File extension

.class

Java class file

.class files contain compiled Java bytecode: one class or interface per file, in the class file format defined by the Java Virtual Machine Specification.

Known meanings

Formats and reference material for .class

Java class file

Compiled Java class or interface in JVM bytecode format

  • binaries
  • Oracle
Open link

Details

Format notes

CLASS (Java Class File)

A .class file is produced by a Java compiler and holds the compiled form of one class or interface. It contains bytecode for the Java Virtual Machine (JVM), which loads and runs it on any platform that provides a JVM. Other languages for the JVM, such as Kotlin, Scala, and Groovy, also compile to this format. Class files are usually distributed together in .jar archives.

Structure

Every class file begins with the magic number 0xCAFEBABE, followed by a minor and a major version number. The major version identifies the Java release that produced it; for example, 52 corresponds to Java 8, and each later release increases the number by one. A JVM refuses class files with a version newer than it supports. After the header come the constant pool, access flags, the class and superclass references, interfaces, fields, methods (including their bytecode), and attributes such as debug information and annotations. All multi-byte values are stored big-endian.

Tooling

The JDK includes javap to disassemble class files. Many decompilers can reconstruct source code approximately from them, because bytecode retains much structural information. The 0xCAFEBABE magic is also used by Mach-O universal binaries, so the first bytes alone do not identify the format.

Preservation And Security Notes

Keep the source code and the JDK version used to build when preserving Java software, since class file versions are not backward-compatible with older JVMs. The JVM verifies bytecode when loading, but class files from untrusted sources should still be treated as executable code.

Further Reading

  • The class File Format, Java Virtual Machine Specification: https://docs.oracle.com/javase/specs/jvms/se21/html/jvms-4.html