Java多线程高并发编程代码笔记(四)

男娘i 2023-06-27 08:47 27阅读 0赞

### 文章目录 ###

*   *  Executor
     *  ExecutorService
     *  Callable
     *  Executors
     *  Future
     *  线程池
     *   *  FixedThreadPool
         *  CachedThreadPool
         *  SingleThreadPool
         *  ScheduledThreadPool
         *  WorkStealingPool
         *  ForkJoinPool
     *  线程池的底层实现
     *   *  ThreadPoolExecutor
         *   *  FixedThreadPool
             *  CachedThreadPool
             *  ScheduledThreadPool
             *  SingleThreadExecutor
     *  参考
     *  源代码

## Executor ##

参考：[https://blog.csdn.net/qq\_34993631/article/details/82713550][https_blog.csdn.net_qq_34993631_article_details_82713550]

> 执行器，这是一个接口，内部维护了一个方法execute它负责执行一项任务。参数为Runnable，方法的具体实现由我们自己来执行。如下面的代码，我们既可以使用单纯的方法调用也可以新启一个新的线程去执行Runnable的run方法。

package demo34;
    
    import java.util.concurrent.Executor;
    
    public class MyExecutor implements Executor { 
    
        public static void main(String[] args) { 
            new MyExecutor().execute(() -> System.out.println(Thread.currentThread().getName() + "hello executor"));
        }
    
        @Override
        public void execute(Runnable command) { 
            new Thread(command).start();
    // command.run();
        }
    
    }

运行结果

Thread-0hello executor

## ExecutorService ##

代表着启动一系列的线程为用户提供服务(本质上也是一个执行器，继承自Executor)  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Nja2V2aW5jeWg_size_16_color_FFFFFF_t_70]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Nja2V2aW5jeWg_size_16_color_FFFFFF_t_70 1]

> 同时它可以提交Callable与Runnable的对象返回一个未来的执行结果对象Future。这里顺便说一下，Callable是一个增强版的Runnable，它的call方法可以抛出异常可以有返回值。其中它的返回值放在了Future对象中，我们可以使用Future对象的get方法来获得返回值。同时它也是一系列线程池的接口比如说ForkJoinPool、ScheduledThreadPoolExecutor,、ThreadPoolExecutor等。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Nja2V2aW5jeWg_size_16_color_FFFFFF_t_70 2]

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Nja2V2aW5jeWg_size_16_color_FFFFFF_t_70 3]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Nja2V2aW5jeWg_size_16_color_FFFFFF_t_70 4]

## Callable ##

Callable是一个增强版的Runnable，它的call方法可以抛出异常可以有返回值。其中它的返回值放在了Future对象中，我们可以使用Future对象的get方法来获得返回值。

## Executors ##

是一个操作Executor的工具方法。

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Nja2V2aW5jeWg_size_16_color_FFFFFF_t_70 5]

## Future ##

Future常与Callable联合使用，Future可以获得Callable执行后的返回值。如果想新建一个线程执行一个这个Callable中的call方法而且获得返回值的话我们可以使用以下的思路。

方案一：new Thread(new FutureTask(一个实现了Callable的类的对象)).start();使用FutureTask来接收任务的返回值。

方案二：new一个线程池然后然后提交Callable的实现的对象。使用Future来获得Callable的返回值。

package demo34;
    
    import java.util.concurrent.ExecutionException;
    import java.util.concurrent.FutureTask;
    import java.util.concurrent.TimeUnit;
    
    /** * new Thread(new FutureTask(一个实现了Callable的类的对象)).start();使用FutureTask来接收任务的返回值 */
    public class MyFuture01 { 
        public static void main(String[] args) throws InterruptedException, ExecutionException { 
    
            FutureTask<Integer> task = new FutureTask<>(() -> { 
                TimeUnit.MILLISECONDS.sleep(500);
                return 1000;
            }); //new Callable () { Integer call();}
    
            new Thread(task).start();
    
            System.out.println(task.get()); //阻塞
    
        }
    }

运行结果

1000

package demo34;
    
    import java.util.concurrent.*;
    
    /** * new一个线程池然后然后提交Callable的实现的对象。使用Future来获得Callable的返回值。 */
    public class MyFuture02 { 
        public static void main(String[] args) throws InterruptedException, ExecutionException { 
    
