Java ExecutorService 例子--教程
ExecutorService is an interface that extends Executor class and represents an asynchronous execution. It provides us mechanisms to manage the end and detect progress of the asynchronous tasks.
ExecutorService 是一个继承自Executor的接口， 代表一个异步的执行； 为我们提供了一个管理线程执行终端和发现异步任务执行进程的机制。
In this example we are going to see some basic functionalities of ExecutorService, as well as handle the Future object, the result of asynchronous computation.
在这个例子里面，我们来看一下ExecutorService 的基本功能， 同时来认识一下如何处理Future 对象， 异步执行结果等。
1. 创建一个Runnable 对象
We are going to create a Runnable that is intended to be executed by the ExecutorService. Create a java class named myThread and paste the following code.
我们来创建一个Runnable 接口，用来被ExecutorService 来调用。 创建一个名为myThread Java 类:
package com.javacodegeeks.core.concurrency.executorservicetest;
public class MyThread implements Runnable {
private String myName;
private int count;
private final long timeSleep;
MyThread(String name, int newcount, long newtimeSleep) {
this.myName = name;
this.count = newcount;
this.timeSleep = newtimeSleep;
}
@Override
public void run() {
// TODO Auto-generated method stub
int sum = 0;
for (int i = 1; i <= this.count; i++) {
sum = sum + i;
}
System.out.println(myName + " thread has sum = " + sum +
" and is going to sleep for " + timeSleep);
try {
Thread.sleep(this.timeSleep);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}这个Runnable 的功能非常简单， 它计算从给定的参数的和， 并在此期间内休眠。
2. Code the ExecutorService
In this example we will use a factor method of ExecutorService that creates a thread pool of fixed number of threads. For this reason, newFixedThreadPool() method is used where we specify the number of threads in the pool. To execute the thread, we can use either execute() method or submit(), where both of them take Runnable as a parameter. execute() method is depending on the implementation of the Executor class and may perform the Runnable in a new thread, in a pooled thread, or in the calling thread. submit() method extends execute(), by returning a Future that represents the submitting task.
The Future can be used to indicate the termination of execution of the thread. For instance, get() method waits for the completion of the computation. If the returning value is null, the task has finished correctly. Otherwise, cancel() method can be called in order to end the execution of this task. It is worth to mention that for bulk or a collection of thread execution, invokeAll() and invokeAny() are used respectively, although there are not used in this example.
To close down the ExecutorService, there are many methods that can be used. In our example we use shutdown() method, in which the submitted tasks are executed before the shutting down but new tasks can not be accepted. Another approach is shutdownNow() method, which stops the executing tasks, pause the waiting ones and returns the list of the awaiting ones. Moreover, awaitTermination() can be used in order to wait until all threads are terminated.
For further understanding of the main functionality of ExecutorService, have a look at the code below. Create ExecutorServiceTest.java file and paste the following.
ExecutorServiceTest.java:
package com.javacodegeeks.core.concurrency.executorservicetest;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
public class ExecutorServiceTest {
private static Future taskTwo = null;
private static Future taskThree = null;
public static void main(String[] args) throws InterruptedException, ExecutionException {
ExecutorService executor = Executors.newFixedThreadPool(2);
// execute the Runnable
Runnable taskOne = new MyThread("TaskOne", 2, 100);
executor.execute(taskOne);
for(int i = 0; i < 2; i++) {
// if this task is not created or is canceled or is completed
if ((taskTwo == null) || (taskTwo.isDone()) || (taskTwo.isCancelled())) {
// submit a task and return a Future
taskTwo = executor.submit(new MyThread("TaskTwo", 4, 200));
}
if ((taskThree == null) || (taskThree.isDone()) || (taskThree.isCancelled())) {
taskThree = executor.submit(new MyThread("TaskThree", 5, 100));
}
// if null the task has finished
if(taskTwo.get() == null) {
System.out.println(i+1 + ") TaskTwo terminated successfully");
} else {
// if it doesn't finished, cancel it
taskTwo.cancel(true);
}
if(taskThree.get() == null) {
System.out.println(i+1 + ") TaskThree terminated successfully");
} else {
taskThree.cancel(true);
}
}
executor.shutdown();
System.out.println("-----------------------");
// wait until all tasks are finished
executor.awaitTermination(1, TimeUnit.SECONDS);
System.out.println("All tasks are finished!");
}
}Now you can see the output of the execution.
Output:
TaskOne thread has sum = 3 and is going to sleep for 100
TaskTwo thread has sum = 10 and is going to sleep for 200
TaskThree thread has sum = 15 and is going to sleep for 100
1) TaskTwo terminated successfully
1) TaskThree terminated successfully
TaskTwo thread has sum = 10 and is going to sleep for 200
TaskThree thread has sum = 15 and is going to sleep for 100
2) TaskTwo terminated successfully
2) TaskThree terminated successfully
-----------------------
All tasks are finished!
Download the source codeThis was an example of ExecutorService in Java. Download the source code of this example: ExecutorServiceTest.zip