StringBuilder由多个线程修改

Question

我问的问题与StringBuilder和StringBuffer之间的差异有关, 但不一样.我想看看如果同时由两个线程修改StringBuilder会发生什么.

我写了以下课程:

public class ThreadTester
{
    public static void main(String[] args) throws InterruptedException
    {
        Runnable threadJob = new MyRunnable();
        Thread myThread = new Thread(threadJob);
        myThread.start();

        for (int i = 0; i < 100; i++)
        {
            Thread.sleep(10);
            StringContainer.addToSb("a");
        }

        System.out.println("1: " + StringContainer.getSb());
        System.out.println("1 length: " + StringContainer.getSb().length());
    }
}

public class MyRunnable implements Runnable
{
    @Override
    public void run()
    {
        for (int i = 0; i < 100; i++)
        {
            try
            {
                Thread.sleep(10);
            }
            catch (InterruptedException e)
            {
                e.printStackTrace();
            }
            StringContainer.addToSb("b");
        }

        System.out.println("2: " + StringContainer.getSb());
        System.out.println("2 length: " + StringContainer.getSb().length());
    }
}

public class StringContainer
{
    private static final StringBuffer sb = new StringBuffer();

    public static StringBuffer getSb()
    {
        return sb;
    }

    public static void addToSb(String s)
    {
        sb.append(s);
    }
}

最初我在StringContainer中保留了一个StringBuffer.由于StringBuffer是线程安全的,一次只有一个线程可以附加到它,因此输出是一致的 - 两个线程都报告缓冲区的长度为200,如:

1: abababababababababbaabababababababbaababababababababababababbabaabbababaabbaababababbababaabbababaabababbaabababbababababaababababababababbababaabbaababbaababababababbaababbababaababbabaabbababababaab
1 length: 200
2: abababababababababbaabababababababbaababababababababababababbabaabbababaabbaababababbababaabbababaabababbaabababbababababaababababababababbababaabbaababbaababababababbaababbababaababbabaabbababababaab
2 length: 200

或其中一个报告199和另外200个,如:

2: abbabababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababab
2 length: 199
1: abbababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababababa
1 length: 200

关键是最后一个线程要完成报告的长度为200.

现在,我将StringContainer更改为StringBuilder而不是StringBuffer即

public class StringContainer
{
    private static final StringBuilder sb = new StringBuilder();

    public static StringBuilder getSb()
    {
        return sb;
    }

    public static void addToSb(String s)
    {
        sb.append(s);
    }
}

我希望有些写操作会被覆盖,这种情况正在发生.但是StringBuilder的内容和长度有时不匹配:

1: ababbabababaababbaabbabababababaab
1 length: 137
2: ababbabababaababbaabbabababababaab
2 length: 137

正如您所看到的,打印内容只有34个字符,但长度为137.为什么会发生这种情况？

@Extreme Coders - 我刚做了一次测试:

2: ababbabababaabbababaabbababaababaabbaababbaaababbaabbabbabbabababbabababbbabbbbbabababbaabababbabaabaaabaababbaabaababababbaabbbabbbbbababababbababaab
1: ababbabababaabbababaabbababaababaabbaababbaaababbaabbabbabbabababbabababbbabbbbbabababbaabababbabaabaaabaababbaabaababababbaabbbabbbbbababababbababaab
1 length: 150
2 length: 150

Java版本:1.6.0_45我正在使用eclipse版本:用于Web开发人员的Eclipse Java EE IDE.版本:Juno Service Release 2 Build id:20130225-0426

更新1:我在eclipse之外运行它,现在它们似乎匹配,但有时我得到ArrayIndexOutOfBoundsException:

$ java -version
java version "1.6.0_27"
OpenJDK Runtime Environment (IcedTea6 1.12.5) (6b27-1.12.5-0ubuntu0.12.04.1)
OpenJDK Server VM (build 20.0-b12, mixed mode)

$ java ThreadTester
1: ababbbbbabbabababababaababbaabbbaabababbbababbabababbabbababbbbbbabaabaababbbbbbabbbbbaabbaaabbbbaabbbababababbbbabbababab
1 length: 123
2: ababbbbbabbabababababaababbaabbbaabababbbababbabababbabbababbbbbbabaabaababbbbbbabbbbbaabbaaabbbbaabbbababababbbbabbababab
2 length: 123

$ java ThreadTester 
2: abbabaabbbbbbbbbababbbbbabbbabbbabaaabbbbbbbabababbbbbbbbbabbbbbbbababababbabbbbaabbbaaabbabaaababaaaabaabbaabbbb
2 length: 115
1: abbabaabbbbbbbbbababbbbbabbbabbbabaaabbbbbbbabababbbbbbbbbabbbbbbbababababbabbbbaabbbaaabbabaaababaaaabaabbaabbbb
1 length: 115

$ java ThreadTester 
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException
    at java.lang.System.arraycopy(Native Method)
    at java.lang.String.getChars(String.java:862)
    at java.lang.AbstractStringBuilder.append(AbstractStringBuilder.java:408)
    at java.lang.StringBuilder.append(StringBuilder.java:136)
    at StringContainer.addToSb(StringContainer.java:14)
    at ThreadTester.main(ThreadTester.java:14)
2: abbbbbbababbbbabbbbababbbbaabbabbbaaabbbababbbbabaabaabaabaaabababaabbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
2 length: 114

从eclipse运行时也会发生ArrayIndexOutOfBoundsException.

更新2: 发生了两个问题.StringBuilder的内容与长度不匹配的第一个问题仅在Eclipse中发生,而不是在命令行中运行时(至少100次我在命令行上运行它从未发生过).

ArrayIndexOutOfBoundsException的第二个问题应该是StringBuilder类的内部实现,它保留了一个字符数组,并Arrays.copyOf在扩展大小时执行.但是无论执行的顺序是什么,它在扩展大小之前如何进行写操作仍然比我好.

顺便说一句,我倾向于同意@ GreyBeardedGeek的答案,即整个演习是浪费时间:-).有时我们只能看到症状,即某些代码的输出,并想知道出了什么问题.这个问题先验地宣称两个线程正在修改(非常着名的)线程不安全对象.

更新3:这是Java Concurrency in Practice p 的官方答案.35:

• 在没有同步的情况下,编译器,处理器和运行时可以对操作看起来执行的顺序做一些彻头彻尾的奇怪事情.尝试推断在不充分同步的多线程程序中"必须"发生内存操作的顺序几乎肯定是不正确的.

• 关于并发程序不充分同步的推理非常困难.

NoVisibility关于p的书中也有一个很好的例子.34.

Answer 1

当多个线程并发访问时,非线程安全类的行为根据定义是"未定义的".

在这种情况下,任何确定确定性行为的企图都是恕我直言,这只是浪费大量时间.