Skip to main content

The Java classloader #1

This week I am starting a series on the Java classloader. Before we start talking about the classloader here's a little foundation.

If you have programmed with C++, you will remember there are two ways of compiling and linking programs.

  1. Static

  2. Dynamic

Any large software consists of many classes and dependencies on external libraries.

Statically compiling a program means creating one large executable file containing all the classes and dependencies. Since all that it needs is bundled in one file which is loaded when the program starts, such a program does not need to locate any files when running. However, because statically linked programs have a very large footprint, this approach is not favored, and an approach known as dynamic linking is used.

The output of a dynamically compiled program is an executable file containing the core program, and many libraries files. When the program starts running only the main executable is loaded in memory. Dependencies are loaded as and when required. Some of these dependencies may be distributed with the software while others may already exist in the underlying operating system, and the program simply expects them to be there. When the program needs to invoke a class that is in an external library, the runtime locates the library and loads the class.

Java uses an approach closer to the latter. In fact Java programs can NEVER be statically linked (unless you compile them into statically linked native code). Software written in Java is distributed as a bundle of class or jar files. When the program is executing, the JVM has to find, load, and invoke methods of various classes. All the classes are not loaded at once, instead they are located and loaded as they are required. The task of loading these class files is the responsiility of the Java classloader.

We will discuss  the Java classloader in greater detail in the next post.

 



Note: This text was originally posted on my earlier blog at http://www.adaptivelearningonline.net

Comments

Popular posts from this blog

Five Reasons Why Your Product Needs an Awesome User Guide

Photo Credit: Peter Merholz ( Creative Commons 2.0 SA License ) A user guide is essentially a book-length document containing instructions for installing, using or troubleshooting a hardware or software product. A user guide can be very brief - for example, only 10 or 20 pages or it can be a full-length book of 200 pages or more. -- prismnet.com As engineers, we give a lot of importance to product design, architecture, code quality, and UX. However, when it comes to the user manual, we often only manage to pay lip service. This is not good. A usable manual is as important as usable software because it is the first line of help for the user and the first line of customer service for the organization. Any organization that prides itself on great customer service must have an awesome user manual for the product. In the spirit of listicles - here are at least five reasons why you should have an awesome user manual! Enhance User Satisfaction In my fourteen years as a

Inheritance vs. composition depending on how much is same and how much differs

I am reading the excellent Django book right now. In the 4th chapter on Django templates , there is an example of includes and inheritance in Django templates. Without going into details about Django templates, the include is very similar to composition where we can include the text of another template for evaluation. Inheritance in Django templates works in a way similar to object inheritance. Django templates can specify certain blocks which can be redefined in subtemplates. The subtemplates use the rest of the parent template as is. Now we have all learned that inheritance is used when we have a is-a relationship between classes, and composition is used when we have a contains-a relationship. This is absolutely right, but while reading about Django templates, I just realized another pattern in these relationships. This is really simple and perhaps many of you may have already have had this insight... We use inheritance when we want to allow reuse of the bulk of one object in other

Planning a User Guide - Part 3/5 - Co-ordinate the Team

Photo by  Helloquence  on  Unsplash This is the third post in a series of five posts on how to plan a user guide. In the first post , I wrote about how to conduct an audience analysis and the second post discussed how to define the overall scope of the manual. Once the overall scope of the user guide is defined, the next step is to coordinate the team that will work on creating the manual. A typical team will consist of the following roles. Many of these roles will be fulfilled by freelancers since they are one-off or intermittent work engagements. At the end of the article, I have provided a list of websites where you can find good freelancers. Creative Artist You'll need to work with a creative artist to design the cover page and any other images for the user guide. Most small to mid-sized companies don't have a dedicated creative artist on their rolls. But that's not a problem. There are several freelancing websites where you can work with great creative ar