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
Labels:

Comments

Post a Comment

Popular posts from this blog

My HSQLDB schema inspection story

This is a simple story of my need to inspect the schema of an HSQLDB database for a participar FOREIGN KEY, and the interesting things I had to do to actually inspect it. I am using an HSQLDB 1.8 database in one of my web applications. The application has been developed using the Play framework , which by default uses JPA and Hibernate . A few days back, I wanted to inspect the schema which Hibernate had created for one of my model objects. I started the HSQLDB database on my local machine, and then started the database manager with the following command java -cp ./hsqldb-1.8.0.7.jar org.hsqldb.util.DatabaseManagerSwing When I tried the view the schema of my table, it showed me the columns and column types on that table, but it did not show me columns were FOREIGN KEYs. Image 1: Table schema as shown by HSQLDB's database manager I decided to search on StackOverflow and find out how I could view the full schema of the table in question. I got a few hints, and they all pointed to ...
Post a Comment
Read more

Commenting your code

Comments are an integral part of any program, even though they do not contribute to the logic. Appropriate comments add to the maintainability of a software. I have heard developers complain about not remembering the logic of some code they wrote a few months back. Can you imagine how difficult it can be to understand programs written by others, when we sometimes find it hard to understand our own code. It is a nightmare to maintain programs that are not appropriately commented. Java classes should contain comments at various levels. There are two types of comments; implementation comments and documentation comments. Implementation comments usually explain design desicisions, or a particularly intricate peice of code. If you find the need to make a lot of implementation comments, then it may signal overly complex code. Documentation comments usually describe the API of a program, they are meant for developers who are going to use your classes. All classes, methods and variables ...
Post a Comment
Read more

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 ...
3 comments
Read more