线程创建与终止

线程创建

Thread类与Runnable接口的关系

public interface Runnable {
　　public abstract void run();
}public class Thread implements Runnable {
    /* What will be run. */
　　private Runnable target;
　　......
　　/**
　　 * Causes this thread to begin execution; the Java Virtual Machine
 　　* calls the <code>run</code> method of this thread.
 　　*/
　　public synchronized void start() {......}　　......
    @Override
    public void run() {
        if (target != null) {
            target.run();
        }
　　}
　　......
}

Thread类与Runnable接口都位于java.lang包中。从上面我们可以看出，Runnable接口中只定义了run()方法，Thread类实现了Runnable 接口并重写了run()方法。当调用Thread 类的start()方法时，实际上Java虚拟机就去调用Thread 类的run()方法，而Thread 类的run()方法中最终调用的是Runnable类型对象的run()方法。

继承Thread并重写run方法

public class ThreadTest1 extends Thread {
    @Override
    public void run() {
        while(true) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("thread 1:" + Thread.currentThread().getName());
        }
    }    public static void main(String[] args) {
        ThreadTest1 thread = new ThreadTest1 ();
        thread.start();
    }//main end
}

可以写成内部类的形式，new Thread(){@Override run(…)}.start();

实现Runnable接口并重写run方法

public class ThreadTest2  implements Runnable {
    @Override
    public void run() {
        while(true) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("thread 3:" + Thread.currentThread().getName());
        }
    }
    public static void main(String[] args) {
        ThreadTest2  thread3 = new ThreadTest2();
        Thread thread = new Thread(thread3);
        thread.start();
    }//main end
}

可以写成内部类的形式，new Thread(new Runnable(){@Override run(…)}).start();

线程终止

当调用Thread类的start()方法时，将会创建一个线程，这时刚创建的线程处于就绪状态（可运行状态），并没有运行，处于就绪状态的线程就可以等JVM调度。当JVM调度该线程时，该线程进入运行状态，即执行Thread类的run()方法中的内容。run()方法执行完，线程结束，线程进入死亡状态。这是线程自然终止的过程，我们也可以通过Thread类提供的一些方法来终止线程。

interrupt()\isInterrupted()\interrupted()方法介绍

stop()方法没有做任何的清除操作就粗暴终止线程，释放该线程所持有的对象锁（下文将介绍），受该对象锁保护的其它对象对其他线程可见，因此具有不安全性。

suspend()方法会使目标线程会停下来，但仍然持有在这之前获得的对象锁，对任何线程来说，如果它们想恢复目标线程，同时又试图使用任何一个锁定的资源，就会造成死锁。

终上所述，不建议使用stop()方法和suspend()方法来终止线程，通常我们通过interrupt()方法来终止处于阻塞状态和运行状态的线程。

需要注意的是，interrupt()方法不会中断一个正在运行的线程，仅仅是将线程的中断标记设为true，当调用了阻塞方法之后，线程会不断监听中断标志，如果为true，则产生一个InterruptedException异常，将InterruptedException放在catch中就能终止线程。

isInterrupted()方法可以返回中断标记，常用循环判断条件。

interrupted()方法测试当前线程是否已经中断，线程的中断标志由该方法清除。interrupted()除了返回中断标记之外，它还会清除中断标记。

interrupt()用法

看下面例子

public class ThreadInterruptedTest extends Thread {
    @Override
    public void run() {
            try {
                int i = 0;
                while(!isInterrupted()) {
                    i ++ ;
                    Thread.sleep(1000);
                    System.out.println(this.getName() + " is looping,i=" + i);
                }
            } catch (InterruptedException e) {
                System.out.println(this.getName() +
                        " catch InterruptedException,state:" + this.getState());
                e.printStackTrace();
            }
    }    public static void main(String[] args) throws Exception {        ThreadInterruptedTest thread = new ThreadInterruptedTest();
        System.out.println(thread.getName()
                + " state:" + thread.getState());          thread.start();
        System.out.println(thread.getName()
                + " state:" + thread.getState());          Thread.sleep(5000);        System.out.println("flag: " + thread.isInterrupted());        //发出中断指令
        thread.interrupt();        System.out.println("flag: " + thread.isInterrupted());        System.out.println(thread.getName()
                + " state:" + thread.getState());          System.out.println(thread.interrupted());
    }
}

