并发编程

最新推荐文章于 2025-09-11 20:53:34 发布

weixin_33730836

最新推荐文章于 2025-09-11 20:53:34 发布

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CC 4.0 BY-SA版权

文章标签：数据库 python

原文链接：https://my.oschina.net/u/3342652/blog/1088692

2019独角兽企业重金招聘Python工程师标准>>>

并发编程

并发编程的目的是让程序执行更快，但是多线程并不一定比单线程执行快。比如多线程会遇到的问题有：上下文切换<单核、时间片>、死锁等。

上下文切换：

public class JavaMain {

    private static final int count = 1000;

    public static void main(String[] args) throws InterruptedException {
        work1();
        work2();
    }

    private static void work1() throws InterruptedException {
        System.out.println("start1 = " + System.currentTimeMillis());
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                int b = 0;
                for (int i = 0; i < 0; i++) {
                    b -= i;
                }
            }
        });
        thread.start();
        int a = 0;
        for (int i = 0; i < 0; i++) {
            a += i;
        }
        thread.join();
        System.out.println("end  1 = " + System.currentTimeMillis());
    }

    private static void work2() {
        System.out.println("start2 = " + System.currentTimeMillis());
        int b = 0;
        for (int i = 0; i < 0; i++) {
            b -= i;
        }
        int a = 0;
        for (int i = 0; i < 0; i++) {
            a += i;
        }
        System.out.println("end  2 = " + System.currentTimeMillis());
    }
}

运行结果：

start1 = 1498834716175
end 1 = 1498834716178
start2 = 1498834716179
end 2 = 1498834716179

如何减少上下文切换：
无锁并发编程、CAS算法、使用最少线程和使用协程。

死锁：

public class JavaMain {

    private static final String a = "A";
    private static final String b = "B";

    public static void main(String[] args) {
        new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (a) {
                    try {
                        Thread.sleep(300);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    synchronized (b) {
                        System.out.println("end 1");
                    }
                }
            }
        }).start();
        new Thread(new Runnable() {
            @Override
            public void run() {
                synchronized (b) {
                    try {
                        Thread.sleep(200);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    synchronized (a) {
                        System.out.println("end 2");
                    }
                }
            }
        }).start();
    }
}