Is the Java classpath final after JVM startup?

Question

I have read a lot about the Java class loading process lately. Often I came across texts that claimed that it is not possible to add classes to the classpath during runtime and

Answer 1

There are two concepts here that are being intermixed: The classpath and the class files in the classpath.

If you point the classpath to a directory, you will generally have no issue adding a file to the directory and having it picked up as part of the classpath. Due to the potential size of all classes in the classpath it isn't really feasible for a modern JVM to load them all at startup. However this is of limited value as it will not include Jar files.

However, changing the classpath itself (which directories, jars, etc. are searched) on a running JVM will depend very much on the implementation. As far as I know, on standard Sun JVMs there is no documented (as in guaranteed to work) method of accomplishing this.

In general, if this is something you need to do (have a dynamic classpath that changes at runtime) then you want to be implementing a ClassLoader, if for no other reason than to be able to throw it away and make a new one that doesn't reference those classes anymore if they need to be unloaded.

However, for a small amount of dynamic loading there are better ways. In Java 1.6 you can specify all the jar files in a directory (*.jar) so you can tell users to put additional libraries in a specified location (although they have to be there at startup).

You also have the option of including a jar file or other location in the classpath even though you don't need it, as a placeholder for someone to put an optional jar or resource file there (such as a log configuration file).

But if you need serious dynamic class loading and especially unloading while the application is running, a Classloader implementation is required.