运行结果

Thread-0 state:NEW
Thread-0 state:RUNNABLE
Thread-0 is looping,i=1
Thread-0 is looping,i=2
Thread-0 is looping,i=3
Thread-0 is looping,i=4
flag: false
flag: true
Thread-0 state:TIMED_WAITING
Thread-0 catch InterruptedException,state:RUNNABLE
false
java.lang.InterruptedException: sleep interrupted
    at java.lang.Thread.sleep(Native Method)
    at com.itpsc.thread.ThreadInterruptedTest.run(ThreadInterruptedTest.java:11)

从运行结果可以看出，调用interrupt() 发出中断指令前，中断标志位false，发出中断指令后中断标志位为true，而调用interrupted()方法后则中断标志被清除。从发出的异常来看，是在一个sleep interrupted，且发出异常后线程被唤醒，以便线程能从异常中正常退出。

线程运行状态图

线程从创建到终止可能会经历各种状态。在Java基础加强之集合篇（模块记忆、精要分析））。而ThreadLocalMap中的Entry 继承了WeakReference，弱引用是不能保证不被垃圾回收器回收的，这就是前文提到的在线程消失之后，线程局部变量的所有副本都会被垃圾回收。此外，Entry 中使用ThreadLocal作为key，线程局部变量作为value。如果threadLocals不为空，则设值否者调用createMap方法创建threadLocals。注意设值的时候传的是this而不是当前线程t。

/**
 * ThreadLocalMap is a customized hash map suitable only for
 * maintaining thread local values. No operations are exported
 * outside of the ThreadLocal class. The class is package private to
 * allow declaration of fields in class Thread.  To help deal with
 * very large and long-lived usages, the hash table entries use
 * WeakReferences for keys. However, since reference queues are not
 * used, stale entries are guaranteed to be removed only when
 * the table starts running out of space.
 */
static class ThreadLocalMap {    /**
     * The entries in this hash map extend WeakReference, using
     * its main ref field as the key (which is always a
     * ThreadLocal object).  Note that null keys (i.e. entry.get()
     * == null) mean that the key is no longer referenced, so the
     * entry can be expunged from table.  Such entries are referred to
     * as "stale entries" in the code that follows.
     */
    static class Entry extends WeakReference<ThreadLocal> {
        /** The value associated with this ThreadLocal. */
        Object value;        Entry(ThreadLocal k, Object v) {
            super(k);
            value = v;
        }
    } 

接下来我们看看createMap方法

/**
 * Create the map associated with a ThreadLocal. Overridden in
 * InheritableThreadLocal.
 *
 * @param t the current thread
 * @param firstValue value for the initial entry of the map
 * @param map the map to store.
 */
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
} 

createMap方法其实就是为当前线程的threadLocals变量分配空间并存储线程的第一个变量。现在我们已经知道线程是如何初始化并设值自己的局部变量了，下面我们看看取值。

Get

/**
 * Returns the value in the current thread's copy of this
 * thread-local variable.  If the variable has no value for the
 * current thread, it is first initialized to the value returned
 * by an invocation of the {@link #initialValue} method.
 *
 * @return the current thread's value of this thread-local
 */
public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null)
            return (T)e.value;
    }
    return setInitialValue();
}  

先获取当前的线程，然后通过getMap(t)方法获取当前线程存变量的对象threadLocals，如果threadLocals不为空则取值并返回（注意传入的key是this对象而不是当前线程t），否则调用setInitialValue方法初始化。setInitialValue和set方法唯一不同的是调用了initialValue进行初始化，也就是在获取变量之前要初始化。

/**
 * Variant of set() to establish initialValue. Used instead
 * of set() in case user has overridden the set() method.
 *
 * @return the initial value
 */
private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}  

总的来讲，每创建一个线程（Thread对象），该线程即拥有存储线程本地变量的threadLocals对象，threadLocals对象初始为null，当通过ThreadLocal对象调用set/get方法时，就会对线程的threadLocals对象进行初始化，并且以当前ThreadLocal对象为键值，以ThreadLocal要保存的变量为value，存到threadLocals。看下面的例子。

