Bytecode

Java Bytecode Manipulation & JVM Languages

The Java bytecode format (class files) is a stable, documented target. Because the JVM executes bytecode rather than source, any JVM language can compile to it, and runtime libraries can generate or transform classes after compilation. This is what powers proxies, ORMs, mocking frameworks and most modern frameworks' cleverness.

If you need to read or transform class files directly, ASM is the low-level Swiss-army knife with the smallest dependency footprint. If you want to generate classes at runtime with a friendlier API, Byte Buddy raises the level far above raw ASM. For whole programs on the JVM, the alternative-language ecosystem (Kotlin, Groovy, Scala, Clojure) is the practical way to express more with less.

What you'll find here: Low-level class-file reading & rewriting with ASM · Runtime code generation & proxies with Byte Buddy · Understand the class-file & verification model · The JVM language ecosystem compiled to bytecode

Bytecode tools

ASM
The low-level, battle-tested bytecode library used across the whole ecosystem (every JDK since JDK 9 uses it internally) for reading and rewriting class files.
open-sourceBSD-3
Byte Buddy
A high-level runtime class-generation library with a fluent API; the easiest way to create dynamic proxies, agents, and generated types without writing assembly.
open-sourceApache-2.0
Javassist
A bytecode toolkit that lets you edit class files using Java-source-level snippets rather than low-level opcodes; handy when ASM feels too low-level.
open-sourceApache-2.0 + LGPL
cglib
The older, classic bytecode-generation library famous for powering Spring proxies; still broadly deployed, though mostly superseded by Byte Buddy.
open-sourceApache-2.0
Kotlin
The most successful JVM language besides Java; a modern, null-safe, concise language that compiles to bytecode and fully interoperates with existing Java.
open-sourceApache-2.0
Groovy
A dynamic, optionally-typed JVM language with a scripting-friendly feel; compiles to bytecode and is the base of the Grails web framework.
open-sourceApache-2.0
Scala
A statically-typed, functional-first JVM language; its compiler emits bytecode that interoperates with Java while enabling richer type systems.
open-sourceApache-2.0
GraalVM (Truffle & native-image)
Oracle's high-performance JDK featuring a polyglot interpreter (Truffle) and native-image, which compiles JVM applications ahead-of-time to native binaries.
open-sourceGPLv2 with CE

In-depth guides in this category

Byte Buddy
Byte Buddy tutorial: create dynamic proxies and classes at runtime, intercept methods, and understand when it beats raw ASM.
ASM
ASM tutorial: the low-level bytecode library used across the JVM ecosystem — read class files, generate method code, and transform loaded classes.