Java多线程Runnable与Callable区别与拓展

我们先来分别看一下这两个接口

Runnable:

1 //
2 // Source code recreated from a .class file by IntelliJ IDEA
3 // (powered by Fernflower decompiler)
4 //
5
6 package java.lang;
7
8 @FunctionalInterface
9 public interface Runnable {
10     void run();
11 }

只有一个方法run(),表示执行任务的逻辑。

Callable:

1 //
2 // Source code recreated from a .class file by IntelliJ IDEA
3 // (powered by Fernflower decompiler)
4 //
5
6 package java.util.concurrent;
7
8 @FunctionalInterface
9 public interface Callable<V> {
10     V call() throws Exception;
11 }

也只有一个方法call(),但是是一个有返回值的方法，这给我们提供了获取方法执行结果的可能，即使它是个异步的任务，它可以获取异常，给我们极大地便利知道任务执行失败的原因。

Runnable例子Demo:

1 package cn.concurrent.executor;
2
3 /**
4  * Created by spark on 17-9-24.
5  */
6 public class RunnabelDemo implements Runnable {
7
8     public RunnabelDemo(String acceptStr) {
9         this.acceptStr = acceptStr;
10     }
11
12     private String acceptStr;
13
14     @Override
15     public void run() {
16         try {
17             // 线程阻塞 1 秒，此时有异常产生，只能在方法内部消化，无法上抛
18             Thread.sleep(1000);
19         } catch (InterruptedException e) {
20             e.printStackTrace();
21         }
22         // 最终处理结果无法返回
23         System.out.println("hello : " + this.acceptStr);
24     }
25
26
27     public static void main(String[] args) {
28         Runnable runnable = new RunnabelDemo("my runable test!");
29         long beginTime = System.currentTimeMillis();
30         new Thread(runnable).start();
31         long endTime = System.currentTimeMillis();
32         System.out.println("cast : " + (endTime - beginTime) / 1000 + " second!");
33     }
34 }

结果：

1 /usr/lib/jvm/java-1.8.0-openjdk-amd64/bin/java -javaagent:/usr/local/idea/lib/idea_rt.jar=41745:/usr/local/idea/bin -Dfile.encoding=UTF-8 -classpath /usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/charsets.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/cldrdata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/dnsns.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/icedtea-sound.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/jaccess.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/localedata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/nashorn.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunec.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunjce_provider.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunpkcs11.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/zipfs.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jce.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jsse.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/management-agent.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/resources.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/rt.jar:/home/spark/IdeaProjects/thread/thread/target/classes:/home/spark/.m2/repository/com/lmax/disruptor/3.3.2/disruptor-3.3.2.jar cn.concurrent.executor.RunnabelDemo
2 cast : 0 second!
3 hello : my runable test!
4
5 Process finished with exit code 0

Callable的例子:

1 package cn.concurrent.executor;
2
3 import java.util.concurrent.Callable;
4 import java.util.concurrent.ExecutionException;
5 import java.util.concurrent.Future;
6 import java.util.concurrent.FutureTask;
7
8 /**
9  * Created by spark on 17-9-24.
10  */
11 public class CallableDemo implements Callable<String> {
12
13     private String name;
14
15     public CallableDemo(String name) {
16         this.name = name;
17     }
18
19     @Override
20     public String call() throws Exception {
21         //执行任务的相关逻辑
22         Thread.sleep(1000);
23         return name + " : this is task is successed.";
24     }
25
26     public static void main(String[] args) throws ExecutionException, InterruptedException {
27         CallableDemo callableDemo = new CallableDemo("wade");
28         //异步执行的结果
29         FutureTask<String> future = new FutureTask<String>(callableDemo);
30         new Thread(future).start();
31         //調用get()方法--阻塞
32         String result=future.get();
33         System.out.println("hello : " + result);
34     }
35 }

结果:

1 /usr/lib/jvm/java-1.8.0-openjdk-amd64/bin/java -javaagent:/usr/local/idea/lib/idea_rt.jar=37879:/usr/local/idea/bin -Dfile.encoding=UTF-8 -classpath /usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/charsets.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/cldrdata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/dnsns.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/icedtea-sound.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/jaccess.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/localedata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/nashorn.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunec.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunjce_provider.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunpkcs11.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/zipfs.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jce.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jsse.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/management-agent.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/resources.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/rt.jar:/home/spark/IdeaProjects/thread/thread/target/classes:/home/spark/.m2/repository/com/lmax/disruptor/3.3.2/disruptor-3.3.2.jar cn.concurrent.executor.CallableDemo
2 hello : wade : this is task is successed.
3
4 Process finished with exit code 0

