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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN"> <html> <head> <title>VM Spec The class File Format</title> </head> <body BGCOLOR=#eeeeff text=#000000 LINK=#0000ff VLINK=#000077 ALINK=#ff0000> <table width=100%><tr> <td><a href="VMSpecTOC.doc.html">Contents</a> | <a href="Overview.doc.html">Prev</a> | <a href="ConstantPool.doc.html">Next</a> | <a href="VMSpecIX.fm.html">Index</a></td><td align=right>The JavaTM Virtual Machine Specification</td> </tr></table> <hr> <a name="80959"></a> CHAPTER 4 <a name="80961"></a> <h1>The <code>class</code> File Format</h1> <hr> <a name="80963"></a> This chapter describes the Java virtual machine <code>class</code> file format. Each <code>class</code> file contains the definition of a single class or interface. Although a class or interface need not have an external representation literally contained in a file (for instance, because the class is generated by a class loader), we will colloquially refer to any valid representation of a class or interface as being in the <code>class</code> file format. <a name="14580"></a> A <code>class</code> file consists of a stream of 8-bit bytes. All 16-bit, 32-bit, and 64-bit quantities are constructed by reading in two, four, and eight consecutive 8-bit bytes, respectively. Multibyte data items are always stored in big-endian order, where the high bytes come first. In the Java and Java 2 platforms, this format is supported by interfaces <code>java.io.DataInput</code> and <code>java.io.DataOutput</code> and classes such as <code>java.io.DataInputStream</code> and <code>java.io.DataOutputStream</code>. <a name="6289"></a> This chapter defines its own set of data types representing <code>class</code> file data: The types <code>u1</code>, <code>u2</code>, and <code>u4</code> represent an unsigned one-, two-, or four-byte quantity, respectively. In the Java and Java 2 platforms, these types may be read by methods such as <code>readUnsignedByte</code>, <code>readUnsignedShort</code>, and <code>readInt</code> of the interface <code>java.io.DataInput</code>. <a name="41600"></a> This chapter presents the <code>class</code> file format using pseudostructures written in a C-like structure notation. To avoid confusion with the fields of classes and class instances, etc., the contents of the structures describing the <code>class</code> file format are referred to as items. Successive items are stored in the <code>class</code> file sequentially, without padding or alignment. <a name="74328"></a> Tables, consisting of zero or more variable-sized items, are used in several <code>class</code> file structures. Although we use C-like array syntax to refer to table items, the fact that tables are streams of varying-sized structures means that it is not possible to translate a table index directly to a byte offset into the table. <a name="74331"></a> Where we refer to a data structure as an array, it consists of zero or more contiguous fixed-sized items and can be indexed like an array. <a name="74353"></a> <hr><h2>4.1 The <code>ClassFile</code> Structure</h2> <a name="44115"></a> A <code>class</code> file consists of a single <code>ClassFile</code> structure: <pre> <a name="19050"></a>    <code>ClassFile { </code>    <code> u4 magic; </code>    <code> u2 minor_version; </code>    <code> u2 major_version; </code>    <code> u2 constant_pool_count; </code>    <code> cp_info constant_pool[constant_pool_count-1]; </code>    <code> u2 access_flags; </code>    <code> u2 this_class; </code>    <code> u2 super_class; </code>    <code> u2 interfaces_count; </code>    <code> u2 interfaces[interfaces_count]; </code>    <code> u2 fields_count; </code>    <code> field_info fields[fields_count]; </code>    <code> u2 methods_count; </code>    <code> method_info methods[methods_count]; </code>    <code> u2 attributes_count; </code>    <code> attribute_info attributes[attributes_count]; </code><a name="19084"></a>    <code>} </code> </pre><a name="7456"></a> The items in the <code>ClassFile</code> structure are as follows: <a name="19719"></a> <dl><dt><code>magic</code><a name="21106"></a> <dd> The <code>magic</code> item supplies the magic number identifying the <code>class</code> file format; it has the value <code>0xCAFEBABE</code>. <a name="13448"></a> <dt><code>minor_version</code>, <code>major_version</code><a name="70425"></a> <dd> The values of the <code>minor_version</code> and <code>major_version</code> items are the minor and major version numbers of this <code>class</code> file.Together, a major and a minor version number determine the version of the <code>class</code> file format. If a <code>class</code> file has major version number M and minor version number m, we denote the version of its <code>class</code> file format as M.m. Thus, <code>class</code> file format versions may be ordered lexicographically, for example, 1.5 < 2.0 < 2.1. A Java virtual machine implementation can support a <code>class</code> file format of version v if and only if v lies in some contiguous range Mi.0 <img src="chars/lt_equal.gif"> v <img src="chars/lt_equal.gif"> Mj.m. Only Sun can specify what range of versions a Java virtual machine implementation conforming to a certain release level of the Java platform may support.<a href="#75883">1</a> <a name="13480"></a> <dt><code>constant_pool_count</code><a name="31689"></a> <dd> The value of the <code>constant_pool_count</code> item is equal to the number of entries in the <code>constant_pool</code> table plus one. A <code>constant_pool</code> index is considered valid if it is greater than zero and less than <code>constant_pool_count</code>, with the exception for constants of type <code>long</code> and <code>double</code> noted in <a href="ClassFile.doc.html#1348">§4.4.5</a>. <a name="29202"></a> <dt><code>constant_pool[]</code><a name="19096"></a> <dd> The <code>constant_pool</code> is a table of structures <a href="ClassFile.doc.html#20080">(§4.4)</a> representing various string constants, class and interface names, field names, and other constants that are referred to within the <code>ClassFile</code> structure and its substructures. The format of each <code>constant_pool</code> table entry is indicated by its first "tag" byte. The <code>constant_pool</code> table is indexed from <code>1</code> to <code>constant_pool_count</code>-<code>1</code>. <a name="13484"></a> <dt><code>access_flags</code><a name="23242"></a> <dd> The value of the <code>access_flags</code> item is a mask of flags used to denote access permissions to and properties of this class or interface. The interpretation of each flag, when set, is as shown in <a href="ClassFile.doc.html#75734">Table 4.1</a>. </dl> <a name="75734"></a><Table Border="1"> <tr><td><a name="75687"></a> Flag Name <td><a name="75689"></a> Value <td><a name="75691"></a> Interpretation <tr><td><a name="75695"></a> <code>ACC_PUBLIC</code> <td><a name="75697"></a> <code>0x0001</code> <td><a name="75699"></a> Declared <code>public</code>; may be accessed from outside its package. <tr><td><a name="75703"></a> <code>ACC_FINAL</code> <td><a name="75705"></a> <code>0x0010</code> <td><a name="75707"></a> Declared <code>final</code>; no subclasses allowed. <tr><td><a name="75711"></a> <code>ACC_SUPER</code> <td><a name="75713"></a> <code>0x0020</code> <td><a name="75715"></a> Treat superclass methods specially wh