Using URLClassLoader to load .class file

Question

I\'m aware that this question has been asked before:

How to use URLClassLoader to load a *.class file?

However I don\'t really understand due to the lack of

Answer 1

In the directory /home/shackle/somedir/classes/pkg I have a file Test.class created from a java file with package pkg; eg :

package pkg;

public class Test {

    public String toString() {
        return "secret_string";
    }
}

Then I load it with :

System.out.println(new URLClassLoader(
        new URL[]{new File("/home/shackle/somedir/classes").toURI().toURL()}
).loadClass("pkg.Test").newInstance().toString());

Notice that I do not put the pkg/Test in the URL string but the load class argument has the pkg. prefix.

You can get the class name directly from the file like this:

Class clsReaderClss = ClassLoader.getSystemClassLoader().loadClass("jdk.internal.org.objectweb.asm.ClassReader");
System.out.println("clsReaderClss = " + clsReaderClss);
Constructor con = clsReaderClss.getConstructor(InputStream.class);
Object reader = con.newInstance(new FileInputStream(directFile));
Method m = clsReaderClss.getMethod("getClassName");
String name = m.invoke(reader).toString().replace('/', '.');
System.out.println("name = " + name);

An alternative that doesn't require access to internal classes.

String pathToClassFile = "/home/shackle/somedir/classes/pkg/Test.class";
ProcessBuilder pb = new ProcessBuilder("javap",pathToClassFile);
Process p = pb.start();
String classname = null;
try(BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()))) {
   String line;
   while(null != (line = br.readLine())) {
       if(line.startsWith("public class")) {
           classname = line.split(" ")[2];
           break;
       }
   }
}
System.out.println("classname = " + classname);

Class can then be loaded with:

String pathToPackageBase = pathToClassFile.substring(0, pathToClassFile.length() - (classname + ".class").length());
System.out.println("pathToPackagBase = " + pathToPackageBase);
Class clss = new URLClassLoader(
        new URL[]{new File(pathToPackageBase).toURI().toURL()}
).loadClass(classname);
System.out.println(clss.newInstance().toString());