java高并发编程(五)线程池


摘自马士兵java并发编程

一、认识Executor、ExecutorService、Callable、Executors

/**
 * 认识Executor
 */
package yxxy.c_026;

import java.util.concurrent.Executor;

public class T01_MyExecutor implements Executor {

    public static void main(String[] args) {
        new T01_MyExecutor().execute(new Runnable(){

            @Override
            public void run() {
                System.out.println("hello executor");
            }
            
        });
    }

    @Override
    public void execute(Runnable command) {
        //new Thread(command).run();
        command.run();
    }

}
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Executor执行器是一个接口,只有一个方法execute执行任务,在java的线程池的框架里边,这个是最顶层的接口;
ExecutorService:从Executor接口继承。
Callable:里面call方法,和Runnable接口很像,设计出来都是被其他线程调用的;但是Runnable接口里面run方法是没有返回值的也不能抛出异常;而call方法有返回值可以抛异常;
Executors: 操作Executor的一个工具类;以及操作ExecutorService,ThreadFactory,Callable等;
 
二、ThreadPool:      
/**
 * 线程池的概念
 */
package yxxy.c_026;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class T05_ThreadPool {
    public static void main(String[] args) throws InterruptedException {
        ExecutorService service = Executors.newFixedThreadPool(5); //execute submit
        for (int i = 0; i < 6; i++) {
            service.execute(() -> {
                try {
                    TimeUnit.MILLISECONDS.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
            });
        }
        System.out.println(service);
        
        service.shutdown();
        System.out.println(service.isTerminated());
        System.out.println(service.isShutdown());
        System.out.println(service);
        
        TimeUnit.SECONDS.sleep(5);
        System.out.println(service.isTerminated());
        System.out.println(service.isShutdown());
        System.out.println(service);
    }
}
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console:

java.util.concurrent.ThreadPoolExecutor@53d8d10a[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
false
true
java.util.concurrent.ThreadPoolExecutor@53d8d10a[Shutting down, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
pool-1-thread-1
pool-1-thread-3
pool-1-thread-2
pool-1-thread-5
pool-1-thread-4
pool-1-thread-1
true
true
java.util.concurrent.ThreadPoolExecutor@53d8d10a[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 6]
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创建了一个线程池,扔了5个线程,接下来要执行6个任务,扔进去线程池里面就启一个线程帮你执行一个,因为这里最多就起5个线程,接下来扔第6个任务的时候,不好意思,它排队了,排在线程池所维护的一个任务队列里面,任务队列大多数使用的都是BlockingQueue,这是线程池的概念;
有什么好处?好处在于如果这个任务执行完了,这个线程不会消失,它执行完任务空闲下来了,如果有新的任务来的时候,直接交给这个线程来运行就行了,不需要新启动线程;从这个概念上讲,如果你的任务和线程池线程数量控制的比较好的情况下,你不需要启动新的线程就能执行很多很多的任务,效率会比较高,并发性好;
 
service.shutdown():关闭线程池,shutdown是正常的关闭,它会等所有的任务都执行完才会关闭掉;还有一个是shutdownNow,二话不说直接就给关了,不管线程有没有执行完;
service.isTerminated(): 代表的是这里所有执行的任务是不是都执行完了。isShutdown()为true,注意它关了但并不代表它执行完了,只是代表正在关闭的过程之中(注意打印Shutting down)
打印5个线程名字,而且第一个线程执行完了之后,第6个任务来了,第1个线程继续执行,不会有线程6;
 
当所有线程全部执行完毕之后,线程池的状态为Terminated,表示正常结束,complete tasks=6
 
线程池里面维护了很多线程,等着你往里扔任务,而扔任务的时候它可以维护着一个任务列表,还没有被执行的任务列表,同样的它还维护着另外一个队列,complete tasks,结束的任务队列,任务执行结束扔到这个队列里,所以,一个线程池维护着两个队列;
 
 
三、Future                                  
/**
 * 认识future
 */
package yxxy.c_026;

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;
import java.util.concurrent.TimeUnit;

public class T06_Future {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        /*FutureTask<Integer> task = new FutureTask<Integer>(new Callable<Integer>(){
            @Override
            public Integer call() throws Exception {
                TimeUnit.MILLISECONDS.sleep(3000);
                return 1000;
            }
        });*/
        
