基本的线程机制

[TOC]

1. 定义任务 (run)

//LiftOff.java
//定义线程任务类
public class LiftOff implements Runnable {
  protected int countDown = 10; // Default
  private static int taskCount = 0;
  private final int id = taskCount++;
  public LiftOff() {}
  public LiftOff(int countDown) {
    this.countDown = countDown;
  }
  public String status() {
    return "#" + id + "(" +
      (countDown > 0 ? countDown : "Liftoff!") + "), ";
  }
  public void run() {
    while(countDown-- > 0) {
      System.out.print(status());
      Thread.yield();// yield:让步，对线程调度器的一种建议：我已经执行完生命周期中最重要的部分，此刻正是切换线程的好时机。
    }
  }

//BasicThreads.java
//执行
public class BasicThreads {
  public static void main(String[] args) {
    Thread t = new Thread(new LiftOff());
    t.start();
    System.out.println("Waiting for LiftOff");
  }
} 

/* Output: (90% match)
Waiting for LiftOff
#0(9), #0(8), #0(7), #0(6), #0(5), #0(4), #0(3), #0(2), #0(1), #0(Liftoff!),
*///:~

2. 使用Executor

Java SE5 的 java.util.conrurrnt包中的执行器（Executor）将为你管理Thread对象，从而简化并发编程。

ExcutorService：具有服务生命周期的Executor

//LiftOff.java
//定义线程任务类
public class LiftOff implements Runnable {
  protected int countDown = 10; // Default
  private static int taskCount = 0;
  private final int id = taskCount++;
  public LiftOff() {}
  public LiftOff(int countDown) {
    this.countDown = countDown;
  }
  public String status() {
    return "#" + id + "(" +
      (countDown > 0 ? countDown : "Liftoff!") + "), ";
  }
  public void run() {
    while(countDown-- > 0) {
      System.out.print(status());
      Thread.yield();//yield:屈服，对线程调度器的一种建议：我已经执行完生命周期中最重要的部分，此刻正是切换线程的好时机。
    }
  }

CachedThreadPool: 为每个任务创建一个线程

// CachedThreadPool.java
public class CachedThreadPool {
  public static void main(String[] args) {
    ExecutorService exec = Executors.newCachedThreadPool();
    for(int i = 0; i < 5; i++)
      exec.execute(new LiftOff());
    exec.shutdown();//防止新任务被提交给这个Executor
  }
} /* Output: (Sample)
#0(9), #0(8), #1(9), #2(9), #3(9), #4(9), #0(7), #1(8), #2(8), #3(8), #4(8), #0(6), #1(7), #2(7), #3(7), #4(7), #0(5), #1(6), #2(6), #3(6), #4(6), #0(4), #1(5), #2(5), #3(5), #4(5), #0(3), #1(4), #2(4), #3(4), #4(4), #0(2), #1(3), #2(3), #3(3), #4(3), #0(1), #1(2), #2(2), #3(2), #4(2), #0(Liftoff!), #1(1), #2(1), #3(1), #4(1), #1(Liftoff!), #2(Liftoff!), #3(Liftoff!), #4(Liftoff!),
*///:~

FixedThreadPool: 可一次性预先执行代价高昂的线程分配，从而可以限制线程数量。

// FixedThreadPool.java
public class FixedThreadPool {
  public static void main(String[] args) {
    // Constructor argument is number of threads:
    ExecutorService exec = Executors.newFixedThreadPool(5);
    for(int i = 0; i < 5; i++)
      exec.execute(new LiftOff());
    exec.shutdown();
  }
} /* Output: (Sample)
#0(9), #0(8), #1(9), #2(9), #3(9), #4(9), #0(7), #1(8), #2(8), #3(8), #4(8), #0(6), #1(7), #2(7), #3(7), #4(7), #0(5), #1(6), #2(6), #3(6), #4(6), #0(4), #1(5), #2(5), #3(5), #4(5), #0(3), #1(4), #2(4), #3(4), #4(4), #0(2), #1(3), #2(3), #3(3), #4(3), #0(1), #1(2), #2(2), #3(2), #4(2), #0(Liftoff!), #1(1), #2(1), #3(1), #4(1), #1(Liftoff!), #2(Liftoff!), #3(Liftoff!), #4(Liftoff!),
*///:~

