Constant Pool

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.

All constant_pool table entries have the following general format:

typedef struct
{
    u1 tag;
    void * data;
} cp_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 as below:

Constant Type	Value
CONSTANT_Utf8	1
CONSTANT_Integer	3
CONSTANT_Float	4
CONSTANT_Long	5
CONSTANT_Double	6
CONSTANT_Class	7
CONSTANT_String	8
CONSTANT_Fieldref	9
CONSTANT_Methodref	10
CONSTANT_InterfaceMethodref	11
CONSTANT_NameAndType	12
CONSTANT_MethodHandle	15
CONSTANT_MethodType	16
CONSTANT_InvokeDynamic	18

CONSTANT_Class_info

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

typedef struct
{
    u1 tag;
    struct
    {
        u2 name_index;
    } * data;
} CONSTANT_Class_info;

CONSTANT_Fieldref_info, CONSTANT_Methodref_info, and CONSTANT_InterfaceMethodref_info

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

typedef struct
{
    u1 tag;
    struct
    {
        u2 class_index;
        u2 name_and_type_index;
    } * data;
} CONSTANT_Fieldref_info,
        CONSTANT_Fieldref_info,
        CONSTANT_InterfaceMethodref_info;

CONSTANT_String_info

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

typedef struct
{
    u1 tag;
    struct
    {
        u2 string_index;
    } * data;
} CONSTANT_String_info;

CONSTANT_Integer_info and CONSTANT_Float_info

The CONSTANT_Integer_info and CONSTANT_Float_info structures represent 4-byte numeric (int and float) constants:

typedef struct
{
    u1 tag;
    struct
    {
        u4 bytes;
    } * data;
} CONSTANT_Integer_info,
        CONSTANT_Float_info;

CONSTANT_Long_info and CONSTANT_Double_info

The CONSTANT_Long_info and CONSTANT_Double_info represent 8-byte numeric (long and double) constants:

typedef struct
{
    u1 tag;
    struct
    {
        u4 high_bytes;
        u4 low_bytes;
    } * data;
} CONSTANT_Long_info,
        CONSTANT_Double_info;

CONSTANT_NameAndType_info

The CONSTANT_NameAndType_info structure is used to represent a field or method, without indicating which class or interface type it belongs to:

typedef struct
{
    u1 tag;
    struct
    {
        u2 name_index;
        u2 descriptor_index;
    } * data;
} CONSTANT_NameAndType_info;

CONSTANT_Utf8_info

The CONSTANT_Utf8_info structure is used to represent constant string values:

typedef struct
{
    u1 tag;
    struct
    {
        u2 length;
        u1 *bytes;
    } * data;
} CONSTANT_Utf8_info;

CONSTANT_MethodHandle_info

The CONSTANT_MethodHandle_info structure is used to represent a method handle:

typedef struct
{
    u1 tag;
    struct
    {
        u1 reference_kind;
        u2 reference_index;
    } * data;
} CONSTANT_MethodHandle_info;

CONSTANT_MethodType_info

The CONSTANT_MethodType_info structure is used to represent a method type:

typedef struct
{
    u1 tag;
    struct
    {
        u2 descriptor_index;
    } * data;
} CONSTANT_MethodType_info;

CONSTANT_InvokeDynamic_info

The CONSTANT_InvokeDynamic_info structure is used by an invokedynamic instruction to specify a bootstrap method, the dynamic invocation name, the argument and return types of the call, and optionally, a sequence of additional constants called static arguments to the bootstrap method.

typedef struct
{
    u1 tag;
    struct
    {
        u2 bootstrap_method_attr_index;
        u2 name_and_type_index;
    } * data;
} CONSTANT_InvokeDynamic_info;