问题:如何把一个Runnbale的任务转变为有返回值的Callable的任务。

^_^我们有工具类Executors可以帮我们实现,如下图：



其中Executors.callable(Runnable),就是我们需要的方法，是不是很简单呢。

代码如下:

1 package cn.concurrent.executor;
2
3 import java.util.concurrent.Callable;
4 import java.util.concurrent.ExecutionException;
5 import java.util.concurrent.Executors;
6 import java.util.concurrent.FutureTask;
7
8 /**
9  * Created by spark on 17-9-24.
10  */
11 public class RunnableToCallable implements Runnable {
12
13     String name;
14
15     public RunnableToCallable(String name) {
16         this.name = name;
17     }
18
19     @Override
20     public void run() {
21         try {
22             Thread.sleep(1000);
23             System.out.println(name + " :任务执行完毕！");
24         } catch (InterruptedException e) {
25             e.printStackTrace();
26         }
27     }
28
29     public static void main(String[] args) throws ExecutionException, InterruptedException {
30         RunnableToCallable task=new RunnableToCallable("wade");
31         //转变为有返回值的任务
32         Callable<Object> future=Executors.callable(task);
33         FutureTask<Object> futureTask=new FutureTask<Object>(future);
34         new Thread(futureTask).start();
35         //調用get()方法--阻塞
36         Object result=futureTask.get();
37         System.out.println("hello : " + result);
38     }
39 }

结果如下：

1 /usr/lib/jvm/java-1.8.0-openjdk-amd64/bin/java -javaagent:/usr/local/idea/lib/idea_rt.jar=36136:/usr/local/idea/bin -Dfile.encoding=UTF-8 -classpath /usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/charsets.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/cldrdata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/dnsns.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/icedtea-sound.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/jaccess.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/localedata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/nashorn.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunec.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunjce_provider.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunpkcs11.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/zipfs.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jce.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jsse.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/management-agent.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/resources.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/rt.jar:/home/spark/IdeaProjects/thread/thread/target/classes:/home/spark/.m2/repository/com/lmax/disruptor/3.3.2/disruptor-3.3.2.jar cn.concurrent.executor.RunnableToCallable
2 wade :任务执行完毕！
3 hello : null
4
5 Process finished with exit code 0

说明:因为该任务没有返回结果，因此返回的为null.

接下来我们讨论一下ExecutorService.executr()与ExecutorService.submit()方法的区别。

类图如下：



从类图可以看到：

execute只能接受Runnable类型的任务，并且没有返回值。

submit不管是Runnable还是Callable类型的任务都可以接受，但是Runnable返回值均为void，所以使用Future的get()获得的还是null

然后来看异常处理：

对于Runnable 只能通过try-catch来捕获异常，对于Callable,直接抛出就可以，然后在get的时候捕获异常进行处理。

如下：

1 package cn.concurrent.executor;
2
3 import java.util.concurrent.*;
4
5 /**
6  * Created by spark on 17-9-24.
7  */
8 public class ThreadExceptionTest {
9
10
11     public static void main(String[] args) {
12         ExecutorService executorService = Executors.newCachedThreadPool();
13         CallableTest callableTest = new CallableTest();
14         Future<Boolean> task = executorService.submit(callableTest);
15         try {
16             Boolean b = task.get();
17         } catch (InterruptedException e) {
18             e.printStackTrace();
19         } catch (ExecutionException e) {
20             e.printStackTrace();
21         }
22
23     }
24
25     static class CallableTest implements Callable<Boolean> {
26
27         @Override
28         public Boolean call() throws Exception {
29             int num = 3 / 0;
30             return false;
31         }
32     }
33 }

结果：

1 /usr/lib/jvm/java-1.8.0-openjdk-amd64/bin/java -javaagent:/usr/local/idea/lib/idea_rt.jar=42960:/usr/local/idea/bin -Dfile.encoding=UTF-8 -classpath /usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/charsets.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/cldrdata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/dnsns.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/icedtea-sound.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/jaccess.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/localedata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/nashorn.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunec.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunjce_provider.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunpkcs11.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/zipfs.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jce.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jsse.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/management-agent.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/resources.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/rt.jar:/home/spark/IdeaProjects/thread/thread/target/classes:/home/spark/.m2/repository/com/lmax/disruptor/3.3.2/disruptor-3.3.2.jar cn.concurrent.executor.ThreadExceptionTest
2 java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
3     at java.util.concurrent.FutureTask.report(FutureTask.java:122)
4     at java.util.concurrent.FutureTask.get(FutureTask.java:192)
5     at cn.concurrent.executor.ThreadExceptionTest.main(ThreadExceptionTest.java:16)
6 Caused by: java.lang.ArithmeticException: / by zero
7     at cn.concurrent.executor.ThreadExceptionTest$CallableTest.call(ThreadExceptionTest.java:29)
8     at cn.concurrent.executor.ThreadExceptionTest$CallableTest.call(ThreadExceptionTest.java:25)
9     at java.util.concurrent.FutureTask.run(FutureTask.java:266)
10     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
11     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
12     at java.lang.Thread.run(Thread.java:748)
13
14 Process finished with exit code 0

get()方法会捕获异常。

接下来，我们详细了解一下Future接口，实现类FutureTask(它有三种状态，分别为未启动，已启动，已完成);

