https://docs.oracle.com/javase/specs/jvms/se8/jvms8.pdf
2.5 Run-Time Data Areas
运行时数据
PC
STACK
HEAP
The Java Virtual Machine has a method area that is shared among all Java Virtual Machine threads
方法区是所有线程共享的，不过和heap的区别是，堆存储的是实例和数组
类似于，操作系统的正文段，也就是编译后的方法代码
It stores per-class structures such as the run-time constant pool, field and method data, and the code for methods and constructors, including the special methods (§2.9) used in class and instance initialization and interface initialization.
2.5.5 Run-Time Constant Pool
运行时常量池
A run-time constant pool is a per-class or per-interface run-time representation of the constant_pool table in a class file (§4.4). It contains several kinds of constants, ranging from numeric literals known at compile-time to method and field references that must be resolved at run-time. The run-time constant pool serves a function similar to that of a symbol table for a conventional programming language, although it contains a wider range of data than a typical symbol table.
Each run-time constant pool is allocated from the Java Virtual Machine's method area (§2.5.4).
其实就是一个表，具体哪个表，参考：
IDEA使用jclasslib-bytecode-viewer插件查看字节码_newbaby2012的博客-CSDN博客
首先这些不是实例，所以不在heap，其次是所有线程共享的，所以不会再stack里
A class file consists of a single ClassFile structure:
类描述
The field descriptor of an instance variable of the multidimensional array type double[] [][] is [[[D
方法描述
常量池由两部分组成，一个标记，一个信息，常见的标记如下
CONSTANT_Class_info
name_INDEX是一个index，必须指向CONSTANT_Utf8_info结构
The constant_pool entry at that index must be a CONSTANT_Utf8_info structure
而CONSTANT_Utf8_info如下，哦这么看。class其实指向一个字节数组，里面其实就是类信息对应的字节码，所以常量池就是CONSTANT_Utf8_info这一类CONSTANT开头的东西？
field
method 咋感觉和field一模一样
里面的attributes_info就是
总共有23种attributes
The number of method parameters is limited to 255 by the definition of a method descriptor (§4.3.3), where the limit includes one unit for this in the case of instance or interface method invocations.
The size of an operand stack in a frame (§2.6) is limited to 65535 values by the max_stack field of the Code attribute (§4.7.3).
Loading, Linking, and Initializing
Loading就是找到二进制文件(classloader的功能)，并且据此创建class字节码文件
符号引用（ symbolic reference）：就是不是直接指向内存地址，类似于spring中的${ocr.port}
The Run-Time Constant Pool
运行时常量池，其实首先取决于常量池，会有很多符号引用，而这些符号引用都来自于constant_pool的各种info
All references in the run-time constant pool are initially symbolic.
A symbolic reference to a field of a class or an interface is derived from a CONSTANT_Fieldref_info structure (§4.4.2) in the binary representation of a class or interface. Such a reference gives the name and descriptor of the field, as well as a symbolic reference to the class or interface in which the field is to be found.
一个引用会给出Field的名字和描述符，并且给出另一个引用，这个引用指向Field对应的class
Run-time constant values are derived from CONSTANT_Integer_info, CONSTANT_Float_info, CONSTANT_Long_info, or CONSTANT_Double_info structures (§4.4.4, §4.4.5) in the binary representation of a class or interface.
看到5.2
