Skip to main content

Security and the Java classloader

The Java classloader plays an important part in the overall security of a Java application.

The classloader works along with the SecurityManager and the Access Controller to make a Java system secure. The classloader is important because it is the entity that first loads the classes. The classloader knows the codebase from where the code was loaded, and if the code was signed. The classloader works in three ways to help make a Java system secure:

  1. It helps the JVM to maintain the integrity of namespaces.
  2. It maps permissions with each class. The permissions associated with every class are also known as the protection domain of the class. This mapping helps the access controller determine which classes have which permissions.
  3. It ensures that code which accesses or defines classes has the appropriate permissions to do so. The classloader works in conjunction with the SecurityManager to enforce this.
Each of these topics are fairly long, and I will not go into intricate details. I will however explain the first topic briefly below.

Definition of namespaces:
We all know that each class in Java is uniquely identified by it's package name. But that's not all. In reality a class is uniquely identified by it's fully qualified name and the classloader that loaded it. Can you imagine why this is needed? It is needed to prevent the integrity of code that is running in Applets. Assume you load Applets from and some malicious website in your browser. If the malicious website uses package names such as com.sun.*, then it is possible that their classes might be used when the Applet from tries to invoke a class, thus causing damage. However this will not happen because classes are unique to the package name and the classloader that loaded them. A different instance of the classloader is used to load the Applet from and the Applet from the malicious website. Hence even if both of them use classes with the same package names, unique instances of these classes will be created for both the Applets amd they will not be able to access each other's classes.

Mapping permissions of classes:
We can configure the Java policy file to restrict or allow certain actions to code loaded from a particular codebase. We can also grant certain priviledges to code that has been signed by a trusted entity. For example we might want to provide I/O access to Applet code that has been signed by It is the classloader that helps in managing this information. The classloader works in conjunction with the security manager and the access controller to enforce the permissions.
You might want to read the links below to better understand how the classloader works.


Notes: This text was originally posted on my earlier blog at


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- 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

Fuctional Programming Principles in Scala - Getting Started

Sometime back I registered for the Functional Programming Principles in Scala , on Coursera. I have been meaning to learn Scala from a while, but have been putting it on the back burner because of other commitments. But  when I saw this course being offered by Martin Odersky, on Coursera , I just had to enroll in it. This course is a 7 week course. I will blog my learning experience and notes here for the next seven weeks (well actually six, since the course started on Sept 18th). The first step was to install the required tools: JDK - Since this is my work machine, I already have a couple of JDK's installed SBT - SBT is the Scala Build Tool. Even though I have not looked into it in detail, it seems like a replacement for Maven. I am sure we will use it for several things, however upto now I only know about two uses for it - to submit assignments (which must be a feature added by the course team), and to start the Scala console. Installed sbt from here , and added the path

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