URLClassLoader can't find classes

Question

I want to use an URLClassLoader to load classes in a particular directory from a jar-archive.

The project structure is as follows

/
 application.jar
 /lib/
  mysql.jar
  log4j.jar
  ...
 /myClasses/
  class1.class
  class2.class
  ...

The jar has the following manifest:

Manifest-Version: 1.0
Ant-Version: Apache Ant 1.8.3
Created-By: 1.7.0_07-b10 (Oracle Corporation)
Main-Class: de.application.start
Class-Path: lib/mysql.jar lib/log4j.jar lib/....jar

What I do currently is iterating over all files within /myClasses/ and trying to load that particular class (they all have the same package) like this:

File classDir = new File("/path/to/my/root/folder/myClasses/");
URL[] url = { classdir.toURI().toURL() };
ClassLoader loader = new URLClassLoader(url);
for (File file : classDir.listFiles()) {
    String filename = StringUtil.getFilenameWithoutExtension(file.getName());
    loader.loadClass("de.myClasses." + filename).getConstructor().newInstance();

But, even though the files obviously exist, I always get a java.lang.ClassNotFoundException: de.myClasses.class1 error. What am I doing wrong?

You write tjat you want to load classes from jar archive, but in your code you give raw directory ("/path/to/my/root/folder/myClasses/") as a parameter to URLClassLoader. You must specify jar file here. — korifey, Sep 12 '12 at 12:54
Yes, that's correct. My application resides in a jar archive and out of that archive I want to load other classes in a directory — F.P, Sep 12 '12 at 12:55
Is your goal to access a class that's stored within a jar that's stored within a jar? That doesn't work, and I'm sure that a search will turn up lots of posts that tell you why and how to work around it. — parsifal, Sep 12 '12 at 13:20

score 1 · Answer 1 · answered Sep 12 '12 at 12:53

1

file.getName() returns "Class1.class" instead of "Class1", right? remove last 6 chars.

Also, if Class1 is in package de.myClasses it should be at

<your_base_dir>/de/myClasses

and if Class1 is in top package (no package) then you should ask for it as

loader.loadClass("Class1") // no de.mClasses. prefix

answered Sep 12 '12 at 12:53

helios

13,574
2
45
55

No, I have done that in my application. I wrote it here this way to save space, I will change it. – F.P Sep 12 '12 at 12:56

score 0 · Accepted Answer · answered Sep 12 '12 at 13:16

0

You should put your compiled classes in directories according its packages. So class de.myClasses.Class1 should be in directory /myClasses/de/myClasses/.

answered Sep 12 '12 at 13:16

michael

185
1
9

URLClassLoader can't find classes

2 Answers2