JAVA 多线程设计模式

JAVA 多线程设计模式
^^^^^^^^^^^^^^^^^^^
- 作者：臭豆腐[trydofor.com]
- 日期：2008-12-24
- 授权：署名-非商业-保持一致 1.0 协议
- 声明：拷贝、分发、呈现和表演本作品，请保留以上全部信息。

0. 文档目录
^^^^^^^^^^
[[<=$INDEX]]

1. 适用范围与知识背景
^^^^^^^^^^^^^^^^^^^^^
本文是对"Java多线程设计模式",中国铁道出版社 (ISBN 7-113-06402-7)
一书的学习备忘,适用于java1.4.
实际项目中应该尽量回避多线程,如果必须使用,最好使用DougLee大师的杰作.

让我也具有DougLee一样能执行多线程的计算机头脑吧

2. Thread 状态图
^^^^^^^^^^^^^^^^
================= txt figure-1 : Thread Status =================
       (_)                Thread Status /jdk1.4
        | new          www.trydofor.com 2008-12-21
  +------------+      =============================  
  |  initial   |       * Object's method/status      
  +-----+------+       @ Thread's method/status      
        | @start()     synchronized can not timeout  
  +------------+                                     
  | executable |<------------------------------------+
  +-+----------+   @sleep() +----------+ @timeout    |
    |        ^    +-------->|@sleeping +------------>|
    | @yield |    |         +----------+ @interrupt()|
    v        |    |                                  |
  +----------+-+  |*wait()  +----------+             |
  |  running   |--+-------->|*wait-set | @timeout    |
  +-----+------+  |         +----+-----+             |
        |         |              | *notify/All()     |
        | @run()  |              | @interrupt()      |
        |         | synch-       v                   |
        v         | ronized +----------+  acquire    |
  +------------+  +-------->|*race-set |------------>|
  |   final    |  |         +----------+             |
  +-----+------+  |         +----------+  done       |
        | sys gc  +-------->| blocking +------------>|
       (8)        block-io  +----------+
================================================================

3. 脆弱独木桥(SingleThreadExecution)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
脆弱的独木桥,又称危险区(Critical Section),只能承重一个人,
因此一次只能允许一个线程执行.
当共享资源可以被多个线程访问,且资源状态会改变时,就可以考虑该模式.

对危险区的划分是重点,以人脑模拟电脑思考程序的执行过程.
同时要注意死锁和继承等对安全性的破坏.

如果看到 synchronized 相互嵌套,基本上就会死锁了.
以下代码和Thread.sleep()很像 :0

================== java :ThreadSleep.java ======================
public class ThreadSleep {
    public static void sleep(long ms) throws InterruptedException{
        if(ms>0){
            Object obj = new Object();
            synchronized(obj){
                obj.wait(ms);
            }
        }
    }
}
================================================================

4. 只许参观不许摸(Immutable)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^
像java.lang.String一样,实例一旦产生,状态就不再改变.
对于一个只读的东东,即便同时有无数个线程访问,也没什么问题.

final class/method/field/parameter 可以很大的提高安全性.

值得注意的是,不要出现以下代码中的问题,错把 mutable 数据暴露出去.

====================== java : Mutable.java =====================
public final class Mutable {

    private final StringBuffer sb;
   
    public Mutable(String str){
        sb = new StringBuffer(str);
    }
   
    public StringBuffer getText(){
        return sb;
    }
}
================================================================

5. 不见不散(GuardedSuspension)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
如果条件不满足,就一直等,不见不散.
便于记忆,可以理解成多线程版的 if

====================== java : Queue.java =======================
import java.util.LinkedList;

public final class Queue {
    private final LinkedList queue = new LinkedList();
   
    public synchronized Object get(){ // 为什么同步this
        while(queue.size()<=0){ // 为什么不用If
            try{
                wait(); // 为什么不是Thread.sleep()
            }catch(InterruptedException e){
                // can not be interrupted
            }
        }
        return queue.removeFirst();
    }
   
    public synchronized void put(Object obj){
        queue.addLast(obj);
        notifyAll();
    }
}
================================================================

6. 你们聊,我有事先走了(Balking)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
不见不散固然为人所敬仰,但不是所有时候都需要不见不散.比如,

* 不需要去执行时.
   比如我经常神经性的Ctrl+s,但编辑器只有当文件变更时,才真正保持.
* 不想等到条件成立时.
   小兔说:我妈妈叫我小兔兔.
   小猪说:我妈妈叫我小猪猪.
   小鸡说:你们聊,我有事先走了!
* 只能执行一次时.
   发工资了,如果能多领几份该多好啊

把Balking的状态通知给调用者的方式.
* 忽略balk的发生.
* 以返回值表达,比如true/false
* 以异常的形式通知.

