Getting LinkageError with class loader

Question

I've written a class loader which extends java.net.URLClassLoader. My class is called EntryClassLoader. It only ovverides findClass() so that it will search a user-supplied path for a class. Beloww is the code:
public Class findClass(String className) throws ClassNotFoundException
try {
// First check to see if the user has prepended some
// directory path
if (className.indexOf(File.separatorChar) != -1) {
String path = className.substring(0,
className.lastIndexOf(File.separatorChar));
className = className.substring(className.lastIndexOf(File.separatorChar)
+ 1);
addURL(new File(path).toURL());
return super.findClass(className);
catch (MalformedURLException e) {
e.printStackTrace();
return null;
Here's a sample test program I've written:
public static void main(String args[]) throws Exception {
EntryClassLoader loader = new EntryClassLoader();
loader.findClass("TestPlayer");
loader.findClass("TestPlayer");
If I comment out the 2nd call to findClass(), the program runs fine, and I know that TestPlayer is successfully loaded. With the code as is however, I get the following error:
Exception in thread "main" java.lang.LinkageError: duplicate class definition: TestPlayer
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:509)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:123)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:246)
at java.net.URLClassLoader.access$100(URLClassLoader.java:54)
at java.net.URLClassLoader$1.run(URLClassLoader.java:193)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:186)
at nirex.EntryClassLoader.findClass(EntryClassLoader.java:33)
at TestLoader.main(TestLoader.java:8)
Could someone please explain why I might be getting this error. And I really don't understand why it works if I only call findClass() once. Thanks.
John

Answer 1

Check if findLoadedClass(className) is null and load only then. If findLoadedClass() returns a non-null value, the class has already been loaded.