Java URLClassLoader: select which classes to import

Question

Currently I'm using this line to load a 3rd party JAR and add its packages/classes to my program

URL [] urls = new URL [] { "http://..." };
new URLClassLoader(urls);

The problem I have with this approach is that the whole JAR is loaded, meaning all packages and all classes are imported. How can I tell URLClassLoaded to load only a few selected classes?

An example would be a JAR hierarchy like this

package A
- class 1
- class 2
package B
- class 1
- class 2
- class 3
- class 4

I'd like to do something like "import only A.* and B.class2"

You will have to implement custom class loader as I have depicted in my answer and set that as the default class loader for loading the classes — Narendra Pathai, Aug 31 '13 at 15:02

Narendra Pathai · Accepted Answer · 2013-08-31T15:09:59.647

1

Provide a custom implementation of ClassLoader.

Override the findClass() method of the classloader and apply the business logic for selecting the classes that you want to be loaded.

class CustomClassLoader extends ClassLoader {

         public Class findClass(String name) {
             if(shouldBeLoaded)
                return defineClass(name, b, 0, b.length);
         }
    }

Setting this as the default class loader for loading (optional)

java -Djava.system.class.loader
    =com.test.CustomClassLoader

edited Aug 31 '13 at 15:09

answered Aug 31 '13 at 14:55

Narendra Pathai

41,187
18
82
120

Do I really need to set it as default? Shouldn't it be the same if I always call new CustomClassLoader() instead of new URLClassLoader()? – Flavio Aug 31 '13 at 15:07
No you can do CustomClassLoader() too. You need not make that default. – Narendra Pathai Aug 31 '13 at 15:08

Java URLClassLoader: select which classes to import

1 Answers1