java多线程创建-Thread,Runnable,callable和threadpool

Posted 2020-09-20

java创建多线程的方式有许多种，这里简要做个梳理

1. 继承Thread类
继承java.lang.Thread类，创建本地多线程的类，重载run()方法，调用Thread的方法启动线程。示例代码如下：

MyThread.java

public class MyThread extends Thread {

    public void run(){
        private int copy = 0;
        System.out.println("Thread id:" + Thread.currentThread().getName()+" == print time:" + System.currentTimeMillis());
        System.out.println("Thread serialnum:" + ++copy);
        try {

            sleep(1000);

        } catch (InterruptedException e) {

            e.printStackTrace();

        }

        System.out.println("Thread id:" + Thread.currentThread().getName()+" == print time:" + System.currentTimeMillis());

    }



    public static void main(String[] args){

        MyThread mt1 = new MyThread();

        MyThread mt2 = new MyThread();

        MyThread mt3 = new MyThread();

        System.out.println("Start all Threads:");

        mt1.start();

        mt2.start();

        mt3.start();
    }
}

输出结果：

Start all Threads:
Thread id:Thread-0 == print time:1495975884383
Thread serialnum:1
Thread id:Thread-1 == print time:1495975884383
Thread serialnum:1
Thread id:Thread-2 == print time:1495975884383
Thread serialnum:1
Thread id:Thread-1 == print time:1495975885385
Thread id:Thread-0 == print time:1495975885385
Thread id:Thread-2 == print time:1495975885385

2. 实现Runnable接口

实现java.lang.Runnable接口的run方法，使用实现的对象实例化Thread类，调用Thread对象的start()方法。
示例代码：

public class MyRunnable implements Runnable {

    private int copy = 0;

    public void run(){
        System.out.println("Thread id:" + Thread.currentThread().getName()+" == print time:" + System.currentTimeMillis());
        System.out.println("Thread serialnum:" + ++copy);
        try {
            sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Thread id:" + Thread.currentThread().getName()+" == print time:" + System.currentTimeMillis());
    }

    public static void main(String[] args){
        MyRunnable mr = new MyRunnable();
        System.out.println("Start all Threads:");
        new Thread(mr).start();
        new Thread(mr).start();
        new Thread(mr).start();
    }
}

输出结果：

Start all Threads:
Thread id:Thread-0 == print time:1495975833588
Thread id:Thread-1 == print time:1495975833588
Thread serial:1
Thread id:Thread-2 == print time:1495975833588
Thread serial:2
Thread serial:3
Thread id:Thread-0 == print time:1495975834603
Thread id:Thread-1 == print time:1495975834603
Thread id:Thread-2 == print time:1495975834603

对比Thread方法和Runnable方法的使输出结果不难发现：
. 二者均可实现多线程并发效果
. Runnable的方法可以使用一个Runnable对象创建多个线程，这多个线程共享其依赖的Runnable对象的资源，适用于多个线程处理相同资源的场景（网上举得较多的例子：卖票）；Thread的方法则是完全相互独立的线程

事实上，嫌贵而言，当前Runnable使用的要比Thread方法普遍的多，除了上面说的资源共享的原因之外，Java的单继承特性也增大了Thread方法的局限性。

3. 实现Callable接口
相对于上述继承Thread类的方法和实现Runnable接口的方法，实现Callable接口的方法不但可以可以实现多线程并发，还能够处理线程的返回值或者获取执行异常。使用Callable实现多线程编程需要实现Callable的call方法(在call方法中指定返回值，抛出异常)，构造与之关联的FutureTask对象，启动线程后，线程执行的结果会存储在FutureTask对象中，可以使用FutureTask的get方法获取。示例代码：

import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;
import static java.lang.Thread.sleep;

public class MyCallable implements Callable<Integer> {
    private int flag = 0;

