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4.7. Attributes

Attributes are used in the ClassFile, field_info, method_info, and Code_attribute structures of the class file format (§4.1, §4.5, §4.6, §4.7.3).

All attributes have the following general format:

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

For all attributes, the attribute_name_index must be a valid unsigned 16-bit index into the constant pool of the class. The constant_pool entry at attribute_name_index must be aCONSTANT_Utf8_info structure (§4.4.7) representing the name of the attribute. The value of the attribute_length item indicates the length of the subsequent information in bytes. The length does not include the initial six bytes that contain the attribute_name_index and attribute_length items.

23 attributes are predefined by this specification. They are listed three times, for ease of navigation:

  • Table 4.7-A is ordered by the attributes' section numbers in this chapter. Each attribute is accompanied by the first version of the class file format in which it was defined, and the corresponding version of the Java SE platform. (For old class file versions, the JDK release is used instead of the Java SE platform version).
  • Table 4.7-B is orderd by the first version of the class file format in which each attribute was defined.
  • Table 4.7-C is ordered by the location in a class file where each attribute is defined to appear.

Within the context of their use in this specification, that is, in the attributes tables of the class file structures in which they appear, the names of these predefined attributes are reserved.

Any conditions on the presence of a predefined attribute in an attributes table are specified explicitly in the section which describes the attribute. If no conditions are specified, then the attribute may appear any number of times in an attributes table.

The predefined attributes are categorized into three groups according to their purpose:

  1. Five attributes are critical to correct interpretation of the class file by the Java Virtual Machine:

    • ConstantValue
    • Code
    • StackMapTable
    • Exceptions
    • BootstrapMethods

    In a class file of version V, each of these attributes must be recognized and correctly read by an implementation of the Java Virtual Machine if the implementation recognizes class files of version V, and V is at least the version where the attribute was first defined, and the attribute appears in a location where it is defined to appear.

  2. Twelve attributes are critical to correct interpretation of the class file by the class libraries of the Java SE platform:

    • InnerClasses
    • EnclosingMethod
    • Synthetic
    • Signature
    • RuntimeVisibleAnnotations
    • RuntimeInvisibleAnnotations
    • RuntimeVisibleParameterAnnotations
    • RuntimeInvisibleParameterAnnotations
    • RuntimeVisibleTypeAnnotations
    • RuntimeInvisibleTypeAnnotations
    • AnnotationDefault
    • MethodParameters

    Each of these attributes in a class file of version V must be recognized and correctly read by an implementation of the class libraries of the Java SE platform if the implementation recognizes class files of version V, and V is at least the version where the attribute was first defined, and the attribute appears in a location where it is defined to appear.

  3. Six attributes are not critical to correct interpretation of the class file by either the Java Virtual Machine or the class libraries of the Java SE platform, but are useful for tools:

    • SourceFile
    • SourceDebugExtension
    • LineNumberTable
    • LocalVariableTable
    • LocalVariableTypeTable
    • Deprecated

    Use of these attributes by an implementation of the Java Virtual Machine or the class libraries of the Java SE platform is optional. An implementation may use the information that these attributes contain, or otherwise must silently ignore these attributes.

Table 4.7-A. Predefined class file attributes (by section)

Attribute Section class file Java SE
ConstantValue §4.7.2 45.3 1.0.2
Code §4.7.3 45.3 1.0.2
StackMapTable §4.7.4 50.0 6
Exceptions §4.7.5 45.3 1.0.2
InnerClasses §4.7.6 45.3 1.1
EnclosingMethod §4.7.7 49.0 5.0
Synthetic §4.7.8 45.3 1.1
Signature §4.7.9 49.0 5.0
SourceFile §4.7.10 45.3 1.0.2
SourceDebugExtension §4.7.11 49.0 5.0
LineNumberTable §4.7.12 45.3 1.0.2
LocalVariableTable §4.7.13 45.3 1.0.2
LocalVariableTypeTable §4.7.14 49.0 5.0
Deprecated §4.7.15 45.3 1.1
RuntimeVisibleAnnotations §4.7.16 49.0 5.0
RuntimeInvisibleAnnotations §4.7.17 49.0 5.0
RuntimeVisibleParameterAnnotations §4.7.18 49.0 5.0
RuntimeInvisibleParameterAnnotations §4.7.19 49.0 5.0
RuntimeVisibleTypeAnnotations §4.7.20 52.0 8
RuntimeInvisibleTypeAnnotations §4.7.21 52.0 8
AnnotationDefault §4.7.22 49.0 5.0
BootstrapMethods §4.7.23 51.0 7
MethodParameters §4.7.24 52.0 8

Table 4.7-B. Predefined class file attributes (by class file version)

Attribute class file Java SE Section
ConstantValue 45.3 1.0.2 §4.7.2
Code 45.3 1.0.2 §4.7.3
Exceptions 45.3 1.0.2 §4.7.5
SourceFile 45.3 1.0.2 §4.7.10
LineNumberTable 45.3 1.0.2 §4.7.12
LocalVariableTable 45.3 1.0.2 §4.7.13
InnerClasses 45.3 1.1 §4.7.6
Synthetic 45.3 1.1 §4.7.8
Deprecated 45.3 1.1 §4.7.15
EnclosingMethod 49.0 5.0 §4.7.7
Signature 49.0 5.0 §4.7.9
SourceDebugExtension 49.0 5.0 §4.7.11
LocalVariableTypeTable 49.0 5.0 §4.7.14
RuntimeVisibleAnnotations 49.0 5.0 §4.7.16
RuntimeInvisibleAnnotations 49.0 5.0 §4.7.17
RuntimeVisibleParameterAnnotations 49.0 5.0 §4.7.18
RuntimeInvisibleParameterAnnotations 49.0 5.0 §4.7.19
AnnotationDefault 49.0 5.0 §4.7.22
StackMapTable 50.0 6 §4.7.4
BootstrapMethods 51.0 7 §4.7.23
RuntimeVisibleTypeAnnotations 52.0 8 §4.7.20
RuntimeInvisibleTypeAnnotations 52.0 8 §4.7.21
MethodParameters 52.0 8 §4.7.24

Table 4.7-C. Predefined class file attributes (by location)

Attribute Location class file
SourceFile ClassFile 45.3
InnerClasses ClassFile 45.3
EnclosingMethod ClassFile 49.0
SourceDebugExtension ClassFile 49.0
BootstrapMethods ClassFile 51.0
ConstantValue field_info 45.3
Code method_info 45.3
Exceptions method_info 45.3
RuntimeVisibleParameterAnnotations, RuntimeInvisibleParameterAnnotations method_info 49.0
AnnotationDefault method_info 49.0
MethodParameters method_info 52.0
Synthetic ClassFile, field_info, method_info 45.3
Deprecated ClassFile, field_info, method_info 45.3
Signature ClassFile, field_info, method_info 49.0
RuntimeVisibleAnnotations, RuntimeInvisibleAnnotations ClassFile, field_info, method_info 49.0
LineNumberTable Code 45.3
LocalVariableTable Code 45.3
LocalVariableTypeTable Code 49.0
StackMapTable Code 50.0
RuntimeVisibleTypeAnnotations, RuntimeInvisibleTypeAnnotations ClassFile, field_info, method_info, Code 52.0