Java使用Thread类代表线程，所有的线程对象都必须是Thread类或其子类的实例。Java可以用四种方式来创建线程：

• 继承Thread创建线程
• 实现Runnable接口创建线程
• 实现callable接口实现线程
• 使用线程池Executor创建线程

1.继承Thread实现线程

　　我们先来看一下Thread的源码，它是一个类，同样也实现了Runnable接口

public
class Thread implements Runnable {
    /* Make sure registerNatives is the first thing <clinit> does. */
    private static native void registerNatives();
    static {
        registerNatives();
    }

    private volatile String name;
    private int            priority;
    private Thread         threadQ;
    private long           eetop;

    /* Whether or not to single_step this thread. */
    private boolean     single_step;

    /* Whether or not the thread is a daemon thread. */
    private boolean     daemon = false;

    /* JVM state */
    private boolean     stillborn = false;

    /* What will be run. */
    private Runnable target;

    /* The group of this thread */
    private ThreadGroup group;

    /* The context ClassLoader for this thread */
    private ClassLoader contextClassLoader;

    /* The inherited AccessControlContext of this thread */
    private AccessControlContext inheritedAccessControlContext;

    /* For autonumbering anonymous threads. */
    private static int threadInitNumber;
    private static synchronized int nextThreadNum() {
        return threadInitNumber++;
    }

    /* ThreadLocal values pertaining to this thread. This map is maintained
     * by the ThreadLocal class. */
    ThreadLocal.ThreadLocalMap threadLocals = null;

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

    /*
     * The requested stack size for this thread, or 0 if the creator did
     * not specify a stack size.  It is up to the VM to do whatever it
     * likes with this number; some VMs will ignore it.
     */
    private long stackSize;

    /*
     * JVM-private state that persists after native thread termination.
     */
    private long nativeParkEventPointer;

    /*
     * Thread ID
     */
    private long tid;

    /* For generating thread ID */
    private static long threadSeqNumber;

    /* Java thread status for tools,
     * initialized to indicate thread 'not yet started'
     */

    private volatile int threadStatus = 0;

    //......

}

通过继承Thread类来创建并启动多线程的一般步骤如下

• 1】d定义Thread类的子类，并重写该类的run()方法，该方法的方法体就是线程需要完成的任务，run()方法也称为线程执行体。
• 2】创建Thread子类的实例，也就是创建了线程对象
• 3】启动线程，即调用线程的start()方法

代码示例：

public class  ThreadTest {

    public static void main(String[] args) {
        new MyThread().start();
    }
    
   static class MyThread extends Thread {//继承Thread
        public void run() {
            System.out.println("我是继承Thread类！！ ");
        }
    }

}

2.实现Runnable接口创建线程

　　我们来看一下Runnable的源码，它是一个接口：

@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

由于run()方法返回值为void类型，所以在执行完任务之后无法返回任何结果。

通过实现Runnable接口创建并启动线程一般步骤如下：

• 1】定义Runnable接口的实现类，一样要重写run()方法，这个run（）方法和Thread中的run()方法一样是线程的执行体
• 2】创建Runnable实现类的实例，并用这个实例作为Thread的target来创建Thread对象，这个Thread对象才是真正的线程对象
• 3】第三部依然是通过调用线程对象的start()方法来启动线程

代码示例：

public class RunnableTest {

    public static void main(String[] args) {
        MyThread2 myThread=new MyThread2();
        Thread thread = new Thread(myThread);
        thread.start();
    }

    static class MyThread2 implements Runnable {
        @Override
        public void run() {
            System.out.println("我是实现Runnable接口！！ ");
        }
    }
}

3.实现callable接口实现线程

　　我们来看一下callable源码，它是一个接口：

@FunctionalInterface
public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

它和Runnable接口不一样的是，call()方法提供了2个额外功能：

• call()方法可以有返回值
• all()方法可以声明抛出异常

java5提供了Future接口来代表Callable接口里call()方法的返回值，并且为Future接口提供了一个实现类FutureTask，这个实现类既实现了Future接口，还实现了Runnable接口，因此可以作为Thread类的target。在Future接口里定义了几个公共方法来控制它关联的Callable任务。