SingleThreadExecutor: 线程数为1的FixedThreadPool。向SingleThreadExecutor提交多任务，这些任务会排队。

// SingleThreadExecutor.java
public class SingleThreadExecutor {
  public static void main(String[] args) {
    ExecutorService exec =
      Executors.newSingleThreadExecutor();
    for(int i = 0; i < 5; i++)
      exec.execute(new LiftOff());
    exec.shutdown();
  }
} /* Output:
#0(9), #0(8), #0(7), #0(6), #0(5), #0(4), #0(3), #0(2), #0(1), #0(Liftoff!), #1(9), #1(8), #1(7), #1(6), #1(5), #1(4), #1(3), #1(2), #1(1), #1(Liftoff!), #2(9), #2(8), #2(7), #2(6), #2(5), #2(4), #2(3), #2(2), #2(1), #2(Liftoff!), #3(9), #3(8), #3(7), #3(6), #3(5), #3(4), #3(3), #3(2), #3(1), #3(Liftoff!), #4(9), #4(8), #4(7), #4(6), #4(5), #4(4), #4(3), #4(2), #4(1), #4(Liftoff!),
*///:~

3. 从任务中返回值 (call)

Runnable是执行工作的独立任务，它无法返回值。此时，可以实现Callable接口，实现call(),并且必须使用ExecutorService.submit()调用它。

// CallableDemo.java
import java.util.concurrent.*;
import java.util.*;

class TaskWithResult implements Callable<String> {
  private int id;
  public TaskWithResult(int id) {
    this.id = id;
  }
  public String call() {
    return "result of TaskWithResult " + id;
  }
}

public class CallableDemo {
  public static void main(String[] args) {
    ExecutorService exec = Executors.newCachedThreadPool();
    ArrayList<Future<String>> results =
      new ArrayList<Future<String>>();
    for(int i = 0; i < 10; i++)
      results.add(exec.submit(new TaskWithResult(i)));
    for(Future<String> fs : results)
      try {
        // get() blocks until completion:
        System.out.println(fs.get());
      } catch(InterruptedException e) {
        System.out.println(e);
        return;
      } catch(ExecutionException e) {
        System.out.println(e);
      } finally {
        exec.shutdown();
      }
  }
} /* Output:
result of TaskWithResult 0
result of TaskWithResult 1
result of TaskWithResult 2
result of TaskWithResult 3
result of TaskWithResult 4
result of TaskWithResult 5
result of TaskWithResult 6
result of TaskWithResult 7
result of TaskWithResult 8
result of TaskWithResult 9
*///:~

submit() 会产生Future对象，它用Callable返回结果的特定类型进行参数化。可以使用isDone()来查询Future是否已完成。完成后可调用get()获取结果，否则，get()将阻塞，直到结果完成。

4. 休眠 (sleep)

//: SleepingTask.java
// Calling sleep() to pause for a while.
import java.util.concurrent.*;

public class SleepingTask extends LiftOff {
  public void run() {
    try {
      while(countDown-- > 0) {
        System.out.print(status());
        // Old-style:
        // Thread.sleep(100);
        // Java SE5/6-style:
        TimeUnit.MILLISECONDS.sleep(100);
      }
    } catch(InterruptedException e) {
      System.err.println("Interrupted");
    }
  }
  public static void main(String[] args) {
    ExecutorService exec = Executors.newCachedThreadPool();
    for(int i = 0; i < 5; i++)
      exec.execute(new SleepingTask());
    exec.shutdown();
  }
} /* Output:
#0(9), #1(9), #2(9), #3(9), #4(9), #0(8), #1(8), #2(8), #3(8), #4(8), #0(7), #1(7), #2(7), #3(7), #4(7), #0(6), #1(6), #2(6), #3(6), #4(6), #0(5), #1(5), #2(5), #3(5), #4(5), #0(4), #1(4), #2(4), #3(4), #4(4), #0(3), #1(3), #2(3), #3(3), #4(3), #0(2), #1(2), #2(2), #3(2), #4(2), #0(1), #1(1), #2(1), #3(1), #4(1), #0(Liftoff!), #1(Liftoff!), #2(Liftoff!), #3(Liftoff!), #4(Liftoff!),
*///:~

