本文共 7931 字，大约阅读时间需要 26 分钟。

Attributes

Attributes are used in the ClassFile, field_info, method_info, and Code_attribute structures of the class file format (, , , ).

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 a CONSTANT_Utf8_info structure () representing the name of the attribute. The value of theattribute_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:

•  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).

•  is orderd by the first version of the class file format in which each attribute was defined.

•  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)

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

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

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 (). 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.

转载地址：http://xkawb.baihongyu.com/