In the Java programming language, source files (.java files) are compiled into class files which have a .class extension. Since Java is a platform-independent language, source code is compiled into an output file known as bytecode, which it stores in a .class file. If a source file has more than one class, each class is compiled into a separate .class file. These .class files can be loaded by any Java Virtual Machine (JVM).
Since JVMs are available for many platforms, the .class file compiled in one platform will execute in a JVM of another platform. This makes Java platform-independent.
There is a handy mnemonic for remembering these 10: My Very Cute Animal Turns Savage In Full Moon Areas.
Magic, Version, Constant, Access, This, Super, Interfaces, Fields, Methods, Attributes (MVCATSIFMA)
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 =|
|magic number (CAFEBABE) used to identify file as conforming to the class file format|
|1|| u1 =|
|2|| u1 =|
|3|| u1 =|
|4||2 bytes||u2||minor version number of the class file format being used|
|6||2 bytes||u2||major version number of the class file format being used. J2SE 6.0=50, J2SE 5.0=49, JDK 1.4=48, JDK 1.3=47, JDK 1.2=46, JDK 1.1=45. For details of earlier version numbers see footnote 1 at The JavaTM Virtual Machine Specification 2nd edition|
|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.|
|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|
|12+cpsize||2 bytes||u2||identifies this class, index into the constant pool to a "Class"-type entry|
|14+cpsize||2 bytes||u2||identifies super class, index into the constant pool to a "Class"-type entry|
|16+cpsize||2 bytes||u2||interface count, number of entries in the following interface table|
|18+cpsize||isize (variable)||table||interface table, an array of variable-sized interfaces|
|18+cpsize+isize||2 bytes||u2||field count, number of entries in the following field table|
|20+cpsize+isize||fsize (variable)||table||field table, variable length array of fields|
|20+cpsize+isize+fsize||2 bytes||u2||method count, number of entries in the following method table|
|22+cpsize+isize+fsize||msize (variable)||table||method table, variable length array of methods|
|22+cpsize+isize+fsize+msize||2 bytes||u2||attribute count, number of entries in the following attribute table|
|24+cpsize+isize+fsize+msize||asize (variable)||table||attribute table, variable length array of attributes|
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. All indexes, or references, to specific constants in the constant pool table are given by 16-bit (type u2) numbers, where index value 1 refers to the first constant in the table (index value 0 is invalid).
Due to historic choices made during the file format development, the number of constants in the constant pool table is not actually the same as the constant pool count which precedes the table. First, the table is indexed starting at 1 (rather than 0), so the count should actually be interpreted as the maximum index. Additionally a couple types of constants, namely longs and doubles, take up two consecutive slots in the table, although the second such slot is a phantom index that is never directly used.
The type of each item (constant) in the constant pool is identified by an initial byte tag. The number of bytes following this tag and their interpretation are then dependent upon the tag value. The legal constant types and their tag values are:
|tag byte||additional bytes||description of constant|
|1|| 2+x bytes|
|UTF-8 (Unicode) string: a character string prefixed by a 16-bit number (type u2) indicating the number of bytes in the encoded string which immediately follows (which may be different than the number of characters). Note that the encoding used is not actually UTF-8, but involves a slight modification of the Unicode standard encoding form.|
|3||4 bytes||Integer: a signed 32-bit two's complement number in big-endian format|
|4||4 bytes||Float: a 32-bit single-precision IEEE 754 floating-point number|
|5||8 bytes||Long: a signed 64-bit two's complement number in big-endian format (takes two slots in the constant pool table)|
|6||8 bytes||Double: a 64-bit double-precision IEEE 754 floating-point number (takes two slots in the constant pool table)|
|7||2 bytes||Class reference: an index within the constant pool to a UTF-8 string containing the fully-qualified class name (in internal format)|
|8||2 bytes||String reference: an index within the constant pool to a UTF-8 string|
|9||4 bytes||Field reference: two indexes within the constant pool, the first pointing to a Class reference, the second to a Name and Type descriptor.|
|10||4 bytes||Method reference: two indexes within the constant pool, the first pointing to a Class reference, the second to a Name and Type descriptor.|
|11||4 bytes||Interface method reference: two indexes within the constant pool, the first pointing to a Class reference, the second to a Name and Type descriptor.|
|12||4 bytes||Name and type descriptor: two indexes to UTF-8 strings within the constant pool, the first representing a name (identifier) and the second a specially-encoded type descriptor.|
There are only two integral constant types, integer and long. Other integral types appearing in the high-level language, such as boolean, byte, and short must be represented as an integer constant.
Class names in Java, when fully qualified, are traditionally dot-separated, such as "java.lang.Object". However within the low-level Class reference constants, an internal form appears which uses slashes instead, such as "java/lang/Object".
The Unicode strings, despite the moniker "UTF-8 string", are not actually encoded according to the Unicode standard, although it is similar. There are two differences (see UTF-8 for a complete discussion). The first is that the codepoint U+0000 is encoded as the two-byte sequence
C0 80 (in hex) instead of the standard single-byte encoding
00. The second difference is that supplementary characters (those outside the BMP at U+10000 and above) are encoded using a surrogate-pair construction similar to UTF-16 rather than being directly encoded using UTF-8. In this case each of the two surrogates is encoded separately in UTF-8. For example U+1D11E is encoded as the 6-byte sequence
ED A0 B4 ED B4 9E, rather than the correct 4-byte UTF-8 encoding of
f0 9d 84 9e.
"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."