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使用Thread的join方法

package com.qcy.testThreadFinish; 
 
/** 
 * @author qcy 
 * @create 2020/09/09 17:05:23 
 */ 
public class Case1 { 
    public static void main(String[] args) throws InterruptedException { 
 
        Thread t1 = new Thread(() -> { 
            try { 
                Thread.sleep(3000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
        }); 
        t1.start(); 
 
        Thread t2 = new Thread(() -> { 
            try { 
                Thread.sleep(3000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
        }); 
        t2.start(); 
 
        t1.join(); 
        t2.join(); 
        System.out.println("主線程結(jié)束"); 
    } 
} 

 join()方法使得主線程等待子線程執(zhí)行結(jié)束，阻塞的是主線程。其底層原理，可以參考我的這篇文章你真得懂Thread.join嗎?

使用線程池的isTerminated方法

package com.qcy.testThreadFinish; 
 
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 
 
/** 
 * @author qcy 
 * @create 2020/09/09 17:05:23 
 */ 
public class Case2 { 
    public static void main(String[] args) { 
 
        ExecutorService pool = Executors.newFixedThreadPool(3); 
 
        pool.execute(() -> { 
            try { 
                Thread.sleep(2000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
        }); 
 
        pool.execute(() -> { 
            try { 
                Thread.sleep(2000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
        }); 
 
        //不再接受新的任務(wù) 
        pool.shutdown(); 
         
        while (true) { 
            //手動(dòng)循環(huán)確實(shí)效率很低，不推薦 
            if (pool.isTerminated()) { 
                System.out.println("線程池中的任務(wù)執(zhí)行結(jié)束"); 
                break; 
            } 
        } 
        System.out.println("主線程結(jié)束"); 
    } 
} 

isTerminated，當(dāng)調(diào)用shutdown()方法后，并且所有提交的任務(wù)完成后才會(huì)返回為true

這里直接使用了固定大小的線程池，線程池的參數(shù)在面試中也經(jīng)常被問(wèn)到，對(duì)線程池不熟悉的同學(xué)，可以參考我的這篇文章說(shuō)說(shuō)線程池

使用Future機(jī)制

package com.qcy.testThreadFinish; 
 
import java.util.concurrent.ExecutionException; 
import java.util.concurrent.ExecutorService; 
import java.util.concurrent.Executors; 
import java.util.concurrent.Future; 
 
/** 
 * @author qcy 
 * @create 2020/09/09 17:05:23 
 */ 
public class Case4 { 
    public static void main(String[] args) throws ExecutionException, InterruptedException { 
 
        ExecutorService pool = Executors.newFixedThreadPool(3); 
 
        Future<Integer> task1 = pool.submit(() -> { 
            try { 
                Thread.sleep(2000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
            return 2; 
        }); 
 
        Future<Integer> task2 = pool.submit(() -> { 
            try { 
                Thread.sleep(2000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
            return 3; 
        }); 
 
        //不再接受新的任務(wù) 
        pool.shutdown(); 
         
        //get方法為阻塞獲取 
        System.out.println("task1的運(yùn)行結(jié)果:" + task1.get()); 
        System.out.println("task2的運(yùn)行結(jié)果:" + task2.get()); 
 
        System.out.println("主線程結(jié)束"); 
    } 
} 

Future機(jī)制，可以參考我的另外一篇博客談?wù)凢uture、Callable、FutureTask關(guān)系

使用CountDownLatch

package com.qcy.testThreadFinish; 
 
import java.util.concurrent.CountDownLatch; 
 
/** 
 * @author qcy 
 * @create 2020/09/09 17:05:23 
 */ 
public class Case5 { 
    public static void main(String[] args) throws InterruptedException { 
 
        CountDownLatch latch = new CountDownLatch(2); 
 
        Thread t1 = new Thread(() -> { 
            try { 
                Thread.sleep(3000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } finally { 
                latch.countDown(); 
            } 
        }); 
        t1.start(); 
 
        Thread t2 = new Thread(() -> { 
            try { 
                Thread.sleep(3000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } finally { 
                latch.countDown(); 
            } 
        }); 
        t2.start(); 
 
        latch.await(); 
        System.out.println("主線程結(jié)束"); 
    } 
} 

每調(diào)用一次countDown方法，計(jì)數(shù)器會(huì)減1，在計(jì)數(shù)器減為0之前，await方法將會(huì)阻塞主線程。有關(guān)CountDownLatch的底層原理，可以參考我的另外一篇博客CountDownLatch實(shí)現(xiàn)原理

使用CompletableFuture

package com.qcy.testThreadFinish; 
 
import java.util.concurrent.CompletableFuture; 
import java.util.concurrent.ExecutionException; 
 
/** 
 * @author qcy 
 * @create 2020/09/09 17:05:23 
 */ 
public class Case6 { 
    public static void main(String[] args) throws InterruptedException, ExecutionException { 
 
        CompletableFuture<Integer> cf1 = CompletableFuture.supplyAsync(() -> { 
            try { 
                Thread.sleep(3000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
            return 2; 
        }); 
 
        CompletableFuture<Integer> cf = CompletableFuture.supplyAsync(() -> { 
            try { 
                Thread.sleep(3000); 
            } catch (InterruptedException e) { 
                e.printStackTrace(); 
            } 
            return 3; 
        }).thenCombine(cf1, (result1, result2) -> result1 * result2); 
 
        //get方法為阻塞獲取 
        System.out.println("計(jì)算結(jié)果為" + cf.get()); 
        System.out.println("主線程結(jié)束"); 
    } 
} 

等到兩個(gè)子任務(wù)都完成后，輸出兩數(shù)之積，再執(zhí)行主線程。對(duì)CompletableFuture不熟悉的同學(xué)，可以參考我的這一篇文章什么，你還不會(huì)用CompletableFuture?