Thread-Specific Storage - Design Patterns

資料來源：	結城浩 - The Essence of Programming
作者：	結城浩
翻譯：	koji

Thread-Specific Storage pattern
假設有一個如下的interface,當我呼叫完call以後,想知道是否有發生error,
或者發生什麼error,那麼就用errno method來調查.(這個前提聽起來有一點笨,
但是在實際的系統中卻常存在)

LegacySystem.java

interface LegacySystem {
    public void call(int parameter);
    public int errno();
}

在這邊我們實做一下上面的interface,
為了簡單一點,在這邊不做任何複雜動作,只單純把parameter
傳給errno

LegacySystemImpl.java

class LegacySystemImpl implements LegacySystem {
    private int errno;
    public void call(int parameter) { errno = parameter; }
    public int errno() { return errno; }
}

上面的code很簡單只是傳入參數
還有印出的動作

如果在single thread下,上面的class不會有任何問題.
但是當LegacySystemImpl物件被複數的thread存取,
那麼將會發生問題.也就是說其他的thread呼叫call會影響到自己呼叫errno method的結果.

主要的如下
現在兩個thread去呼叫
第一個thread一直去呼叫call(0),
然後5秒以後第二個thread一直呼叫call(1)
如果發生傳入call的值和errno method回傳的值不同時,
印出???並呼叫System.exit結束

Main1.java

class Main1 extends Thread {
    private static LegacySystem system = new LegacySystemImpl();
    private int value = 0;
    public Main1(int value) {
        this.value = value;
    }
    public void run() {
        while (true) {
            System.out.println(Thread.currentThread().getName() + " checks.");
            system.call(value);
            try { Thread.sleep(100); } catch (InterruptedException e) {}
            int errno = system.errno();
            System.out.println(Thread.currentThread().getName() + ": value = " + value + ", errno = " + errno);
            if (value != errno) {
                System.out.println("???");
                System.exit(0);
            }
        }
    }
    public static void main(String[] args) {
        new Main1(0).start();
        try { Thread.sleep(5000); } catch (InterruptedException e) {}
        new Main1(1).start();
    }
}

結果如下
Thread-0 checks.
Thread-0: value = 0, errno = 0
Thread-0 checks.
Thread-0: value = 0, errno = 0
Thread-0 checks.
Thread-0: value = 0, errno = 0
（中略）
Thread-0 checks.
Thread-0: value = 0, errno = 0
Thread-0 checks.
Thread-1 checks. ←Thread-1啟動以後...
Thread-0: value = 0, errno = 1 ←結果亂七八糟囉。
???

所以現在改寫成如下的proxy

LegacySystemProxy.java

class LegacySystemProxy implements LegacySystem {
    private ThreadLocal thlocal = new ThreadLocal();
    public void call(int parameter) {
        getImpl().call(parameter);
    }
    public int errno() {
        return getImpl().errno();
    }
    private LegacySystemImpl getImpl() {
        LegacySystemImpl impl = (LegacySystemImpl)thlocal.get();
        if (impl == null) {
            impl = new LegacySystemImpl();
            thlocal.set(impl);
        }
        return impl;
    }
}

在這邊使用的java.lang.ThreadLocal,讓現在的thread保有其固有的領域(對thread來說specific的領域),為了讓thread有固有的LegacySystemImpl,所以使用threadlocal.

使用threadlocal的get和set method可以去讀寫現在的thread的固有領域

寫個新的main

Main2.java

class Main2 extends Thread {
    private static LegacySystem system = new LegacySystemProxy();
    private int value = 0;
    public Main2(int value) {
        this.value = value;
    }
    public void run() {
        while (true) {
            System.out.println(Thread.currentThread().getName() + " checks.");
            system.call(value);
            try { Thread.sleep(100); } catch (InterruptedException e) {}
            int errno = system.errno();
            System.out.println(Thread.currentThread().getName() + ": value = " + value + ", errno = " + errno);
            if (value != errno) {
                System.out.println("???");
                System.exit(0);
            }
        }
    }
    public static void main(String[] args) {
        new Main2(0).start();
        try { Thread.sleep(5000); } catch (InterruptedException e) {}
        new Main2(1).start();
    }
}

結果如下

Thread-0 checks.
Thread-0: value = 0, errno = 0
（中略）
Thread-0 checks.
Thread-1 checks.
Thread-0: value = 0, errno = 0
Thread-0 checks.
Thread-1: value = 1, errno = 1
Thread-1 checks.
Thread-0: value = 0, errno = 0
Thread-0 checks. ←就像這樣
Thread-1 checks. ←就算這樣也不怕。
Thread-0: value = 0, errno = 0
Thread-0 checks.
Thread-1: value = 1, errno = 1
Thread-1 checks.
Thread-0: value = 0, errno = 0
（後略）

以上pattern在以下書籍的p475中介紹,解說文章及程式則由結城先生新撰寫的.

[POSA2] Pattern-Oriented Software Architecture, Volume 2: Patterns for Concurrent and Networked Objects

http://www.amazon.co.jp/exec/obidos/ASIN/0471606952/249-1714634-2037127

此pattern在以下書籍也有
http://www.hyuki.com/dp/dp2.html (日文)