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3、线程的并发工具类

一、Fork-Join框架

1、分而治之

规模为N的问题，N<阈值，直接解决，N>阈值，将N分解为K个小规模子问题，子问题互相对立，与原问题形式相同，将子问题的解合并得到原问题的解

动态规范

3、线程的并发工具类-LMLPHP

2、工作密取 workStealing

就是在任务分割的时候，前面的任务执行可能会比后面的执行速度快，当前面的执行完，后面的还没执行的时候，执行完前面的任务的线程不会停止，而是从后面的任务的尾部取出子任务继续工作。Fork-Join就是实现了这样的机制。

3、线程的并发工具类-LMLPHP

Fork/Join使用的标准范式

3、线程的并发工具类-LMLPHP

代码如下:


import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.RecursiveTask;
/**
 * @Auther: BlackKingW
 * @Date: 2019/4/14 12:09
 * @Description:
 */
public class SumArray {

    private static class SumTask extends RecursiveTask<Integer>{

        private final static int THRESHOLD = MakeArray.ARRAY_LENGTH/10;
        private int[] src; //表示我们要实际统计的数组
        private int fromIndex;//开始统计的下标
        private int toIndex;//统计到哪里结束的下标

        public SumTask(int[] src, int fromIndex, int toIndex) {
            this.src = src;
            this.fromIndex = fromIndex;
            this.toIndex = toIndex;
        }

		@Override
		protected Integer compute() {
			if(toIndex-fromIndex < THRESHOLD) {
				int count = 0;
				for(int i=fromIndex;i<=toIndex;i++) {
			    	//SleepTools.ms(1);
			    	count = count + src[i];
				}
				return count;
			}else {
				//fromIndex....mid....toIndex
				//1...................70....100
				int mid = (fromIndex+toIndex)/2;
				SumTask left = new SumTask(src,fromIndex,mid);
				SumTask right = new SumTask(src,mid+1,toIndex);
				invokeAll(left,right);
				return left.join()+right.join();
			}
		}
    }


    public static void main(String[] args) {

        ForkJoinPool pool = new ForkJoinPool();
        int[] src = MakeArray.makeArray();

        SumTask innerFind = new SumTask(src,0,src.length-1);

        long start = System.currentTimeMillis();

        pool.invoke(innerFind);//同步调用
        System.out.println("Task is Running.....");

        System.out.println("The count is "+innerFind.join()
                +" spend time:"+(System.currentTimeMillis()-start)+"ms");

    }
}

public class MakeArray {
    //数组长度
    public static final int ARRAY_LENGTH  = 100000000;

    public static int[] makeArray() {

        //new一个随机数发生器
        Random r = new Random();
        int[] result = new int[ARRAY_LENGTH];
        for(int i=0;i<ARRAY_LENGTH;i++){
            //用随机数填充数组
            result[i] =  r.nextInt(ARRAY_LENGTH*3);
        }
        return result;

    }
}

二、常用的并发工具类

1、CountDownLatch

作用：是一组线程等待其他的线程完成工作以后在执行，加强版join

await用来等待，countDown负责计数器的减一

代码示例：


import java.util.concurrent.CountDownLatch;

/**
 * @Auther: BlackKingW
 * @Date: 2019/4/14 12:09
 * @Description:
 */
public class UseCountDownLatch {

	static CountDownLatch latch = new CountDownLatch(6);

	//初始化线程(只有一步，有4个)
    private static class InitThread implements Runnable{

        @Override
        public void run() {
        	System.out.println("Thread_"+Thread.currentThread().getId()
        			+" ready init work......");
        	latch.countDown();//初始化线程完成工作了，countDown方法只扣减一次；
            for(int i =0;i<2;i++) {
            	System.out.println("Thread_"+Thread.currentThread().getId()
            			+" ........continue do its work");
            }
        }
    }

