Chapter 4. The class File Format

4.1. The ClassFile Structure

ClassFile {
    u4             magic;
    u2             minor_version;
    u2             major_version;
    u2             constant_pool_count;
    cp_info        constant_pool[constant_pool_count-1];
    u2             access_flags;
    u2             this_class;
    u2             super_class;
    u2             interfaces_count;
    u2             interfaces[interfaces_count];
    u2             fields_count;
    field_info     fields[fields_count];
    u2             methods_count;
    method_info    methods[methods_count];
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}

4.3. Descriptors

4.3.2. 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

4.4. The Constant Pool

cp_info {
    u1 tag;
    u1 info[];
}
Constant Type Value
CONSTANT_Class 7
CONSTANT_Fieldref 9
CONSTANT_Methodref 10
CONSTANT_InterfaceMethodref 11
CONSTANT_String 8
CONSTANT_Integer 3
CONSTANT_Float 4
CONSTANT_Long 5
CONSTANT_Double 6
CONSTANT_NameAndType 12
CONSTANT_Utf8 1
CONSTANT_MethodHandle 15
CONSTANT_MethodType 16
CONSTANT_InvokeDynamic 18

4.5. Fields

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

4.6. Methods

method_info {
    u2             access_flags;
    u2             name_index;
    u2             descriptor_index;
    u2             attributes_count;
    attribute_info attributes[attributes_count];
}
Flag Name Value Interpretation
ACC_PUBLIC 0x0001 Declared public; may be accessed from outside its package.
ACC_PRIVATE 0x0002 Declared private; accessible 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; must not be overridden (§5.4.5).
ACC_SYNCHRONIZED 0x0020 Declared synchronized; invocation is wrapped by a monitor use.
ACC_BRIDGE 0x0040 A bridge method, generated by the compiler.
ACC_VARARGS 0x0080 Declared with variable number of arguments.
ACC_NATIVE 0x0100 Declared native; implemented in a language other than Java.
ACC_ABSTRACT 0x0400 Declared abstract; no implementation is provided.
ACC_STRICT 0x0800 Declared strictfp; floating-point mode is FP-strict.
ACC_SYNTHETIC 0x1000 Declared synthetic; not present in the source code.

4.7. Attributes

attribute_info {
    u2 attribute_name_index;
    u4 attribute_length;
    u1 info[attribute_length];
}