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Understanding Java Components like JVM, JRE, JDK and OpenJDK

This post explains the most relevant concepts around the software needed to run and/or compile Java in a computer.

Java Virtual Machine

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 need compile your *.java files to obtain *.class files that contain the bytecodes understood 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, see the Wikipedia list), the aim is that with the same bytecodes they all give the same results.

Java Runtime Environment (JRE)

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.

Java Development Kit (JDK)

Java Development Kit (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.

There are different JDK providers in the market. The official one is Oracle JDK, but there are many others.

OpenJDK

Open Java Development Kit (OpenJDK) is an open-source implementation of the JDK and the base for the Oracle JDK. There is almost no difference between the Oracle JDK and the OpenJDK.

Official instructions about how to install OpenJDK can be found here.

What are the differences between Oracle JDK and OpenJDK?

According to Oracle, the differences are close. Build process for Oracle JDK releases builds on OpenJDK 7 by adding just a couple of pieces, like the deployment code, which includes Oracle’s implementation of the Java Plugin and Java WebStart, as well as some closed source third party components like a graphics rasterizer, some open source third party components, like Rhino, and a few bits and pieces here and there, like additional documentation or third party fonts. Moving forward, Oracle’s intent is to open source all pieces of the Oracle JDK except those that they consider commercial features such as JRockit Mission Control (not yet available in Oracle JDK), and replace encumbered third party components with open source alternatives to achieve closer parity between the code bases.

External References

pmgallardo

About pmgallardo

I studied Computer Science at University of Salamanca. Since then, I have been working first as developer and then as SAP consutant. This blog is about problems I dealt when using computers, and more important, the solutions I found. Whenever I am on an issue and suddenlly I have a flash that leads me to a solution, I document my discoveries in a post.

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