How to find the package name given a class name?

Question

Given a class name as a string, how do I get the package name of it at run time ? I do not have the fully qualified name with package name + class name. Simply only the clas

Answer 1

The only way would be to navigate the directories/jars in classpath and an entry with the class name.

Here is some code that almost does what you want. This code is part of a class I have that searches for implementations/factory methods for creating instances of interfaces. Sorry, don't have the time to change this to look for a named class, should be an easy change, and as I mentioned elsewhere you don't need to load the class, just check the name.

public Class locateImplementation(Class type) {
     Class c = null;
     String[] cp = System.getProperty("java.class.path").split(File.pathSeparator);

     for (int i = 0; (c == null) && (i < cp.length); ++i) {
         File f = new File(cp[i]);
         if (f.exists() && f.canRead()) {
             if (isJar(f)) {
                 try {
                     c = searchJar(type, new FileInputStream(f));
                 } catch (Throwable t) {
                     // Nothing to worry about
                 }
             } else {
                 c = searchFile(type, f);
             }
         }
     }

     return c;
 }

private boolean isClass(String path) {
    return path.matches(".+\\.class$") && !path.contains("$");
}

private Class searchFile(Class type, File f) {
    return searchFile(type, f, f.getPath());
}

private Class searchFile(Class type, File f, String root) {
    Class implementation = null;

    if (f.isDirectory()) {
        File[] files = f.listFiles();
        for (int i = 0; i < files.length; i++) {
            implementation = searchFile(type, files[i], root);
            if (implementation != null) {
                break;
            }
        }
    } else if (isClass(f.getPath())) {
        String path = f.getPath().substring(root.length() + 1);
        Class c = getClass(path);
        if ((c != null) && !c.isInterface() &&
                type.isAssignableFrom(c)) {
            implementation = c;
        }
    }
    return implementation;
}

private Class getClass(String name) {
    Class c;
    String className = name.replaceAll("[/\\\\]", ".")
        .replaceFirst("^\\.", "").replace(".class", "");
    try {
        c = Class.forName(className);
    } catch (Throwable e) {
        c = null;
    }

    return c;
}


private Class searchJar(Class type, InputStream in)
        throws Exception {
    ZipInputStream zin = new ZipInputStream(in);
    Class implementation = null;

    ZipEntry ze;
    while ((implementation == null)
            && ((ze = zin.getNextEntry()) != null)) {
        String name = ze.getName();
        if (name.endsWith("class")
                && name.matches("^com.xxx.+")
                && !name.contains("$")) {
            try {
                Class c = getClass(name);
                if ((c != null) && !c.isInterface()
                        && type.isAssignableFrom(c)) {
                    implementation = c;
                }
            } catch (Throwable t) {
                // Nothing to worry about
            }
        }
    }

    return implementation;
}

private boolean isJar(File f) {
    return f.getPath().endsWith(".jar");
}