    public Integer call() throws Exception {
        System.out.println("Thread id:" + Thread.currentThread().getName() + "  == print time:"+System.currentTimeMillis());
        sleep(10000);
        System.out.println("Thread id:" + Thread.currentThread().getName() + "  == print time:"+System.currentTimeMillis());
        ++flag;
        return flag;
    }

    public static void main(String[] args) throws Exception{
        MyCallable myCallable = new MyCallable();
        FutureTask<Integer> future = new FutureTask<Integer>(myCallable);
        FutureTask<Integer> future2 = new FutureTask<Integer>(myCallable);
        new Thread(future).start();
        new Thread(future2).start();
        int Result,Result2;
//        while (!future.isDone());
        Result = future.get();
//
//        while(!future2.isDone());
        Result2 = future2.get();

        System.out.println("GET THREAD NAME:"+ Result );
        System.out.println("GET THREAD NAME 2:"+Result2);
    }
}

执行结果：

Thread id:Thread-0  == print time:1495997744617
Thread id:Thread-1  == print time:1495997744617
Thread id:Thread-0  == print time:1495997754625
Thread id:Thread-1  == print time:1495997754625
GET THREAD NAME:1
GET THREAD NAME 2:2

 

上述代码执行逻辑与前面Runnale的示例代码类似，通过构造Thread对象来启动多线程。区别在于此处构造了futureTask方法用来用来存储线程返回值。需要注意的是，线程的Callable泛型接口里的类型和call()方法的返回值类型和FutureTask的泛型接口类型要一致。

4. 使用线程池和Executor框架
前面的三种方案，我们创建了Runnable或者Callable或者Thread的任务，扔到Thread中去启动，我们需要手动去管理各种多线程的参数，比如线程数量，线程复用，线程安全的控制等。Java5以后的版本提供了Executor技术，能够提供搭建好的线程池，为多线程提供了完整的解决方案，能够有效的管理线程数量，线程复用策略，保证线程安全，避免this逃逸问题等。
Executor提供的线程池方案包括newCachedThreadPool,newFixedThreadPool,newScheduledThreadPool和SingleThreadExecutor(Fork/Join框架还提供了一个newWorkStealingPool)，这些线程池分别提供了不同的线程管理方案，执行返回一个ExecutorService对象，这个对象可以调用execute()方法或者submit()方法，将前面定义的Runnable任务或者Callable任务或者Thread任务提交到各个线程去执行

我们以比较常用的newCachedThreadPool()为例，下面示例代码使用ExecutorService执行一个Callable对象任务，获取每个线程的返回值。

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

import static java.lang.Thread.sleep;

public class MyCallable2 implements Callable<String> {

    public String call() throws Exception{
        sleep(1000);
        System.out.println("terminate the thread : "+Thread.currentThread().getName());
        return Thread.currentThread().getName();
    }

    public static void main(String[] args){
        ExecutorService es = Executors.newCachedThreadPool();
        List<Future<String>> futureList = new ArrayList<Future<String>>();
        for(int i=0;i<5;i++){
            Future future = es.submit(new MyCallable2());
            futureList.add(future);
        }

        for (Future f: futureList
             ) {
            while(!f.isDone());
            try {
                System.out.println("EXECUTE RESULT:"+f.get());
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
            finally {
                es.shutdown();
            }
        }
    }
}

执行结果如下：

terminate the thread : pool-1-thread-5
terminate the thread : pool-1-thread-4
terminate the thread : pool-1-thread-3
terminate the thread : pool-1-thread-2
terminate the thread : pool-1-thread-1
EXECUTE RESULT:pool-1-thread-1
EXECUTE RESULT:pool-1-thread-2
EXECUTE RESULT:pool-1-thread-3
EXECUTE RESULT:pool-1-thread-4
EXECUTE RESULT:pool-1-thread-5

使用Executors创建Runnable线程与创建Callable线程的方法类似，且不用管理返回值，相对更加简单，此处不再赘述。