What is the bytecode structure of JVM in Java?

1. Magic number

The first four bytes of all .class files are magic numbers, and the fixed value of the magic number is 0xCAFEBABE. The magic number is placed at the beginning of the file, and the JVM can determine whether the file is likely to be a .class file based on the beginning of the file. If this is the case, subsequent operations will continue.

2. Version number

The version number is the 4 bytes after the magic number. The first 2 bytes show the next version number, and the last 2 bytes Displays the major version number.

The bytes after the major version number are the constant pool entries. There are two types of constants stored in the constant pool: literals and symbolic references. Literals are declared as Final constant values ​​in the code, and symbols refer to the globally qualified names of categories and interfaces, the names and specifiers of fields, and the names and specifiers of methods. The constant pool is divided into two parts: the constant pool counter and the constant pool data area.

3. Constant pool

The two bytes after the end of the constant pool describe whether the Class is a class or an interface, and whether it uses Public, Abstract, Final, etc. Decorator decoration.

4. Access flag

The two bytes after the end of the constant pool describe whether the Class is a class or an interface, and whether it is modified by Public, Abstract, Final, etc. Talisman modification.

5. Current class name

The two bytes after the access flag describe the fully qualified name of the current class. The value saved by these two bytes is the index value in the constant pool. According to the index value, the fully qualified name of this class can be found in the constant pool.

6. Parent class name

The two bytes after the current class name describe the fully qualified name of the parent class, the same as above, and are also saved in the constant pool. index value.

7. Interface information

After the parent class name is a two-byte interface counter, describing the number of interfaces implemented by this class or the parent class. The next n bytes are the index values ​​of string constants for all interface names.

8. Field table

The field table is used to describe variables declared in classes and interfaces, including class-level variables and instance variables, but does not include method internal declarations local variables.

What are the basic data types of java

The basic data types of Java are divided into:

1. Integer type, used to represent the data type of integer.

2. Floating point type, a data type used to represent decimals.

3. Character type. The keyword of character type is "char".

4. Boolean type is the basic data type that represents logical values.

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