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java并发编程学习笔记之线程池等源码小析

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      在java并发编程中,线程池是一个比较重要的点,什么时候需要使用线程池,什么时候不需要使用线程池,看不同的需求,众所周知,新增一个线程是比较耗资源的,因此如果每次新增一个任务就添加一个线程,在分时系统中,这不仅会造成每个线程所获得的执行时间大大降低,同时也会使cpu和内存大大消耗,线程池是一种比较合适的处理办法,一方面缓解资源紧张,一方面又能获得不错的性能,但是,对于批处理作业和耗费资源不是很多的任务,选择线程池不是一个很好地设计办法。

     首先看看两个新的接口,Callable和Future源码如下

     

public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

    

package java.util.concurrent;


public interface Future<V> {

    
    boolean cancel(boolean mayInterruptIfRunning);

    
    boolean isCancelled();

   
    boolean isDone();

    
    V get() throws InterruptedException, ExecutionException;

    
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

    简而言之,callable接口类似Runnable 接口,其call()方法和Runnable的run()方法很相似,但是Callable有返回值,而Runnable没有返回值。Future保存异步计算的结果。可以启动一个计算,将Future对象交给某个线程,然后忘掉它,也就是当他是一个返回值。

    通常在一般线程中会使用FutureTask类,FutureTask接口继承自RunnableFuture,而Runnable接口继承Runnable和Future。

    首先看下FutureTask的简单用法:

package com.luchi.thread.threadpool;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.Future;
import java.util.concurrent.FutureTask;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;

public class TestFutureRCallable implements Callable<Integer>{

	private int counter=0;
	@Override
	public Integer call() throws Exception {
		// TODO Auto-generated method stub
		
		System.out.println("i am on the running");
		return 1;
	}
	public  static  void main(String[]args) throws InterruptedException, ExecutionException{
		
		TestFutureRCallable testThread=new TestFutureRCallable();
		FutureTask<Integer>futureTask=new FutureTask<Integer>(testThread);
		Thread thread=new Thread(futureTask);
		thread.start();
		System.out.println("future returns:"+futureTask.get());
		
	}

	
	
}

      上面程序把Callable的继承类当做FutureTask构造函数参数,然后运行Thread,最后FutureTask能够得到返回值。

      FutureTask有几个构造函数,来看源码

 

     

 public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

    /**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Runnable}, and arrange that {@code get} will return the
     * given result on successful completion.
     *
     * @param runnable the runnable task
     * @param result the result to return on successful completion. If
     * you don't need a particular result, consider using
     * constructions of the form:
     * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
     * @throws NullPointerException if the runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

  一个是FutureTask(Callable callbale),接受Callable对象,另一个是FutureTask(Runnable runnable,V result),接受Runnable对象。但是从源码可以看出,不管是Callable或者是Runnable,FutureTask都将其转化成Callable对象,Executors.callable(runnable, result);这个方法使用了适配器模式,将Runnable对象转换成Callable对象,看一眼源码:

 public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }



  static final class RunnableAdapter<T> implements Callable<T> {
        final Runnable task;
        final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }


   从源码可以看出,适配器将Runnable对象的run方法放在了Callable对象的call接口中

   也就是说,无论是Callable还是Runnable对象,在FutureTask中都是当做Callable对象使用,由于FutureTask继承了Runnable接口,看一眼其实现的run方法

   

  public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

   其核心就是执行callable对象的call方法,这也和上面的分析对应。

   然后看一眼FutureTask的get方法

   

  public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

 如果计算没有结束,则阻塞,如果已经完成则返回计算结果

 

 说了这么多,最后来看看线程池。

 首先看下线程池的简单用法:

 

package com.luchi.thread.threadpool;

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 TestThreadPool implements Callable<Integer>{

	@Override
	public Integer call() throws Exception {
		// TODO Auto-generated method stub

		System.out.println("the thread is running");
		return 10;
	}
	
	public static void main(String args[]) throws InterruptedException, ExecutionException{
		

		ExecutorService excutor =Executors.newCachedThreadPool();
		TestFutureRCallable test=new TestFutureRCallable();
		Future<Integer> future=excutor.submit(test);
		System.out.println("  "+future.get());
		excutor.shutdown();
	}
	

	
	
}

 

 

 

 上面的程序中,简单的使用了线程池,常见的获取线程池的方法有两种,一种是 Executors.newCachedThreadPool()一种是Executors.newFixedThreadPool();看一眼两者的源码

 

 

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

 

   两者都返回了ThreadPoolExecutor对象,ThreadPoolExecutor构造函数的意义简单解释下,第一个和第二个参数指的是线程池中线程的线程数量最小M和最大的值N,第三个是多长时间空闲线程回收,第四个参数是第三个的时间单位,第五个参数是表示使用的阻塞Queue,线程池开设线程的方法如下:

   假如新任务来了,如果当前线程数少于最小的M,则新增一个线程,如果在M~N之间,则把任务丢进等待队列中,如果等待队列满了之后,则再新增一个线程,直到到最大的值N。

   newFixecThreadPool中使用了M值和N值相同,也就是新任务来了会一直增开线程数到M,然后再丢进LinkedBlockingQueue中,LinkedBlockingQueue是一个大小无限的阻塞队列,当然这个无限是相对于当前的资源情况,newCachedThreadPool的线程数是从0到无限个,而SynchronousQueue容量为0,意味着任务来了就新开一个线程?(这里不是很了解,有待研究)

   再来看一下其submit()方法

 

 public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    /**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

   summit接受Callable和Runnable方法,返回执行的Future对象,本文不去探讨实现细节。

 

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