一文搞懂Java创建线程的五种方法

2022-11-13 10:11:16 线程 一文 五种

题目描述

Java创建线程的几种方式

Java使用Thread类代表线程,所有线程对象都必须是Thread类或者其子类的实例。Java可以用以下5种方式来创建线程

  • 继承Thread类创建线程;
  • 实现Runnable接口创建线程;
  • 实现Callable接口,通过FutureTask包装器来创建Thread线程;
  • 使用ExecutorService、Callable(或者Runnable)、Future实现由返回结果的线程。
  • 使用CompletableFuture类创建异步线程,且是据有返回结果的线程。 jdk8新支持的

实现:使用这5种方式创建线程,体验其中的妙处。 

解题思路

继承Thread类创建线程

Thread类本质上是实现了Runnable接口的一个实例,代表一个线程的实例。启动线程的唯一方法就是通过Thread类的start()实例方法。start()方法是一个native方法,它将启动一个新线程,并执行run()方法。这种方式实现多线程很简单,通过自己的类直接extends Thread,并复写run()方法,就可以启动新线程并执行自己定义的run()方法。

实现Runnable接口创建线程

如果自己的类已经extends另一个类,就无法直接extends Thread,此时,可以实现一个Runnable接口

实现Callable接口,通过FutureTask包装器来创建Thread线程

实现一个Callable接口(它是一个具有返回值的)

使用ExecutorService、Callable(或者Runnable)、Future实现由返回结果的线程

Executors类,提供了一系列工厂方法用于创建线程池,返回的线程池都实现了ExecutorService接口:

Executors类,提供了一系列工厂方法用于创建线程池,返回的线程池都实现了ExecutorService接口:

//创建固定数目线程的线程池。
public static ExecutorService newFixedThreadPool(int nThreads) ;
//创建一个可缓存的线程池,调用execute 将重用以前构造的线程(如果线程可用)。如果现有线程没有可用的,则创建一个新线程并添加到池中。终止并从缓存中移除那些已有 60 秒钟未被使用的线程。
public static ExecutorService newCachedThreadPool();
//创建一个单线程化的Executor。
public static ExecutorService newSingleThreadExecutor();
//创建一个支持定时及周期性的任务执行的线程池,多数情况下可用来替代Timer类。
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize);

ExecutoreService提供了submit()方法,传递一个Callable,或Runnable,返回Future。如果Executor后台线程池还没有完成Callable的计算,这调用返回Future对象的get()方法,会阻塞直到计算完成。

使用CompletableFuture类创建异步线程,且是据有返回结果的线程

Future模式的缺点

Future虽然可以实现获取异步执行结果的需求,但是它没有提供通知的机制,我们无法得知Future什么时候完成。

要么使用阻塞,在future.get()的地方等待future返回的结果,这时又变成同步操作。要么使用isDone()轮询地判断Future是否完成,这样会耗费CPU的资源。

CompletableFuture 介绍

JDK1.8新加入的一个实现类CompletableFuture,实现了Future, CompletionStage两个接口。

CompletableFuture中4个异步执行任务静态方法:

public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {
        return asyncSupplyStage(asyncPool, supplier);
    }
    
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier,Executor executor) {
    return asyncSupplyStage(screenExecutor(executor), supplier);
}
    
public static CompletableFuture<Void> runAsync(Runnable runnable) {
    return asyncRunStage(asyncPool, runnable);
}
    
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor) {
    return asyncRunStage(screenExecutor(executor), runnable);
}

其中supplyAsync用于有返回值的任务,runAsync则用于没有返回值的任务。Executor参数可以手动指定线程池,否则默认ForkJoinPool.commonPool()系统级公共线程池

代码详解

第一种 继承Thread类创建线程

package cn.xiaoxuzhu.daily;

import java.util.concurrent.CountDownLatch;
 

 
 
public class ThreadDemo1 extends Thread {
    CountDownLatch countDownLatch;
 
    public ThreadDemo1(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
 
    @Override
    public void run() {
        try {
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getName() + ":my thread ");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            countDownLatch.countDown();
        }
    }
 
    public static void main(String[] args) {
        // 第一种:使用extends Thread方式
        CountDownLatch countDownLatch1 = new CountDownLatch(2);
        for (int i = 0; i < 2; i++) {
            ThreadDemo1 myThread1 = new ThreadDemo1(countDownLatch1);
            myThread1.start();
        }
 
        try {
            countDownLatch1.await();
            System.out.println("thread complete...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

 
}

第二种:实现Runnable接口创建线程

package cn.xiaoxuzhu.daily;
 
import java.util.concurrent.CountDownLatch;
 

 
 
 
public class ThreadDemo2  implements Runnable{
    CountDownLatch countDownLatch;
 
    public ThreadDemo2(CountDownLatch countDownLatch) {
        this.countDownLatch = countDownLatch;
    }
    @Override
    public void run() {
        try {
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getName() + ":my runnable ");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            countDownLatch.countDown();
        }
    }
 
    public static void main(String[] args) {
        // 第二种:使用implements Runnable方式
        CountDownLatch countDownLatch2 = new CountDownLatch(2);
        ThreadDemo2 myRunnable = new ThreadDemo2(countDownLatch2);
        for (int i = 0; i < 2; i++) {
            new Thread(myRunnable).start();
        }
 
        try {
            countDownLatch2.await();
            System.out.println("runnable complete...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

