new AtomicInteger(1);
@Override
public Thread newThread(Runnable r) {
Thread t = new Thread(r, "my-thread-" + mThreadNum.getAndIncrement());
System.out.println(t.getName() + " has been created");
return t;
}
}
public static class MyIgnorePolicy implements RejectedExecutionHandler {
public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
doLog(r, e);
}
private void doLog(Runnable r, ThreadPoolExecutor e) {
// 可做日誌記錄等
System.err.println( r.toString() + " rejected");
// System.out.println("completedTaskCount: " + e.getCompletedTaskCount());
}
}
static class MyTask implements Runnable {
private String name;
public MyTask(String name) {
this.name = name;
}
@Override
public void run() {
try {
System.out.println(this.toString() + " is running!");
Thread.sleep(3000); //讓任務執行慢點
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public String getName() {
return name;
}
@Override
public String toString() {
return "MyTask [name=" + name + "]";
}
}
}
輸出結果如下：
其中執行緒執行緒1-4先佔滿了核心執行緒和最大執行緒數量，然後4、5執行緒進入等待佇列，7-10執行緒被直接忽略拒絕執行，等1-4執行緒中有執行緒執行完後通知4、5執行緒繼續執行。