    //业务线程
    private static class BusiThread implements Runnable{

        @Override
        public void run() {
        	try {
				latch.await();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
            for(int i =0;i<3;i++) {
            	System.out.println("BusiThread_"+Thread.currentThread().getId()
            			+" do business-----");
            }
        }
    }

    public static void main(String[] args) throws InterruptedException {
    	//单独的初始化线程,初始化分为2步，需要扣减两次
        new Thread(new Runnable() {
            @Override
            public void run() {
            	SleepTools.ms(1); //休眠1s
                System.out.println("Thread_"+Thread.currentThread().getId()
            			+" ready init work step 1st......");
                latch.countDown();//每完成一步初始化工作，扣减一次
                System.out.println("begin step 2nd.......");
                SleepTools.ms(1);
                System.out.println("Thread_"+Thread.currentThread().getId()
            			+" ready init work step 2nd......");
                latch.countDown();//每完成一步初始化工作，扣减一次
            }
        }).start();
        new Thread(new BusiThread()).start();
        for(int i=0;i<=3;i++){
            Thread thread = new Thread(new InitThread());
            thread.start();
        }

        latch.await();
        System.out.println("Main do ites work........");
    }
}

2、CyclicBarrier

让一组线程达到某个屏障，被阻塞，一直到组内最后一个线程达到屏障时，屏障开放，所有被阻塞的线程会继续运行CyclicBarrier(int parties)

CyclicBarrier(int parties, Runnable barrierAction)，屏障开放，barrierAction定义的任务会执行

代码示例：

import java.util.Map;
import java.util.Random;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CyclicBarrier;

/**
 * @Auther: BlackKingW
 * @Date: 2019/4/14 12:09
 * @Description:
 */
public class UseCyclicBarrier {

	private static CyclicBarrier barrier
		= new CyclicBarrier(5,new CollectThread());

    private static ConcurrentHashMap<String,Long> resultMap
            = new ConcurrentHashMap<>();//存放子线程工作结果的容器

    public static void main(String[] args) {
        for(int i=0;i<=4;i++){
            Thread thread = new Thread(new SubThread());
            thread.start();
        }

    }

    //负责屏障开放以后的工作
    private static class CollectThread implements Runnable{

        @Override
        public void run() {
            StringBuilder result = new StringBuilder();
            for(Map.Entry<String,Long> workResult:resultMap.entrySet()){
            	result.append("["+workResult.getValue()+"]");
            }
            System.out.println(" the result = "+ result);
            System.out.println("do other business........");
        }
    }

    //工作线程
    private static class SubThread implements Runnable{

        @Override
        public void run() {
        	long id = Thread.currentThread().getId();//线程本身的处理结果
            resultMap.put(Thread.currentThread().getId()+"",id);
            Random r = new Random();//随机决定工作线程的是否睡眠
            try {
                if(r.nextBoolean()) {
                	Thread.sleep(2000+id);
                	System.out.println("Thread_"+id+" ....do something ");
                }
                System.out.println(id+"....is await");
                barrier.await();
            	Thread.sleep(1000+id);
                System.out.println("Thread_"+id+" ....do its business ");
            } catch (Exception e) {
                e.printStackTrace();
            }

        }
    }
}

CountDownLatch和CyclicBarrier辨析

1、countdownlatch放行由第三者控制，CyclicBarrier放行由一组线程本身控制
2、countdownlatch放行条件》=线程数，CyclicBarrier放行条件=线程数

3、Semaphore

控制同时访问某个特定资源的线程数量，用在流量控制


import java.sql.Connection;
import java.util.LinkedList;
import java.util.concurrent.Semaphore;
/**
 * @Auther: BlackKingW
 * @Date: 2019/4/14 12:09
 * @Description:
 */
public class DBPoolSemaphore {

	private final static int POOL_SIZE = 10;
	private final Semaphore useful,useless;//useful表示可用的数据库连接，useless表示已用的数据库连接

	public DBPoolSemaphore() {
		this. useful = new Semaphore(POOL_SIZE);
		this.useless = new Semaphore(0);
	}

