Find all classes in package (& call static methods) at runtime

Question

I have a package in which there are many classes, and these classes all have a static method called onLoad. I want to call the onLoad method when the program starts, however I do not want to hard-wire each and every one, i.e. classA.onLoad(); classB.onLoad(); etc.

How can I list all classes in package com.foo.bar.asd and run onLoad() on all of them?

Thanks in advance

Sorry, I messed up the title. It said that I wanted to create instances of these classes which I do not. I want to call static methods in them, and I have edited the title. — Jachdich, Oct 26 '19 at 16:30
Core reflection doesn't offer a way to list all classes in a package. However, there are libraries which can help with this, many of which are showcased [here](https://stackoverflow.com/questions/520328/). From there you'll likely either have the `Class` object or the class name. If you end up with a class name then you can [get the `Class` object using reflection](https://stackoverflow.com/questions/1438420/). After you have the `Class` object you just [have to find and invoke the static method](https://stackoverflow.com/questions/2467544/). — Slaw, Oct 26 '19 at 16:46

Alex R · Accepted Answer · 2019-10-26T17:32:23.160

I found this question quite interesting so I came up with a solution. The tricky part here is to actually find all classes that are in a given package.

In this example I assume that all classes are in the same package where the class C is and that all classes except C have an onLoad method which may be inaccessible (i.e. private). Then you can use the following example code.

public final class C {

    public static void main(String[] args) throws Exception {
        for (Class<?> cls : getClasses(C.class)) {
            if (cls != C.class) {
                Method onLoad = cls.getDeclaredMethod("onLoad");
                onLoad.setAccessible(true);
                onLoad.invoke(null);
            }
        }
    }

    private static List<Class<?>> getClasses(Class<?> caller)
            throws IOException, URISyntaxException {
        return Files.walk(getPackagePath(caller))
                .filter(Files::isRegularFile)
                .filter(file -> file.toString().endsWith(".class"))
                .map(path -> mapPathToClass(path, caller.getPackage().getName()))
                .collect(Collectors.toList());
    }

    private static Class<?> mapPathToClass(Path clsPath, String packageName) {
        String className = clsPath.toFile().getName();
        className = className.substring(0, className.length() - 6);
        return loadClass(packageName + "." + className);
    }

    private static Path getPackagePath(Class<?> caller)
            throws IOException, URISyntaxException {
        String packageName = createPackageName(caller);
        Enumeration<URL> resources = caller.getClassLoader()
                .getResources(packageName);
        return Paths.get(resources.nextElement().toURI());
    }

    private static String createPackageName(Class<?> caller) {
        return caller.getPackage().getName().replace(".", "/");
    }

    private static Class<?> loadClass(String name) {
        try {
            return Class.forName(name);
        } catch (ClassNotFoundException e) {
            return null;
        }
    }
}

I've omitted all exception checking and statements like resources.nextElement() may even throw an exception. So if you want to cover those cases, you need to add a few checks here and there.

I shall try this when I have time. Is there a way to have the class be in a different package? Not really needed but it neatens up the source tree a bit — Jachdich, Oct 26 '19 at 18:30
@Jachdich Sure, in the main method, pass one of the classes that are inside the package and have an `onLoad` method to `getClasses` (instead of `C.class`). Then you can safely remove the `if` statement, too. — Alex R, Oct 26 '19 at 18:34
Yes it does work thank you! however when I try to export as a JAR, it fails with "FileSystemNotFoundException". A full log can be found [here](https://pastebin.com/yY0Ee854) — Jachdich, Oct 28 '19 at 22:04
do you know why it doesn't work inside a jar? That's kinda important for a program that needs to be distributed — Jachdich, Nov 13 '19 at 08:47

SternK · Answer 2 · 2019-10-26T18:07:24.167

You can use the reflections library.

...
import org.reflections.Reflections;
import org.reflections.scanners.ResourcesScanner;
import org.reflections.scanners.SubTypesScanner;
import org.reflections.util.ClasspathHelper;
import org.reflections.util.ConfigurationBuilder;
import org.reflections.util.FilterBuilder;

public static void main(String[] args)
   throws NoSuchMethodException, IllegalAccessException, IllegalArgumentException, InvocationTargetException 
{
  Reflections reflections = new Reflections(new ConfigurationBuilder()
    .setScanners(new SubTypesScanner(false /* don't exclude Object.class */), new ResourcesScanner())
    .setUrls(ClasspathHelper.forClassLoader(ClasspathHelper.contextClassLoader()))
    .filterInputsBy(new FilterBuilder().include(FilterBuilder.prefix("com.foo.bar.asd"))));

  Set<Class<?>> classes = reflections.getSubTypesOf(Object.class);


  for (Class<?> clazz : classes)
  {
     Method method = clazz.getMethod("onLoad");
     method.invoke(null);
  }
}

Please note that this library also has dependencies (guava.jar and javassist.jar).

I'd really like to keep away from guava, as it's google, but other people will no doubt find this useful — Jachdich, Oct 26 '19 at 18:29

Find all classes in package (& call static methods) at runtime

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