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Java面试题2:多线程与synchronized


2 有一个静态变量num,初始值为0。现在开了1000个线程,每个线程内循环1000次,每循环对num自加1,问最后的值是大于、等于还是小于1000000?
答:
(1)小于1000000,因为有1000个线程,不同的线程有可能同时访问num,导致自加的次数变少。

import java.util.concurrent.TimeUnit;

public class Test implements Runnable{
private static int num = 0;

@Override
public void run() {
for(int i = 1; i <=1000; i++){
num++;
System.out.println(Thread.currentThread().getName() + ", num = " + num );
}
}

public static void main(String[] args) throws InterruptedException {
for (int i = 1; i <=1000; i++) {
Thread thread = new Thread(new Test());
thread.setName("Thread:"+i);
thread.start();
}

try {
// 等待全部子线程执行完毕
TimeUnit.SECONDS.sleep(30);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Finally, num = "

运行结果:
Thread:19, num = 999981
Thread:19, num = 999982
Thread:19, num = 999983
Thread:975, num = 999367
Thread:975, num = 999984
Thread:975, num = 999985
Thread:975, num = 999986
Thread:975, num = 999987
Thread:975, num = 999988
Thread:975, num = 999989
Thread:975, num = 999990
Finally, num = 999990

(2)若要防止此现象,要用static synchronized关键字对数据进行同步保护。

import java.util.concurrent.TimeUnit;

public class Test implements Runnable{
private static int num = 0;

static synchronized private void increaseNumber() {
num++;
}

@Override
public void run() {
for(int i = 1; i <=1000; i++){
increaseNumber();
System.out.println(Thread.currentThread().getName() + ", num = " + num );
}
}

public static void main(String[] args) throws InterruptedException {
for (int i = 1; i <=1000; i++) {
Thread thread = new Thread(new Test());
thread.setName("Thread:"+i);
thread.start();
}

try {
// 等待全部子线程执行完毕
TimeUnit.SECONDS.sleep(30);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("Finally, num = "

运行结果:
Thread:3, num = 999993
Thread:3, num = 999994
Thread:3, num = 999995
Thread:3, num = 999996
Thread:3, num = 999997
Thread:3, num = 999998
Thread:3, num = 999999
Thread:3, num = 1000000
Thread:788, num = 999985
Finally, num = 1000000


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