调用sleep可抛出InterruptedException。异常在run()中被捕获，因为异常不能跨线程转播到main()中，所以必须本地处理。但也可以使用Excutor的异常捕获。

5. 优先级 (priorities)

线程优先级将该线程的重要性传递给了调度器，优先级仅仅是执行频率的高低。
通常任务都有默认的优先级。

你可以使用getPriority()读取优先级，并用setPriority()来修改他；

//: SimplePriorities.java
// Shows the use of thread priorities.
import java.util.concurrent.*;

public class SimplePriorities implements Runnable {
  private int countDown = 5;
  private volatile double d; // No optimization
  private int priority;
  public SimplePriorities(int priority) {
    this.priority = priority;
  }
  public String toString() {
    return Thread.currentThread() + ": " + countDown;
  }
  public void run() {
    Thread.currentThread().setPriority(priority);
    while(true) {
      // An expensive, interruptable operation:
      for(int i = 1; i < 100000; i++) {
        d += (Math.PI + Math.E) / (double)i;
        if(i % 1000 == 0)
          Thread.yield();
      }
      System.out.println(this);
      if(--countDown == 0) return;
    }
  }
  public static void main(String[] args) {
    ExecutorService exec = Executors.newCachedThreadPool();
    for(int i = 0; i < 5; i++)
      exec.execute(
        new SimplePriorities(Thread.MIN_PRIORITY));
    exec.execute(
        new SimplePriorities(Thread.MAX_PRIORITY));
    exec.shutdown();
  }
} /* Output: (70% match)
Thread[pool-1-thread-6,10,main]: 5
Thread[pool-1-thread-6,10,main]: 4
Thread[pool-1-thread-6,10,main]: 3
Thread[pool-1-thread-6,10,main]: 2
Thread[pool-1-thread-6,10,main]: 1
Thread[pool-1-thread-3,1,main]: 5
Thread[pool-1-thread-2,1,main]: 5
Thread[pool-1-thread-1,1,main]: 5
Thread[pool-1-thread-5,1,main]: 5
Thread[pool-1-thread-4,1,main]: 5
...
*///:~

尽管JDK有10个优先级，但它与大多数操作系统都不能映射好。通常只使用MIN_PRIORITY 、NORM_PRIORITY、 MAX_PRIORITY 三种级别。

6. 让步 (yield)

通过yield()方法暗示cpu，建议具有相同优先级的其他线程可以运行。

7. 后台线程 (daemon)

所谓后台线程，就是指在程序运行的时候在后台提供的一种通用的服务线程，且这种线程不属于程序中不可获取的部分。因此，所以的非后台线程结束时，程序就终结了，同时会杀死所以后台线程。反过来说，只要有任何非后台线程在运行，程序就不会终止。

//: SimpleDaemons.java
// Daemon threads don't prevent the program from ending.
import java.util.concurrent.*;

public class SimpleDaemons implements Runnable {
  public void run() {
    try {
      while(true) {
        TimeUnit.MILLISECONDS.sleep(100);
        System.out.println(Thread.currentThread() + " " + this);
      }
    } catch(InterruptedException e) {
      System.out.println("sleep() interrupted");
    }
  }
  public static void main(String[] args) throws Exception {
    for(int i = 0; i < 10; i++) {
      Thread daemon = new Thread(new SimpleDaemons());
      daemon.setDaemon(true); // Must call before start()
      daemon.start();
    }
    System.out.println("All daemons started");
    TimeUnit.MILLISECONDS.sleep(175);
  }
} /* Output: (Sample)
All daemons started
Thread[Thread-0,5,main] SimpleDaemons@530daa
Thread[Thread-1,5,main] SimpleDaemons@a62fc3
Thread[Thread-2,5,main] SimpleDaemons@89ae9e
Thread[Thread-3,5,main] SimpleDaemons@1270b73
Thread[Thread-4,5,main] SimpleDaemons@60aeb0
Thread[Thread-5,5,main] SimpleDaemons@16caf43
Thread[Thread-6,5,main] SimpleDaemons@66848c
Thread[Thread-7,5,main] SimpleDaemons@8813f2
Thread[Thread-8,5,main] SimpleDaemons@1d58aae
Thread[Thread-9,5,main] SimpleDaemons@83cc67
...
*///:~