        FutureTask<Integer> task = new FutureTask<>(()->{
            TimeUnit.MILLISECONDS.sleep(3000);
            return 1000;
        });
        
        new Thread(task).start();
        
        System.out.println(task.get()); //阻塞
        
        //*******************************
        ExecutorService service = Executors.newFixedThreadPool(5);
        Future<Integer> f = service.submit(()->{
            TimeUnit.MILLISECONDS.sleep(5000);
            return 1;
        });
        System.out.println(f.isDone());
        System.out.println(f.get());
        System.out.println(f.isDone());
        
    }
}
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1000
false
1
true
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Future: ExecutorService里面有submit方法,它的返回值是Future类型,因为你扔一个任务进去需要执行一段时间,未来的某一个时间点上,任务执行完了产生给你一个结果,这个Future代表的就是那个Callable的返回值;
 
 
四、并行计算的例子:      
/**
 * 线程池的概念
 * nasa
 */
package yxxy.c_026;

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;

public class T07_ParallelComputing {
    public static void main(String[] args) throws InterruptedException, ExecutionException {
        long start = System.currentTimeMillis();
        List<Integer> results = getPrime(1, 200000); 
        long end = System.currentTimeMillis();
        System.out.println(end - start);
        
        final int cpuCoreNum = 4;
        
        ExecutorService service = Executors.newFixedThreadPool(cpuCoreNum);
        
        MyTask t1 = new MyTask(1, 80000); //1-5 5-10 10-15 15-20
        MyTask t2 = new MyTask(80001, 130000);
        MyTask t3 = new MyTask(130001, 170000);
        MyTask t4 = new MyTask(170001, 200000);
        
        Future<List<Integer>> f1 = service.submit(t1);
        Future<List<Integer>> f2 = service.submit(t2);
        Future<List<Integer>> f3 = service.submit(t3);
        Future<List<Integer>> f4 = service.submit(t4);
        
        start = System.currentTimeMillis();
        f1.get();
        f2.get();
        f3.get();
        f4.get();
        end = System.currentTimeMillis();
        System.out.println(end - start);
    }
    
    static class MyTask implements Callable<List<Integer>> {
        int startPos, endPos;
        
        MyTask(int s, int e) {
            this.startPos = s;
            this.endPos = e;
        }
        
        @Override
        public List<Integer> call() throws Exception {
            List<Integer> r = getPrime(startPos, endPos);
            return r;
        }
        
    }
    
    //判断是否是质数
    static boolean isPrime(int num) {
        for(int i=2; i<=num/2; i++) {
            if(num % i == 0) return false;
        }
        return true;
    }
    
    static List<Integer> getPrime(int start, int end) {
        List<Integer> results = new ArrayList<>();
        for(int i=start; i<=end; i++) {
            if(isPrime(i)) results.add(i);
        }
        
        return results;
    }
}
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console:

2280
818
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第二种方式使用了一个线程池,一般线程池有多少个线程,数量多少合适是需要调整的,大多数情况下cpu有几个核至少就应该起多少个线程,可以多起一个但不能少于cpu核数,将20万分成了4段;
这里为什么不将20万平均分呢?
 
 
五、CachedThreadPool
package yxxy.c_026;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class T08_CachedPool {
    public static void main(String[] args) throws InterruptedException {
        ExecutorService service = Executors.newCachedThreadPool();
        System.out.println(service);
        
        for (int i = 0; i < 2; i++) {
            service.execute(() -> {
                try {
                    TimeUnit.MILLISECONDS.sleep(500);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println(Thread.currentThread().getName());
            });
        }
        
        System.out.println(service);
        
        TimeUnit.SECONDS.sleep(80); //cachedthreadPool里面的线程空闲状态默认60s后销毁,这里保险起见
        
        System.out.println(service);
        
