[Go deep into Java Virtual Machine] 2: Class file structure

 

 

The unsigned number belongs to the basic data type. u1, u2, u4, and u8 represent the unsigned number of 1, 2, 4, and 8 bytes respectively. A table is a data type that consists of multiple unsigned numbers or other tables as data items. All Tables habitually end with "_ info.

(), Its only function is to determine whether the file is a Class file that can be accepted by the virtual machine. Its value is fixed to 0 xCAFEBABE. The four bytes of magic store the minor version number and major version number of the Class file. The later version of JDK can be backward compatible with the lower version of the Class file, but cannot run a later version of the Class file.

(), Which is the data type most associated with other projects in the Class file and one of the Data projects that occupy the largest Class file space.

The literal volume is similar to the Java-level constant concepts, such as text strings and constants declared as final. Symbolic references include the following three constants:

• A symbolic reference is a set of symbols used to describe the referenced object. A symbolic reference can be a literal of any form and can be used to locate the object without ambiguity. Symbol reference has nothing to do with the memory layout implemented by the virtual machine. The referenced target is not necessarily loaded into the memory.
• : Direct reference can be a pointer to the target, a relative offset, or a handle that can be indirectly located to the target. Direct reference is related to the memory layout implemented by virtual machines. The direct reference translated by the same symbol reference on different virtual machine instances is generally not the same. If direct reference is available, the referenced target must already exist in the memory.

CONSTANT_Utf8_info			The value is
	Length	U2	UF-8
	Bytes	U1	The length is
CONSTANT_Integer_info	Tag	U1	The value is
	Bytes	U4	Stored in front
CONSTANT_Float_info	Tag	U1	The value is
	Bytes	U4	Stored in front
CONSTANT_Long_info	Tag	U1	The value is
	Bytes	U8	Stored in front
CONSTANT_Double_info	Tag	U1	The value is
	Bytes	U8	Stored in front
CONSTANT_Class_info	Tag	U1	The value is
	Index	U2	Index pointing to a constant with a fully qualified name
CONSTANT_String_info	Tag	U1	The value is
	Index	U2	Index pointing to the string literal
CONSTANT_Fieldref_info	Tag	U1	The value is
	Index	U2	Class or interface descriptor pointing to declared Fields
	Index	U2	Field name and type descriptor
CONSTANT_Methodref_info	Tag	U1	The value is
	Index	U2	Class descriptor pointing to the declared Method
	Index	U2	Method Name and type descriptor
CONSTANT_InrerfaceMethodref_info	Tag	U1	The value is
	Index	U2	Interface descriptor pointing to the declared Method
	Index	U2	Method Name and type descriptor
CONSTANT_NameAndType_info	Tag	U1	The value is
	Index	U2	Index of a constant project pointing to a field or method name
	Index	U2	Index pointing to this field or constant item of the method Descriptor

After the constant pool ends, the next two bytes represent the access_flag, which is used to identify access information at the Class or interface level, including whether the Class is a Class or an interface, whether it is defined as public or abstract, whether it is declared as final if it is a class, and so on. Each type of access information is represented by a hexadecimal flag value. If there are multiple types of access information at the same time, the obtained flag value is the logic or of the Flag values of these types of access information.

This_class, super_class, interfaces

Below is the most format of the field table:

Int m = 2; then there may be an attribute named ConstantValue, which points to constant 2. For more information about attribute_info, see the following description of the Attribute Table project.

For example, in the internal class, in order to maintain access to the external class, the field pointing to the external class instance is automatically added.

If the method table has the Code attribute, its structure will be shown in the following table:

) An exception occurs when the type is catch_type or its subclass (catch_type is the index pointing to a CONSTANT_Class_info constant ).The situation in this article.

). If the variable is not modified by final, or is not of the basic type or string, initialize the variable in the <clinit> method.

In addition, the attribute value of ConstantValue is only limited to the basic type and String type, which is obvious because it can only reference the literal values of the basic type and String type from the constant pool.

The following briefly describes the differences among final, static, and final modified field values: