Question

I’m looking a utility method so that given a class will return the full classpath required to run this class externally. This means the jar the class is in as well as all jars (or folders) of classes that it uses.

UPDATE: there are tools that analyze .class files to find dependencies. This is not what I’m looking for. I’m looking for something that uses Java’s reflection API on an already loaded class. I’ll settle for something that analyzes byte code, if it goes recursively into classes it finds through the class loader

Answer 1

Reflection will not help you a lot for this one. You will need to analyse the byte code to find dependencies.

UPDATE:

Alright then. I am using a library that I made years ago, that you can download here.

The following code:

package classdep;

import java.io.InputStream;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import org.jedo.classfile.ClassFile;
import org.jedo.classfile.ConstantPool;

public class Main {

    public static void main(String[] args) {
        try {
            ClassLoader cl = Thread.currentThread().getContextClassLoader();
            List<String> classes = new ArrayList<String>();
            classes.add(args[0].replace('.', '/'));
            for (int i = 0; i < classes.size(); ++i) {
                String className = classes.get(i);
                URL url = cl.getResource(className + ".class");
                if (url == null) {
                    System.out.println("--- class not found " + className);
                } else {
                    System.out.println(url);
                    ClassFile classFile = new ClassFile();
                    InputStream in = url.openStream();
                    try {
                        classFile.load(in);
                    } finally {
                        in.close();
                    }
                    ConstantPool cp = classFile.getConstantPool();
                    for (String name: cp.getClassNames()) {
                        if (!classes.contains(name)) {
                            classes.add(name);
                        }
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Will give you all the dependencies of a class. When applied to org.jedo.classfile.ClassFile, it produces the following output:

file:/D:/projects/casagrande/jedo/build/classes/org/jedo/classfile/ClassFile.class
file:/D:/projects/casagrande/jedo/build/classes/org/jedo/classfile/ConstantPool.class
file:/D:/projects/casagrande/jedo/build/classes/org/jedo/classfile/FieldInfo.class
file:/D:/projects/casagrande/jedo/build/classes/org/jedo/classfile/MethodInfo.class
file:/D:/projects/casagrande/jedo/build/classes/org/jedo/classfile/AttributeInfo.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/io/File.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/io/FileInputStream.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/io/DataInputStream.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/io/StreamCorruptedException.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/io/FileOutputStream.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/io/DataOutputStream.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/lang/StringBuilder.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/lang/StringBuffer.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/lang/Object.class
jar:file:/C:/Program%20Files/Java/jdk1.6.0_18/jre/lib/rt.jar!/java/io/IOException.class
...

Followed by a lot of system classes. You need to filter out system classes, and parse the other urls to extract either the .jar file if it is a jar: url, or the directory if it is a file: url.