JAVA多线程

Posted 2020-12-12 doge-elder

直接上代码去跑发现其中的规律即可

public class ThreadPollUtil {
    private static int corePoolSize = Runtime.getRuntime().availableProcessors();

    /**
     * corePoolSize 用于指定核心线程数量
     * maximumPoolSize 指定最大线程数
     * keepAliveTime和TimeUnit指定线程空闲后的最大存活时间
     */
    public static ThreadPoolExecutor executor  = new ThreadPoolExecutor(corePoolSize, corePoolSize+1, 10l, TimeUnit.SECONDS,
            new LinkedBlockingQueue<Runnable>(1000));

/*
    让主线程等待  CountDownLatch 任务计数器

    countDownLatch这个类使一个线程等待其他线程各自执行完毕后再执行。

    是通过一个计数器来实现的，计数器的初始值是线程的数量。每当一个线程执行完毕后，计数器的值就-1，当计数器的值为0时，表示所有线程都执行完毕，然后在闭锁上等待的线程就可以恢复工作了。
*/


    public static void main(String[] args) throws InterruptedException {
        //创建一个三个子线程 初始化计数器
        Map<Object, Object> map = new HashMap<>();
        CountDownLatch countDownLatch = new CountDownLatch(3);

        Long m=1000000000L;
        long time11 = System.currentTimeMillis();
        ThreadPollUtil.executor.submit(new Runnable() {
            @Override
            public void run() {
                long time1 = System.currentTimeMillis();
                int i=0;
                for (int i1 = 0; i1 < m; i1++) {
                    i=i1+i;
                }
                map.put("1",i);
                //计数器-1
                countDownLatch.countDown();

                long time2 = System.currentTimeMillis();
                System.out.println("线程一"+(time1 - time2) + "毫秒。");
            }
        });
        ThreadPollUtil.executor.submit(new Runnable() {
            @Override
            public void run() {
                long time1 = System.currentTimeMillis();
                int i=0;

                for (int i1 = 0; i1 < m; i1++) {
                    i=i1+i;
                }
                map.put("1",i);
                //计数器-1
                countDownLatch.countDown();

                long time2 = System.currentTimeMillis();
                System.out.println("线程二"+(time1 - time2) + "毫秒。");
            }
        });
        ThreadPollUtil.executor.submit(new Runnable() {
            @Override
            public void run() {
                long time1 = System.currentTimeMillis();
                int i=0;
                for (int i1 = 0; i1 < m; i1++) {
                    i=i1+i;
                }
                map.put("1",i);
                //计数器-1
                countDownLatch.countDown();

                long time2 = System.currentTimeMillis();
                System.out.println("线程三"+(time1 - time2) + "毫秒。");
            }
        });

        /**
         * 通过await方法让主线程等待
         */
        countDownLatch.await();
        long time22 = System.currentTimeMillis();
        System.out.println("总线程"+(time11 - time22) + "毫秒。");

    }
}

 

再做个比较没有使用多线程的自上而下的逻辑

class Xxx{
    public static void main(String[] args) throws InterruptedException {
        

                long time1 = System.currentTimeMillis();
                int i=0;
                Long m=1000000000L;
                for (int i1 = 0; i1 < m; i1++) {
                    i=i1+i;
                }
        System.out.println("执行一");
                for (int i1 = 0; i1 < m; i1++) {
                    i=i1+i;
                }
        System.out.println("执行二");
                for (int i1 = 0; i1 < m; i1++) {
                    i=i1+i;
                }
        System.out.println("执行三");
        long time2 = System.currentTimeMillis();
        System.out.println("总线程"+(time1 - time2) + "毫秒。");

    }

}