那么怎么使用Callable呢？一般情况下是配合ExecutorService来使用的，在ExecutorService接口中声明了若干个submit方法的重载版本：

<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);

第一个submit方法里面的参数类型就是Callable。

暂时只需要知道Callable一般是和ExecutorService配合来使用的，具体的使用方法讲在后面讲述。

一般情况下我们使用第一个submit方法和第三个submit方法，第二个submit方法很少使用。

3.1 Future

我们来看一下Future的源码，它是一个接口：

public interface Future<V> {

    /**
     * Attempts to cancel execution of this task.  This attempt will
     * fail if the task has already completed, has already been cancelled,
     * or could not be cancelled for some other reason. If successful,
     * and this task has not started when {@code cancel} is called,
     * this task should never run.  If the task has already started,
     * then the {@code mayInterruptIfRunning} parameter determines
     * whether the thread executing this task should be interrupted in
     * an attempt to stop the task.
     *
     * <p>After this method returns, subsequent calls to {@link #isDone} will
     * always return {@code true}.  Subsequent calls to {@link #isCancelled}
     * will always return {@code true} if this method returned {@code true}.
     *
     * @param mayInterruptIfRunning {@code true} if the thread executing this
     * task should be interrupted; otherwise, in-progress tasks are allowed
     * to complete
     * @return {@code false} if the task could not be cancelled,
     * typically because it has already completed normally;
     * {@code true} otherwise
     */
    boolean cancel(boolean mayInterruptIfRunning);

    /**
     * Returns {@code true} if this task was cancelled before it completed
     * normally.
     *
     * @return {@code true} if this task was cancelled before it completed
     */
    boolean isCancelled();

    /**
     * Returns {@code true} if this task completed.
     *
     * Completion may be due to normal termination, an exception, or
     * cancellation -- in all of these cases, this method will return
     * {@code true}.
     *
     * @return {@code true} if this task completed
     */
    boolean isDone();

    /**
     * Waits if necessary for the computation to complete, and then
     * retrieves its result.
     *
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     */
    V get() throws InterruptedException, ExecutionException;

    /**
     * Waits if necessary for at most the given time for the computation
     * to complete, and then retrieves its result, if available.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return the computed result
     * @throws CancellationException if the computation was cancelled
     * @throws ExecutionException if the computation threw an
     * exception
     * @throws InterruptedException if the current thread was interrupted
     * while waiting
     * @throws TimeoutException if the wait timed out
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

我们来看一下它的各个方法：

• boolean cancel(boolean mayInterruptIfRunning)：用来取消任务，如果取消任务成功则返回true，如果取消任务失败则返回false。参数mayInterruptIfRunning表示是否允许取消正在执行却没有执行完毕的任务，如果设置true，则表示可以取消正在执行过程中的任务。如果任务已经完成，则无论mayInterruptIfRunning为true还是false，此方法肯定返回false，即如果取消已经完成的任务会返回false；如果任务正在执行，若mayInterruptIfRunning设置为true，则返回true，若mayInterruptIfRunning设置为false，则返回false；如果任务还没有执行，则无论mayInterruptIfRunning为true还是false，肯定返回true。

• boolean isCancelled()：如果在Callable任务正常完成前被取消，返回True

• boolean isDone()：若Callable任务完成，返回True

• V get() throws InterruptedException, ExecutionException：返回Callable里call（）方法的返回值，调用这个方法会导致程序阻塞，必须等到子线程结束后才会得到返回值

• V get(long timeout, TimeUnit unit)
      throws InterruptedException, ExecutionException, TimeoutException：用来获取执行结果，如果在指定时间内，还没获取到结果，就直接返回null

因为Future只是一个接口，所以是无法直接用来创建对象使用的，因此就有了下面的FutureTask

3.2 FutureTask

我们先来看一下FutureTask的实现：

public class FutureTask<V> implements RunnableFuture<V> {

FutureTask类实现了RunnableFuture接口，我们看一下RunnableFuture接口的实现：

public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

可以看出RunnableFuture继承了Runnable接口和Future接口，而FutureTask实现了RunnableFuture接口。所以它既可以作为Runnable被线程执行，又可以作为Future得到Callable的返回值。

FutureTask提供了2个构造器：

public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

事实上，FutureTask是Future接口的一个唯一实现类。

3.3 使用FutureTask对象作为Thread对象的target创建并启动线程

接下来我们看如何创建并启动有返回值的线程：

• 1】创建Callable接口的实现类，并实现call()方法，然后创建该实现类的实例（从java8开始可以直接使用Lambda表达式创建Callable对象）。
• 2】使用FutureTask类来包装Callable对象，该FutureTask对象封装了Callable对象的call()方法的返回值
• 3】使用FutureTask对象作为Thread对象的target创建并启动线程（因为FutureTask实现了Runnable接口）
• 4】调用FutureTask对象的get()方法来获得子线程执行结束后的返回值

代码示例：

public class CallableAndFuture {
    public static void main(String[] args) {

        Callable<Integer> call = new Callable<Integer>() {
            public Integer call() throws Exception {
                System.out.println("计算线程正在计算结果...");
                Thread.sleep(3000);
                return 1;
            }
        };
        FutureTask<Integer> future = new FutureTask<>(call);

        new Thread(future,"有返回值的线程").start();//实质上还是以Callable对象来创建并启动线程

        try {
            System.out.println("子线程的返回值：" + future.get());//get()方法会阻塞，直到子线程执行结束才返回
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

    }
}

3.4 使用executor创建线程

3.4.1.使用Callable+Future获取执行结果

代码示例：

public class CallableAndFuture {

    public static void main(String[] args) {
        /** Executors提供了一系列工厂方法用于创先线程池，返回的线程池都实现了ExecutorService接口。   */
        ExecutorService executor = Executors.newCachedThreadPool();
        Task task = new Task();
        Future<Integer> result = executor.submit(task);

        executor.shutdown();

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }

        System.out.println("主线程在执行任务");

        try {
            System.out.println("task运行结果"+result.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

        System.out.println("所有任务执行完毕");
    }

    static class Task implements Callable<Integer> {
        @Override
        public Integer call() throws Exception {
            System.out.println("子线程在进行计算");
            Thread.sleep(3000);
            int sum = 0;
            for(int i=0;i<100;i++)
                sum += i;
            return sum;
        }
    }
}

3.4.2.使用Callable+FutureTask获取执行结果

public class CallableAndFuture2 {
    public static void main(String[] args) {
        //第一种方式
        ExecutorService executor = Executors.newCachedThreadPool();
        Task task = new Task();
        FutureTask<Integer> futureTask = new FutureTask<Integer>(task);
        executor.submit(futureTask);
        executor.shutdown();

        //第二种方式，注意这种方式和第一种方式效果是类似的，只不过一个使用的是ExecutorService，一个使用的是Thread
        /*Task task = new Task();
        FutureTask<Integer> futureTask = new FutureTask<Integer>(task);
        Thread thread = new Thread(futureTask);
        thread.start();*/

        try {
            Thread.sleep(1000);
        } catch (InterruptedException e1) {
            e1.printStackTrace();
        }

        System.out.println("主线程在执行任务");

        try {
            System.out.println("task运行结果"+futureTask.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

        System.out.println("所有任务执行完毕");
    }

    static class Task implements Callable<Integer> {
        @Override
        public Integer call() throws Exception {
            System.out.println("子线程在进行计算");
            Thread.sleep(3000);
            int sum = 0;
            for(int i=0;i<100;i++)
                sum += i;
            return sum;
        }
    }
}

