How can you check for the existance of system packages?

[Steve Bentley]

Hi,

I need to be able to check whether certain system packages exist when my Java app loads. For my specific use-case the system packages are listed in a .properties file. I assumed that I could use for example ClassLoader.getSystemResource("javax/swing"); to test whether a specific package exists, but this doesn't work. However the following works for other packages on my classpath: myClassLoader.getResource("com/example/mypackage");

This is a simple runnable example:

package com.example.mypackage;

import java.net.URL;

public class FindPackagesTest {

  public static void main(String[] args) throws Exception 
  {
    String[] packages = new String[] {"com.example.mypackage", "javax.swing"};
    ClassLoader myClassLoader = FindPackagesTest.class.getClassLoader();
    for (String pack : packages) {
      String packPath = pack.replace('.', '/');
      URL success = myClassLoader.getResource(packPath);
      if (success == null) {
        success = ClassLoader.getSystemResource(packPath);
        if (success == null) System.out.println("Could not find [" + packPath + "]");
        else System.out.println("Found [" + packPath + "] with system class loader");
      } else {
        System.out.println("Found [" + packPath + "] with myClassLoader");
      }
    }
  }
}

The console output you will see when you run this program is:
Found [com/example/mypackage] with myClassLoader
Could not find [javax/swing]

Is there any way to check for system packages without having to know the classes they contain?

For the curious, my application is embedding the Felix OSGi runtime into a webapp. The system classes which my OSGi bundles need are specified in a .properties file and I would like to be able to check that all the packages exist before I boot OSGi. Without the check, the only way I know if a system package doesn't exist (for example, I miss spelt it in the .properties file) is when an OSGi bundle that needs it tries to load a class and gets a NoClassDefFoundError.

Thanks in advance for your help.
Steve Bentley

[Rob Spoor]

Check out java.lang.Package