What are JDK, JRE and JVM?

JDK, JRE and JVM

JDK, JRE and JVM are most commonly heard in the Java programming language.

Many people get confused with these 3 terms or they think that they are one and the same, unfortunately they are not synonyms, and each one has got its own responsibility.



What is JVM?


1.      It stands for Java Virtual Machine
2.      JVM is responsible for converting byte code into machine specific code because of which we have different JVM for different Operating system, like different JVM for Windows, Linux or Solaris.
3.      But all the JVMs understand the byte code given to it no matter from which machine the byte code got created. This makes Java platform independent.
4.      The Java Virtual machine (JVM) is the virtual machine that runs the Java bytecodes. The JVM doesn't understand Java source code, that's why you compile your *.java files to obtain *.class files that contain the bytecodes understandable by the JVM. It's also the entity that allows Java to be a "portable language" (write once, run anywhere). Indeed there are specific implementations of the JVM for different systems (Windows, Linux, MacOS).
5.      As we all aware when we compile a Java file, output is not an ‘exe’ but it’s a ‘.class’ file. ‘.class’ file consists of Java byte codes which are understandable by JVM. Java Virtual Machine interprets the byte code into the machine code depending upon the underlying operating system and hardware combination. It is responsible for all the things like garbage collection, array bounds checking, etc… JVM is platform dependent.
6.      JVM also provides several paramount features including Memory Management, Garbage Collection and Security etc.
7.      We can customize the JVM by using Java options.
8.      We can allocate maximum and minimum memory for Heap residing in JVM.
9.      JVM is Virtual because it does not exist physically; we can’t install JVM as it comes with JRE.
10.  The JVM is called “virtual” because it provides a machine interface that does not depend on the underlying operating system and machine hardware architecture. This independence from hardware and operating system is a cornerstone of the write-once run-anywhere value of Java programs.
11.  There are different JVM implementations are there. These may differ in things like performance, reliability, speed, etc. These implementations will differ in those areas where Java specification doesn’t mention how to implement the features, like how the garbage collection process works is JVM dependent, Java spec doesn’t define any specific way to do this.
12. The JVM performs following main tasks:
o    Loads code
o    Verifies code
o    Executes code
o    Provides runtime environment

 

What is JRE?


1.      It stands for Java Runtime Environment
2.      It is the implementation of JVM, it physically exists, and hence we can install JRE.
3.      It provides the platform to execute/run the java programs.
4.      It consists of JVM and other libraries like rt.jar which are required to execute the Java program.
5.      We can run any Java program using JRE but we can’t compile the java code using JRE as it does not contain any compiler, debugger etc.
6.      We can see JRE installed in many places like Broswers, TV, Mobile, set-top boxes and most of the electronic gadgets.
7.      JRE in these places uses the JVM and executes the byte code provides in the form of JAR.
8.      JRE = JVM + Required Library to run Application
9.      The Java Runtime Environment (JRE) provides the libraries, the Java Virtual Machine, and other components to run applets and applications written in the Java programming language. In addition, two key deployment technologies are part of the JRE: Java Plug-in, which enables applets to run in popular browsers; and Java Web Start, which deploys standalone applications over a network. It is also the foundation for the technologies in the Java 2 Platform, Enterprise Edition (J2EE) for enterprise software development and deployment. The JRE does not contain tools and utilities such as compilers or debuggers for developing applets and applications.
10.  Java Runtime Environment contains JVM, class libraries, and other supporting files. It does not contain any development tools such as compiler, debugger, etc. Actually JVM runs the program, and it uses the class libraries, and other supporting files provided in JRE. If you want to run any java program, you need to have JRE installed in the system
11.  The Java Virtual Machine provides a platform-independent way of executing code; that mean compile once in any machine and run it anywhere (any machine).

What is JDK?

1.      It stands for Java Development Kit and it physically exists and we can install it.
2.      It contains JRE + Development tools like compiler, debugger etc.
3.      Since it contains JRE along with compiler, we can write java code and compile the java code.
4.      Similar to JRE, JDK is also platform specific, we need to use separate JDK for different Operating system like Windows, Linux etc.
5.      All the libraries like Java, Javac etc exist inside the bin folder of JDK installed path.
6.      The JDK is a superset of the JRE, and contains everything that is in the JRE, plus tools such as the compilers and debuggers necessary for developing applets and applications.
7.      Java Developer Kit contains tools needed to develop the Java programs, and JRE to run the programs. The tools include compiler (javac.exe), Java application launcher (java.exe), Appletviewer, etc…
8.      Compiler converts java code into byte code. Java application launcher opens a JRE, loads the class, and invokes its main method.
9.      You need JDK, if at all you want to write your own programs, and to compile them. For running java programs, JRE is sufficient.
10.  JRE is targeted for execution of Java files
11.  i.e. JRE = JVM + Java Packages Classes (like util, math, lang, awt,swing etc)+runtime libraries.
12.  JDK is mainly targeted for java development. You can create a Java file (with the help of Java packages), compile a Java file and run a java file.

 

Difference between JDK, JRE and JVM


·         JDK is a super set of JRE which contains the JRE along with compiler; hence it can be used for development purposes.

·         JRE contains JVM and other run time libraries required to execute the program.
·         It does not contain other development tools like compiler, debugger etc.
·         Hence we can install JRE only to run the Java applications not to develop them.

·         JVM cannot be installed as it does not have any physical existence.
·         It comes with JRE and JDK installation.
·         It provides platform independence by converting the bytecode obtained from any machine into corresponding machine instructions but it is created only when we execute the java program.

What is JIT Compiler?


1.      It stands for Just In Time Compiler, it was introduced to improve the performance of Java Virtual machine.
2.      As we know JVM converts the bytes code into machine instructions, JIT helps to improve this operation by converting similar byte codes into machine instruction at the same time.
3.      Hence it reduces the overall execution time of program.
4.      JIT is also a part of JVM.

The below chart shows the different features of each of the Java technologies.




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