java并发框架Executor介绍

Executor框架是指java 5中引入的一系列并发库中与executor相关的一些功能类,其中包括线程池,Executor,Executors,ExecutorService,CompletionService,Future,Callable等。

他们的关系为:

 

并发编程的一种编程方式是把任务拆分为一些列的小任务,即Runnable,然后在提交给一个Executor执行,Executor.execute(Runnalbe) 。Executor在执行时使用内部的线程池完成操作。

一、创建线程池

Executors类,提供了一系列工厂方法用于创先线程池,返回的线程池都实现了ExecutorService接口。

public static ExecutorService newFixedThreadPool(int nThreads)

创建固定数目线程的线程池。

public static ExecutorService newCachedThreadPool()

创建一个可缓存的线程池,调用execute 将重用以前构造的线程(如果线程可用)。如果现有线程没有可用的,则创建一个新线程并添加到池中。终止并从缓存中移除那些已有 60 秒钟未被使用的线程。

public static ExecutorService newSingleThreadExecutor()

创建一个单线程化的Executor。

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize)

创建一个支持定时及周期性的任务执行的线程池,多数情况下可用来替代Timer类。

Java代码  收藏代码
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Executor executor = Executors.newFixedThreadPool(10); 
Runnable task = new Runnable() { 
    @Override 
    public void run() { 
        System.out.println("task over"); 
    
}; 
executor.execute(task); 
   
executor = Executors.newScheduledThreadPool(10); 
ScheduledExecutorService scheduler = (ScheduledExecutorService) executor; 
scheduler.scheduleAtFixedRate(task, 10, 10, TimeUnit.SECONDS); 

  

 二、ExecutorService与生命周期

ExecutorService扩展了Executor并添加了一些生命周期管理的方法。一个Executor的生命周期有三种状态,运行 ,关闭 ,终止。Executor创建时处于运行状态。当调用ExecutorService.shutdown()后,处于关闭状态,isShutdown()方法返回true。这时,不应该再想Executor中添加任务,所有已添加的任务执行完毕后,Executor处于终止状态,isTerminated()返回true。

如果Executor处于关闭状态,往Executor提交任务会抛出unchecked exception RejectedExecutionException。

Java代码  收藏代码
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ExecutorService executorService = (ExecutorService) executor; 
while (!executorService.isShutdown()) { 
    try
        executorService.execute(task); 
    } catch (RejectedExecutionException ignored) { 
           
    
executorService.shutdown(); 

  

 三、使用Callable,Future返回结果

Future<V>代表一个异步执行的操作,通过get()方法可以获得操作的结果,如果异步操作还没有完成,则,get()会使当前线程阻塞。FutureTask<V>实现了Future<V>和Runable<V>。Callable代表一个有返回值得操作。

Java代码  收藏代码
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Callable<Integer> func = new Callable<Integer>(){ 
    public Integer call() throws Exception { 
        System.out.println("inside callable"); 
        Thread.sleep(1000); 
        return new Integer(8); 
    }        
};       
FutureTask<Integer> futureTask  = new FutureTask<Integer>(func); 
Thread newThread = new Thread(futureTask); 
newThread.start(); 
   
try
    System.out.println("blocking here"); 
    Integer result = futureTask.get(); 
    System.out.println(result); 
} catch (InterruptedException ignored) { 
} catch (ExecutionException ignored) { 
 ExecutoreService提供了submit()方法,传递一个Callable,或Runnable,返回Future。如果Executor后台线程池还没有完成Callable的计算,这调用返回Future对象的get()方法,会阻塞直到计算完成。
 
例子:并行计算数组的和。
 
Java代码  收藏代码
package executorservice; 
   
import java.util.ArrayList; 
import java.util.List; 
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; 
import java.util.concurrent.FutureTask; 
   
public class ConcurrentCalculator { 
   
    private ExecutorService exec; 
    private int cpuCoreNumber; 
    private List<Future<Long>> tasks = new ArrayList<Future<Long>>(); 
   
    // 内部类 
    class SumCalculator implements Callable<Long> { 
        private int[] numbers; 
        private int start; 
        private int end; 
   
        public SumCalculator(final int[] numbers, int start, int end) { 
            this.numbers = numbers; 
            this.start = start; 
            this.end = end; 
        
   
        public Long call() throws Exception { 
            Long sum = 0l; 
            for (int i = start; i < end; i++) { 
                sum += numbers[i]; 
            
            return sum; 
        
