JDK, JRE & JVM

JDK builds it, JRE runs it, JVM executes it. The JDK contains the JRE, and the JRE contains the JVM.

Java source becomes bytecode once, then the JVM loads, verifies, interprets, and JIT-compiles the hot paths into native code. It interprets first, compiles the busy parts.

The specification is the universal rulebook. The implementation is the software that follows the rulebook. Bytecode is portable across all implementations, but each implementation is a native program for a specific platform.

Java compiles to platform-neutral bytecode instead of native machine code. Each platform has its own JVM that translates that bytecode into native instructions at runtime. That is the real meaning of write once, run anywhere.