Java is platform independent language How?
Java programming language provides platform independence, what does it mean? It means same Java program can be run on any platform or operating system e.g. Windows, Linux or Solaris without any change. This is the great benefit for someone coming from platform dependent programming language like C or C++ whose code needs to be ported for every single platform because they use native libraries, which differ in every platform. Now question comes how Java achieve platform independence, what makes Java programs running on every platform without any change? This is one of the most basis question Java programmer ask when they start learning Java programming language. If you read further you will come to know about class files, bytecode and Java virtual machine which together provides platform independence to Java. One of the simplest analogies I can associate with platform independence is the person taking red carpet with him and instead of walking on floor, he always walks on red carpet, no matter where he is walking. That red carpet is the JVM; your Java program runs on JVM rather on any particular platform or machine.
Java Runtime Environment (JRE)
Java is everywhere in browser, in mobile, in TV or in set-top boxes and if you are into Java programming language than you know that Java code which is bundled in JAR (Java archive) file require Java virtual machine JVM to execute it. Now JVM is an executable or program like any other program and you can install that into your machine. You have seen browser often suggesting download JRE to run a Java Applet downloaded from Internet. Various version of JRE are available in java.oracle.com and most of the user who just want to execute Java program inside browser or standalone downloads JRE. All browsers including Internet Explorer, Firefox and Chrome can work with JRE.
Java Virtual Machine (JVM)
When you download JRE and install on your machine you got all the code required to create JVM. Java Virtual Machine is get created when you run a java program using java command e.g. java Hello World. JVM is responsible for converting byte code into machine specific code and that’s why you have different JVM for Windows, Linux or Solaris but one JAR can run on all this operating system. Java Virtual machine is at heart of Java programming language and provide several feature to Java programmer including Memory Management and Garbage Collection, Security and other system level services. Java Virtual Machine can be customized e.g we can specify starting memory or maximum memory of heap size located inside JVM at the time of JVM creation. If we supplied invalid argument to java command it may refuse to create Java Virtual Machine by saying “failed to create Java virtual machine: invalid argument”. In short Java Virtual Machine or JVM is the one who provides Platform independence to Java.
Java Development Kit (JDK)
JDK is also loosely referred as JRE but its lot more than JRE and it provides all the tools and executable require to compile debug and execute Java Program. Just like JRE, JDK is also platform specific and you need to use separate installer for installing JDK on Linux and Windows. Current Version of JDK is 1.7 which is also referred as Java7 and it contains javac (java compiler) based on programming rules of Java7 and Java which can execute java7 code with new features like String in Switch, fork-join framework or Automatic Resource Management. When you install JDK, installation folder is often referred as JAVA_HOME. All binaries are located inside JAVA_HOME/bin which includes javac, java and other binaries and they must be in your system PATH in order to compile and execute Java programs.
DIFFERENCE BETWEEN JRE, JDK AND JVM
In short here are few differences between JRE, JDK and JVM:
1) JRE and JDK come as installer while JVM are bundled with them.
2) JRE only contain environment to execute java program but doesn’t contain other tool for compiling java program.
3) JVM comes along with both JDK and JRE and created when you execute Java program by giving “java” command.
Just in Time Compiler (JIT)
Initially Java has been accused of poor performance because it’s both compiles and interpret instruction. Since compilation or Java file to class file is independent of execution of Java program do not confuse. Here compilation word is used for byte code to machine instruction translation. JIT are advanced part of Java Virtual machine which optimize byte code to machine instruction conversion part by compiling similar byte codes at same time and thus reducing overall execution time. JIT is part of Java Virtual Machine and also performs several other optimizations such as in-lining function.