深入理解Java虚拟机这个知识点是真的说错了
好几年前要和团队小伙伴讲JVM，于是买了周志明老师的《深入理解java虚拟机》，结合jvms来准备培训材料。在对比着jvms看时，发现周老师书中关于运行时常量池和字符串常量池的描述可能存在错误，文中说道：
运行时常量池相对于Class文件常量池的另外一个重要特征是具备动态性，Java语言并不要求常量 一定只有编译期才能产生，也就是说，并非预置入Class文件中常量池的内容才能进入方法区运行时常 量池，运行期间也可以将新的常量放入池中，这种特性被开发人员利用得比较多的便是String类的 intern()方法。
这句话我是不认同的，我再怎么读jvms,也读不出来何处有说String.intern会放入运行时常量池。于是通过书中给的勘误邮箱发邮件说明，但始终石沉大海。有一天发现周老师在github上开通了深入理解Java虚拟机的ussue专区，还挺活跃，心想这次有终于有望了，赶紧去提issue,我当时的想法是，周老师看到issue后会意识到到自己的疏忽，然后在下一版修正。没成想，我两唇枪舌战了好一番，不得不说周老师的知识储备和逻辑能力很强，差点就把我说服了。最后周老师看说服不了我，也就不理我了，我也是同样的想法。
有兴趣的朋友可以看看issue的讨论
https://github.com/fenixsoft/jvm_book/issues/112
没成想又过了半年，朋友给我发来Hollis的一个帖子，文中引用了我的观点，但是他站在周老师那边的，而且说我混淆了jvms和jls。
引文：https://mp.weixin.qq.com/s?__biz=MzI4NzczMDgxNQ==&mid=2247484071&idx=1&sn=46fc487f5842e609c4e639d2d8fb67de&chksm=ebc87fb7dcbff6a1cc3db8e011fd1e4001c960fc59488b01f46fab3127d70d4f043a9dcd091a&token=2074503721&lang=zh_CN#rd
在官方的「Java虚拟机规范」中，明确的表示过，字符串字面量以及 intern 过的字符串内容，都是要进到运行时常量池的。规范是这么说的，所以，不管咋说，字符串常量就是要存储在运行时常量池的。
这句话就真的是大错特错，如果jvms中真的有这句话，周老师估计理都不会理我，直接回我一句，Read The Fucking Manual就行了。于是我又和Hollis唇枪舌战一番。
周老师的核心观点在于
1.字面量（String Literal）以CONSTANT_String_info结构存储在运行时常量池中,所以字符串常量池就是虚拟机中存储字面量的集合。2.String str = String.valueOf(6666666).intern()，str和相同CONSTANT_String_info结构的string literal指向的一个对象，所以字符串常量池是运行时常量池的一部分。3.HotSpot源码中String.intern会调用到stringTable.cpp里的intern方法，jvm在解析contant pool时，也会调用stringTable.cpp里的intern方法，所以字符串常量池是运行时常量池的一部分。
周老师的观点在我看来，等同于以下逻辑：粉丝A喜欢易烊千玺，易烊千玺的对象喜欢易烊千玺，所以粉丝A=易烊千玺的媳妇。我也很纳闷，我们看的是一样的规范，上述三句话前面我都同意，可是怎么就推导出后面的结论了。
jvms11白纸黑字写的清清楚楚
?The Java Virtual Machine maintains a run-time constant pool for each class and interface (§2.5.5). This data structure serves many of the purposes of the symbol table of a conventional programming language implementation. The constant_pool table in the binary representation of a class or interface (§4.4) is used to construct the run-time constant pool upon class or interface creation (§5.3).
?A string constant is a reference to an instance of class String, and is derived from a CONSTANT_String_info structure (§4.4.3). To derive a string constant, the Java Virtual Machine examines the sequence of code points given by the CONSTANT_String_info structure:
?If the method String.intern has previously been invoked on an instance of class String containing a sequence of Unicode code points identical to that given by the CONSTANT_String_info structure, then the string constant is a reference to that same instance of class String.
?Otherwise, a new instance of class String is created containing the sequence of Unicode code points given by the CONSTANT_String_info structure. The string constant is a reference to the new instance. Finally, the method String.intern is invoked on the new instance.
这怎么也推导不出来String.intern会放到运行时常量池中，规范中明确说了JVM会为每个calss或者interface单独维护一个Runtime Constant pool，class中的constant_pool表就是用来构造Runtime Constant pool的。
既然互相说服不了，我就去statckoverflow问了一下，也找了几个同事沟通，大家还是站在我这边的。
?stackoverflow的回复：
?https://stackoverflow.com/questions/70346805/does-string-intern-has-anything-to-to-with-jvm-run-time-constant-pool?noredirect=1#comment124353086_70346805
?So formally, the String instances corresponding to the string constants used in a class are part of that class’s runtime constant pool but initialized in terms of String.intern to ensure that each class has canonicalized string instances in its pool.
?But this relationship has only one direction. When application code invokes String.intern() explicitly, it won’t access a class’s run-time constant pool. It wouldn’t even be clear, which run-time constant pool we shall expect to be accessed.?So intern() has nothing do to with the run-time constant pool, at least not more than it has to do with every other caller.
?A source of confusion is the fact that the data structure used by the JVM to implement intern() has no name in the JVMS or JLS at all. So, without a formal name, different names appear in different media. E.g., the API documentation of intern() says
?A pool of strings, initially empty, is maintained privately by the class String.?it’s typically some kind of hash table, but the term “pool” matches its purpose and since it exists at runtime, it’s not surprising that people come up with terms easy to confuse with the run-time constant pools of JVMS §5.1.