How to load Classes at runtime from a folder or JAR?

java class reflection jar load
Sam Stern picture Sam Stern · Jun 13, 2012 · Viewed 97.8k times · Source

I am trying to make a Java tool that will scan the structure of a Java application and provide some meaningful information. To do this, I need to be able to scan all of the .class files from the project location (JAR/WAR or just a folder) and use reflection to read about their methods. This is proving to be near impossible.

I can find a lot of solutions based on URLClassloader that allow me to load specific classes from a directory/archive, but none that will allow me to load classes without having any information about the class name or package structure.

EDIT: I think I phrased this poorly. My issue is not that I can't get all of the class files, I can do that with recursion etc. and locate them properly. My issue is obtaining a Class object for each class file.

Answer

Apfelsaft picture Apfelsaft · Jun 13, 2012

The following code loads all classes from a JAR file. It does not need to know anything about the classes. The names of the classes are extracted from the JarEntry.

JarFile jarFile = new JarFile(pathToJar);
Enumeration<JarEntry> e = jarFile.entries();

URL[] urls = { new URL("jar:file:" + pathToJar+"!/") };
URLClassLoader cl = URLClassLoader.newInstance(urls);

while (e.hasMoreElements()) {
    JarEntry je = e.nextElement();
    if(je.isDirectory() || !je.getName().endsWith(".class")){
        continue;
    }
    // -6 because of .class
    String className = je.getName().substring(0,je.getName().length()-6);
    className = className.replace('/', '.');
    Class c = cl.loadClass(className);

}

edit:

As suggested in the comments above, javassist would also be a possibility. Initialize a ClassPool somewhere before the while loop form the code above, and instead of loading the class with the class loader, you could create a CtClass object:

ClassPool cp = ClassPool.getDefault();
...
CtClass ctClass = cp.get(className);

From the ctClass, you can get all methods, fields, nested classes, .... Take a look at the javassist api: https://jboss-javassist.github.io/javassist/html/index.html

Related questions

How to read the value of a private field from a different class in Java?

I have a poorly designed class in a 3rd-party JAR and I need to access one of its private fields. For example, why should I need to choose private field is it necessary? class IWasDesignedPoorly { private Hashtable stuffIWant; } IWasDesignedPoorly obj = ...; …

Read More
Check if a Class Object is subclass of another Class Object in Java

I'm playing around with Java's reflection API and trying to handle some fields. Now I'm stuck with identifying the type of my fields. Strings are easy, just do myField.getType().equals(String.class). The same applies for other non-derived classes. …

Read More
Getting Class type from String

I have a String which has a name of a class say "Ex" (no .class extension). I want to assign it to a Class variable, like this: Class cls = (string).class How can i do that?

Read More