当前线程运行完毕，再运行后续逻辑

小咪咪 2023-06-17 09:55 20阅读 0赞

# 一、问题描述 #

在程序设计中，可能存在这样的情景：主线程中存在一个子线程，子线程需要在执行完毕后为后续代码逻辑提供参数。但在代码执行时，子进程还没执行完毕，后续的代码已经开始执行了，这时候就会出现参数为空的异常，或是直接报错。

public class ThreadTest {
        public static void main(String[] args) {
            String str;
            Thread thread = null;
            thread = new Thread(new Runnable() {
                @Override
                public void run() {
                    while(true){
                        System.out.println("Thread is running...");
                        break;
                    }
                }
            });
            thread.start();
    
            System.out.println("Thread is finished...");
        }
    }

执行结果：

![format_png][]

因此，如何才能使得子进程执行完毕后才开始运行后续代码呢？

# 二、解决方法 #

## 1. join ##

Thread类提供了一个join( )方法，其原理是不断检查子进程是否执行完毕，当子进程未执行完成时，调用该子进程的进程被阻塞，直到子进程执行完毕后，再继续运行调用进程。

public class ThreadTest {
        public static void main(String[] args) {
            String str;
            Thread thread = null;
            thread = new Thread(new Runnable() {
                @Override
                public void run() {
                    while(true){
                        System.out.println("Thread is running...");
                        break;
                    }
                }
            });
            thread.start();
            try {
                thread.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
    
            System.out.println("Thread is finished...");
        }
    }

执行结果：

![format_png 1][]

## 2. CountDownLatch ##

###  CountDownLatch允许一个或多个线程等待其他线程执行完毕后再运行。 ###

*   CountDownLatch的构造函数接收int类型的参数作为计数器，若要等待N个点再执行后续逻辑，就传入N。
 *  这里的N可以是N个线程，也可以是N个执行步骤。
 *  当我们调用countDown( )方法时，N会减一。
 *  调用await( ) 方法来阻塞当前线程，直到N减为0。

public class ThreadTest {
        public static void main(String[] args) {
            CountDownLatch count = new CountDownLatch(2);
            String str;
            Thread thread1, thread2 = null;
            thread1 = new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println("Thread1" + " is running...");
                    count.countDown();
                    System.out.println("Thread1 " + " is finished...");
                }
            });
            thread2 = new Thread(new Runnable() {
                @Override
                public void run() {
                    System.out.println("Thread2" + " is running...");
                    count.countDown();
                    System.out.println("Thread2" + " is finished...");
                }
            });
            thread1.start();
            thread2.start();
            try {
                count.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("All is finished.");
        }
    }

执行结果：

![format_png 2][]

[format_png]: https://imgconvert.csdnimg.cn/aHR0cHM6Ly9pbWFnZXMyMDE4LmNuYmxvZ3MuY29tL2Jsb2cvOTU2MzUyLzIwMTgwNS85NTYzNTItMjAxODA1MTQwOTQ0NDY3ODQtMTAyNTgxNzQzOC5wbmc?x-oss-process=image/format,png
[format_png 1]: https://imgconvert.csdnimg.cn/aHR0cHM6Ly9pbWFnZXMyMDE4LmNuYmxvZ3MuY29tL2Jsb2cvOTU2MzUyLzIwMTgwNS85NTYzNTItMjAxODA1MTQwOTQ2MDk2NTYtNTIwODI2NjA2LnBuZw?x-oss-process=image/format,png
[format_png 2]: https://imgconvert.csdnimg.cn/aHR0cHM6Ly9pbWFnZXMyMDE4LmNuYmxvZ3MuY29tL2Jsb2cvOTU2MzUyLzIwMTgwNS85NTYzNTItMjAxODA1MTQxMDAzMDg3OTItMTYwODUxODE0Ny5wbmc?x-oss-process=image/format,png