Problem reloading a jar using URLClassLoader

Question

I need to add plugin functionality to an existing application for certain parts of the application. I want to be able to add a jar at runtime and the application should be able to load a class from the jar without restarting the app. So far so good. I found some samples online using URLClassLoader and it works fine.

I also wanted the ability to reload the same class when an updated version of the jar is available. I again found some samples and the key to achieving this as I understand is that I need to use a new classloader instance for each new load.

I wrote some sample code but hit a NullPointerException. First let me show you guys the code:

package test.misc;

import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;

import plugin.misc.IPlugin;

public class TestJarLoading {

    public static void main(String[] args) {

        IPlugin plugin = null;

        while(true) {
            try {
                File file = new File("C:\\plugins\\test.jar");
                String classToLoad = "jartest.TestPlugin";
                URL jarUrl = new URL("jar", "","file:" + file.getAbsolutePath()+"!/");
                URLClassLoader cl = new URLClassLoader(new URL[] {jarUrl}, TestJarLoading.class.getClassLoader());
                Class loadedClass = cl.loadClass(classToLoad);
                plugin = (IPlugin) loadedClass.newInstance();
                plugin.doProc();
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
                try {
                    Thread.sleep(30000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

IPlugin is a simple interface with just one method doProc:

public interface IPlugin {
    void doProc();
}

and jartest.TestPlugin is an implementation of this interface where doProc just prints out some statements.

Now, I package the jartest.TestPlugin class into a jar called test.jar and place it under C:\plugins and run this code. The first iteration runs smoothly and the class loads without issues.

When the program is executing the sleep statement, I replace C:\plugins\test.jar with a new jar containing an updated version of the same class and wait for the next iteration of while. Now here's what I don't understand. Sometimes the updated class gets reloaded without issues i.e. the next iteration runs fine. But sometimes, I see an exception thrown:

java.lang.NullPointerException
at java.io.FilterInputStream.close(FilterInputStream.java:155)
at sun.net.www.protocol.jar.JarURLConnection$JarURLInputStream.close(JarURLConnection.java:90)
at sun.misc.Resource.getBytes(Resource.java:137)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:256)
at java.net.URLClassLoader.access$000(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:307)
at java.lang.ClassLoader.loadClass(ClassLoader.java:252)
at test.misc.TestJarLoading.main(TestJarLoading.java:22)

I have searched on the net and scratched my head but can't really arrive at any conclusion as to why this exception is thrown and that too - only sometimes, not always.

I need your experience and expertise to understand this. What's wrong with this code? Please help!!

Let me know if you need any more info. Thanks for looking!

Answer 1

For everyone's benefit, let me summarize the real problem and the solution that worked for me.

As Ryan pointed out, there is a bug in JVM, which affects Windows Platform. URLClassLoader does not close the open jar files after it opens them for loading classes, effectively locking the jar files. The jar files can't be deleted or replaced.

The solution is simple: close the open jar files after they've been read. However, to get a handle to the open jar files, we need to use reflection since the properties we need to traverse down are not public. So we traverse down this path

URLClassLoader -> URLClassPath ucp -> ArrayList<Loader> loaders
JarLoader -> JarFile jar -> jar.close()

The code to close the open jar files can be added to a close() method in a class extending URLClassLoader:

public class MyURLClassLoader extends URLClassLoader {

public PluginClassLoader(URL[] urls, ClassLoader parent) {
    super(urls, parent);
}

    /**
     * Closes all open jar files
     */
    public void close() {
        try {
            Class clazz = java.net.URLClassLoader.class;
            Field ucp = clazz.getDeclaredField("ucp");
            ucp.setAccessible(true);
            Object sunMiscURLClassPath = ucp.get(this);
            Field loaders = sunMiscURLClassPath.getClass().getDeclaredField("loaders");
            loaders.setAccessible(true);
            Object collection = loaders.get(sunMiscURLClassPath);
            for (Object sunMiscURLClassPathJarLoader : ((Collection) collection).toArray()) {
                try {
                    Field loader = sunMiscURLClassPathJarLoader.getClass().getDeclaredField("jar");
                    loader.setAccessible(true);
                    Object jarFile = loader.get(sunMiscURLClassPathJarLoader);
                    ((JarFile) jarFile).close();
                } catch (Throwable t) {
                    // if we got this far, this is probably not a JAR loader so skip it
                }
            }
        } catch (Throwable t) {
            // probably not a SUN VM
        }
        return;
    }
}

(This code was taken from the second link that Ryan posted. This code is also posted on the bug report page.)

However, there's a catch: For this code to work and be able to get a handle to the open jar files to close them, the loader used to load the classes from the file by URLClassLoader implementation has to be a JarLoader. Looking at the source code of URLClassPath (method getLoader(URL url)), I noticed that it uses a JARLoader only if the file string used to create the URL does not end in "/". So, the URL must be defined like this:

URL jarUrl = new URL("file:" + file.getAbsolutePath());

The overall class loading code should look something like this:

void loadAndInstantiate() {
    MyURLClassLoader cl = null;
    try {
        File file = new File("C:\\jars\\sample.jar");
        String classToLoad = "com.abc.ClassToLoad";
        URL jarUrl = new URL("file:" + file.getAbsolutePath());
        cl = new MyURLClassLoader(new URL[] {jarUrl}, getClass().getClassLoader());
        Class loadedClass = cl.loadClass(classToLoad);
        Object o = loadedClass.getConstructor().newInstance();
    } finally {
        if(cl != null)
            cl.close();
    } 
}

Update: JRE 7 has introduced a close() method in the class URLClassLoader which may have solved this issue. I haven't verified it.