======================= java : Data.java =======================
public abstract class Data {

    private boolean changed;
    public final synchronized void change(){
        changed = true;
    }
    public final synchronized void save(){
        if(!changed) return;
       
        doSomething();
        changed = false;
    }
    protected abstract void doSomething();
}
================================================================

7. 生产者消费者(ProducerConsumer)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
生产者和消费者在不同的线程上运行,一般情况下,两者的处理速度存在很大差异.
因此在两者之间存在一个隐藏的"桥梁"来缓解这样的差异.管道(pipe)模式,是该
模式的简化形式.

所有参与者,
* Data 被传来传去的东西.
* Producer 生产Data的家伙.
* Customer 消灭Data的家伙.
* Channel  以上三者的纽带,是传送,调度的核心.

为什么Producer不能直接把Data给Customer呢?
因为这样的话,就是Producer线程来完成Customer的动作了.

参与者多了,有必要设置超时和可取消.好的习惯是让Chanel互斥的方法 throws
InterruptedException,隐式的告诉调用者,该方法具有以下含义,
* 我可能很耗时
* 我可以被取消
* 我可能调用了Object.wait(),Thread.sleep(),Thread.join().
   注意,Object.wait()被interrupt的时候,需要重新获得锁,才会被执行.

====================== java : Channel.java =====================
import java.util.LinkedList;

public class Channel {
    private final int capacity;
    private final LinkedList datas;
   
    public Channel(int capacity){
        this.capacity = capacity;
        this.datas = new LinkedList();
    }
   
    public synchronized void put(Object obj) throws InterruptedException{
        while(datas.size()>=capacity){
            wait();
        }
        datas.addLast(obj);
        notifyAll();
    }
   
    public synchronized Object get() throws InterruptedException{
        while(datas.size()<= 0){
            wait();
        }
       
        Object obj = datas.removeFirst();
        notifyAll();
        return obj;
    }
}
================================================================

8. 读写锁(ReadWriteLock)
^^^^^^^^^^^^^^^^^^^^^^^^
为了解决以下两种冲突,
* 读写(read-write conflict)
* 写写(write-write conflict)

在read和write的时候,都采用以下的结构,
===================== java : Example.java ======================
    ReadWriteLock lock = new ReadWriteLock();
   
    public String read() throws InterruptedException{
        lock.readLock(); // 为什么不可以在try内呢?
        try{
            return doRead();
        }finally{
            lock.readUnlock();
        }
    }
   
    public void write(String str) throws InterruptedException{
        lock.writeLock();
        try{
            doWrite(str);
        }finally{
            lock.writeUnlock();
        }
    }
================================================================

================== java : ReadWriteLock.java ===================
public final class ReadWriteLock {
    private int readingReaders = 0;
    private int waitingWriters = 0;      // 不计数会有什么问题?
    private int writingWriters = 0;
    private boolean preferWriter = true; // 为什么添加这个标记?
   
    public synchronized void readLock() throws InterruptedException{
        while(writingWriters>0 || (preferWriter && waitingWriters>0)){
            wait();
        }
        readingReaders++;
    }
   
    public synchronized void readUnlock() throws InterruptedException{
        readingReaders --;
        preferWriter = true;
        notifyAll(); // 可能唤醒哪些线程(Reader/Writer)?
    }
   
    public synchronized void writeLock() throws InterruptedException{
        waitingWriters ++;
        try{
            while(readingReaders>0|| writingWriters>0){
                wait();
            }
        }finally{
            waitingWriters --;
        }
        writingWriters ++;
    }
   
    public synchronized void writeUnlock() throws InterruptedException{
        writingWriters --;
        preferWriter = false;
        notifyAll();  // 可能唤醒哪些线程(Reader/Writer)?
    }
}
================================================================

9. 临时工/合同工(ThreadPerMessage/WokerThread)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
这两种模式的基本特点,就是把工作交给其他的线程去处理,
把调用(invocation)和执行(execution)放在不同个线程中,以提高响应能力.