        
    }
}
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console:

java.util.concurrent.ThreadPoolExecutor@7852e922[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0]
java.util.concurrent.ThreadPoolExecutor@7852e922[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 0]
pool-1-thread-2
pool-1-thread-1
java.util.concurrent.ThreadPoolExecutor@7852e922[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 2]
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FixedThreadPool为固定个数的线程池;
CachedThreadPool:刚开始一个线程都没有,来一个任务就起一个线程,假设起了两个线程A,B,如果来了第三个任务,这时候恰好线程B任务执行完了,线程池里面有空闲的,这时候直接让线程池里空闲的线程B来执行;最多起多少个线程?你的系统能支撑多少个为止;默认的情况下,只要一个线程空闲的状态超过60s,这个线程就自动的销毁了,alivetime=60s;这个值也可以自己指定。
 
 
六、SingleThreadExecutor
package yxxy.c_026;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class T09_SingleThreadPool {
    public static void main(String[] args) {
        ExecutorService service = Executors.newSingleThreadExecutor();
        for(int i=0; i<5; i++) {
            final int j = i;
            service.execute(()->{
                
                System.out.println(j + " " + Thread.currentThread().getName());
            });
        }
    }
}
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console:

0 pool-1-thread-1
1 pool-1-thread-1
2 pool-1-thread-1
3 pool-1-thread-1
4 pool-1-thread-1
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SingleThreadExecutor:线程池里就1个线程;扔5个任务,也永远只有1个线程执行;
它能保证任务前后一定是顺序执行,先扔的任务一定先执行完;只有等第一个任务执行完才执行第二个任务。
 
 
七、ScheduledThreadPool
package yxxy.c_026;

import java.util.Random;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

public class T10_ScheduledPool {
    public static void main(String[] args) {
        ScheduledExecutorService service = Executors.newScheduledThreadPool(4);
        service.scheduleAtFixedRate(()->{
            try {
                TimeUnit.MILLISECONDS.sleep(new Random().nextInt(1000));
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName());
        }, 0, 500, TimeUnit.MILLISECONDS);
    }
}
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ScheduledThreadPool: 执行定时的任务,定时器线程池,一般可以用来替代timer,而且它里面的线程是可以复用的,第一个线程执行完了之后,任务来了如果第一个线程是空闲的,还可以拿第一个线程来执行。而Timer每次都是new一个新的线程。
scheduleAtFixedRate( Runnable command, long initialDelay, long period, TimeUnit unit),第1个参数是任务,第1个任务马上执行,每隔500毫秒这个任务重复执行。
 
 
八、WorkStealingPool
/**
 *
 */
package yxxy.c_026;

import java.io.IOException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

public class T11_WorkStealingPool {
    public static void main(String[] args) throws IOException {
        ExecutorService service = Executors.newWorkStealingPool();
        int count = Runtime.getRuntime().availableProcessors();    //看cpu多少核的;如果是4核,默认就帮你起4个线程
        System.out.println(count);    
        
        service.execute(new R(1000));
        for(int i=0; i<count; i++){
            service.execute(new R(2000));
        }
        
        //由于产生的是精灵线程(守护线程、后台线程),主线程不阻塞的话,看不到输出
        System.in.read(); 
    }

    static class R implements Runnable {
        int time;

        R(int t) {
            this.time = t;
        }

        @Override
        public void run() {
            try {
                TimeUnit.MILLISECONDS.sleep(time);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            
            System.out.println(time  + " " + Thread.currentThread().getName());
        }
    }
}
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console:

8
1000 ForkJoinPool-1-worker-1
2000 ForkJoinPool-1-worker-2
2000 ForkJoinPool-1-worker-0
2000 ForkJoinPool-1-worker-5
2000 ForkJoinPool-1-worker-3
2000 ForkJoinPool-1-worker-6
2000 ForkJoinPool-1-worker-7
2000 ForkJoinPool-1-worker-4
2000 ForkJoinPool-1-worker-1
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WorkStealingPool:工作窃取,假设有3个线程A、B、C在运行,workStealing可以简单这么认为,每个线程都维护自己的一个队列,线程A的队列里头积累了5个任务,线程B的队列里1个任务,C的队列里2个任务;那么当线程B执行完任务之后,他会去别的线程池所维护的队列里面把任务偷过来继续执行,主动的找活干。
本质上是使用ForkJoinPool来实现的:
public static ExecutorService newWorkStealingPool() {
        return new ForkJoinPool
            (Runtime.getRuntime().availableProcessors(),
             ForkJoinPool.defaultForkJoinWorkerThreadFactory,
             null, true);
    }
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例子解释:cpu多少核默认的起多少个线程,(这里是8),前面几个任务都扔给1-8个线程了,第9个任务来的时候在那里等着了,谁会去执行它呢?先执行完任务的这个线程会去执行。第1个线程只睡1s钟,首先执行完,所以第9个任务一定是第一个线程1去运行它,他会主动的把任务拿过去运行。
workStealing的线程是精灵线程,daemon线程,特点就是主线程main方法一旦结束了,它后台可能还在运行,但是你是看不到它任务输出的;这里Syetem.in.read()让主函数阻塞才能看到输出。debug的时候能看到Daemon Thread[ForkJoinPool-1-worker-1]。为什么用精灵线程?它是在后台不断的运行的,只要虚拟机不退出,这个线程就不会退出,你有任务来了之后,它永远会主动去拿。
 
