Differences between JDK, JRE and JVM

In the Java programming language, all source code is first written in plain text files ending with the .java extension. You can write it in any notepad or text editor or IDE. Those source files are then compiled into .class files by the javac compiler.

A .class file does not contain code that is native to your processor,  it instead contains byte codes , the machine language of the Java Virtual Machine (Java VM). The java launcher tool then runs your application with an instance of the Java Virtual Machine.

To understand the difference between JDK, JRE AND JVM please consider below diagram:

                  JDK = JRE +DEVELOPMENT TOOLS

                  JRE = JVM + LIBRARY CLASSES

JAVA DEVELOPMENT KIT (JDK):

The Java Development Kit (JDK) is a software development environment used for developing Java applications and applets. The JDK contains a private Java Virtual Machine (JVM) and a few other resources such as, an interpreter/loader (Java), a compiler (javac), an archiver (jar), a documentation generator (Javadoc) and other tools needed in Java development.

JDK is an acronym for Java Development Kit.  It physically exists. It contains JRE + development tools.

JDK is an implementation of any one of the below given Java Platforms released by Oracle Corporation:

Standard Edition Java Platform

Enterprise Edition Java Platform

Micro Edition Java Platform

 

Java Development Kit (JDK) is a superset of a JRE and contains tools for Java programmers, e.g. a javac compiler. The Java Development Kit is provided free of charge either by Oracle Corporation directly, or by the OpenJDK open source project, which is governed by Oracle.

Java Development Kit (in short JDK) is Kit which provides the environment to develop and execute(run) the Java program. JDK is a kit(or package) which includes two things

Development Tools(to provide an environment to develop your java programs)

JRE (to execute your java program).

Note : JDK is only used by Java Developers.

Java Runtime Environment (JRE):

JRE is an acronym for Java Runtime Environment. It is also written as Java RTE. The Java Runtime Environment is a set of software tools which are used for developing Java applications. It is used to provide the runtime environment. It is the implementation of JVM. It physically exists. It contains a set of libraries + other files that JVM uses at runtime.

Java Runtime Environment (JRE) is a software package that contains what is required to run a Java program. It includes a Java Virtual Machine implementation together with an implementation of the Java Class Library. The Oracle Corporation, which owns the Java trademark, distributes a Java Runtime environment with their Java Virtual Machine called HotSpot.

 JRE is an installation package which provides environment to only run(not develop) the java program(or application)onto your machine. JRE is only used by them who only wants to run the Java Programs i.e. end users of your system.

JRE consists of the following components:

• Deployment technologies, including deployment, Java Web Start and Java Plug-in.
• User interface toolkits, including Abstract Window Toolkit (AWT), Swing, Java 2D, Accessibility, Image I/O, Print Service, Sound, drag and drop (DnD) and input methods.
• Integration libraries, including Interface Definition Language (IDL), Java Database Connectivity (JDBC), Java Naming and Directory Interface (JNDI), Remote Method Invocation (RMI), Remote Method Invocation Over Internet Inter-Orb Protocol (RMI-IIOP) and scripting.
• Other base libraries, including international support, input/output (I/O), extension mechanism, Beans, Java Management Extensions (JMX), Java Native Interface (JNI), Math, Networking, Override Mechanism, Security, Serialization and Java for XML Processing (XML JAXP).
• Lang and util base libraries, including lang and util, management, versioning, zip, instrument, reflection, Collections, Concurrency Utilities, Java Archive (JAR), Logging, Preferences API, Ref Objects and Regular Expressions.
• Java Virtual Machine (JVM), including Java HotSpot Client and Server Virtual Machines.

Java virtual machine (JVM):

A Java virtual machine (JVM) is an abstract computing machine that enables a computer to run a Java program. It is called a virtual machine because it doesn't physically exist. It is a specification that provides a runtime environment in which Java byte code can be executed. It can also run those programs which are written in other languages and compiled to Java byte code.

A JVM (Java Virtual Machine) is available on many different operating systems, the same .class files are capable of running on Microsoft Windows, the Solaris™ Operating System (Solaris OS), Linux, or Mac OS. Some virtual machines, such as the Java SE HotSpot at a Glance, perform additional steps at runtime to give your application a performance boost. This includes various tasks such as finding performance bottlenecks and recompiling (to native code) frequently used sections of code.

JVMs are available for many hardware and software platforms. JVM, JRE, and JDK are platform dependent because the configuration of each OS is different from each other. However, Java is platform independent. There are three notions of the JVM: specification, implementation, and instance.The implementation of JVM is also actively released by other companies besides Sun Micro Systems.

Java Virtual machine(JVM) is a very important part of both JDK and JRE because it is contained or inbuilt in both. Whatever Java program you run using JRE or JDK goes into JVM and JVM is responsible for executing the java program line by line hence it is also known as interpreter.

 The specification is a document that formally describes what is required of a JVM implementation. Having a single specification ensures all implementations are interoperable. A JVM implementation is a computer program that meets the requirements of the JVM specification. An instance of a JVM is an implementation running in a process that executes a computer program compiled into Java bytecode.

The JVM performs the following main tasks:

• Loads code (Class Loader)
• Verifies code (Byte Code Verifier)
• Executes code (Interpreter)
• Provides runtime environment (Run Time Environment)

Class Loader: The Class Loader loads all necessary classes needed for the execution of a program. It provides security by separating the namespaces of the local file system from that imported through the network. These files are loaded either from a hard disk, a network or from other sources.

Byte Code Verifier: The JVM puts the code through the Byte Code Verifier that checks the format and checks for an illegal code. Illegal code, for example, is code that violates access rights on objects or violates the implementation of pointers.

Intrepreter: At runtime the Byte Code is loaded, checked and run by the interpreter. The interpreter has the following two functions:

• Execute the Byte Code
• Make appropriate calls to the underlying hardware