JRuby
By the JRuby project
JRuby is an implementation of the Ruby language that runs on the Java Virtual Machine, translating Ruby source into JVM bytecode instead of the bytecode produced by the reference MRI (Matz's Ruby Interpreter). It allows Ruby code to call…
Definition
JRuby is an implementation of the Ruby language that runs on the Java Virtual Machine, translating Ruby source into JVM bytecode instead of the bytecode produced by the reference MRI (Matz's Ruby Interpreter). It allows Ruby code to call Java libraries directly and lets Java applications host Ruby as an embedded scripting language, giving Ruby developers access to the JVM's threading model and its large ecosystem of compiled libraries.
Overview
JRuby was built to give Ruby developers a path onto the Java Virtual Machine, a platform with mature garbage collection, native OS threading, and an enormous existing library of production-grade code that Ruby's reference implementation could not reach directly. Rather than running Ruby and Java as separate processes that talk over a socket or shell interface, JRuby compiles Ruby source into JVM bytecode at runtime, so Ruby objects and Java objects can call each other's methods within the same call stack. Internally, JRuby's compiler parses Ruby source into an abstract syntax tree, applies successive intermediate representations, and emits JVM bytecode that the JVM's just-in-time compiler can then optimize over the life of a running program, often outperforming MRI on long-running, compute-heavy workloads once the JIT has warmed up. Because the JVM provides true native OS threads rather than a global interpreter lock, JRuby can execute Ruby code across multiple CPU cores concurrently, a capability the standard MRI interpreter has historically restricted. JRuby occupies the same conceptual niche for Ruby that Jython occupies for Python: both compile a dynamic scripting language to JVM bytecode to gain interoperability and platform reach. It is distinct from TruffleRuby, a newer Ruby implementation built on GraalVM's Truffle framework that targets even higher peak performance through more aggressive dynamic optimization, at the cost of a different tuning and deployment profile. In practice, JRuby is used to run Ruby web applications, including Rails applications, on JVM application servers so they can be deployed alongside existing Java infrastructure and monitored with standard JVM tooling. It is also used to call Java libraries that have no Ruby equivalent, and in enterprises that standardize on the JVM for operational reasons but want development teams to keep writing in Ruby. The trade-offs are startup time and native-extension compatibility: JVM startup and JIT warm-up make JRuby slower to begin executing short-lived scripts than MRI, and Ruby C extensions written against MRI's native API generally do not work unmodified on JRuby. Teams running short command-line scripts or depending heavily on MRI-specific C extensions are usually better served by MRI, while long-running server workloads that benefit from JVM concurrency and tooling are where JRuby's design pays off. Operationally, teams running JRuby also inherit standard JVM concerns such as heap sizing and garbage collector tuning, which is unfamiliar territory for Ruby developers used to MRI's simpler memory model but becomes routine once a team has JVM operations experience in-house.
Key Features
- Compiles Ruby source into JVM bytecode for execution
- Allows Ruby code to call Java libraries and classes directly
- Supports true concurrent multithreading using native JVM threads
- Benefits from the JVM's mature garbage collector and JIT compiler
- Enables deployment of Ruby and Rails apps on Java application servers
- Provides interoperability with the broader Java library ecosystem
- Ships as an open-source project independent of MRI Ruby
- Integrates with standard JVM monitoring and profiling tools