Is it possible to "add" to classpath dynamically in java?

Question

java -classpath ../classes;../jar;. parserTester

How can i get the functionality in the above command programmatically? Like, is it possible to run as:

java parserTester

and get the same result? I tried using URLClassLoader but it modifies the classpath and does not add to it.

Thanx!

---

Thanks for the response Milhous. But that is what i am trying to do.. How is it possible to get the jar into the classpath first? I tried using a custom classloader too :(

That works.. But sorry that i need to run it only as: java parserTester I would like to know if such a thing is possible???

It needs to be so bcoz i have parserTester.java and .class in a separate folder. I need to retain the file structure. The parserTester makes use of a jar in a separate jar folder.

Answer 1

You can use a java.net.URLClassLoader to load classes with any program defined list of URL's you wish:

public class URLClassLoader extends SecureClassLoader

This class loader is used to load classes and resources from a search path of URLs referring to both JAR files and directories. Any URL that ends with a '/' is assumed to refer to a directory. Otherwise, the URL is assumed to refer to a JAR file which will be opened as needed.

The AccessControlContext of the thread that created the instance of URLClassLoader will be used when subsequently loading classes and resources.

The classes that are loaded are by default granted permission only to access the URLs specified when the URLClassLoader was created.

Since: 1.2

And a little fancy footwork can extend it to support using wildcarded pathnames to pick up entire directories of JARs (this code has some references to utility methods, but their implementation should be obvious in the context):

/**
 * Add classPath to this loader's classpath.
 * <p>
 * The classpath may contain elements that include a generic file base name.  A generic basename
 * is a filename without the extension that may begin and/or end with an asterisk.  Use of the
 * asterisk denotes a partial match. Any files with an extension of ".jar" whose base name match
 * the specified basename will be added to this class loaders classpath.  The case of the filename is ignored.
 * For example "/somedir/*abc" means all files in somedir that end with "abc.jar", "/somedir/abc*"
 * means all files that start with "abc" and end with ".jar", and "/somedir/*abc*" means all files
 * that contain "abc" and end with ".jar".
 *
 */
public void addClassPath(String cp) {
    String                              seps=File.pathSeparator;                // separators

    if(!File.pathSeparator.equals(";")) { seps+=";"; }                          // want to accept both system separator and ';'
    for(StringTokenizer st=new StringTokenizer(cp,seps,false); st.hasMoreTokens(); ) {
        String pe=st.nextToken();
        File   fe;
        String bn=null;

        if(pe.length()==0) { continue; }

        fe=new File(pe);
        if(fe.getName().indexOf('*')!=-1) {
            bn=fe.getName();
            fe=fe.getParentFile();
            }

        if(!fe.isAbsolute() && pe.charAt(0)!='/' && pe.charAt(0)!='\\') { fe=new File(rootPath,fe.getPath()); }
        try { fe=fe.getCanonicalFile(); }
        catch(IOException thr) {
            log.diagln("Skipping non-existent classpath element '"+fe+"' ("+thr+").");
            continue;
            }
        if(!GenUtil.isBlank(bn)) {
            fe=new File(fe,bn);
            }
        if(classPathElements.contains(fe.getPath())) {
            log.diagln("Skipping duplicate classpath element '"+fe+"'.");
            continue;
            }
        else {
            classPathElements.add(fe.getPath());
            }

        if(!GenUtil.isBlank(bn)) {
            addJars(fe.getParentFile(),bn);
            }
        else if(!fe.exists()) {                                                 // s/never be due getCanonicalFile() above
            log.diagln("Could not find classpath element '"+fe+"'");
            }
        else if(fe.isDirectory()) {
            addURL(createUrl(fe));
            }
        else if(fe.getName().toLowerCase().endsWith(".zip") || fe.getName().toLowerCase().endsWith(".jar")) {
            addURL(createUrl(fe));
            }
        else {
            log.diagln("ClassPath element '"+fe+"' is not an existing directory and is not a file ending with '.zip' or '.jar'");
            }
        }
    log.diagln("Class loader is using classpath: \""+classPath+"\".");
    }

/**
 * Adds a set of JAR files using a generic base name to this loader's classpath.  See @link:addClassPath(String) for
 * details of the generic base name.
 */
public void addJars(File dir, String nam) {
    String[]                            jars;                                   // matching jar files

    if(nam.endsWith(".jar")) { nam=nam.substring(0,(nam.length()-4)); }

    if(!dir.exists()) {
        log.diagln("Could not find directory for Class Path element '"+dir+File.separator+nam+".jar'");
        return;
        }
    if(!dir.canRead()) {
        log.error("Could not read directory for Class Path element '"+dir+File.separator+nam+".jar'");
        return;
        }

    FileSelector fs=new FileSelector(true).add("BaseName","EG",nam,true).add("Name","EW",".jar",true);
    if((jars=dir.list(fs))==null) {
        log.error("Error accessing directory for Class Path element '"+dir+File.separator+nam+".jar'");
        }
    else if(jars.length==0) {
        log.diagln("No JAR files match specification '"+new File(dir,nam)+".jar'");
        }
    else {
        log.diagln("Adding files matching specification '"+dir+File.separator+nam+".jar'");
        Arrays.sort(jars,String.CASE_INSENSITIVE_ORDER);
        for(int xa=0; xa<jars.length; xa++) { addURL(createUrl(new File(dir,jars[xa]))); }
        }
    }

private URL createUrl(File fe) {
    try {
        URL url=fe.toURI().toURL();
        log.diagln("Added URL: '"+url.toString()+"'");
        if(classPath.length()>0) { classPath+=File.pathSeparator; }
        this.classPath+=fe.getPath();
        return url;
        }
    catch(MalformedURLException thr) {
        log.diagln("Classpath element '"+fe+"' could not be used to create a valid file system URL");
        return null;
        }
    }