How to print class and interface name from a java jar file

Question

I have a Jar in java which is containing 2 classes and 1 Interface. How can i get the interface and class names from the jar. Currently I am able to get the class names, but not the interface name.

   List jClasses = getClasseNames("D://Test.jar");

            System.out.println(jClasses.size());

            for (int i = 0; i < jClasses.size(); i++) {

                System.out.println("Print Classes ::" + jClasses.get(i));

                if(( null != jClasses.getClass().getInterfaces()[i])) {
                    System.out.println(jClasses.getClass().getInterfaces()[i]);
                } else {
                    System.out.println("No connection");
                }
             }

   public static List getClasseNames(String jarName) {
        ArrayList classes = new ArrayList();

        try {
            JarInputStream jarFile = new JarInputStream(new FileInputStream(
                    jarName));
            JarEntry jarEntry;

            while (true) {
                jarEntry = jarFile.getNextJarEntry();
                if (jarEntry == null) {
                    break;
                }
                if (jarEntry.getName().endsWith(".class")) {

                    classes.add(jarEntry.getName().replaceAll("/", "\\."));
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
        return classes;
    }

output :

Print Classes ::com.java.testclient.PTest1.class
interface java.util.List
 ====== 
Print Classes ::com.java.testclient.ClassSpy.class
interface java.util.RandomAccess
 ====== 
Print Classes ::com.java.testclient.myInt.class
interface java.lang.Cloneable
 ====== 
Print Classes ::com.java.testclient.PTest.class
interface java.io.Serializable

Please suggest.

What is getClasseNames and this jClasses.getClass().getInterfaces()[i] is suspicious, jClasses.get(i).getClass().getInterfaces() maybe — user180100
– user180100, Commented Dec 26, 2016 at 16:52
You should really avoid using raw types. That would auto-document your code. jClasses is a List<String>. Calling getClass() on it returns List.class, and does nothing with the actual class names in the list. You would need to use a ClassLoader, that would load classes from the jar by name (and hope they don't have dependencies from another jar), and then check if the loaded class is a regular class or an interface. Or you would need to actually analyze the bytecode of the entries of the jar file to decide if the class is a regular class or an interface. — JB Nizet
– JB Nizet, Commented Dec 26, 2016 at 17:01

GabrielBB · Accepted Answer · 2016-12-26 20:30:57Z

You can use this class:

package io.github.gabrielbb.java.utilities;

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.List;
import java.util.jar.JarEntry;
import java.util.jar.JarInputStream;

/**
 * @author Gabriel Basilio Brito
 * @since 12/26/2016
 * @version 1.1
 */
public class ClassesAndInterfacesFromJar {

    public static List<Class> getJarClasses(String jarPath) throws IOException, ClassNotFoundException {
        File jarFile = new File(jarPath);
        return getJarClasses(jarFile);
    }

    public static List<Class> getJarClasses(File jar) throws IOException, ClassNotFoundException {
        ArrayList<Class> classes = new ArrayList();
        JarInputStream jarInputStream = null;
        URLClassLoader cl;

        try {
            cl = URLClassLoader.newInstance(new URL[]{new URL("jar:file:" + jar + "!/")}); // To load classes inside the jar, after getting their names

            jarInputStream = new JarInputStream(new FileInputStream(
                    jar)); // Getting a JarInputStream to iterate through the Jar files 

            JarEntry jarEntry = jarInputStream.getNextJarEntry();

            while (jarEntry != null) {
                if (jarEntry.getName().endsWith(".class")) { // Avoiding non ".class" files
                    String className = jarEntry.getName().replaceAll("/", "\\."); // The ClassLoader works with "." instead of "/"
                    className = className.substring(0, jarEntry.getName().length() - 6);  // Removing ".class" from the string
                    Class clazz = cl.loadClass(className); // Loading the class by its name
                    classes.add(clazz);
                }

                jarEntry = jarInputStream.getNextJarEntry(); // Next File
            }
        } finally {
            if (jarInputStream != null) {
                jarInputStream.close(); // Closes the FileInputStream
            }
        }
        return classes;
    }

    // Main Method for testing purposes
    public static void main(String[] args) {
        try {
            String jarPath = "C://Test.jar";
            List<Class> classes = getJarClasses(jarPath);

            for (Class c : classes) {
                // Here we can use the "isInterface" method to differentiate an Interface from a Class
                System.out.println(c.isInterface() ? "Interface: " + c.getName() : "Class: " + c.getName());
            }
        } catch (Exception ex) {
            System.err.println(ex);
        }
    }

It can be found at:

https://github.com/GabrielBB/Java-Utilities/blob/master/ClassesAndInterfacesFromJar.java

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