JVM, JDK and JRE are most commonly used in java programming.
Many people think that these are same or gets confused about their differences.
JVM, JDK and JRE are interlinked with each other and works together in
cooperative manner.
- JVM (Java Virtual Machine): Interpreter + JIT
JVM is code executing component of Java
that executes byte code (.class file). The output of Java compilation is byte
code and only JVM can understand the byte code. JVM interprets this byte code
to native code depending upon underlying processor and architecture. JVM has
two main components interpreter and Just In Time (JIT) compiler. JIT is
compiler which helps for optimizing byte code to machine code conversion and
hence results into reduced execution time.
Note that Java is platform independent but JVM is not platform
independent. Each JVM has its own structure of executable which differs from
platform to platform. For each OS there is different JVM available. So JVM is
highly platform dependent.
- JRE (Java Runtime Environment): JVM + byte code verifier + class loader
JRE contains an environment for execution
of Java program. JRE consist of JVM, class loader and byte code verifier. Class
loader module loads all the .class files (byte code) and required libraries
which are then verified by byte code verifier. Then this verified code is
interpreted by JVM which creates native code.
JDK contains tools like compiler and debugger
to develop Java program. It provides predefined set of libraries, API classes
and other files which is used by JVM at runtime. It also includes JRE,
interpreter (java), compiler (javac), document generator (javadoc).
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