Java 线程池 管理工作线程池。它包含一个队列，用于存储等待执行的任务。我们可以使用 ThreadPoolExecutor 在 Java 中创建线程池。Java 线程池管理 Runnable 线程的集合。工作线程从队列中执行 Runnable 线程。java.util.concurrent.Executors 提供了创建 Java 中线程池的 java.util.concurrent.Executor 接口的工厂和支持方法。Executors 是一个实用类，通过各种工厂方法还提供了与 ExecutorService、ScheduledExecutorService、ThreadFactory 和 Callable 类一起使用的有用方法。让我们编写一个简单的程序来解释它的工作原理。首先，我们需要有一个 Runnable 类，命名为 WorkerThread.java
package com.journaldev.threadpool;
public class WorkerThread implements Runnable {
private String command;
public WorkerThread(String s){
this.command=s;
}
@Override
public void run() {
System.out.println(Thread.currentThread().getName()+" Start. Command = "+command);
processCommand();
System.out.println(Thread.currentThread().getName()+" End.");
}
private void processCommand() {
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
@Override
public String toString(){
return this.command;
}
}
ExecutorService 示例
这是测试程序类 SimpleThreadPool.java，我们在其中使用 Executors 框架 创建固定大小的线程池。
package com.journaldev.threadpool;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
public class SimpleThreadPool {
public static void main(String[] args) {
ExecutorService executor = Executors.newFixedThreadPool(5);
for (int i = 0; i < 10; i++) {
Runnable worker = new WorkerThread("" + i);
executor.execute(worker);
}
executor.shutdown();
while (!executor.isTerminated()) {
}
System.out.println("Finished all threads");
}
}
在上面的程序中，我们创建了一个由 5 个工作线程组成的固定大小的线程池。然后，我们提交了 10 个作业给此池，由于池大小为 5，它将开始处理 5 个作业，而其他作业将处于等待状态，一旦完成一个作业，工作线程将从等待队列中选择另一个作业并执行。这是上述程序的输出。
pool-1-thread-2 Start. Command = 1
pool-1-thread-4 Start. Command = 3
pool-1-thread-1 Start. Command = 0
pool-1-thread-3 Start. Command = 2
pool-1-thread-5 Start. Command = 4
pool-1-thread-4 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
pool-1-thread-3 End.
pool-1-thread-3 Start. Command = 8
pool-1-thread-2 End.
pool-1-thread-2 Start. Command = 9
pool-1-thread-1 Start. Command = 7
pool-1-thread-5 Start. Command = 6
pool-1-thread-4 Start. Command = 5
pool-1-thread-2 End.
pool-1-thread-4 End.
pool-1-thread-3 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
Finished all threads
輸出確認池中有五個線程，命名從“pool-1-thread-1”到“pool-1-thread-5”，它們負責執行提交到池中的任務。
ThreadPoolExecutor範例
Executors類提供了使用ExecutorService的簡單實現，使用ThreadPoolExecutor，但ThreadPoolExecutor提供了更多功能。我們可以指定在創建ThreadPoolExecutor實例時活躍的線程數，還可以限制線程池的大小，並創建自己的RejectedExecutionHandler實現來處理無法放入工作隊列的任務。這是我們的RejectedExecutionHandler接口的自定義實現。
package com.journaldev.threadpool;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;
public class RejectedExecutionHandlerImpl implements RejectedExecutionHandler {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
System.out.println(r.toString() + " is rejected");
}
}
ThreadPoolExecutor提供了幾種方法，我們可以通過這些方法找出執行器的當前狀態、池大小、活動線程數和任務數。所以我有一個監視線程，它將以一定的時間間隔打印執行器信息。
package com.journaldev.threadpool;
import java.util.concurrent.ThreadPoolExecutor;
public class MyMonitorThread implements Runnable
{
private ThreadPoolExecutor executor;
private int seconds;
private boolean run=true;
public MyMonitorThread(ThreadPoolExecutor executor, int delay)
{
this.executor = executor;
this.seconds=delay;
}
public void shutdown(){
this.run=false;
}
@Override
public void run()
{
while(run){
System.out.println(
String.format("[monitor] [%d/%d] Active: %d, Completed: %d, Task: %d, isShutdown: %s, isTerminated: %s",
this.executor.getPoolSize(),
this.executor.getCorePoolSize(),
this.executor.getActiveCount(),
this.executor.getCompletedTaskCount(),
this.executor.getTaskCount(),
this.executor.isShutdown(),
this.executor.isTerminated()));
try {
Thread.sleep(seconds*1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}
以下是使用ThreadPoolExecutor的線程池實現範例。
package com.journaldev.threadpool;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class WorkerPool {
public static void main(String args[]) throws InterruptedException{
//RejectedExecutionHandler 實現
RejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();
//取得 ThreadFactory 實現以使用
ThreadFactory threadFactory = Executors.defaultThreadFactory();
//建立 ThreadPoolExecutor
ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue(2), threadFactory, rejectionHandler);
//啟動監控執行緒
MyMonitorThread monitor = new MyMonitorThread(executorPool, 3);
Thread monitorThread = new Thread(monitor);
monitorThread.start();
//提交工作至執行緒池
for(int i=0; i<10; i++){
executorPool.execute(new WorkerThread("cmd"+i));
}
Thread.sleep(30000);
//關閉池
executorPool.shutdown();
//關閉監視執行緒
Thread.sleep(5000);
monitor.shutdown();
}
}
請注意，當初始化 ThreadPoolExecutor 時，我們將初始池大小設置為 2，最大池大小設置為 4，工作隊列大小設置為 2。因此，如果有 4 個運行中的任務且提交了更多任務，工作隊列將僅保留其中 2 個，其餘將由 RejectedExecutionHandlerImpl 處理。以下是確認上述語句的程式輸出。
pool-1-thread-1 Start. Command = cmd0
pool-1-thread-4 Start. Command = cmd5
cmd6 is rejected
pool-1-thread-3 Start. Command = cmd4
pool-1-thread-2 Start. Command = cmd1
cmd7 is rejected
cmd8 is rejected
cmd9 is rejected
[monitor] [0/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-4 End.
pool-1-thread-1 End.
pool-1-thread-2 End.
pool-1-thread-3 End.
pool-1-thread-1 Start. Command = cmd3
pool-1-thread-4 Start. Command = cmd2
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-1 End.
pool-1-thread-4 End.
[monitor] [4/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true
請注意執行者的活動、已完成和總已完成任務計數的變化。我們可以調用 shutdown() 方法來完成所有已提交任務的執行並終止執行緒池。如果您想要安排一個具有延遲或定期運行的任務，則可以使用 ScheduledThreadPoolExecutor 類。在 Java 調度執行緒池執行者 中了解更多關於它們的信息。