问题描述

我正在尝试为这个问题找到第三种解决方案.

I'm trying to find a third solution to this question.

我不明白为什么这不打印 false.

I can't understand why this doesn't print false.

public class MyClass {

    public MyClass() {
        try {
            Field f = String.class.getDeclaredField("value");
            f.setAccessible(true);
            f.set("true", f.get("false"));
        } catch (Exception e) {
        }
    }

    public static void main(String[] args) {
        MyClass m = new MyClass();
        System.out.println(m.equals(m));
    }
}

当然，由于字符串实习，被修改的 "true" 实例与 PrintStream 的 print 方法中使用的实例完全相同?

Surely, because of string interning, the "true" instance being modified is exactly the same one used in the print method of PrintStream?

public void print(boolean b) {
    write(b ? "true" : "false");
}

我错过了什么?

编辑

@yshavit 的一个有趣的观点是，如果您添加该行

An interesting point by @yshavit is that if you add the line

System.out.println(true);

在try之前，输出是

true
false

推荐答案

这可能是一个 HotSpot JVM 错误.

This is arguably a HotSpot JVM bug.

问题出在字符串字面量实习机制上.

• java.lang.String 字符串文字的实例是在常量池解析期间延迟创建的.
• 最初，常量池中的字符串文字由 CONSTANT_String_info 结构指向 CONSTANT_Utf8_info.
• 每个类都有自己的常量池.也就是说，MyClass 和 PrintStream 有自己的一对 CONSTANT_String_info/CONSTANT_Utf8_info 用于文字 的 cpool 条目'真实'.
• 当CONSTANT_String_info第一次被访问时，JVM启动解析过程.字符串实习是这个过程的一部分.
• 为了找到匹配的文本，JVM 将 CONSTANT_Utf8_info 的内容与 StringTable 中的字符串实例的内容进行比较.
• ^^^ 这就是问题所在.来自 cpool 的原始 UTF 数据与 Java char[] 数组内容进行比较，用户可以通过反射进行欺骗.

• java.lang.String instances for the string literals are created lazily during constant pool resolution.
• Initially a string literal is represented in the constant pool by CONSTANT_String_info structure that points to CONSTANT_Utf8_info.
• Each class has its own constant pool. That is, MyClass and PrintStream have their own pair of CONSTANT_String_info / CONSTANT_Utf8_info cpool entries for the literal 'true'.
• When CONSTANT_String_info is accessed for the first time, JVM initiates the process of resolution. String interning is the part of this process.
• To find a match for a literal being interned, JVM compares the contents of CONSTANT_Utf8_info with the contents of string instances in the StringTable.
• ^^^ And here is the problem. Raw UTF data from cpool is compared to Java char[] array contents that can be spoofed by a user via Reflection.

那么，您的测试中发生了什么?

1. f.set("true", f.get("false")) 启动MyClass'true'的解析代码>.
2. JVM在StringTable中没有发现匹配序列'true'的实例，并创建一个新的java.lang.String，将其存储在 StringTable 中.
StringTable 中该字符串的
3. value 被反射替换.
4. System.out.println(true) 启动PrintStream 类中文字'true' 的解析.
5. JVM 将 UTF 序列 'true' 与来自 StringTable 的字符串进行比较，但没有找到匹配项，因为该字符串已经具有 'false' 值.'true' 的另一个字符串被创建并放置在 StringTable 中.

public class Test {

    static class Inner {
        static String trueLiteral = "true";
    }

    public static void main(String[] args) throws Exception {
        Field f = String.class.getDeclaredField("value");
        f.setAccessible(true);
        f.set("true", f.get("false"));

        if ("true" == Inner.trueLiteral) {
            System.out.println("OK");
        } else {
            System.out.println("BUG!");
        }
    }
}