Java class file

Java class file
Internet media type	application/java-vm, application/x-httpd-java, application/x-java, application/java, application/java-byte-code, application/x-java-class, application/x-java-vm
Developed by	Sun Microsystems

A Java class file is a file (with the .class filename extension) containing Java bytecode that can be executed on the Java Virtual Machine (JVM). A Java class file is usually produced by a Java compiler from Java programming language source files (

On 11 December 2006, the class file format was modified under Java Specification Request (JSR) 202.

There are 10 basic sections to the Java class file structure:

• Magic Number: 0xCAFEBABE
• Version of Class File Format: the minor and major versions of the class file
• Constant Pool: Pool of constants for the class
• Access Flags: for example whether the class is abstract, static, etc.
• This Class: The name of the current class
• Super Class: The name of the super class
• Interfaces: Any interfaces in the class
• Fields: Any fields in the class
• Methods: Any methods in the class
• Attributes: Any attributes of the class (for example the name of the sourcefile, etc.)

Class files are identified by the following 4 byte header (in hexadecimal): CA FE BA BE (the first 4 entries in the table below). The history of this magic number was explained by James Gosling referring to a restaurant in Palo Alto:

"We used to go to lunch at a place called St Michael's Alley. According to local legend, in the deep dark past, the Grateful Dead used to perform there before they made it big. It was a pretty funky place that was definitely a Grateful Dead Kinda Place. When Jerry died, they even put up a little Buddhist-esque shrine. When we used to go there, we referred to the place as Cafe Dead. Somewhere along the line it was noticed that this was a HEX number. I was re-vamping some file format code and needed a couple of magic numbers: one for the persistent object file, and one for classes. I used CAFEDEAD for the object file format, and in grepping for 4 character hex words that fit after "CAFE" (it seemed to be a good theme) I hit on BABE and decided to use it. At that time, it didn't seem terribly important or destined to go anywhere but the trash-can of history. So CAFEBABE became the class file format, and CAFEDEAD was the persistent object format. But the persistent object facility went away, and along with it went the use of CAFEDEAD - it was eventually replaced by RMI."

Because the class file contains variable-sized items and does not also contain embedded file offsets (or pointers), it is typically parsed sequentially, from the first byte toward the end. At the lowest level the file format is described in terms of a few fundamental data types:

• u1: an unsigned 8-bit integer
• u2: an unsigned 16-bit integer in big-endian byte order
• u4: an unsigned 32-bit integer in big-endian byte order
• table: an array of variable-length items of some type. The number of items in the table is identified by a preceding count number (the count is a u2), but the size in bytes of the table can only be determined by examining each of its items.

Some of these fundamental types are then re-interpreted as higher-level values (such as strings or floating-point numbers), depending on context. There is no enforcement of word alignment, and so no padding bytes are ever used. The overall layout of the class file is as shown in the following table.

Byte offset	Size	Type or value	Description
0	4 bytes	u1 = 0xCA hex	magic number (CAFEBABE) used to identify file as conforming to the class file format
1		u1 = 0xFE hex
2		u1 = 0xBA hex
3		u1 = 0xBE hex
4	2 bytes	u2	minor version number of the class file format being used
5
6	2 bytes	u2	major version number of the class file format being used. Java SE 28 = 72 (0x48 hex), Java SE 27 = 71 (0x47 hex), Java SE 26 = 70 (0x46 hex), Java SE 25 = 69 (0x45 hex), Java SE 24 = 68 (0x44 hex), Java SE 23 = 67 (0x43 hex), Java SE 22 = 66 (0x42 hex), Java SE 21 = 65 (0x41 hex), Java SE 20 = 64 (0x40 hex), Java SE 19 = 63 (0x3F hex), Java SE 18 = 62 (0x3E hex), Java SE 17 = 61 (0x3D hex), Java SE 16 = 60 (0x3C hex), Java SE 15 = 59 (0x3B hex), Java SE 14 = 58 (0x3A hex), Java SE 13 = 57 (0x39 hex), Java SE 12 = 56 (0x38 hex), Java SE 11 = 55 (0x37 hex), Java SE 10 = 54 (0x36 hex), Java SE 9 = 53 (0x35 hex), Java SE 8 = 52 (0x34 hex), Java SE 7 = 51 (0x33 hex), Java SE 6.0 = 50 (0x32 hex), Java SE 5.0 = 49 (0x31 hex), JDK 1.4 = 48 (0x30 hex), JDK 1.3 = 47 (0x2F hex), JDK 1.2 = 46 (0x2E hex), JDK 1.1 = 45 (0x2D hex). For details of earlier version numbers see footnote 1 at The JavaTM Virtual Machine Specification 2nd edition
7
8	2 bytes	u2	constant pool count, number of entries in the following constant pool table. This count is at least one greater than the actual number of entries; see following discussion.
9
10	cpsize (variable)	table	constant pool table, an array of variable-sized constant pool entries, containing items such as literal numbers, strings, and references to classes or methods. Indexed starting at 1, containing (constant pool count - 1) number of entries in total (see note).
...
...
...
10+cpsize	2 bytes	u2	access flags, a bitmask
11+cpsize
12+cpsize	2 bytes	u2	identifies this class, index into the constant pool to a "Class"-type entry
13+cpsize
14+cpsize	2 bytes	u2	identifies super class, index into the constant pool to a "Class"-type entry
15+cpsize
16+cpsize	2 bytes	u2	interface count, number of entries in the following interface table
17+cpsize
18+cpsize	isize (variable)	table	interface table: a variable-length array of constant pool indexes describing the interfaces implemented by this class
...
...
...
18+cpsize+isize	2 bytes	u2	field count, number of entries in the following field table
19+cpsize+isize
20+cpsize+isize	fsize (variable)	table	field table, variable length array of fields each element is a field_info structure defined in https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.5
...
...
...
20+cpsize+isize+fsize	2 bytes	u2	method count, number of entries in the following method table
21+cpsize+isize+fsize
22+cpsize+isize+fsize	msize (variable)	table	method table, variable length array of methods each element is a method_info structure defined in https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.6
...
...
...
22+cpsize+isize+fsize+msize	2 bytes	u2	attribute count, number of entries in the following attribute table
23+cpsize+isize+fsize+msize
24+cpsize+isize+fsize+msize	asize (variable)	table	attribute table, variable length array of attributes each element is an attribute_info structure defined in https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.7
...
...
...

The following is a representation of a "JDK 10 Release Notes".

↑ Tim Lindholm, Frank Yellin (1999). The Java Virtual Machine Specification (Second ed.). Prentice Hall. ISBN 0-201-43294-3. Retrieved 2008-10-13. The official defining document of the Java Virtual Machine, which includes the class file format. Both the first and second editions of the book are freely available online for viewing and/or download.

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