What is package level restriction?

[Himanshu Gupta]

Yesterday I was just to add a file in package which comes with java. Like java.lang
I wrote following code

  package java.lang;
 
public class Ttest {
 
/**
 * @param args
 */
public static void main(String[] args) {
// TODO Auto-generated method stub
 
}
 
}
  

and got following error when tried to run that class.

  java.lang.SecurityException: Prohibited package name: java.lang
at java.lang.ClassLoader.preDefineClass(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
Exception in thread "main"   

I want to know that how they achieved package level restriction? I know that some may feel this question absolute baseless because naming a package which is already there makes no sense. But i just want to know that what really happens and how they have done it? If someone can help me that will be very helpful to me.

Thanks a lot for reading this.

[Ernest Friedman-Hill]

The stack trace makes it pretty clear that it's implemented by java.security.SecureClassLoader .

[Peter Lawrey]

The Sun license agreement, which you agreed to when you downloaded Java states you cannot modify or add to standard packages such as java.* and javax.*, sun.* and com.sun.*

Solution: Use your own package.

[Himanshu Gupta]

Thanks Peter.

Have you gone through the whole agreement before accepting it?