Sergey Malenkov's Blog

Sergey Malenkov is an engineer on the Swing team from Saint-Petersburg, Russia. He is responsible engineer for JavaBeans technologies: introspection, customization and long term persistence. Also he is interested in game development.

How to load classes from JAR or ZIP?

I needed to load the classes from the dt.jar archive on the fly. The path to the archive was generated automatically based on the "java.home" system property. The original idea was to use the URLClassLoader, but it could not find classes. I had to write a custom class loader which read an archive and loaded classes on demand. At that instant I realized why the URLClassLoader did not work: I had incorrectly generated the path to the archive and the URLClassLoader for a wonder provided no warning that the archive was not found.

Below is the code of the custom class loader for work with jar- and zip-archives:

public final class ZipClassLoader extends ClassLoader {
    private final ZipFile file;

    public ZipClassLoader(String filename) throws IOException {
        this.file = new ZipFile(filename);
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        ZipEntry entry = this.file.getEntry(name.replace('.', '/') + ".class");
        if (entry == null) {
            throw new ClassNotFoundException(name);
        }
        try {
            byte[] array = new byte[1024];
            InputStream in = this.file.getInputStream(entry);
            ByteArrayOutputStream out = new ByteArrayOutputStream(array.length);
            int length = in.read(array);
            while (length > 0) {
                out.write(array, 0, length);
                length = in.read(array);
            }
            return defineClass(name, out.toByteArray(), 0, out.size());
        }
        catch (IOException exception) {
            throw new ClassNotFoundException(name, exception);
        }
    }
}

Note that it is not necessary to cache classes here, because the ClassLoader caches loaded classes automatically. Also you can override the following methods to search resources in the archive:

    @Override
    protected URL findResource(String name) {
        ZipEntry entry = this.file.getEntry(name);
        if (entry == null) {
            return null;
        }
        try {
            return new URL("jar:file:" + this.file.getName() + "!/" + entry.getName());
        }
        catch (MalformedURLException exception) {
            return null;
        }
    }

    @Override
    protected Enumeration<URL> findResources(final String name) {
        return new Enumeration<URL>() {
            private URL element = findResource(name);

            public boolean hasMoreElements() {
                return this.element != null;
            }

            public URL nextElement() {
                if (this.element != null) {
                    URL element = this.element;
                    this.element = null;
                    return element;
                }
                throw new NoSuchElementException();
            }
        };
    }

Perhaps someone may need it because it slightly faster. But I recommend to use the URLClassLoader which is able to work with remote archives and supports security:

URL[] urls = { new URL("jar:file:" + path + "!/") };
return URLClassLoader.newInstance(urls);

If the classes cannot be loaded, double check the correctness of the path.

July 2008

Search this blog:

Categories

Archives

July 2008
December 2007
November 2007
October 2007
March 2007
October 2006
August 2006

Recent Entries

How to load classes from JAR or ZIP?

It is time to choose Color

How to veto a property change?

This work is licensed under a Creative Commons License.