翻译：JVM虚拟机规范1.7中的运行时常量池部分（一）

原文链接：

https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4

Java Virtual Machine instructions do not rely on the run-time layout of classes, interfaces, class instances, or arrays. Instead, instructions refer to symbolic information in the constant_pool table.

JVM虚拟机指令不依赖于类、接口、类实例、或者数组的运行时布局。反而，指令引用在常量池中的符号信息。

All constant_pool table entries have the following general format:

所有常量池表中条目有下面的通用格式：

cp_info {
    u1 tag;
    u1 info[];
}

Each item in the constant_pool table must begin with a 1-byte tag indicating the kind of cp_info entry. The contents of the info array vary with the value of tag. The valid tags and their values are listed in Table 4.4-A. Each tag byte must be followed by two or more bytes giving information about the specific constant. The format of the additional information varies with the tag value.

在常量表中的每个条目必须以一个字节的标签开始，标识条目的类型。info数组的内容根据tag的值而发生变化。下面的表格列出了所有有效的标记和他们的值。

每个标记类型必须跟随两个或更多字节的信息，标识特定的常量。额外信息的格式随tag的值而变化。

4.4.1. The CONSTANT_Class_info Structure

CONSTANT_Class_info 的结构：

The CONSTANT_Class_info structure is used to represent a class or an interface:

CONSTANT_Class_info结构代表一个类或者一个接口

The items of the CONSTANT_Class_info structure are as follows:

CONSTANT_Class_info 结构的条目如下：

tag

The tag item has the value CONSTANT_Class (7).

tag项的值为CONSTANT_Class (7).

name_index

The value of the name_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info structure (§4.4.7) representing a valid binary class or interface name encoded in internal form (§4.2.1).

name_index的值必须是一个有效的constant_pool表中条目的索引。constant_pool表中的该索引对应的项必须是一个CONSTANT_Utf8_info结构，代表一个有效的二进制类或者接口名（内部格式编码）。

Because arrays are objects, the opcodes anewarray and multianewarray - but not the opcode new - can reference array "classes" via CONSTANT_Class_info structures in the constant_pool table. For such array classes, the name of the class is the descriptor of the array type (§4.3.2).

因为数组是对象，操作码anewarray 和multianewarray -但不是操作码new -能够通过本结构体来引用数组类型。对于这样的数组类型，类的名字是数组类型的描述符。

比如，代表二维数组int[][]的类的名字是[[I，代表线程数组Thread[]的类的名字是[Ljava/lang/Thread;

An array type descriptor is valid only if it represents 255 or fewer dimensions.

数组描述符只在数组维数少于255时有效。

 

4.4.2. The CONSTANT_Fieldref_info, CONSTANT_Methodref_info, and CONSTANT_InterfaceMethodref_info Structures

Fields, methods, and interface methods are represented by similar structures:

域，方法和接口方法用类似的结构来代表：

The items of these structures are as follows:

结构体中的条目如下：

tag

The tag item of a CONSTANT_Fieldref_info structure has the value CONSTANT_Fieldref (9).

The tag item of a CONSTANT_Methodref_info structure has the value CONSTANT_Methodref (10).

The tag item of a CONSTANT_InterfaceMethodref_info structure has the valueCONSTANT_InterfaceMethodref (11).

CONSTANT_Fieldref_info 中的tag值为CONSTANT_Fieldref 。

CONSTANT_Methodref_info 中的tag值为CONSTANT_Methodref 。

CONSTANT_InterfaceMethodref_info 中的tag值为valueCONSTANT_InterfaceMethodref。

class_index

The value of the class_index item must be a valid index into the constant_pool table. Theconstant_pool entry at that index must be a CONSTANT_Class_info structure (§4.4.1) representing a class or interface type that has the field or method as a member.

The class_index item of a CONSTANT_Methodref_info structure must be a class type, not an interface type.

The class_index item of a CONSTANT_InterfaceMethodref_info structure must be an interface type.

The class_index item of a CONSTANT_Fieldref_info structure may be either a class type or an interface type.

class_index的值必须是常量表中有效的索引。常量表中该索引所指向的条目必须是CONSTANT_Class_info类型的结构体，代表拥有该field或者方法作为其成员的类或接口。

　　　CONSTANT_Methodref_info中class_index的值必须是class类型，不能是接口。

　　　CONSTANT_InterfaceMethodref_info 中的class_index必须是接口。

　　　CONSTANT_Fieldref_info中的class_index可以是接口，也可以是类。

name_and_type_index

　　The value of the name_and_type_index item must be a valid index into the constant_pool table. Theconstant_pool entry at that index must be a CONSTANT_NameAndType_info structure (§4.4.6). This constant_pool entry indicates the name and descriptor of the field or method.

　　In a CONSTANT_Fieldref_info, the indicated descriptor must be a field descriptor (§4.3.2). Otherwise, the indicated descriptor must be a method descriptor (§4.3.3).

　　If the name of the method of a CONSTANT_Methodref_info structure begins with a '<' ('\u003c'), then the name must be the special name <init>, representing an instance initialization method (§2.9). The return type of such a method must be void.

　　name_and_type_index 的值必须是常量表中的有效索引。其值必须是CONSTANT_NameAndType_info结构体。这个条目标识字段或者方法的名字和描述符。

　　在CONSTANT_Fieldref_info中，指示的描述符必须是一个字段描述符。否则，必须是一个方法描述符。

　　如果CONSTANT_Methodref_info 的方法名字以'<' 开头，那么这个名字一定是<init>，代表对象的初始化方法。返回值必须为void。

4.4.3. The CONSTANT_String_info Structure

The CONSTANT_String_info structure is used to represent constant objects of the type String:

CONSTANT_String_info 结构体用来代表字符串常量：

The items of the CONSTANT_String_info structure are as follows:

tag

The tag item of the CONSTANT_String_info structure has the value CONSTANT_String (8).

tag值为CONSTANT_String

string_index

The value of the string_index item must be a valid index into the constant_pool table. The constant_pool entry at that index must be a CONSTANT_Utf8_info structure (§4.4.7) representing the sequence of Unicode code points to which the String object is to be initialized.

值必须为常量表有效索引。必须是一个CONSTANT_Utf8_info 结构体，代表unicode序列，指向将被初始化的String对象