java - 将 ThreadLocal 与包含静态成员的现有类一起使用

Question

我正在尝试使用 ThreadLocal 为预先存在的非线程安全类提供线程安全，但遇到了问题。似乎没有执行隔离 - 线程仍然共享静态，而不是每个线程的本地静态。

我相信我的用法几乎与 this StackOverflow question 中描述的 SimpleDateFormatter 的示例本地化完全平行。 ，但它并没有按照我希望的方式进行。

我所希望的是，有人使用过它会指出我必须犯的极其愚蠢的错误......所以我想我的问题是:你能发现我在这里做错了什么吗？

这是我的简单类:

public class SimpleClassWithStaticMembers {
    private static String theStaticString =
        "StaticStringInClassWithStaticMember";
    public void setTheStaticString (String val) {
        SimpleClassWithStaticMembers.theStaticString = val; 
    }
    public String getTheStaticString () {
        return SimpleClassWithStaticMembers.theStaticString;
    }
}

这是创建 SimpleClassWithStaticMembers 线程本地实例的线程类:

public class SimpleTesterThread extends Thread {
    private void showMsg (String msg) {  
        System.out.println (msg); 
        System.out.flush(); 
    }
    public SimpleTesterThread (String threadId) {
        super(threadId);
    }
    public void run() {
        try { Thread.sleep(2000); } catch (InterruptedException ex) { }
        ThreadLocal<SimpleClassWithStaticMembers> localizedClass =
            new ThreadLocal<SimpleClassWithStaticMembers>();
        localizedClass.set(new SimpleClassWithStaticMembers());
        // repeating here to be sure we overlap all with all
        for (int ii=0; ii < 3; ii++) { 
            localizedClass.get().setTheStaticString ("Setby_" + this.getName());
            try { Thread.sleep(2000); } catch (InterruptedException ex) { }
            showMsg("            Thread [" + this.getName() + "] - " 
                + localizedClass.get().getTheStaticString() + "'.");
        }
        showMsg ("Thread [" + this.getName() 
            + "] complete. Our 'threadlocal' string is now - " 
            + localizedClass.get().getTheStaticString() + "'.");
        localizedClass.remove();
    }
}

当 SimpleTesterThread 的十个实例被创建(给它们不同的线程名称，如“AAAAAAAAAA”、“BBBBBBBBB”等)，然后启动时，输出清楚地表明它们正在共享实例。日志输出包括:

...            Thread [JJJJJJJJJJ] - Setby_CCCCCCCCCC'.            Thread [DDDDDDDDDD] - Setby_JJJJJJJJJJ'.            Thread [IIIIIIIIII] - Setby_DDDDDDDDDD'.            Thread [GGGGGGGGGG] - Setby_IIIIIIIIII'.            Thread [EEEEEEEEEE] - Setby_GGGGGGGGGG'.Thread [EEEEEEEEEE] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.            Thread [HHHHHHHHHH] - Setby_GGGGGGGGGG'.            Thread [BBBBBBBBBB] - Setby_GGGGGGGGGG'.            Thread [FFFFFFFFFF] - Setby_GGGGGGGGGG'.Thread [FFFFFFFFFF] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'....            Thread [JJJJJJJJJJ] - Setby_GGGGGGGGGG'.Thread [JJJJJJJJJJ] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.Thread [HHHHHHHHHH] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.            Thread [GGGGGGGGGG] - Setby_GGGGGGGGGG'.Thread [GGGGGGGGGG] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.            Thread [IIIIIIIIII] - Setby_GGGGGGGGGG'.            Thread [CCCCCCCCCC] - Setby_GGGGGGGGGG'.Thread [CCCCCCCCCC] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.Thread [AAAAAAAAAA] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.            Thread [DDDDDDDDDD] - Setby_GGGGGGGGGG'.Thread [DDDDDDDDDD] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.Thread [IIIIIIIIII] complete. Our 'threadlocal' string is now - Setby_GGGGGGGGGG'.============== all threads complete.

我没有包括创建、启动和加入线程的类 - 如果觉得这有帮助，我很乐意编辑以添加它。

Answer 1

拥有单独的实例没有帮助，因为所有实例都具有相同的静态字段。可变静态是邪恶的。

如果您真的不能更改类，您可能只想使用锁，这样每个客户端都可以一次使用一个静态字段。如果你想要静态字段的不同实例，那么你可能需要使用类加载器(另一个明显的解决方案是重写字节码，这更不愉快)。