ThreadLocal应用

public class ThreadLocalShareVariable {    public static void main(String[] args) {
        //创建3个线程
        for(int i=0; i<3;i++) {
            //创建线程
            new Thread(new Runnable(){
                @Override
                public void run() {
                    //线程设置自己的变量
                    int age = new Random().nextInt(100);
                    String name = getRandomString(5);
                    System.out.println("Thread " + Thread.currentThread().getName()
                            + " has put data:" + name + " " + age);                    //存储与当前线程有关的变量
                    Passenger.getInstance().setName(name);
                    Passenger.getInstance().setAge(age);                    //线程访问共享变量
                    new ModuleA().getData();
                    new ModuleB().getData();
                }
            }).start();
        }
    }    static class ModuleA {
        public void getData(){
            //获取与当前线程有关的变量
            String name = Passenger.getInstance().getName();
            int data = Passenger.getInstance().getAge();
            System.out.println("moduleA get data from "
            + Thread.currentThread().getName() + ":" + name + " "+ data);
        }
    }    static class ModuleB {
        public void getData(){
            //获取与当前线程有关的变量
            String name = Passenger.getInstance().getName();
            int data = Passenger.getInstance().getAge();
            System.out.println("moduleB get data from "
            + Thread.currentThread().getName() + ":" + name + " "+ data);
        }
    }    /**
     * 随机生成字符串
     * @param length
     * @return
     */
    public static String getRandomString(int length){
        final String str = "abcdefghijklmnopqrstuvwxyz";
        StringBuffer sb = new StringBuffer();
        int len = str.length();
        for (int i = 0; i < length; i++) {
            sb.append(str.charAt(
                    (int) Math.round(Math.random() * (len-1))));
        }
        return sb.toString();
    }}class Passenger {
    private String name;
    private int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public Passenger(){}    //ThreadLocal存储线程变量
    public static ThreadLocal<Passenger> thsd = new ThreadLocal<Passenger>();    public static Passenger getInstance() {
        //获取当前线程范围内的共享变量实例
        Passenger passenger = thsd.get();
        //懒汉模式创建实例
        if(passenger == null) {
            passenger = new Passenger();
            thsd.set(passenger);
        }
        return passenger;
    }}

运行结果

Thread Thread-1 has put data:vwozg 33
Thread Thread-2 has put data:hubdn 30
Thread Thread-0 has put data:mkwrt 35
moduleA get data from Thread-2:hubdn 30
moduleA get data from Thread-0:mkwrt 35
moduleA get data from Thread-1:vwozg 33
moduleB get data from Thread-1:vwozg 33
moduleB get data from Thread-0:mkwrt 35
moduleB get data from Thread-2:hubdn 30

创建3个线程，每个线程要保存一个Passenger 对象，并且通过ModuleA 、ModuleB来访问每个线程对应保存的Passenger 对象。

多线程之间共享变量

上面我们讨论的是多线程之间如何访问自己的变量。那么多线程之间共享变量时如何的呢，看下的例子，线程1对共享变量进行减一操作，线程2对共享变量进行加2操作。

public class MutilThreadShareVariable {
    static volatile int count = 100;
    public static void main(String[] args) throws Exception{
        final ShareDataDec sdDec = new ShareDataDec();
        final ShareDataInc sdInc = new ShareDataInc();
        //线程1
        new Thread(new Runnable() {
            @Override
            public void run() {
                for(int i=0;i<5;i++) {
                    sdDec.dec();
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();
        //线程2
        new Thread(new Runnable(){
            @Override
            public void run() {
                for(int i=0;i<5;i++) {
                    sdInc.inc();
                    try {
                        Thread.sleep(1000);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }).start();;
    }    static class ShareDataDec {
        public synchronized void dec() {
            count --;
            System.out.println("Thread " + Thread.currentThread().getName()
                    + " dec 1 from count,count remain " + count);
        }
    }    static class ShareDataInc {
        public synchronized void inc() {
            count = count + 2;
            System.out.println("Thread " + Thread.currentThread().getName()
                    + " inc 2 from count,count remain " + count);
        }
    }
}

运行结果

Thread Thread-0 dec 1 from count,count remain 99
Thread Thread-1 inc 2 from count,count remain 101
Thread Thread-0 dec 1 from count,count remain 100
Thread Thread-1 inc 2 from count,count remain 102
Thread Thread-0 dec 1 from count,count remain 101
Thread Thread-1 inc 2 from count,count remain 103
Thread Thread-0 dec 1 from count,count remain 102
Thread Thread-1 inc 2 from count,count remain 104
Thread Thread-0 dec 1 from count,count remain 103
Thread Thread-1 inc 2 from count,count remain 105

线程共享变量，只要对要对共享变量进行修改的代码进行同步即可。