workStealing用于什么场景:就说任务分配的不是很均匀,有的线程维护的任务队列比较长,有些线程执行完任务就结束了不太合适,所以他执行完了之后可以去别的线程维护的队列里去偷任务,这样效率更高。
 
 
 
九、ForkJoinPool    
ForkJoinPool: forkjoin的意思就是如果有一个难以完成的大任务,需要计算量特别大,时间特别长,可以把大任务切分成一个个小任务,如果小任务还是太大,它还可以继续分,至于分成多少你可以自己指定,... 分完之后,把结果进行合并,最后合并到一起join一起,产生一个总的结果。而里面任务的切分你可以自己指定,线程的启动根据你任务切分的规则,由ForkJoinPool这个线程池自己来维护。
 
 
例子1:
package yxxy.c_026;

import java.io.IOException;
import java.util.Arrays;
import java.util.Random;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import java.util.concurrent.RecursiveTask;

public class T12_ForkJoinPool {
    static int[] nums = new int[1000000];
    static final int MAX_NUM = 50000;
    static Random r = new Random();
    
    static {
        for(int i=0; i<nums.length; i++) {
            nums[i] = r.nextInt(100);
        }
        
        System.out.println(Arrays.stream(nums).sum()); //stream api 
    }
    
    
    static class AddTask extends RecursiveAction { 
        
        int start, end;
        
        AddTask(int s, int e) {
            start = s;
            end = e;
        }

        @Override
        protected void compute() {
            
            if(end-start <= MAX_NUM) {
                long sum = 0L;
                for(int i=start; i<end; i++) sum += nums[i];
                System.out.println("from:" + start + " to:" + end + " = " + sum);
            } else {
                int middle = start + (end-start)/2;
                AddTask subTask1 = new AddTask(start, middle);
                AddTask subTask2 = new AddTask(middle, end);
                subTask1.fork();
                subTask2.fork();
            }
        }
    }
    
    public static void main(String[] args) throws IOException {
        ForkJoinPool fjp = new ForkJoinPool();
        AddTask task = new AddTask(0, nums.length);
        fjp.execute(task);
        
        System.in.read();
        
    }
}
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console:

49494882
from:906250 to:937500 = 1545274
from:968750 to:1000000 = 1537201
from:593750 to:625000 = 1548289
from:718750 to:750000 = 1546396
from:468750 to:500000 = 1550373
from:843750 to:875000 = 1543421
from:218750 to:250000 = 1549856
from:93750 to:125000 = 1548384
from:562500 to:593750 = 1541814
from:812500 to:843750 = 1547885
from:187500 to:218750 = 1546831
from:687500 to:718750 = 1554064
from:437500 to:468750 = 1547434
from:937500 to:968750 = 1547676
from:875000 to:906250 = 1551839
from:62500 to:93750 = 1548576
from:531250 to:562500 = 1550943
from:656250 to:687500 = 1544991
from:156250 to:187500 = 1548367
from:406250 to:437500 = 1539881
from:125000 to:156250 = 1548128
from:500000 to:531250 = 1545229
from:781250 to:812500 = 1544296
from:625000 to:656250 = 1545283
from:375000 to:406250 = 1553931
from:31250 to:62500 = 1544024
from:750000 to:781250 = 1543573
from:343750 to:375000 = 1546407
from:0 to:31250 = 1539743
from:281250 to:312500 = 1549470
from:312500 to:343750 = 1552190
from:250000 to:281250 = 1543113
View Code
 
