How does the JIT Compiler improve performance?

The JVM starts by interpreting bytecode, and the JIT compiles frequently executed (hot) methods and loops to native machine code at runtime.

HotSpot uses tiered compilation: C1 (client) compiles quickly with light optimization, then C2 (server) recompiles the hottest code with aggressive optimizations. Because it profiles the actual running workload, the JIT applies speculative optimizations — inlining, loop unrolling, dead-code elimination, escape analysis — that a static compiler cannot, and it deoptimizes back to the interpreter if a speculative assumption (e.g. a monomorphic call site) is later violated.