Java 执行器
2018-02-28 16:06 更新
Java线程教程 - Java执行器
框架提供了一种将任务提交与任务执行分离的方法。
java.util.concurrent包中的Executor接口是执行器框架的基础。
它是一个只有一个方法的接口,如图所示:
public interface Executor { void execute (Runnable command); }
import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; class RunnableTask implements Runnable { private int taskId; private int loopCounter; public RunnableTask(int taskId, int loopCounter) { this.taskId = taskId; this.loopCounter = loopCounter; } public void run() { for (int i = 1; i <= loopCounter; i++) { try { System.out.println("Task #" + this.taskId + " - Iteration #" + i); Thread.sleep(1000); } catch (Exception e) { System.out.println("Task #" + this.taskId + " has been interrupted."); break; } } } } public class Main { public static void main(String[] args) { final int THREAD_COUNT = 3; final int LOOP_COUNT = 3; final int TASK_COUNT = 5; // Get an executor with three threads in its thread pool ExecutorService exec = Executors.newFixedThreadPool(THREAD_COUNT); // Create five tasks and submit them to the executor for (int i = 1; i <= TASK_COUNT; i++) { RunnableTask task = new RunnableTask(i, LOOP_COUNT); exec.submit(task); } exec.shutdown(); } }
上面的代码生成以下结果。
结果承载任务
要在任务完成时获取任务的结果,请使用Callable接口的实例。
类型参数V是任务的结果的类型。
Callable接口有一个call()方法。它可以返回任何类型的值。
它允许你抛出异常。它声明如下:
public interface Callable<V> { V call() throws Exception; }
import java.util.concurrent.Callable; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; class CallableTask implements Callable<Integer> { private int taskId; public CallableTask(int taskId) { this.taskId = taskId; } public Integer call() throws InterruptedException { int total = taskId; try { System.out.println("Task #" + this.taskId); Thread.sleep(1000); } catch (InterruptedException e) { System.out.println("Task #" + this.taskId + " has been interupted."); throw e; } total+=taskId; return total; } } public class Main { public static void main(String[] args) throws Exception { // Get an executor with three threads in its thread pool ExecutorService exec = Executors.newFixedThreadPool(3); CallableTask task = new CallableTask(1); // Submit the callable task to executor Future<Integer> submittedTask = exec.submit(task); Integer result = submittedTask.get(); System.out.println("Task"s total sleep time: " + result + " seconds"); exec.shutdown(); } }
上面的代码生成以下结果。
调度任务
执行器框架允许您计划将来运行的任务。
import java.time.LocalDateTime; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; class ScheduledTask implements Runnable { private int taskId; public ScheduledTask(int taskId) { this.taskId = taskId; } public void run() { LocalDateTime currentDateTime = LocalDateTime.now(); System.out.println("Task #" + this.taskId + " ran at " + currentDateTime); } } public class Main { public static void main(String[] args) { // Get an executor with 3 threads ScheduledExecutorService sexec = Executors.newScheduledThreadPool(3); ScheduledTask task1 = new ScheduledTask(1); ScheduledTask task2 = new ScheduledTask(2); // Task #1 will run after 2 seconds sexec.schedule(task1, 2, TimeUnit.SECONDS); // Task #2 runs after 5 seconds delay and keep running every 10 seconds sexec.scheduleAtFixedRate(task2, 5, 10, TimeUnit.SECONDS); try { TimeUnit.SECONDS.sleep(60); } catch (InterruptedException e) { e.printStackTrace(); } sexec.shutdown(); } }
上面的代码生成以下结果。
在任务执行中处理未捕获的异常
执行器框架在任务执行期间处理任何未捕获异常的事件。
如果使用Executor对象的execute()方法执行Runnable任务,任何未捕获的运行时异常将停止任务执行,并且异常堆栈跟踪将打印在控制台上。
import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; public class Main { public static void main(String[] args) { Runnable badTask = () -> { throw new RuntimeException( "Throwing exception from task execution..."); }; ExecutorService exec = Executors.newSingleThreadExecutor(); exec.execute(badTask); exec.shutdown(); } }
上面的代码生成以下结果。
在Callable任务中处理异常
以下代码显示了如何在Callable任务中处理异常。
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 Main { public static void main(String[] args) { Callable<Object> badTask = () -> { throw new RuntimeException( "Throwing exception from task execution..."); }; ExecutorService exec = Executors.newSingleThreadExecutor(); Future submittedTask = exec.submit(badTask); try { Object result = submittedTask.get(); } catch (ExecutionException e) { System.out.println(e.getMessage()); System.out.println(e.getCause().getMessage()); } catch (InterruptedException e) { e.printStackTrace(); } exec.shutdown(); } }
执行完成服务
要将提交的任务的结果提供给执行程序,请使用执行程序的完成服务。
它由CompletionService接口的一个实例表示。
import java.util.concurrent.Callable; import java.util.concurrent.ExecutorCompletionService; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; class MyResult { private int taskId; private int result; public MyResult(int taskId, int result) { this.taskId = taskId; this.result = result; } public int getTaskId() { return taskId; } public int getResult() { return result; } public String toString() { return "Task Name: Task #" + taskId + ", Task Result:" + result + " seconds"; } } class SleepingTask implements Callable<MyResult> { private int taskId; private int loopCounter; public SleepingTask(int taskId, int loopCounter) { this.taskId = taskId; this.loopCounter = loopCounter; } public MyResult call() throws InterruptedException { int totalSleepTime = 0; for (int i = 1; i <= loopCounter; i++) { try { System.out.println("Task #" + this.taskId + " - Iteration #" + i); Thread.sleep(1000); totalSleepTime = totalSleepTime + 1000; } catch (InterruptedException e) { System.out.println("Task #" + this.taskId + " has been interupted."); throw e; } } return new MyResult(taskId, totalSleepTime); } } public class Main { public static void main(String[] args) throws Exception { // Get an executor with three threads in its thread pool ExecutorService exec = Executors.newFixedThreadPool(3); // Completed task returns an object of the TaskResult class ExecutorCompletionService<MyResult> completionService = new ExecutorCompletionService<>( exec); for (int i = 1; i <= 5; i++) { SleepingTask task = new SleepingTask(i, 3); completionService.submit(task); } for (int i = 1; i <= 5; i++) { Future<MyResult> completedTask = completionService.take(); MyResult result = completedTask.get(); System.out.println("Completed a task - " + result); } exec.shutdown(); } }
上面的代码生成以下结果。
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