Java applications have gained use and fame increasingly over the past few years. We can see it all around us on our phones, tablets, and on our computers. This would essentially mean to compile arduous amounts of code to make it at par with the compatibilities of running it on the different operating systems.
This in turn becomes a boon for the developers, as Java is designed to be a platform independent – or “Write once, run anywhere” module. So it becomes a onetime affair that helps in the long run across platforms.
What is the JVM?
Java Virtual Machine (JVM) is a program which is basically an execution place for the Java applications. JVM acts like a versatile machine that processes the interpretation of the bytecodes that are compiled from the Java Source Code, by converting it into machines language. JVM is a part of Java Run Environment (JRE).
Normally, for other programming languages, the compiler would produce the machine code for that particular system. Whereas, as a Java compiler produces code for a Java Machine which is known as Java Virtual Machine.
As explained by Matthew Tyson, Java World in his blog on JVM – he points out the differentiation of the definitions of JVM for the developers as well as the technical terms.
Technical definition: The JVM can be the specification for a software program that executes the code as well as providing the runtime for that given code.
Everyday definition: The JVM is how we conduct the runs for our Java programs. We can set up the JVM settings and then rely on it to manage the program resources while it performs execution.
“Under the Java Development Services, When developers talk about the JVM, they usually are referring to the process running on a machine, especially a server, one that represents and controls resource usage for a Java app. Whereas, the JVM specification describes the requirements that are needed to build a program to perform the tasks.”
The garbage collection system
In the pre-Java era, the program memory was managed by the programmer. But in Java, this task is managed by the JVM – it manages its memory through this process of garbage collection, which periodically identifies and removes the unused memory in Java programs. This all is contained inside a running JVM.
However, this trend caused quite an uproar about this development, as it wasn’t a feature for the other programming languages, much like its other features. Post this, there have been many optimizations to where it has come to now.
The Three Dimensions of JVM
The three parts of the JVM that you need to understand are – Specification, implementation and instance. Let’s look into each of these to understand better.
JVM is a software specification, which means that its implementation details are not defined within the spec itself. To be able to implement the Java virtual machine correctly, there is a pre-requisite need of being able to read the class file format and perform the operations specified.
However, JVM’s only job of running the Java programs, is not as simple as it might seem on the outset. This is due to the power and the flexibility that Java and its scope brings.
JVM the virtual machine
Like the name suggests, Java Virtual Machine is an abstraction from an actual machine – like a server that runs any program. But JVM is a little ahead of the game, where it creates a predictable environment for programs to run within.
JVM, although, doesn’t actually create a virtual operating system, like a proper virtual machine would. To put into better words, JVM is more like a “managed runtime environment” or a “process virtual machine”.
Implementations of JVM specifications, is like producing an actual software program. There are many JVM implementations, both open source as well as proprietary. Hotspot JVM is one of the most frequently used JVM.
Most licensed JVM’s are often created to provide OS-specific performance improvements. Generally, you can download and install the JVM as a bundled part of Java Runtime Environment (JRE).
Basically, as the JVM spec gets implemented and released in the market as a software product – people can download and run it as a program. This downloaded program is an instantiated version of the JVM.
Often in conversations between developers, the JVM that’s the point of the conversation is actually this instance being run in a software or product environment.
JVM Execution Engine
The execution engine in the JVM component handles the execution of the code in each class, this is after the class loader has completed loading classes. This is a crucial to the running of a JVM. It is in fact the JVM instance in the practical lens.
Executing code requires managing the access to the system’s resources. The execution engine of the JVM is the medium between the running program and the OS, which connects the supply and demand for the file, network and memory resources.
The Execution Engine’s Management of System Resources
System resources can be essentially be divided into the two vast categories of – memory and the rest of the components.
JVM has the responsibility of disposing unused memory through garbage collection. It also takes care of allocating and maintaining a ‘referential structure. Even something as small as a new keyword in Java, is turned into an OS – specific request for memory allocation, as a part of its functionality.
Apart from memory collection and disposal, the execution engine maintains the upkeep of the resources for system access and network I/O. This is what sets JVM apart from the others, with its application’s resource needs and being responsive to each OS environment. Thus, it is able to work without qualms in every operating system without glitches.
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This article was originally posted in Computer Tech Reviews.