类图如下：



1.首先可以看到，它也是一个线程，可以直接继承它来运行任务，然后交给ExecutorService执行。

如下：

1 package cn.concurrent.executor;
2
3 import java.util.concurrent.ExecutorService;
4 import java.util.concurrent.Executors;
5 import java.util.concurrent.FutureTask;
6
7 /**
8  * Created by spark on 17-9-24.
9  */
10 public class FutureTaskDemo extends FutureTask<String> {
11     public FutureTaskDemo(Runnable runnable, String s) {
12         super(runnable, s);
13         System.out.println(s);
14     }
15
16     @Override
17     public void run() {
18         try {
19             Thread.sleep(1000);
20             System.out.println("this is running.");
21         } catch (InterruptedException e) {
22             e.printStackTrace();
23         }
24     }
25
26     public static void main(String[] args) {
27         FutureTaskDemo futureTaskDemo = new FutureTaskDemo(new Runnable() {
28             @Override
29             public void run() {
30                 System.out.println("this is wade.");
31             }
32         }, "wade");
33
34         ExecutorService es = Executors.newFixedThreadPool(1);
35         FutureTask<String> task = (FutureTask<String>) es.submit(futureTaskDemo);
36         System.out.println("hello:"+task);
37         es.shutdown();
38     }
39 }

结果如下：

1 /usr/lib/jvm/java-1.8.0-openjdk-amd64/bin/java -javaagent:/usr/local/idea/lib/idea_rt.jar=46018:/usr/local/idea/bin -Dfile.encoding=UTF-8 -classpath /usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/charsets.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/cldrdata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/dnsns.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/icedtea-sound.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/jaccess.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/localedata.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/nashorn.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunec.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunjce_provider.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/sunpkcs11.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/ext/zipfs.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jce.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/jsse.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/management-agent.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/resources.jar:/usr/lib/jvm/java-1.8.0-openjdk-amd64/jre/lib/rt.jar:/home/spark/IdeaProjects/thread/thread/target/classes:/home/spark/.m2/repository/com/lmax/disruptor/3.3.2/disruptor-3.3.2.jar cn.concurrent.executor.FutureTaskDemo
2 wade
3 hello:java.util.concurrent.FutureTask@266474c2
4 this is running.
5
6 Process finished with exit code 0

2.看一下run()方法

1     public void run() {
2             /**
3             *首先判断任务的状态 如果任务的状态不是new 说明任务的状态已经改变（说明他已经走了4种可能变化的一种）
4             * 如果状态是new就会把 当前执行任务的线程付给runner, 这里用的cmpandset如果runner不为空 说明已经有线程在执行
5         * 任务也会退出执行,如果状态是new并且runner为空并且把当前的线程付给了runner那么就继续执行任务(runner state 都是 volatile
6             *类型的变量是一个很轻量机的线程安全操作)
7             *引起state状态变化的原因 就是调用了cancel 或是 run
8             **/
9             if (state != NEW ||
10                 !UNSAFE.compareAndSwapObject(this, runnerOffset,
11                                              null, Thread.currentThread()))
12                 return;
13
14             //开始执行任务
15             try {
16                 Callable<V> c = callable;
17                 /**
18                 * 如果 要执行的任务不为空 并且状态 new 就执行
19
20                 ***/
21                 if (c != null && state == NEW) {
22                     V result;
23                     boolean ran;
24                     try {
25                         //执行任务
26                         result = c.call();
27                         //如果没有意外发生就执行成功了
28                         ran = true;
29                     } catch (Throwable ex) {
30                             //有异常
31                         result = null;
32                         ran = false;
33                         //设置异常
34                         setException(ex);
35                     }
36                     //如果转型成功了 设置结果
37                     if (ran)
38                         (result);
39                 }
40             } finally {
41                 // runner must be non-null until state is settled to
42                 // prevent concurrent calls to run()
43                 //不管是否执行成功了 都把runner设置成null
44                 runner = null;
45                 // state must be re-read after nulling runner to prevent
46                 // leaked interrupts
47                 int s = state;
48                 if (s >= INTERRUPTING)
49                     handlePossibleCancellationInterrupt(s);
50             }
51         }

