java.net.JarURLConnection Example

In this example we shall show you how to make use of JarURLConnection class, This class establishes a connection to a jar URL using the JAR protocol. A JarURLConnection instance can refer to either a JAR archive file or to an entry of such a file. jar URLs are specified as follows: jar:{archive-url}!/{entry} where !/ is called a separator. This separator is important to determine if an archive or an entry of an archive is referred.

Jar entry: jar:http://www.jcg.com/bar/baz.jar!/com/foo/Quux.class
Jar file: jar:http://www.jcg.com/bar/baz.jar!/
Jar directory: jar:http://www.jcg.com/bar/baz.jar!/com/foo/

Now, suppose that we have a main application. Also, we have a small application which is doing some logic for our main application. How we can run our small app which was packaged as a JAR file from inside our main app?

So, Let’s see the below example.



package com.jcg;

 * @author ashraf
public class Bean {
	public static void main( String [] args) {
	public static void sayHello() {
		System.out.println("Hello from loaded JAR class !!!");



package com.jcg;

import java.io.IOException;
import java.lang.reflect.Method;
import java.net.JarURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.Map.Entry;
import java.util.jar.Attributes;
import java.util.jar.JarFile;
import java.util.jar.Manifest;

 * @author ashraf
public class JarURLConnectionTest {
	private final static String JAR_URL = "jar:file:/C:/Users/ashraf_sarhan/simple-bean-1.0.jar!/";
	private final static String JAR_FILE_PATH = "file:/C:/Users/ashraf_sarhan/simple-bean-1.0.jar";
	private static URLClassLoader urlClassLoader;

	 * @param args
	 * @throws Exception 
	public static void main(String[] args) throws Exception {

		try {

			// Create a URL that refers to a jar file in the file system
			URL FileSysUrl = new URL(JAR_URL);

			// Create a jar URL connection object
			JarURLConnection jarURLConnection = (JarURLConnection)FileSysUrl.openConnection();
			// Get the jar file
			JarFile jarFile = jarURLConnection.getJarFile();
			// Get jar file name
			System.out.println("Jar Name: " + jarFile.getName());
			// When no entry is specified on the URL, the entry name is null
			System.out.println("\nJar Entry: " + jarURLConnection.getJarEntry());
			// Get the manifest of the jar
			Manifest manifest = jarFile.getManifest();

			// Print the manifest attributes
			System.out.println("\nManifest file attributes: ");
			for (Entry entry : manifest.getMainAttributes().entrySet()) {
				System.out.println(entry.getKey() +": "+ entry.getValue());
			System.out.println("\nExternal JAR Execution output: ");

			// Get the jar URL which contains target class
			URL[] classLoaderUrls = new URL[]{new URL(JAR_FILE_PATH)};

			// Create a classloader and load the entry point class
			urlClassLoader = new URLClassLoader(classLoaderUrls);

			// Get the main class name (the entry point class)
			String mainClassName = manifest.getMainAttributes().getValue(Attributes.Name.MAIN_CLASS);  

			// Load the target class
			Class beanClass = urlClassLoader.loadClass(mainClassName);

			// Get the main method from the loaded class and invoke it
			Method method = beanClass.getMethod("main", String[].class);

			// init params accordingly
			String[] params = null; 

			// static method doesn't have an instance
			method.invoke(null, (Object) params);

		} catch (MalformedURLException e) {
		} catch (IOException e) {




Jar Name: C:\Users\ashraf_sarhan\simple-bean-1.0.jar

Jar Entry: null

Manifest file attributes: 
Build-Jdk: 1.7.0_40
Built-By: ashraf_sarhan
Manifest-Version: 1.0
Created-By: Apache Maven
Main-Class: com.jcg.Bean
Archiver-Version: Plexus Archiver

External JAR Execution output: 
Hello from loaded JAR class !!!


In the above example, It’s possible you don’t know the main class of the JAR file which you need to run it so we loaded our simple-bean-1.0.jar file which print a message to console using JarURLConnection and reading its Manifest to find the entry point class (i.e., main class name – com.jcg.Bean), then we create a classloader and load the entry point class. Finally, we are using reflection to get its public static void main(String[]) method and invoke it.


When constructing a JAR url via new URL(context, spec), the following rules apply:

  • if there is no context URL and the specification passed to the URL constructor doesn’t contain a separator, the URL is considered to refer to a JarFile.
  • if there is a context URL, the context URL is assumed to refer to a JAR file or a Jar directory.
  • if the specification begins with a ‘/’, the Jar directory is ignored, and the spec is considered to be at the root of the Jar file.


context: jar:http://www.jcg.com/bar/jar.jar!/, spec: baz/entry.txt
url: jar:http://www.jcg.com/bar/jar.jar!/baz/entry.txt

context: jar:http://www.jcg.com/bar/jar.jar!/baz, spec: entry.txt
url: jar:http://www.jcg.com/bar/jar.jar!/baz/entry.txt

context: jar:http://www.jcg.com/bar/jar.jar!/baz, spec: /entry.txt
url: jar:http://www.jcg.com/bar/jar.jar!/entry.txt

Download the Source Code of this example:

This was an example of how to use JarURLConnection class.

You can download the full source code of this example here: java.net.JarURLConnection Example Code

Ashraf Sarhan

Ashraf Sarhan is a passionate software engineer, an open source enthusiast, has a Bsc. degree in Computer and Information Systems from Alexandria University. He is experienced in building large, scalable and distributed enterprise applications/service in multiple domains. He also has a keen interest in JavaEE, SOA, Agile and Big Data technologies.
Notify of

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Inline Feedbacks
View all comments
Back to top button