Java Concurrency 1:Basic

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基本概念

程序、进程、线程
程序是指令、数据及其组织形式的描述。现代操作系统在运行一个程序时，会为其创建一个进程。例如启动一个Java程序就会创建一个Java进程，用jps指令可以查看所有的Java进程。
现代操作系统调度的最小单元是线程，也叫轻量级进程，一个进程里可以创建多个线程，进程是线程的容器。

Java中线程实现方式有两种：
1. 直接继承Thread类（Thread类也是实现了Runnable接口）；
2. 先实现Runnable接口，然后将类传给Thread执行；
实际应用采取该方式：
① 避免由于Java的单继承特性带来的局限；
② 将任务机制和执行机制分离开，属于比较好的设计；

开启线程前设置好线程的名称，方便后续用jstack之类的工具分析，养成良好的规范意识。

线程间通信

线程的几个状态，在Thread类中有枚举：

new
runnable
java线程将就绪和运行两种状态笼统的称作runnable
blocked
waiting
timed-waiting
terminated

如下是源码：

public enum State { /** * Thread state for a thread which has not yet started. */ NEW, /** * Thread state for a runnable thread. A thread in the runnable * state is executing in the Java virtual machine but it may * be waiting for other resources from the operating system * such as processor. */ RUNNABLE, /** * Thread state for a thread blocked waiting for a monitor lock. * A thread in the blocked state is waiting for a monitor lock * to enter a synchronized block/method or * reenter a synchronized block/method after calling * {@link Object#wait() Object.wait}. */ BLOCKED, /** * Thread state for a waiting thread. * A thread is in the waiting state due to calling one of the * following methods: *

{@link Object#wait() Object.wait} with no timeout
{@link #join() Thread.join} with no timeout
{@link LockSupport#park() LockSupport.park}

* *

A thread in the waiting state is waiting for another thread to * perform a particular action. * * For example, a thread that has called Object.wait() * on an object is waiting for another thread to call * Object.notify() or Object.notifyAll() on * that object. A thread that has called Thread.join() * is waiting for a specified thread to terminate. */ WAITING, /** * Thread state for a waiting thread with a specified waiting time. * A thread is in the timed waiting state due to calling one of * the following methods with a specified positive waiting time: *

{@link #sleep Thread.sleep}
{@link Object#wait(long) Object.wait} with timeout
{@link #join(long) Thread.join} with timeout
{@link LockSupport#parkNanos LockSupport.parkNanos}
{@link LockSupport#parkUntil LockSupport.parkUntil}

*/ TIMED_WAITING, /** * Thread state for a terminated thread. * The thread has completed execution. */ TERMINATED; }

Java线程状态变迁

由该图可以看出有些方法是属于根类Object的，有些是属于Thread类的。
执行Thread.sleep(long)和Object.wait(long)方法后都会进入TIMED_WAIT状态，但是sleep()方法不会释放锁，而wait会让出锁，具体源码中都有说明，一看便知。
这里线程终止是等待run()方法执行完成后自动进入TERMINATED状态，那有没有什么方式可以停止线程呢？已经被标记为deprecated的stop()方法自然不能再用，因为该方法会造成多线程下的数据不一致性问题。
书中给出两种比较“优雅”的方式来停止线程。

public class Shutdown {    public static void main(String[] args) throws Exception {        Runner one = new Runner();        Thread countThread = new Thread(one, "CountThread");        countThread.start();        // 睡眠1秒，main线程对CountThread进行中断，使CountThread能够感知中断而结束        TimeUnit.SECONDS.sleep(1);        countThread.interrupt();        Runner two = new Runner();        countThread = new Thread(two, "CountThread");        countThread.start();        // 睡眠1秒，main线程对Runner two进行取消，使CountThread能够感知on为false而结束        TimeUnit.SECONDS.sleep(1);        two.cancel();    }    private static class Runner implements Runnable {        private long             i;     /**         *  这里用volatile是要保证可见性吗         */        private volatile boolean on = true;        @Override        public void run() {            while (on && !Thread.currentThread().isInterrupted()) {                i++;            }            System.out.println("Count i = " + i);        }        public void cancel() {            on = false;        }    }}

参考：
http://www.cnblogs.com/freemanabc/p/5618159.html
http://www.importnew.com/16142.html
《Java并发编程的艺术》

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