临时工相当灵活,呼之即来,挥之即去,不过缺点是浪费资源,不便控制.
合同工则可以弥补上述缺点,让工作持续稳定.

一般来讲,多使用线程池技术,和既定的安全框架,不多累述.来道头脑风暴吧,
====================== java : Test.java ========================
public class Test {
    public void doOther(final Object obj){
        new Thread(){
            public void run(){
                System.out.println(obj);
                // Business Method
            }
        }.start();
    }
}
================================================================

完成method方法,使下面程序只输出"12",而不输出"3";
不可以在method中print任何字符出来.
====================== java : Quest.java =======================
public class Quest {
    public static void haha(Object obj){
        System.out.print("1");
        method(obj);
        System.out.print("2");
        synchronized(obj){
            System.out.print("3");
        }
    }
   
    public static void main(String[] args) {
        haha("");
    }
}
================================================================
一种答案:
==================== aes : 密码是trydofor ======================
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wpjDuAM=
================================================================

10. 拿订单提货(Future)
^^^^^^^^^^^^^^^^^^^^^
要那么长时间才完成啊,我先占个座吧,好了告诉我.
Future模式,可以设置返回值,也可以采用callback,这依赖于订货者的要求.

==================== java : FutureData.java ====================
public class FutureData implements Data {
    private Object data = null; // 真正返回值
    private volatile boolean ready = false;
   
    public synchronized void set(Object obj){
        if(ready){
            return;
        }
        data = obj;
        ready = true;
        notifyAll();
    }
   
    public synchronized Object get(){ // 一直等下去
        while(!ready){
            try {
                wait();
            } catch (InterruptedException e) {
            }
        }
        return data;
    }
   
    public boolean isReady(){ // 可以来检查.
        return ready;
    }
}
================================================================

11. 你还是自裁吧(TwoPhaseTermination)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
从外界终止一个执行中的线程是不安全的,最好的办法是让它自己搞定.

不要使用Thread.stop(),它已经被 deprecated 的了,因为他强制终止线程.

==================== java : DoItYouself.java ===================
public class DoItYouself extends Thread{
    private volatile boolean needShutdown = false; // 为何 volatile
   
    public void shutdown(){
        needShutdown = true;
        interrupt(); // 为什么呢?
    }
   
    public final void run(){
        try{
            while(!needShutdown){
            //doWork();
            }
        }catch(InterruptedException e){
        }finally{
            //doShutdown();
        }
    }
}
================================================================

另外,下面代码会输出"13",因为ShutdownHook Thread.start()会在System.exit()或
所有非Deamon线程结束时被调用.
==================== java : DoItYouself.java ===================
public class ShutdownHook{
    public static void main(String[] args) {
        Runtime.getRuntime().addShutdownHook(
            new Thread(){
                public void run(){
                    System.out.print("3");
                }
            }
        );
        System.out.print("1");
        System.exit(0);
        System.out.print("2");
    }
}
================================================================

[[!拓展内容, interrupt() 的作用.]]
调用interrupt(),会产生以下作用之一,不可兼得中断异常和中断状态.
  1) 当线程sleep/wait/join时,抛出中断异常(InterruptedException).
  2) 非1)时,线程变成中断状态(interrupted),可以使用Thread.interrupted();
======================== java : 相互转换 =======================
    InterruptedException se = null;
    try{
        if(Thread.interrupted()){
            throw new InterruptedException(); // 状态 => 异常
        }
    }catch(InterruptedException e){
        Thread.currentThread().interrupt(); // 异常 => 状态
        se = e; // 异常 => 异常
    }finally{
        //doShutdown();
    }
    if(se != null) throw e;
================================================================

12. 私有保险箱(ThreadLocalStorage)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ThreadLocal模式下的多线程行为表面上看与单线程很像,可以不用考虑互斥.
这种以线程为主键的map,可以很好的分离开线程私有的信息,就像局部变量一样.

------------
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]呵呵，大家现在似乎都不活跃了。加油啊，一起来盖啊。

跑到这个版来了,找了半天.
以后不发基础的东西了,JE都是architecture的.