4.使用线程池Executor创建线程

4.1 Executor执行Runnable

public class Executor执行Runnable {

    /**
     * 从结果中可以看出，pool-1-thread-1和pool-1-thread-2均被调用了两次，这是随机的，execute会首先在线程池中选择
     * 一个已有空闲线程来执行任务，如果线程池中没有空闲线程，它便会创建一个新的线程来执行任务。
     */
    public static void main(String[] args){
        ExecutorService executorService = Executors.newCachedThreadPool();
//      ExecutorService executorService = Executors.newFixedThreadPool(5);
//      ExecutorService executorService = Executors.newSingleThreadExecutor();
        for (int i = 0; i < 5; i++){
            executorService.execute(new TestRunnable());
            System.out.println("************* a" + i + " *************");
        }
        executorService.shutdown();
    }
}

     class TestRunnable implements Runnable {
        public void run() {
            System.out.println(Thread.currentThread().getName() + "线程被调用了。");
        }

}

执行结果：

************* a1 *************
************* a2 *************
pool-1-thread-2线程被调用了。
************* a3 *************
pool-1-thread-1线程被调用了。
pool-1-thread-2线程被调用了。
************* a4 *************
pool-1-thread-3线程被调用了。

4.2Executor执行Callable

public class Executor执行Callable {
    /**
     *  从结果中可以同样可以看出，submit也是首先选择空闲线程来执行任务，如果没有，才会创建新的线程来执行任务。
     *  另外，需要注意：如果Future的返回尚未完成，则get（）方法会阻塞等待，直到Future完成返回，可以通过
     *  调用isDone（）方法判断Future是否完成了返回。
     */
    public static void main(String[] args){
        ExecutorService executorService = Executors.newCachedThreadPool();
        List<Future<String>> resultList = new ArrayList<Future<String>>();

        //创建10个任务并执行
        for (int i = 0; i < 10; i++){
            //使用ExecutorService执行Callable类型的任务，并将结果保存在future变量中
            Future<String> future = executorService.submit(new TaskWithResult(i));
            //将任务执行结果存储到List中
            resultList.add(future);
        }

        //遍历任务的结果
        for (Future<String> fs : resultList){
            try{
                while(!fs.isDone());//Future返回如果没有完成，则一直循环等待，直到Future返回完成
                System.out.println(fs.get());     //打印各个线程（任务）执行的结果
            }catch(InterruptedException e){
                e.printStackTrace();
            }catch(ExecutionException e){
                e.printStackTrace();
            }finally{
                //启动一次顺序关闭，执行以前提交的任务，但不接受新任务
                executorService.shutdown();
            }
        }
    }
}


class TaskWithResult implements Callable<String> {
    private int id;

    public TaskWithResult(int id){
        this.id = id;
    }

    /**
     * 任务的具体过程，一旦任务传给ExecutorService的submit方法，
     * 则该方法自动在一个线程上执行
     */
    public String call() throws Exception {
        System.out.println("call()方法被自动调用！！！    " + Thread.currentThread().getName());
        //该返回结果将被Future的get方法得到
        return "call()方法被自动调用，任务返回的结果是：" + id + "    " + Thread.currentThread().getName();
    }
}

执行结果：

call()方法被自动调用！！！    pool-1-thread-1
call()方法被自动调用，任务返回的结果是：0    pool-1-thread-1
call()方法被自动调用！！！    pool-1-thread-2
call()方法被自动调用，任务返回的结果是：1    pool-1-thread-2
call()方法被自动调用！！！    pool-1-thread-3
call()方法被自动调用，任务返回的结果是：2    pool-1-thread-3
call()方法被自动调用！！！    pool-1-thread-5
call()方法被自动调用！！！    pool-1-thread-6
call()方法被自动调用！！！    pool-1-thread-7
call()方法被自动调用！！！    pool-1-thread-9
call()方法被自动调用！！！    pool-1-thread-4
call()方法被自动调用，任务返回的结果是：3    pool-1-thread-4
call()方法被自动调用，任务返回的结果是：4    pool-1-thread-5
call()方法被自动调用，任务返回的结果是：5    pool-1-thread-6
call()方法被自动调用，任务返回的结果是：6    pool-1-thread-7
call()方法被自动调用！！！    pool-1-thread-8
call()方法被自动调用，任务返回的结果是：7    pool-1-thread-8
call()方法被自动调用，任务返回的结果是：8    pool-1-thread-9
call()方法被自动调用！！！    pool-1-thread-10
call()方法被自动调用，任务返回的结果是：9    pool-1-thread-10