Scan class or jar file in Java

Question

I need to scan class or jar file that implements or extend a specific class.
It's the same mechanism that Eclipse use to scan the plugin presence. There is a way to do this ?

there are ways, but you shouldn't need it ;) – Bozho May 25 '10 at 08:34 — Bozho, May 25 '10 at 08:34
@Bozho: Why not? Don't tell me you can smell it ;). – Adeel Ansari May 25 '10 at 08:38 — Adeel Ansari, May 25 '10 at 08:38

score 1 · Accepted Answer · edited May 23 '17 at 10:32

From exmaple depot: How to load a Class that is not on the classpath:

// Create a File object on the root of the directory 
// containing the class file
File file = new File("c:\\class\\");

try {
    // Convert File to a URL
    URL url = file.toURL();          // file:/c:/class/
    URL[] urls = new URL[]{url};

    // Create a new class loader with the directory
    ClassLoader loader = new URLClassLoader(urls);

    // Load in the class; Class.childclass should be located in
    // the directory file:/c:/class/user/information
    Class cls = loader.loadClass("user.informatin.Class");
} catch (MalformedURLException e) {
} catch (ClassNotFoundException e) {
}

List the files in the directory if necessary, using File.list().

When the class is loaded, you can check if it implements a specific interface by doing clazz.isInstance. From the docs:

Determines if the specified Object is assignment-compatible with the object represented by this Class. This method is the dynamic equivalent of the Java language instanceof operator.

To load classes from a jar-file:

How to load a jar file at runtime

To list files in a jar file: JarFile.entries().

score 0 · Answer 2 · answered Nov 07 '12 at 05:51

You can scan it without classloading it by using a bytecode parser such as ASM or BCEL. With ASM...

byte[] bytecode = [read bytes from class or jar file];
ClassReader reader = new ClassReader( bytecode );
String[] interfaces = reader.getInterfaces();

score 0 · Answer 3 · answered May 25 '10 at 08:33

0

Yes, there are methods just to do that. Look at the docs here.

Scan class or jar file in Java

3 Answers3

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