JVM and bytecode overview

Java source compiles to bytecode—platform-neutral instructions for the JVM. That indirection is why the same JAR can run on macOS dev machines and Linux servers without recompiling source for each OS.

Key pieces

• JDK — Java Development Kit: compiler (javac), tools, and a JRE
• JRE — Java Runtime Environment: JVM + core libraries (mostly superseded by modular JDK images)
• JVM — loads classes, verifies bytecode, executes with interpreter + JIT
• .class files — one per class; packaged into JARs for deployment

Class loading and execution

1. Class loader finds Main.class on the classpath
2. Bytecode verifier checks safety constraints
3. JVM invokes Main.main as the entry point
4. Hot methods get JIT-compiled to native code for speed

Important interview questions and answers

1. Q: JDK vs JRE?
   A: JDK includes development tools (compiler); JRE is runtime-only—modern deployments often use jlink/custom JDK images.
2. Q: What is bytecode?
   A: Intermediate instructions for the JVM—not machine code for a specific CPU.
3. Q: Can other languages run on the JVM?
   A: Yes—Kotlin, Scala, Clojure compile to bytecode and interoperate with Java libraries.

Self-check

1. Which tool compiles .java to .class?
2. Why does bytecode help portability?