必须在线程启动之前调用setDaemon()方法，才能将它设置为后台线程。

//: Daemons.java
// Daemon threads spawn other daemon threads.
import java.util.concurrent.*;

class Daemon implements Runnable {
  private Thread[] t = new Thread[10];
  public void run() {
    for(int i = 0; i < t.length; i++) {
      t[i] = new Thread(new DaemonSpawn());
      t[i].start();
      System.out.println("DaemonSpawn " + i + " started, ");
    }
    for(int i = 0; i < t.length; i++)
      System.out.println("t[" + i + "].isDaemon() = " +
        t[i].isDaemon() + ", ");
    while(true)
      Thread.yield();
  }
}

class DaemonSpawn implements Runnable {
  public void run() {
    while(true)
      Thread.yield();
  }
}

public class Daemons {
  public static void main(String[] args) throws Exception {
    Thread d = new Thread(new Daemon());
    d.setDaemon(true);
    d.start();
    System.out.println("d.isDaemon() = " + d.isDaemon() + ", ");
    // Allow the daemon threads to
    // finish their startup processes:
    TimeUnit.SECONDS.sleep(1);
  }
} /* Output: (Sample)
d.isDaemon() = true, DaemonSpawn 0 started, DaemonSpawn 1 started, DaemonSpawn 2 started, DaemonSpawn 3 started, DaemonSpawn 4 started, DaemonSpawn 5 started, DaemonSpawn 6 started, DaemonSpawn 7 started, DaemonSpawn 8 started, DaemonSpawn 9 started, t[0].isDaemon() = true, t[1].isDaemon() = true, t[2].isDaemon() = true, t[3].isDaemon() = true, t[4].isDaemon() = true, t[5].isDaemon() = true, t[6].isDaemon() = true, t[7].isDaemon() = true, t[8].isDaemon() = true, t[9].isDaemon() = true,
*///:~

后台线程创建的线程都是后台线程。

后台线程不会执行finally子句。因为该线程是随进程戛然而止的。

8.隐式设置全局线程参数

通过编写定制的ThreadFactory（实现接口ThreadFactory的newThread()方法）可以定制由Executor创建的线程的属性（后台，优先级，名称……）。

//: DaemonFromFactory.java
// Using a Thread Factory to create daemons.
import java.util.concurrent.*;

public class DaemonFromFactory implements Runnable {
  public void run() {
    try {
      while(true) {
        TimeUnit.MILLISECONDS.sleep(100);
        System.out.println(Thread.currentThread() + " " + this);
      }
    } catch(InterruptedException e) {
      System.out.println("Interrupted");
    }
  }
  public static void main(String[] args) throws Exception {
    ExecutorService exec = Executors.newCachedThreadPool(
      new DaemonThreadFactory());
    for(int i = 0; i < 10; i++)
      exec.execute(new DaemonFromFactory());
    System.out.println("All daemons started");
    TimeUnit.MILLISECONDS.sleep(500); // Run for a while
  }
}

class DaemonThreadFactory implements ThreadFactory {
  public Thread newThread(Runnable r) {
    Thread t = new Thread(r);
    t.setDaemon(true);
    return t;
  }
}
/* (Execute to see output) *///:~

9. 编码的变体 (extends Thread)

在简单情况下，直接从Thread继承

//: SimpleThread.java
// Inheriting directly from the Thread class.

public class SimpleThread extends Thread {
  private int countDown = 5;
  private static int threadCount = 0;
  public SimpleThread() {
    // Store the thread name:
    super(Integer.toString(++threadCount));
    start();
  }
  public String toString() {
    return "#" + getName() + "(" + countDown + "), ";
  }
  public void run() {
    while(true) {
      System.out.print(this);
      if(--countDown == 0)
        return;
    }
  }
  public static void main(String[] args) {
    for(int i = 0; i < 5; i++)
      new SimpleThread();
  }
} /* Output:
#1(5), #1(4), #1(3), #1(2), #1(1), #2(5), #2(4), #2(3), #2(2), #2(1), #3(5), #3(4), #3(3), #3(2), #3(1), #4(5), #4(4), #4(3), #4(2), #4(1), #5(5), #5(4), #5(3), #5(2), #5(1),
*///:~