第三种:实现Callable接口,通过FutureTask包装器来创建Thread线程

计算1~100的叠加

package cn.xiaoxuzhu.daily;
 
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;
 

 
public class ThreadDemo3 implements Callable<Integer> {
 
    public static void main(String[] args) {
        ThreadDemo3 threadDemo03 = new ThreadDemo3();
        //1、用futureTask接收结果
        FutureTask<Integer> futureTask = new FutureTask<>(threadDemo03);
        new Thread(futureTask).start();
 
        //2、接收线程运算后的结果
        try {
            //futureTask.get();这个是堵塞性的等待
            Integer sum = futureTask.get();
            System.out.println("sum="+sum);
            System.out.println("-------------------");
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
 
    @Override
    public Integer call() throws Exception {
        int sum = 0;
        for (int i = 0; i <101 ; i++) {
            sum+=i;
        }
        return sum;
    }
}

第四种:使用ExecutorService、Callable(或者Runnable)、Future实现返回结果的线程

package cn.xiaoxuzhu.daily;
 
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
 

 
 
public class ThreadDemo4 {
 
    static class MyCallable implements Callable<Integer> {
        private CountDownLatch countDownLatch;
 
        public MyCallable(CountDownLatch countDownLatch) {
            this.countDownLatch = countDownLatch;
        }
 
        public Integer call() {
            int sum = 0;
            try {
               
                for (int i = 0; i <= 100; i++) {
                    sum += i;
                }
                System.out.println("线程执行结果:"+sum);
                
            } finally {
                countDownLatch.countDown();
            }
            return sum;
        }
 
    }
 
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        // 第四种:使用使用线程池方式
        // 接受返回参数
        List<Future> resultItems2 = new ArrayList<Future>();
        // 給线程池初始化5個线程
        ExecutorService executorService = Executors.newFixedThreadPool(5);
        CountDownLatch countDownLatch4 = new CountDownLatch(10);
 
        for (int i = 0; i < 10; i++) {
            MyCallable myCallable = new MyCallable(countDownLatch4);
            Future result = executorService.submit(myCallable);
            resultItems2.add(result);
        }
 
        // 等待线程池中分配的任务完成后才关闭(关闭之后不允许有新的线程加入,但是它并不会等待线程结束),
        // 而executorService.shutdownNow();是立即关闭不管是否线程池中是否有其他未完成的线程。
        executorService.shutdown();
        try {
            countDownLatch4.await();
            Iterator<Future> iterator = resultItems2.iterator();
            System.out.println("----------------------");
            while (iterator.hasNext()) {
                try {
                    System.out.println("线程返回结果:"+iterator.next().get());
                } catch (ExecutionException e) {
                    e.printStackTrace();
                }
            }
            System.out.println("callable complete...");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

第五种:使用ComletetableFuture类创建异步线程,且是据有返回结果的线程

package cn.xiaoxuzhu.daily;
 
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
 
import org.junit.Test;
 

 
public class ThreadDemo5 {
 
    
    @Test
    public void completableFuture1(){
        CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future1 finished!");
            return "future1 finished!";
        });
 
        CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> {
            System.out.println("future2 finished!");
            return "future2 finished!";
        });
 
        CompletableFuture<Void> future3 = CompletableFuture.allOf(future1, future2);
        try {
            future3.get();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
        System.out.println("future1: " + future1.isDone() + " future2: " + future2.isDone());
 
    }
 
    
    @Test
    public void test01() throws Exception {
        ExecutorService service = Executors.newFixedThreadPool(5);
        
        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> {
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "xiaoxuzhu";
        }, service);
        CompletableFuture<Void> data = CompletableFuture.runAsync(() -> System.out.println("xiaoxuzhu"));
        
        future.whenComplete((x,y)-> System.out.println("有延迟3秒:执行当前任务的线程继续执行:"+x+","+y)); //执行当前任务的线程继续执行
        data.whenCompleteAsync((x,y)-> System.out.println("交给线程池另起线程执行:"+x+","+y)); // 交给线程池另起线程执行
        future.exceptionally(Throwable::toString);
        //System.out.println(future.get());
        
        //第二个线程依赖第一个的结果
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> 5).thenApply(x -> x);
 
        
        CompletableFuture<Integer> future2 = future1.handleAsync((x, y) -> x + 2);
        System.out.println(future2.get());//7
        
        future2.thenAccept(System.out::println);
        
        future2.thenRunAsync(()-> System.out.println("继续下一个任务"));
        
        CompletableFuture<Integer> future3 = future1.thenCombine(future2, Integer::sum);
        System.out.println(future3.get()); // 5+7=12
        
        future1.thenAcceptBothAsync(future2,(x,y)-> System.out.println(x+","+y)); //5,7
        
        CompletableFuture<Integer> future4 = future1.applyToEither(future2, x -> x);
        System.out.println(future4.get()); //5
        
        future1.acceptEither(future2, System.out::println);
        
        future1.runAfterEither(future,()-> System.out.println("有一个完成了,我继续"));
        
        future1.runAfterBoth(future,()-> System.out.println("都完成了,我继续"));
        
        future1.thenComposeAsync(x->CompletableFuture.supplyAsync(()->x+1))
                .thenComposeAsync(x->CompletableFuture.supplyAsync(()->x+2))
                .thenCompose(x->CompletableFuture.runAsync(()-> System.out.println("流操作结果:"+x)));
        TimeUnit.SECONDS.sleep(5);//主线程sleep,等待其他线程执行
    }
}

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