Java™ How To Program (Early Objects), Tenth Edition

Phase 5: Execution

In Phase 5, the JVM executes the program’s bytecodes, thus performing the actions specified by the program (Fig. 1.10). In early Java versions, the JVM was simply an interpreter for Java bytecodes. Most Java programs would execute slowly, because the JVM would interpret and execute one bytecode at a time. Some modern computer architectures can execute several instructions in parallel. Today’s JVMs typically execute bytecodes using a combination of interpretation and so-called just-in-time (JIT) compilation. In this process, the JVM analyzes the bytecodes as they’re interpreted, searching for hot spots—parts of the bytecodes that execute frequently. For these parts, a just-in-time (JIT) compiler, such as Oracle’s Java HotSpot™ ...