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JIT means Just In Time compiler.The JIT used to compile java source file and generate class files.The execution engine means Java Virtual Machine(JVM) which executes the class files A software "execution engine" that safely and compatibly executes the byte codes in Java class files on a Processor.
A module that alternately decodes and executes every statement in some body of code. The Java interpreter decodes and executes bytecode for the Java virtual machine The Java interpreter is actually a part of JVM. Virtual machine is not just executing the bytecodes, it has lot of tasks to do. That full-fledged environment is referred to as JVM.