CODEWITHSUNDEEP
javaeclipsereflectionpackage
user3373261
user3373261

Reputation: 55

Method to iterating through classes in a package using Reflection

I am currently learning about Reflection and I have seen most of the similar posts to my question on stack however, I don't feel they fully answer my question on it.

I want to know if I have a package in Eclipse can I use Reflection to iterate through the classes in the package to find which one implements interface. There is only 1 interface so either they implement it or don't.

Could anyone provide a basic clear example as to how I may go about this? I have been told by some that this is just not possible whilst other people say it is possible using Reflection.

Thank you to any one who could help clear this matter.

Upvotes: 0

Views: 2633

Answers (4)

AlexR
AlexR

Reputation: 115328

Reflection API does not provide directly facility to iterate over classes in specific package. It concentrates on discovery of class once you have it.

To achieve what you want to have to read the class path of your application, iterate over the class path, open jars and go into directories, find files that end with *.class and get them as resource like getResource(full_class_name) or get it directly as class using Class.forName().

This method has a limitation: you will not see classes loaded by custom class loaders.

Unless this is an exercise I'd recommend you to use Reflections library that does everything you need and (probably) even more... :)

Some more details

Java system property "java.class.path" contains class path of your application separated with ; on Windows and : on Unix.

So, this is the way you can get list of class path elements: System.getProperty("java.class.path").split(File.pathSeparator)

Here is some code sample that can help you to start:

for (String cpElement : System.getProperty("java.class.path").split(File.pathSeparator)) {
    File cpFile = new File(cpElement);
    if (!cpFile.exists()) {
         continue;
    }
    if (cpFile.isDirectory()) {
        findClassesInDirectory(cpFile);
    } else {
        findClassesInArchive(cpFile); 
    }
}

Etc, etc. I am leaving implementation of findClassesInDirectory() and findClassesInArchive() for you. Nice exercise. Have fun.

Upvotes: 2

Joop Eggen
Joop Eggen

Reputation: 109532

Reflection does not provide all operations for a total inspection.

For a known class one can get the physical URL of a class SomeClass using:

CodeSource codeSource = SomeClass.class.getProtectionDomain().getCodeSource();
if (codeSource == null) {
    // Run-time class; URL into the rt.jar.
} else {
    URL url = codeSource.getLocation();
    // "file:/.... /classes/.../SomeClass.class"
    // "jar:file:/... /someJar.jar!/.../SomeClass.class"
}

For Java run time classes (rt.jar) codeSource will be null. On the URL of the jar you can open a (zip) file system, and walk through folders as any real file system.

You'll probably want to inspect classes without $ in the name (embedded, generated anonymous classes): only fileName.matches("[^$]+\\.class").

Upvotes: 0

Joe Inner
Joe Inner

Reputation: 1470

have you heard of guava-libraries for Java. They provide useful utilities regarding reflection.

For your specific problem, I would have a look a the TypeToken and the method getTypes().

Good luck

Upvotes: 1

Nazgul
Nazgul

Reputation: 1902

yes you can so it. But with eclipse its more of an AST tree translation and not reflections. See if you want to iterate over source code and see which source file implements the interface you probably need AST tree walkers to do that. But on the other hand if you want to introspect at runtime the class files in a given package or a folder which implement the said interface then you need Reflections to do that. Either way its doable. I cant give you the code to try that out as a little time with Google will give you the answers you need though not all at the same place.

Upvotes: 0

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