Attributes of classfile

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:

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

        
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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 a ​​CONSTANT_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. 1. 
      
2. Five attributes are critical to correct interpretation of the ​​class​​ file by the Java Virtual Machine:
3. 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.

• • 
    
• 
  

​​ConstantValue​​

​​Code​​

​​StackMapTable​​

​​Exceptions​​

​​BootstrapMethods​​

• 
  

3. Twelve attributes are critical to correct interpretation of the ​​class​​ file by the class libraries of the Java SE platform:
4. 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.

• • 
    
• 
  

​​InnerClasses​​

​​EnclosingMethod​​

​​Synthetic​​

​​Signature​​

​​RuntimeVisibleAnnotations​​

​​RuntimeInvisibleAnnotations​​

​​RuntimeVisibleParameterAnnotations​​

​​RuntimeInvisibleParameterAnnotations​​

​​RuntimeVisibleTypeAnnotations​​

​​RuntimeInvisibleTypeAnnotations​​

​​AnnotationDefault​​

​​MethodParameters​​

• 
  

5. 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:
6. 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.

• • 
    
• 
  

​​SourceFile​​

​​SourceDebugExtension​​

​​LineNumberTable​​

​​LocalVariableTable​​

​​LocalVariableTypeTable​​

​​Deprecated​​

• 
  

7. 
   

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

4.7.1. Defining and Naming New Attributes

Compilers are permitted to define and emit ​​class​​​ files containing new attributes in the ​​attributes​​​ tables of ​​class​​​ file structures, ​​field_info​​​ structures, ​​method_info​​​ structures, and ​​Code​​​ attributes (​ ​§4.7.3​​​). Java Virtual Machine implementations are permitted to recognize and use new attributes found in these ​​attributes​​​ tables. However, any attribute not defined as part of the ​​class​​​ file specification must not affect the semantics of the ​​class​​ file. Java Virtual Machine implementations are required to silently ignore attributes they do not recognize.

For instance, defining a new attribute to support vendor-specific debugging is permitted. Because Java Virtual Machine implementations are required to ignore attributes they do not recognize, ​​class​​​ files intended for that particular Java Virtual Machine implementation will be usable by other implementations even if those implementations cannot make use of the additional debugging information that the ​​class​​ files contain.

Java Virtual Machine implementations are specifically prohibited from throwing an exception or otherwise refusing to use ​​class​​​ files simply because of the presence of some new attribute. Of course, tools operating on ​​class​​​ files may not run correctly if given ​​class​​ files that do not contain all the attributes they require.

Two attributes that are intended to be distinct, but that happen to use the same attribute name and are of the same length, will conflict on implementations that recognize either attribute. Attributes defined other than in this specification must have names chosen according to the package naming convention described in The Java Language Specification, Java SE 8 Edition (JLS §6.1).

Future versions of this specification may define additional attributes.