关于Java多线程小结

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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;

}

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