关于Java多线程小结

项目场景：数据采集需要多线程执行，在所有子线程结束后主线程写入文件采集时间的结束点，但是在数据采集的时候也读取这个时间点

问题：当多个线程在读文件的时候主线程同时在写文件，导致多个线程读到的时间不一致，或者就是线程读不到时间，出现数据丢失

解决：1.创建线程池代码如下

          ExecutorService executorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors()*5);

           for(int i = 0; i < 5; i++){

               executorService.execute(new ThreadDemo());

          }

           //禁止添加新的线程

           executorService.shutdown();

           //等待所有线程结束后执行

           if(executorService .awaitTermination(Long.MAX_VALUE, TimeUnit.MINUTES )){

              System. out.println("i am main thread" );

          }

     2.创建线程组

          ThreadGroup g= new ThreadGroup("g1" );

           for (int i = 0; i < 2; i++) {

              Thread t= new Thread(g ,new ThreadDemo());

               t.start();

          }

             // 获得当前线程组中线程数目， 包括可运行和不可运行的

          //当前的线程数为零时表示线程组的线程执行结束

           while(g .activeCount() > 0){

//               System.out.println("group1的当前的活跃数量："+g.activeCount());

        }

        System. out.println("==g中所有线程执行结束===" );

     3.使用join(),线程会依次执行，会影响执行效率

     public class JoinDemo {

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

           for (int i = 0 ; i < 3 ; i++){

              JoinFoo foo = new JoinFoo();

               foo.start();

               foo.join();

          }

          System. out .println("main Thread over" );

     }

      static class JoinFoo extends Thread {

           int total ;

           @Override

           public void run(){

              System. out .println("thread 1 is running" ); 

               try {

                   Thread. sleep(2000);

              } catch (InterruptedException e ) {

                    // TODO Auto-generated catch block

                    e.printStackTrace();

              }

            System. out .println("thread 1 is over and set flag to 1"); 

          }

     }

}

public class ThreadDemo extends Thread{

     public int i = 0;

     public ThreadDemo(){

           super();

     }

     

     @Override

     public void run(){

          System. out.println(Thread.currentThread().getName()+ "====="+(++i ));

          System. out.println("子线程" +Thread.currentThread ().getName());

           try {

              Thread. sleep(1000);

          } catch (InterruptedException e ) {

               // TODO Auto-generated catch block

               e.printStackTrace();

          }

     }

     

}

对于单个线程等待的可使用标记

public class TestSync {

      private static int FLAG = 0;

      public static void main(String[] args) {

          Thread1 thread1 = new Thread1();

          System. out .println("thread 1 is start" );

           thread1.start();

           while (FLAG != 1) {

               // System.out.println("main thread is waiting");

          }

           FLAG = 0;

          System. out .println("main thread is back to work");

     }

      static class Thread1 extends Thread {

           int total ;

           public void run() {

              System. out .println("thread 1 is running" );

               for (int i = 0; i < 5; i++) {

                    total = total + i;

                   System. out .println("total = " + total );

              }

              System. out .println("thread 1 is over and set flag to 1");

               FLAG = 1;

          }

     }

}