Question

everyone.

I used openjdk-7 on arch linux. I started to learn Java recently, and encountered such a problem:

I created a file at /home/hqwrong/Code/java/mew/Mouth.java:

package mew;

public class Mouth{
   public static void main(String argv[]){
       pickle.Say s = new pickle.Say();
  }
}

and another one at /home/hqwrong/Code/java/pickle/Say.java :

package pickle;

public class Say{
   public Say(){
      System.out.println("Say");
   }
}

I compiled Say.java to Say.class,using:

$ cd /home/hqwrong/Code/java/pickle
$ javac Say.java

which is successful.

I compiled Mouth.java ,using:

$ cd ../mew
$ export CLASSPATH=.:/home/hqwrong/Code/java/
$ javac Say.java

no error message.

But after I type:

$ java Say

I got:

Exception in thread "main" java.lang.SecurityException: Prohibited package name: java.mew
at java.lang.ClassLoader.preDefineClass(ClassLoader.java:649)
at java.lang.ClassLoader.defineClass(ClassLoader.java:785)
 at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:472)

It’s same when I use:

$ java -cp $CLASSPATH Say

I need your help,please?

Answer 1

Since there is no good answer yet, I’ll post mine.

First, you should really have a separate folder for your classes and your sources. I suggest using java/src for your sources, and java/classes for your classes. Since the classes are stored in the classes folder, this is the one that should be in the classpath.

The folder tree of your sources should then match your package tree. This means that the class mew.Mouth must contain the line package mew, be defined in the Mouth.java file, in the java/src/mew folder.

To compile your classes, put you in the java/src directory, and use the following command:

javac -d ../classes mew/Mouth.java pickle/Say.java

The compiler will automatically generate the folder structure matching the package structure in the classes directory. If you make structural modifications in your source tree, just remove everything in the classes folder, and recompile everything.

To run your classes, you must refer to their fully qualified name. And the folder containing your package tree (the java/classes folder) must be in the classpath. Once this is done, from everywhere, you can use

java mew.Mouth

Note that, as you have discovered, the java and javax packages are reserved. You can’t use them for your own classes.