Class File Structure

Class File Structure

In this tutorial we are going to discuss about Java virtual machine class file structure. Each class file contains the definition of a single class or interface. The class file structure is as shown below.

ClassFile {
    u4               magic_number;
    u2               minor_version;
    u2               major_version;
    u2               constant_pool_count;
    cp_info       constant_pool[constant_pool_count-1];
    u2               access_flags;
    u2               this_class;
    u2               super_class;
    u2               interfaces_count;
    u2               interfaces[interfaces_count];
    u2               fields_count;
    field_info   fields[fields_count];
    u2               methods_count;
    method_info  methods[methods_count];
    u2               attributes_count;
    attribute_info attributes[attributes_count];


  • u1 = unsigned one-byte quantity
  • u2 = unsigned two-byte quantity
  • u4 = unsigned four-byte quantity

Details of the .class file structure as well explained in the Oracle class file format and can be referred by developers for reference. A summary of these fields are as follow.

  • The first 4 bytes of class file is magic number.
  • This is a predefined value to identify the Java class file.
  • This value should be 0xCAFEBABE.
  • JVM will use this value to identify whether the class file is valid or not and also to know whether the class file is generated by valid compiler or not.
  • Whenever we are executing a Java class if JVM unable to find magic_number then we will get runtime error saying java.lang.ClassFormatError : Incompatible magic number
major and minor versions
  • major and minor versions represent class file version
  • JVM will use these versions to identify which version of compiler generates the current .class file
  • If a class file has major version number M and minor version number m, we denote the version of its class file format as M.m.
  • Major and minor versions both are allocates 2 bytes
  • The possible values are
  • When JDK 1.2 introduced support for major version 46, the only minor version supported under that major version was 0. Later JDKs continued the practice of introducing support for a new major version (47, 48, etc) but supporting only a minor version of 0 under the new major version.
major  minor   Java platform version 
 45       3           1.0
 45       3           1.1
 46       0           1.2
 47       0           1.3
 48       0           1.4
 49       0           1.5
 50       0           1.6
 51       0           1.7
 52       0           8
 53       0           9
 54       0           10
 55       0           11
 56       0           12
 57       0           13
 58       0           14
 59       0           15
 60       0           16
 61       0           17
 62       0           18
 63       0           19
 64       0           20
 65       0           21
 66       0           22
 67       0           23

package com.ashok.jvm.test;


 * @author ashok.mariyala
public class ClassVersionChecker {
    private static void checkClassVersion() throws Exception {
        DataInputStream in = new DataInputStream(new FileInputStream("D://Ashok /Test.class"));
        int magic = in.readInt();
        if (magic != 0xcafebabe) {
              System.out.println(" It is not a valid class!");
        int minor = in.readUnsignedShort();
        int major = in.readUnsignedShort();
        System.out.println( major + " . " + minor);
   public static void main(String[] args) throws Exception {


Higher version JVM can always run class files generated by lower version compiler but lower version JVM can’t run class files generated by higher version compiler. If we are trying to run then we will get run time exception saying UnsupportedClassVersionError:Test : unsupported major.minor version.


The value of the constant_pool_count item is equal to the number of entries in the constant_pool table plus one.The constant pool table is where most of the literal constant values are stored. This includes values such as numbers of all sorts, strings, identifier names, references to classes and methods, and type descriptors.


It represents information about constants present in the constant table. The constant_pool is a table of structures representing various string constants, class and interface names, field names, and other constants that are referred to within the ClassFile structure and its substructures. The format of each constant_pool table entry is indicated by its first “tag” byte.


Tag : 7    

Description : The name of a class


Tag : 9   

Description : The name and type of a Field, and the class of which it is a member.  


Tag : 10   

Description : The name and type of a Method, and the class of which it is a member.


Tag : 11  

Description : The name and type of a Interface Method, and the Interface of which it is a member.


Tag : 8    

Description : The index of a CONSTANT_Utf8 entry.


Tag : 3    

Description : 4 bytes representing a Java integer.


Tag : 4    

Description : 4 bytes representing a Java float.


Tag : 5    

Description : 8 bytes representing a Java long.


Tag : 6  

Description : 8 bytes representing a Java double.


Tag : 12  

Description : The Name and Type entry for a field, method, or interface.


Tag : 1  

Description : 2   bytes for the length, then a string in Utf8 (Unicode) format.


Access flags follows the Constant Pool. It is a 2 byte entry that indicates whether the file defines a class or an interface, whether it is public or abstract or final in case it is a class. Below is a list of some of the access flags and their interpretation.

Class File Structure


A class may be marked with the ACC_SYNTHETIC flag to indicate that it was generated by a compiler and does not appear in source code. 


Next 2 bytes after access_flags is this_class. It represents the fully qualified name of the current class.


Next 2 bytes after this_class is super_class. It represents the fully qualified name of the super class.


Next 2 bytes after super_class is interfaces_count. It represents the number of interfaces implemented by the current class.


It represents the names of interfaces implemented by the current class.


It represents the number of fields present in the current class.


It represents field information present in the current class.


It represents the number of methods present in the current class.


It represents method information present in the current class.


It represents the number of attributes present in the current class.


It represents attribute information present in the current class.

That’s all about class file structure in JVM. If you have any queries or feedback, please write us at Enjoy learning, Enjoy JVM..!!

Class File Structure
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