ClassFile之Fields-CFANZ编程社区

Each field is described by a field_info structure.

No two fields in one class file may have the same name and descriptor (§4.3.2).

The structure has the following format:

field_info {
    u2             access_flags;
    u2             name_index;
    u2             descriptor_index;
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}

The items of the field_info structure are as follows:

access_flags

The value of the access_flags item is a mask of flags used to denote access permission to and properties of this field. The interpretation of each flag, when set, is specified in Table 4.5-A.

Table 4.5-A. Field access and property flags

Flag Name	Value	Interpretation
`ACC_PUBLIC`	0x0001	Declared `public`; may be accessed from outside its package.
`ACC_PRIVATE`	0x0002	Declared `private`; usable only within the defining class.
`ACC_PROTECTED`	0x0004	Declared `protected`; may be accessed within subclasses.
`ACC_STATIC`	0x0008	Declared `static`.
`ACC_FINAL`	0x0010	Declared `final`; never directly assigned to after object construction (JLS §17.5).
`ACC_VOLATILE`	0x0040	Declared `volatile`; cannot be cached.
`ACC_TRANSIENT`	0x0080	Declared `transient`; not written or read by a persistent object manager.
`ACC_SYNTHETIC`	0x1000	Declared synthetic; not present in the source code.
`ACC_ENUM`	0x4000	Declared as an element of an `enum`.

Fields of classes may set any of the flags in Table 4.5-A. However, each field of a class may have at most one of its ACC_PUBLIC, ACC_PRIVATE, and ACC_PROTECTED flags set (JLS §8.3.1), and must not have both its ACC_FINAL and ACC_VOLATILE flags set (JLS §8.3.1.4).

Fields of interfaces must have their ACC_PUBLIC, ACC_STATIC, and ACC_FINAL flags set; they may have their ACC_SYNTHETIC flag set and must not have any of the other flags in Table 4.5-A set (JLS §9.3).

The ACC_SYNTHETIC flag indicates that this field was generated by a compiler and does not appear in source code.

The ACC_ENUM flag indicates that this field is used to hold an element of an enumerated type.

All bits of the access_flags item not assigned in Table 4.5-A are reserved for future use. They should be set to zero in generated class files and should be ignored by Java Virtual Machine implementations.

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) which represents a valid unqualified name denoting a field (§4.2.2).

descriptor_index

The value of the descriptor_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) which represents a valid field descriptor (§4.3.2).

attributes_count

The value of the attributes_count item indicates the number of additional attributes of this field.

attributes[]

Each value of the attributes table must be an attribute_info structure (§4.7).

A field can have any number of optional attributes associated with it.

The attributes defined by this specification as appearing in the attributes table of a field_info structure are listed in Table 4.7-C.

The rules concerning attributes defined to appear in the attributes table of a field_info structure are given in §4.7.

The rules concerning non-predefined attributes in the attributes table of a field_info structure are given in §4.7.1.

Field Descriptors

Table 4.3-A. Interpretation of field descriptors

FieldType term	Type	Interpretation
`B`	`byte`	signed byte
`C`	`char`	Unicode character code point in the Basic Multilingual Plane, encoded with UTF-16
`D`	`double`	double-precision floating-point value
`F`	`float`	single-precision floating-point value
`I`	`int`	integer
`J`	`long`	long integer
`L` ClassName `;`	`reference`	an instance of class ClassName
`S`	`short`	signed short
`Z`	`boolean`	`true` or `false`
`[`	`reference`	one array dimension

The field descriptor of an instance variable of type int is simply I.

The field descriptor of an instance variable of type Object is Ljava/lang/Object;. Note that the internal form of the binary name for class Object is used.

The field descriptor of an instance variable of the multidimensional array type double[][][] is [[[D.