10. 加入一个线程 (join)

一个线程可以在其他线程上调用join()，效果是等待一段时间直到第二个线程结束才执行。如果线程t2在线程t1上调用t1.join()，t2将被挂起，直到目标线程t1结束才恢复。

也可以在join()中带上一个超时参数，如果目标线程t1在这段时间内未结束，join()总能返回。

在调用线程t1上调用t1.interrupt() 中断 joint()。

//: Joining.java
// Understanding join().

class Sleeper extends Thread {
  private int duration;
  public Sleeper(String name, int sleepTime) {
    super(name);
    duration = sleepTime;
    start();
  }
  public void run() {
    try {
      sleep(duration);
    } catch(InterruptedException e) {
      System.out.println(getName() + " was interrupted. " +
        "isInterrupted(): " + isInterrupted());
      return;
    }
    System.out.println(getName() + " has awakened");
  }
}

class Joiner extends Thread {
  private Sleeper sleeper;
  public Joiner(String name, Sleeper sleeper) {
    super(name);
    this.sleeper = sleeper;
    start();
  }
  public void run() {
   try {
      sleeper.join();
    } catch(InterruptedException e) {
     System.out.println("Interrupted");
    }
    System.out.println(getName() + " join completed");
  }
}

public class Joining {
  public static void main(String[] args) {
    Sleeper
      sleepy = new Sleeper("Sleepy", 1500),
      grumpy = new Sleeper("Grumpy", 1500);
    Joiner
      dopey = new Joiner("Dopey", sleepy),
      doc = new Joiner("Doc", grumpy);
    grumpy.interrupt();
  }
} /* Output:
Grumpy was interrupted. isInterrupted(): false
Doc join completed
Sleepy has awakened
Dopey join completed
*///:~

11. 捕获异常 (CaptureUncaughtException)

java SE5 后，可以用Executor捕获异常。为了捕获异常，我们修改Executor产生线程的方式。Thread.UncaughtExceptionHandler 接口允许你在每个Thread对象上附着一个异常处理器。Thread.UncaughtExceptionHandler.uncaughtException()会在线程因未捕获的异常而临近死亡时被调用。

//: CaptureUncaughtException.java
import java.util.concurrent.*;

class ExceptionThread2 implements Runnable {
  public void run() {
    Thread t = Thread.currentThread();
    System.out.println("run() by " + t);
    System.out.println(
      "eh = " + t.getUncaughtExceptionHandler());
    throw new RuntimeException();
  }
}

class MyUncaughtExceptionHandler implements
Thread.UncaughtExceptionHandler {
  public void uncaughtException(Thread t, Throwable e) {
    System.out.println("caught " + e);
  }
}

class HandlerThreadFactory implements ThreadFactory {
  public Thread newThread(Runnable r) {
    System.out.println(this + " creating new Thread");
    Thread t = new Thread(r);
    System.out.println("created " + t);
    t.setUncaughtExceptionHandler(
      new MyUncaughtExceptionHandler());
    System.out.println(
      "eh = " + t.getUncaughtExceptionHandler());
    return t;
  }
}

public class CaptureUncaughtException {
  public static void main(String[] args) {
    ExecutorService exec = Executors.newCachedThreadPool(
      new HandlerThreadFactory());
    exec.execute(new ExceptionThread2());
  }
} /* Output: (90% match)
HandlerThreadFactory@de6ced creating new Thread
created Thread[Thread-0,5,main]
eh = MyUncaughtExceptionHandler@1fb8ee3
run() by Thread[Thread-0,5,main]
eh = MyUncaughtExceptionHandler@1fb8ee3
caught java.lang.RuntimeException
*///:~

如果要在代码中处处使用相同的异常处理器，那就在Thread类中设置一个静态域，并把这个处理器设置为默认的未捕获异常处理器：

//: SettingDefaultHandler.java
import java.util.concurrent.*;

public class SettingDefaultHandler {
  public static void main(String[] args) {
    Thread.setDefaultUncaughtExceptionHandler(
      new MyUncaughtExceptionHandler());
    ExecutorService exec = Executors.newCachedThreadPool();
    exec.execute(new ExceptionThread());
  }
} /* Output:
caught java.lang.RuntimeException
*///:~

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