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主线程等待所有子线程执行完成之后再继续往下执行的解决方案
问题背景:
目前需要解析十多个Excel文件并将数据导入到数据库,文件导出完成之后还有后续步骤需要处理,例如更新批次状态。
如果采用单线程的方式顺序去处理每个文件的话,感觉有点慢,虽说不算是特别慢,但是由于用户需要等待导入结果,
考虑到用于体验问题,因此需要坐下性能优化。既然是性能优化,基本都是考虑到使用多线程并发处理。但是多线线程有个问题,
就是启动多个线程后,如何让当前线程等待这些子线程执行完毕,因为需要根据所以子线程的执行结果来更新批次状态
解决方案1:
基本思路是这样:
每个SubThread子线程类实例有个自己的状态99-初始化 0-执行成功 1-执行失败,当执行完毕之后,将状态修改为0或者1
MainThread主线程类中有个List,用来登记所有子线程。子线程的创建通过主线程来创建,每次创建之后,都会将子线程添加到List中。
所有子线程创建完成之后,通过主线程的start方法启动所有子线程,并通过一个while循环来遍历List中的所有子线程的状态,
判断是否存在状态为99的,如果没有,则便是全部子线程执行完毕
/** * 子线程类 * @author Administrator * */ public class SubThread implements Runnable{ private int status = 99; //99-初始化 0-执行成功 1-执行失败 public void run() { System.out.println("开始执行..."); try{ Thread.sleep(2000); }catch(Exception e){ e.printStackTrace(); } status=0; System.out.println("执行完毕..."); } public int getStatus() { return status; } public void setStatus(int status) { this.status = status; } } |
/** * 主线程类 * @author Administrator * */ public class MainThread { private List<SubThread> subThreadList = new ArrayList<SubThread>(); /** * 创建子线程 * @return */ public SubThread createSubThread(){ SubThread subThread = new SubThread(); subThreadList.add(new SubThread()); return subThread; } public boolean start(){ for(SubThread subThread : subThreadList){ new Thread(subThread).start(); } /** * 监控所有子线程是否执行完毕 */ boolean continueFlag = true; while(continueFlag){ for(SubThread subThread : subThreadList){ if(subThread.getStatus()==99){ continueFlag = true; break; } continueFlag = false; } } /** * 判断子线程的执行结果 */ boolean result = true; for(SubThread subThread : subThreadList){ if(subThread.getStatus()!=0){ result = false; break; } } return result; } |
测试代码:
public static void main(String[] args) { MainThread main = new MainThread(); main.createSubThread(); main.createSubThread(); main.createSubThread(); boolean result = main.start(); System.out.println(result); } |
解决方案2:
通过计数器方式解决。基本思路如下:
计数器类CountLauncher负责记录正在执行的子线程的总数,所有的子线程共享该计数器类对象,当子线程执行完毕之后,调用计数器的counDown()方法进行计数器减1.
主线程通过计数器类来判断是否所有子线程都执行完毕。
/** * 计数器类 * @author Administrator * */ public class CountLauncher { private int count; public CountLauncher(int count){ this.count = count; } public synchronized void countDown(){ count --; } public int getCount() { return count; } public void setCount(int count) { this.count = count; } } |
/** * 子线程类 * @author Administrator * */ public class SubThread implements Runnable{ /** * 计数器类对象实例 */ private CountLauncher countLauncher; private int status = 99; //99-初始化 0-执行成功 1-执行失败 public void run() { System.out.println("开始执行..."); try{ Thread.sleep(2000); }catch(Exception e){ e.printStackTrace(); } status=0; System.out.println("执行完毕..."); countLauncher.countDown(); } public int getStatus() { return status; } public void setStatus(int status) { this.status = status; } public CountLauncher getCountLauncher() { return countLauncher; } public void setCountLauncher(CountLauncher countLauncher) { this.countLauncher = countLauncher; } |
/** * 主线程类 * @author Administrator * */ public class MainThread { private List<SubThread> subThreadList = new ArrayList<SubThread>(); /** * 创建子线程 * @return */ public synchronized SubThread createSubThread(){ SubThread subThread = new SubThread(); subThreadList.add(new SubThread()); return subThread; } public boolean start(){ CountLauncher countLauncher = new CountLauncher(subThreadList.size()); for(SubThread subThread : subThreadList){ subThread.setCountLauncher(countLauncher); new Thread(subThread).start(); } while(countLauncher.getCount()>0){ System.out.println(countLauncher.getCount()); } /** * 判断子线程的执行结果 */ boolean result = true; for(SubThread subThread : subThreadList){ if(subThread.getStatus()!=0){ result = false; break; } } return result; } /** * 测试实例 */ public static void main(String[] args) { MainThread main = new MainThread(); main.createSubThread(); main.createSubThread(); main.createSubThread(); boolean result = main.start(); System.out.println(result); } |
解决方案3(推荐):
该方法的处理方式同解决方案2 ,只不过使用的是Java自带的计数器类java.util.concurrent.CountDownLatch。
还有一点就是不需要在主线程中通过while来监控所有子线程,是否通过调用它的await方法进行等待所有子线程的执行完毕。
使用计数器时,需要注意的一点是:子线程中调用countDown()方法时一定要放在最后来执行,否则会出现子线程未执行完毕,主线程就开始往下执行了。
因为一定计数器为0,就会自动唤醒主线程的。
/** * 子线程 * @author Administrator * */ public class SubThread implements Runnable{ private int status =99; private CountDownLatch threadsSignal; @Override public void run(){ System.out.println("开始执行..."); try{ Thread.sleep(2000); }catch(Exception e){ e.printStackTrace(); } status=0; System.out.println("执行完毕..."); threadsSignal.countDown(); } public int getStatus() { return status; } public void setStatus(int status) { this.status = status; } public CountDownLatch getThreadsSignal() { return threadsSignal; } public void setThreadsSignal(CountDownLatch threadsSignal) { this.threadsSignal = threadsSignal; } } |
public class MainThread { public List<SubThread> subThreadList = new ArrayList<SubThread>(); /** * 创建子线程 * @param clazzPath * @param methodName * @return */ public SubThread createSubThread(){ SubThread subThread = new SubThread(); subThreadList.add(subThread); return subThread; } /** * 启动线程 */ public int start() throws Exception{ CountDownLatch threadSignal = new CountDownLatch(subThreadList.size()); for(SubThread subThread :subThreadList){ subThread.setThreadsSignal(threadSignal); Thread thread = new Thread(subThread); thread.start(); } threadSignal.await(); System.out.println("所有子线程执行完毕..."); for(SubThread subThread :subThreadList){ if(subThread.getStatus()!=0){ return 1; } } return 0; } public static void main(String[] args) throws Exception{ MainThread main = new MainThread(); main.createSubThread(); main.createSubThread(); main.createSubThread(); main.createSubThread(); int result = main.start(); System.out.println("执行结果:" + result); } |
来自为知笔记(Wiz)
主线程等待所有子线程执行完成之后再继续往下执行的解决方案
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