例子解释:
对数组中100万个数求和计算,第一种方式是普通的将所有数加在一起(for循环);
第二种方式使用ForkJoinPool计算,分而治之,它里面执行的任务必须是ForkJoinTask,这个任务可以自动进行切分,一般用的时候从RecursiveAction或RecursiveTask继承,RecursiveTask递归任务,因为它切分任务还可以在切分。RecursiveAction没有返回值,RecursiveTask有返回值。
 
例子2:
package yxxy.c_026;

import java.io.IOException;
import java.util.Arrays;
import java.util.Random;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveAction;
import java.util.concurrent.RecursiveTask;

public class T12_ForkJoinPool {
    static int[] nums = new int[1000000];
    static final int MAX_NUM = 50000;
    static Random r = new Random();
    
    static {
        for(int i=0; i<nums.length; i++) {
            nums[i] = r.nextInt(100);
        }
        
        System.out.println(Arrays.stream(nums).sum()); //stream api 
    }
    
    
    static class AddTask extends RecursiveTask<Long> {
        
        int start, end;
        
        AddTask(int s, int e) {
            start = s;
            end = e;
        }

        @Override
        protected Long compute() {
            
            if(end-start <= MAX_NUM) {
                long sum = 0L;
                for(int i=start; i<end; i++) sum += nums[i];
                return sum;
            } 
            
            int middle = start + (end-start)/2;
            
            AddTask subTask1 = new AddTask(start, middle);
            AddTask subTask2 = new AddTask(middle, end);
            subTask1.fork();
            subTask2.fork();
            
            return subTask1.join() + subTask2.join();
        }
    }
    
    public static void main(String[] args) throws IOException {
        ForkJoinPool fjp = new ForkJoinPool();
        AddTask task = new AddTask(0, nums.length);
        fjp.execute(task);
        
        long result = task.join();
        System.out.println(result);
    }
}
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console:

49498457
49498457
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和例子1差不多,唯一的区别是有返回值了,RecursiveTask<V>中的V泛型就是返回值类型。
long result = task.join(),因为join本身就是阻塞的,只有等所有的都执行完了,最后才得出总的执行结果。所以不需要System.in.read了;
 
 
 
十、自定义线程池 ThreadPoolExecutor
ThreadPoolExecutor:大多数的线程池的实现背后调用的都是ThreadPoolExecutor(前面6种就ForkJoinPool不是),它是线程池通用的一个类,可以自定义线程池;
ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)
corePoolSize:核心的线程池里的线程数,自己指定。
maximumPoolSize:最多这个线程池里装多少个线程;
keepAliveTime:线程呆多久没有任务传给它就会消失;
unit:和上面统一指定的;
blockingQueue:真正的装任务的容器,往往都是用blockingQueue;阻塞式的;任务来了就扔进去,什么时候用到了都可以取。
 
例如,fixedThreadPool的实现是:
public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }
View Code

 

十一、parallel stream  

package yxxy.c_026;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class T14_ParallelStreamAPI {
    public static void main(String[] args) {
        List<Integer> nums = new ArrayList<>();
        Random r = new Random();
        for(int i=0; i<10000; i++) nums.add(1000000 + r.nextInt(1000000));
        
        //System.out.println(nums);
        
        long start = System.currentTimeMillis();
        nums.forEach(v->isPrime(v));
        long end = System.currentTimeMillis();
        System.out.println(end - start);
        
        //使用parallel stream api
        
        start = System.currentTimeMillis();
        nums.parallelStream().forEach(T14_ParallelStreamAPI::isPrime);
        end = System.currentTimeMillis();
        
        System.out.println(end - start);
    }
    
    static boolean isPrime(int num) {
        for(int i=2; i<=num/2; i++) {
            if(num % i == 0) return false;
        }
        return true;
    }
}
View Code

console:

1526
337
View Code
paralleStream(): 运用多线程,如果把这1万个数看成是数据流,我们用多线程去访问里面的数,共同来做计算,默认使用多线程。
 
 
 
 
 
 
 
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