            ExecutorService service = Executors.newFixedThreadPool(5);
            Future<Integer> f = service.submit(() -> { 
                TimeUnit.MILLISECONDS.sleep(500);
                return 1;
            });
            System.out.println(f.get());
            System.out.println(f.isDone());
    
        }
    }

运行结果

1
    true

## 线程池 ##

### FixedThreadPool ###

package demo35;
    
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.TimeUnit;
    
    public class MyFixedThreadPool { 
        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);
        }
    }

运行结果

java.util.concurrent.ThreadPoolExecutor@7cca494b[Running, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
    false
    true
    java.util.concurrent.ThreadPoolExecutor@7cca494b[Shutting down, pool size = 5, active threads = 5, queued tasks = 1, completed tasks = 0]
    pool-1-thread-1
    pool-1-thread-2
    pool-1-thread-4
    pool-1-thread-5
    pool-1-thread-3
    pool-1-thread-1
    true
    true
    java.util.concurrent.ThreadPoolExecutor@7cca494b[Terminated, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 6]

>  *  整个程序new了一个5个线程的线程池，使用for循环向这个线程池抛了5个任务。它的执行原则是哪一个线程空闲就由哪个线程来执行这个任务。所以我们看到的线程池的线程序号是不固定的乱序的，但是它有个规则就是先执行完任务的线程会在新线程到来时优先分配到任务。
>  *  线程池shutdown之后程序不会立刻停止而是要等待的所有线程都执行完毕之后再停止服务，所以我们看到的就是Running Shutting down Terminated
>  *  线程池的任务大体上分为两类，等待就绪队列与已完成任务的队列。通过输出结果我们可以看出在开始有5个正在执行的任务1个任务驻留在就绪队列等待执行，在执行结束后我们的已执行队列中就会有6个元素。

利用线程池做并行计算。

/** * 线程池的概念 * 并行计算 */
    package demo35;
     
    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 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);
    		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;
    		}
    		
    	}
    	
    	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;
    	}
    }

> 这是一个质数计算的问题，我们把质数计算划分为不同的数据段是因为越大的质数越难计算，所以直观上计算大量的小数字的质数的时间相当于计算少量的大数字花的时间。这样一来我们就将这个大的任务相对均匀的拆分开来避免了任务分配不均匀造成的等待（也就是时间浪费）。

运行结果

3204
    954

### CachedThreadPool ###

> CachedPool的主要特点就是如果新来的一个任务需要这个线程池来执行的话，如果当前线程池没有闲置的线程那么就新启动一个线程，如果有空闲线程那么就使用其中的一个空闲线程。就是这样的一个有弹性的线程池。默认情况下当一个线程空闲超过60s那么就会销毁，而且线程数量最大不能超过int类型的最大值或者是计算机内存的大小。

package demo35;
    
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.TimeUnit;
    
    public class 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);
    
            System.out.println(service);
    
    
        }
    }

运行结果

java.util.concurrent.ThreadPoolExecutor@677327b6[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 0]
    java.util.concurrent.ThreadPoolExecutor@677327b6[Running, pool size = 2, active threads = 2, queued tasks = 0, completed tasks = 0]
    pool-1-thread-1
    pool-1-thread-2
    java.util.concurrent.ThreadPoolExecutor@677327b6[Running, pool size = 0, active threads = 0, queued tasks = 0, completed tasks = 2]

### SingleThreadPool ###

> 这个线程池中只有一个线程，它的使用场景就是当我们需要保证任务执行的先后顺序的时候就可以使用它。

package demo35;
    
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    
    public class MySingleThreadPool { 
    
        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());
                });
            }
        }
    }

运行结果

0pool-1-thread-1
    1pool-1-thread-1
    2pool-1-thread-1
    3pool-1-thread-1
    4pool-1-thread-1

### ScheduledThreadPool ###

一个定时执行任务的一个线程池它所执行的任务的参数如下：

public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,long initialDelay,long period, TimeUnit unit)

>  *  initialDelay：在开始多少单位时间的时候执行第一个任务。
>  *  Period：每隔多长时间执行下一个任务。
>  *  Unit：时间的单位。
>  *  它的底层基于DelayedWorkQueue。

package demo35;
    
    import java.util.Random;
    import java.util.concurrent.Executors;
    import java.util.concurrent.ScheduledExecutorService;
    import java.util.concurrent.TimeUnit;
    
    /** * 以下代码展示了已启动就开始执行的而且步幅为0.5s的线程执行方式 */
    public class MyScheduledPool { 
        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);
    
        }
    }

运行结果

pool-1-thread-1
    pool-1-thread-1
    pool-1-thread-2
    pool-1-thread-1
    pool-1-thread-3
    pool-1-thread-3
    pool-1-thread-3
    pool-1-thread-3
    
    ....

