Ramblings from a Bihari!

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did u know u can find the location of the .class file from the instance??

In general there is no guarantee that u can always get the location(Basically URL) from where ur instance or object is loaded but luck u can’t completely discard.. 🙂

The following class which i am putting tries to find out the location of teh instance from where it is loaded. All you need to do is call the getClass method on your concerned instance and pass the output to this method.

package com.deepak.classloader;

import java.io.File;
import java.net.MalformedURLException;
import java.net.URL;
import java.security.CodeSource;
import java.security.ProtectionDomain;

public class FindClassLoaction {

public static URL getClassLocation (final Class clazz)
{
URL clazzLocation = null;
final String clazzFileName = clazz.getName().replace (‘.’, ‘/’).concat (“.class”);

final ProtectionDomain pd = clazz.getProtectionDomain ();
if (pd != null)
{
final CodeSource codeSourceOfClazz = pd.getCodeSource();
if (codeSourceOfClazz != null) clazzLocation = codeSourceOfClazz.getLocation ();

if (clazzLocation != null)
{
if (“file”.equals (clazzLocation.getProtocol ()))
{
try
{
if (clazzLocation.toExternalForm ().endsWith (“.jar”) ||
clazzLocation.toExternalForm ().endsWith (“.zip”))
clazzLocation = new URL (“jar:”.concat (clazzLocation.toExternalForm ())
.concat(“!/”).concat (clazzFileName));
else if (new File (clazzLocation.getFile ()).isDirectory ())
clazzLocation = new URL (clazzLocation, clazzFileName);
}
catch (MalformedURLException ignore) {}
}
}
}

if (clazzLocation == null)
{
final ClassLoader clsLoader = clazz.getClassLoader ();

clazzLocation = clsLoader != null ?
clsLoader.getResource (clazzFileName) :
ClassLoader.getSystemResource (clazzFileName);
}

return clazzLocation;
}

}

The luck factor here is that u can’t guarantee the fact that every class is loaded with some ProtectionDomain i.e. It depends on the classloader whether it is creating the ProtectionDomain before loading the class….:(

also the CodeSource can be null depending on the classloader implementation. U can almost be sure that protection domain and code source will be populated for the classloders which are instance of URLClassLoader. So better you can put a check on the claasloader whether its an instance of URLClassLoader to be on safe side.

Sun’s implementation seems to allow finding the class file as resource ..

enjoy….

April 24, 2008 Posted by | techie stuff | 6 Comments