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.
Examples:
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.
Example:
Bean.java:
package com.jcg;
/**
* @author ashraf
*
*/
public class Bean {
public static void main( String [] args) {
sayHello();
}
public static void sayHello() {
System.out.println("Hello from loaded JAR class !!!");
}
}
JarURLConnectionTest.java:
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) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
}
Output:
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 !!!
Explanation:
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.
Tip
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.
Examples:
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