Task执行后如果成功会调用set()方法,如果有异常会调用setException()方法。

我们先看下set方法 ：

1     protected void set(V v) {
2             /**
3             *如过state是new 把state设置成 COMPLETING
4             *
5             **/
6
7           if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
8               outcome = v;
9               //将任务设置成NORMAL   over the task
10               UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
11               finishCompletion();
12           }
13       }

最后，来看一下最重要的方法get();

public V get() throws InterruptedException, ExecutionException {
int s = state;
　　　　　//当线程状态为新建活着执行中时一直调用awaitDone方法
if (s <= COMPLETING)
　　　　　　  //循环判断线程状态是否已经执行成功，如果执行成功返回线程状态；其中还包括线程取消，中断等情况的判断。可参见下方源码。
　　　　　　  //所以这里便是上面例子中为什么线程执行成功后即可立即得到结果，如果还没有执行成功

s = awaitDone(false, 0L);

　　　　　　　//线程状态正常返回结果
return report(s);
}

如果get时,FutureTask的状态为未完成状态，则调用awaitDone方法进行阻塞。awaitDone():

private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
//计算一下需要等待的时间，有可能为0，为0的话就无限期等待。
final long deadline = timed ? System.nanoTime() + nanos : 0L;
//一个等待节点
WaitNode q = null;
//是否加入队列
boolean queued = false;
for (;;) {
//如果当且调用get方法的线程被interrupt 那么就把当前线程从等待队列remove
//然后抛出异常
if (Thread.interrupted()) {
removeWaiter(q);
throw new InterruptedException();
}
int s = state;
//如果任务已经完成 不管是被暂停了 还是出现异常了 只要状态大于COMPLETING就返回。
if (s > COMPLETING) {
if (q != null)
q.thread = null;
return s;
}
else if (s == COMPLETING) // cannot time out yet
//如果任务正在完成中(注意是ING进行时) 那么让出一会CPU时间在继续执行
Thread.yield();
else if (q == null)
//创建等待节点
q = new WaitNode();
else if (!queued)
//第一次创建的等待节点需要加入等待队列，这里加入队列等待。
queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
q.next = waiters, q);
else if (timed) {
//如果设置了等待 那么就得锁住线程等待，如果时间到了就返回状态。
//方法 public V get(long timeout, TimeUnit unit) 这里会根据状态做后续处理。
nanos = deadline - System.nanoTime();
if (nanos <= 0L) {
removeWaiter(q);
return state;
}
LockSupport.parkNanos(this, nanos);
}
else
//否则锁住线程等待其他线程解锁。
LockSupport.park(this);
}
}

awaitDone方法可以看成是不断轮询查看FutureTask的状态。在get阻塞期间：

如果执行get的线程被中断，则移除FutureTask的所有阻塞队列中的线程（waiters）,并抛出中断异常；

如果FutureTask的状态转换为完成状态（正常完成或取消），则返回完成状态；

如果FutureTask的状态变为COMPLETING, 则说明正在set结果，此时让线程等一等；

如果FutureTask的状态为初始态NEW，则将当前线程加入到FutureTask的阻塞线程中去；

如果get方法没有设置超时时间，则阻塞当前调用get线程；如果设置了超时时间，则判断是否达到超时时间，如果到达，则移除FutureTask的所有阻塞列队中的线程，并返回此时FutureTask的状态，如果未到达时间，则在剩下的时间内继续阻塞当前线程。

详细了解这几个方法：

当FutureTask处于未启动或者已启动状态时，执行Future.get()方法，将导致调用线程阻塞，当FutureTask处于已完成状态时，执行Future.get()方法时，将调用线程立即返回结果或者抛出异常。

当FutureTask处于未启动状态时，执行Future.cancel()方法,导致此任务永远不会被执行 ；当FutureTask处于已启动状态时，执行Future.cancel()方法将以中断执行此任务线程的方式来试图停止任务;当FutureTask处于已启动状态时，执行Future.cancel(false)方法不会对正在执行此任务的线程产生影响。

说明：Future.get()方法的实现基于AbstractQueuedSynchronizer(简称为AQS)，并发包中有很多可阻塞类(如ReentrantLock)都是基于AQS实现的，AQS是一个同步框架，它提供了通用机制来原子性管理同步状态，阻塞和唤醒线程，以及维护被阻塞线程的队列，后面有专门博客介绍AQS.

博客有很多的不足，望大家指点。