### WorkStealingPool ###

> 工作窃取线程池，一般情况下CPU是几核的就会启动几个线程，每一个线程都维护者自己的一个执行队列的，当某些线程将自己队列中的任务都执行完毕的时候就会去其他线程的队列中窃取任务来执行以此提高效率。它的底层是基于ForkJoinPool的，常常用于任务分配不均匀的场景中。

![在这里插入图片描述][20200105184336229.png]

package demo35;
    
    import java.io.IOException;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.TimeUnit;
    
    public class MyWorkStealingPool { 
        public static void main(String[] args) throws IOException { 
            ExecutorService service = Executors.newWorkStealingPool();
            System.out.println(Runtime.getRuntime().availableProcessors());
    
            service.execute(new R(1000));
            service.execute(new R(2000));
            service.execute(new R(2000));
            service.execute(new R(2000)); //daemon
            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());
    
            }
    
        }
    }

运行结果

4
    1000 ForkJoinPool-1-worker-1
    2000 ForkJoinPool-1-worker-0
    2000 ForkJoinPool-1-worker-2
    2000 ForkJoinPool-1-worker-3
    2000 ForkJoinPool-1-worker-1

### ForkJoinPool ###

这个线程池设计的思想就与MapReduce极其相似，将一个大的任务分解成一个个小的任务当多个线程来执行。然后将计算的结果汇总得到最终结果。这也是用到了递归的思想。其中它的任务分为两种一种没有返回值是RecursiveAction，一种有返回值RecursiveTask。常常用于大量数据的运算以下为示例代码：

package demo35;
    
    import java.io.IOException;
    import java.util.Arrays;
    import java.util.Random;
    import java.util.concurrent.ForkJoinPool;
    import java.util.concurrent.RecursiveTask;
    
    public class MyForkJoinPool { 
        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(); } } } */
    
        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);
    
            //System.in.read();
    
        }
    }

运行结果

49476407
    49476407

## 线程池的底层实现 ##

### ThreadPoolExecutor ###

我们会发现FixedThreadPool、CachedThreadPool、ScheduledThreadPool、SingleThreadExecutor都是基于ThreadExecutor。而WorkStealingPool与ForkJoinPool的底层都是ForkJoinPool。

#### FixedThreadPool ####

![在这里插入图片描述][20200105184632145.png]

#### CachedThreadPool ####

![在这里插入图片描述][2020010518443296.png]

#### ScheduledThreadPool ####

![在这里插入图片描述][20200105184445776.png]

![在这里插入图片描述][20200105184411355.png]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L2Nja2V2aW5jeWg_size_16_color_FFFFFF_t_70 6]

#### SingleThreadExecutor ####

![在这里插入图片描述][20200105184522387.png]

## 参考 ##

[https://www.bilibili.com/video/av33688545?p=20][https_www.bilibili.com_video_av33688545_p_20]

[https://blog.csdn.net/qq\_34993631/article/details/82713550][https_blog.csdn.net_qq_34993631_article_details_82713550]

## 源代码 ##

[https://gitee.com/cckevincyh/java\_concurrent\_learning][https_gitee.com_cckevincyh_java_concurrent_learning]

[https_blog.csdn.net_qq_34993631_article_details_82713550]: https://blog.csdn.net/qq_34993631/article/details/82713550
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[20200105184336229.png]: https://img-blog.csdnimg.cn/20200105184336229.png
[20200105184632145.png]: https://img-blog.csdnimg.cn/20200105184632145.png
[2020010518443296.png]: https://img-blog.csdnimg.cn/2020010518443296.png
[20200105184445776.png]: https://img-blog.csdnimg.cn/20200105184445776.png
[20200105184411355.png]: https://img-blog.csdnimg.cn/20200105184411355.png
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[20200105184522387.png]: https://img-blog.csdnimg.cn/20200105184522387.png
[https_www.bilibili.com_video_av33688545_p_20]: https://www.bilibili.com/video/av33688545?p=20
[https_gitee.com_cckevincyh_java_concurrent_learning]: https://gitee.com/cckevincyh/java_concurrent_learning