	//存放数据库连接的容器
	private static LinkedList<Connection> pool = new LinkedList<Connection>();
	//初始化池
	static {
        for (int i = 0; i < POOL_SIZE; i++) {
            pool.addLast(SqlConnectImpl.fetchConnection());
        }
	}

	/*归还连接*/
	public void returnConnect(Connection connection) throws InterruptedException {
		if(connection!=null) {
			System.out.println("当前有"+useful.getQueueLength()+"个线程等待数据库连接！！"
					+"可用连接数:"+useful.availablePermits());
			useless.acquire();
			synchronized (pool) {
				pool.addLast(connection);
			}
			useful.release();
		}
	}

	/*从池子拿连接*/
	public Connection takeConnect() throws InterruptedException {
		useful.acquire();
		Connection conn;
		synchronized (pool) {
			conn = pool.removeFirst();
		}
		useless.release();
		return conn;
	}

}

4、Exchange

两个线程间的数据交换， 用的比较少

代码示例：

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.concurrent.Exchanger;
/**
 * @Auther: BlackKingW
 * @Date: 2019/4/14 12:09
 * @Description:
 */
public class UseExchange {
    private static final Exchanger<Set<String>> exchange
    	= new Exchanger<Set<String>>();

    public static void main(String[] args) {

    	//第一个线程
        new Thread(new Runnable() {
            @Override
            public void run() {
            	Set<String> setA = new HashSet<String>();//存放数据的容器
                try {
                	/*添加数据
                	 * set.add(.....)
                	 * */
                	setA = exchange.exchange(setA);//交换set
                	/*处理交换后的数据*/
                } catch (InterruptedException e) {
                }
            }
        }).start();

      //第二个线程
        new Thread(new Runnable() {
            @Override
            public void run() {
            	Set<String> setB = new HashSet<String>();//存放数据的容器
                try {
                	/*添加数据
                	 * set.add(.....)
                	 * set.add(.....)
                	 * */
                	setB = exchange.exchange(setB);//交换set
                	/*处理交换后的数据*/
                } catch (InterruptedException e) {
                }
            }
        }).start();

    }
}

5、Callable、Future和FutureTask

类之间的关系

3、线程的并发工具类-LMLPHP

isDone，结束，正常还是异常结束，或者自己取消，返回true；

isCancelled 任务完成前被取消，返回true；

cancel（boolean）：

  1. 任务还没开始，返回false
  2. 任务已经启动，cancel（true），中断正在运行的任务，中断成功，返回true，cancel（false），不会去中断已经运行的任务
  3. 任务已经结束，返回false

代码示例：



import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;


/**
 * @Auther: BlackKingW
 * @Date: 2019/4/14 12:09
 * @Description:
 */
public class UseFuture {

	/*实现Callable接口，允许有返回值*/
	private static class UseCallable implements Callable<Integer>{

		private int sum;
		@Override
		public Integer call() throws Exception {
			System.out.println("Callable子线程开始计算");
			Thread.sleep(2000);
			for(int i=0;i<5000;i++) {
				sum = sum+i;
			}
			System.out.println("Callable子线程计算完成，结果="+sum);
			return sum;
		}

	}

	public static void main(String[] args)
			throws InterruptedException, ExecutionException {

		UseCallable useCallable = new UseCallable();
		FutureTask<Integer> futureTask = new FutureTask<Integer>(useCallable);
		new Thread(futureTask).start();
		Random r = new Random();
		SleepTools.second(1);
		if(r.nextBoolean()) {//随机决定是获得结果还是终止任务
			System.out.println("Get UseCallable result = "+futureTask.get());
		}else {
			System.out.println("中断计算");
			futureTask.cancel(true);
		}

	}

}

场景举例：包含图片和文字的文档的处理：图片（云上），可以用future去取图片，主线程继续解析文字。

04-14 20:06
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