    
   
    public ConcurrentCalculator() { 
        cpuCoreNumber = Runtime.getRuntime().availableProcessors(); 
        exec = Executors.newFixedThreadPool(cpuCoreNumber); 
    
   
    public Long sum(final int[] numbers) { 
        // 根据CPU核心个数拆分任务,创建FutureTask并提交到Executor 
        for (int i = 0; i < cpuCoreNumber; i++) { 
            int increment = numbers.length / cpuCoreNumber + 1
            int start = increment * i; 
            int end = increment * i + increment; 
            if (end > numbers.length) 
                end = numbers.length; 
            SumCalculator subCalc = new SumCalculator(numbers, start, end); 
            FutureTask<Long> task = new FutureTask<Long>(subCalc); 
            tasks.add(task); 
            if (!exec.isShutdown()) { 
                exec.submit(task); 
            
        
        return getResult(); 
    
   
    /**
     * 迭代每个只任务,获得部分和,相加返回
     
     * @return
     */ 
    public Long getResult() { 
        Long result = 0l; 
        for (Future<Long> task : tasks) { 
            try
                // 如果计算未完成则阻塞 
                Long subSum = task.get(); 
                result += subSum; 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } catch (ExecutionException e) { 
                e.printStackTrace(); 
            
        
        return result; 
    
   
    public void close() { 
        exec.shutdown(); 
    

  

 

 Main

Java代码  收藏代码
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int[] numbers = new int[] { 1, 2, 3, 4, 5, 6, 7, 8, 10, 11 }; 
ConcurrentCalculator calc = new ConcurrentCalculator(); 
Long sum = calc.sum(numbers); 
System.out.println(sum); 
calc.close(); 

  

 四、CompletionService

在刚在的例子中,getResult()方法的实现过程中,迭代了FutureTask的数组,如果任务还没有完成则当前线程会阻塞,如果我们希望任意字任务完成后就把其结果加到result中,而不用依次等待每个任务完成,可以使CompletionService。生产者submit()执行的任务。使用者take()已完成的任务,并按照完成这些任务的顺序处理它们的结果 。也就是调用CompletionService的take方法是,会返回按完成顺序放回任务的结果,CompletionService内部维护了一个阻塞队列BlockingQueue,如果没有任务完成,take()方法也会阻塞。修改刚才的例子使用CompletionService:

Java代码  收藏代码
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public class ConcurrentCalculator2 { 
   
    private ExecutorService exec; 
    private CompletionService<Long> completionService; 
   
   
    private int cpuCoreNumber; 
   
    // 内部类 
    class SumCalculator implements Callable<Long> { 
        ...... 
    
   
    public ConcurrentCalculator2() { 
        cpuCoreNumber = Runtime.getRuntime().availableProcessors(); 
        exec = Executors.newFixedThreadPool(cpuCoreNumber); 
        completionService = new ExecutorCompletionService<Long>(exec); 
   
   
    
   
    public Long sum(final int[] numbers) { 
        // 根据CPU核心个数拆分任务,创建FutureTask并提交到Executor 
        for (int i = 0; i < cpuCoreNumber; i++) { 
            int increment = numbers.length / cpuCoreNumber + 1
            int start = increment * i; 
            int end = increment * i + increment; 
            if (end > numbers.length) 
                end = numbers.length; 
            SumCalculator subCalc = new SumCalculator(numbers, start, end);  
            if (!exec.isShutdown()) { 
                completionService.submit(subCalc); 
   
   
            
               
        
        return getResult(); 
    
   
    /**
     * 迭代每个只任务,获得部分和,相加返回
     
     * @return
     */ 
    public Long getResult() { 
        Long result = 0l; 
        for (int i = 0; i < cpuCoreNumber; i++) {             
            try
                Long subSum = completionService.take().get(); 
                result += subSum;            
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } catch (ExecutionException e) { 
                e.printStackTrace(); 
            
        
        return result; 
    
   
    public void close() { 
        exec.shutdown(); 
    

  

 五、使用场景

1.可用于替代timer做定时任务

2.改造项目中原先运行慢的代码,切割成多个块,在多个线程中运行,最后结果再整合在一起

3.代码异步执行,先返回主要部分

等等

posted @   兔老霸夏  阅读(975)  评论(0编辑  收藏  举报
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