Creating an instance of a class at runtime?

Question

Does anyone know how to create an instance of a class at RunTime? For example, I want to load classes from a JAR file at RunTime, and then create instances of certain classes of each JAR. I ask this because I do not see how to create an instance of one of those classes the traditional way, SomeClass var = new SomeClass(). I am pretty sure that someone out there has done this and succeeded in doing it. All of the post on this stuff only talk about loading the class from a JAR file, but I have already loaded them using URLClassLoader�s findClass() method. I have also created an instance of the class that findClass() returns using newInstance(), but newInstance() returns an object of type Object. How can I convert this object to an object of type SomeClass? You cannot cast it because the compiler will complain due to the import statement issue. So if you cannot include the import statement because that classpath does not exist, then how the heck would you cast the newInstance() object into a SomeClass object?

Answer 1

You can cast the returned object to the type you need...that is what I do in my applet. The trick is that you must get the instance of the applets class loader (or you get a classCastException). Pay attention to this line below - that's the real key here.
"// Get a reference to the applets own classloader"
protected CBaseQuestionnaireFile m_BaseObject = null;// declared up front
m_BaseObject = loadAndRunClass("com.cpm.dataentry.questionnaire.CQuestionnaireFile");// fully qualified base object
Here is the load and run method:
   CBaseQuestionnaireFile loadAndRunClass(String classname)
      com.cpm.common.base.CBaseQuestionnaireFile questBase = null;
      Class cClass = null;
      try
         // first we open the jar file with the classes we need
         String[] aJarList = new String[10];
         // Questionnaire format file
         File fURL1 = new File(GetMainFrame().m_strQuestionnaireFileName);
         URL url1   = new URL(fURL1.toURL(),"");
         URL urlNew = new URL("jar:" + url1.toExternalForm() + "!/" );
         // Server base class directory
         String strServerDirectory = "http://" + GetMainFrame().m_strHost + "/APPLETS/";
         File fURL2 = new File(strServerDirectory);
         URL url2 = new URL(fURL2.toURL(),"");
         URL urlNew2 = new URL("jar:" + url2.toExternalForm() + "!/" );
         // Local base class directory
         String strLocalDirectory = CSystem.GetBasePath() + "/Research/bin/";
         File fURL3 = new File(strLocalDirectory);
         URL url3 = new URL(fURL3.toURL(),"");
         URL urlNew3 = new URL("jar:" + url3.toExternalForm() + "!/" );
         File fURLBase = new File(CSystem.GetBasePath() + "/Research/bin/base.jar");
         URL urlBase = new URL(fURLBase.toURL(),"");
         URL urlNewBase = new URL("jar:" + urlBase.toExternalForm() + "!/" );
         File fURLControl = new File(CSystem.GetBasePath() + "/Research/bin/controls.jar");
         URL urlControl = new URL(fURLControl.toURL(),"");
         URL urlNewControl = new URL("jar:" + urlControl.toExternalForm() + "!/" );
         File fURLUtil = new File(CSystem.GetBasePath() + "/Research/bin/utlities.jar");
         URL urlUtil= new URL(fURLUtil.toURL(),"");
         URL urlNewUtil = new URL("jar:" + urlUtil.toExternalForm() + "!/" );
         // Determine where to look
         URL[] urlList = null;
         if(GetMainFrame().m_isStandalone == false) {
            // From a browser
            CSystem.PrintDebugMessage("Running as an applet");
            if(GetMainFrame().m_bOnLineMode == true) {
               // On line
               CSystem.PrintDebugMessage("*** On Line Mode ***");
               urlList = new URL[2];
               urlList[0] = urlNew;
               urlList[1] = urlNew2;
            else {
               // Off line
               CSystem.PrintDebugMessage("*** Off Line Mode ***");
               urlList = new URL[4];
               urlList[0] = urlNew;
               urlList[1] = urlNewBase;
               urlList[2] = urlNewControl;
               urlList[3] = urlNewUtil;
          else {
             CSystem.PrintDebugMessage("*** Stand Alone Mode ***");
             urlList = new URL[1];
             urlList[0] = urlNew;
CSystem.PrintDebugMessage("Question file/path: " + GetMainFrame().m_strQuestionnaireFileName);
        // Set the wait cursor
        GetMainFrame().SetWaitCursor();
        // Get a reference to the applets own classloader
        Class myClass = getClass();
        ClassLoader appletClassLoader = myClass.getClassLoader();
        // Call our multi-jar class loader
        JarClassLoader jarLoader = new JarClassLoader(urlList,appletClassLoader);
CSystem.PrintDebugMessage("CPM Test - passed Jar Loader" + jarLoader.toString());
         // Load the classname from the jarfile
         try
              cClass = jarLoader.loadClass(classname);
         catch(ClassNotFoundException cnfe)
            Object[] optionsConfirm = { "Ok" };
            JOptionPane.showOptionDialog(GetMainFrame(),"Questionnaire file is either damaged or has been tampered with.", "Questionnaire File Error",
            JOptionPane.DEFAULT_OPTION, JOptionPane.WARNING_MESSAGE, null, optionsConfirm, optionsConfirm[0]);
            GetMainFrame().m_strQuestionnaireFileName = "";
            // Clear the wait cursor
            GetMainFrame().ClearWaitCursor();
            return null;
         Class cSuperclass = cClass.getSuperclass();
CSystem.PrintDebugMessage("Test Superclass type is: " + cSuperclass.toString());
         Object o = cClass.newInstance();
CSystem.PrintDebugMessage("Test plain class type is: " + o.toString());
         // Never remove this line of code
         // without it, a crafty user could use our load routine
         // to load/and run some nasty code
         // This test makes SURE that *ONLY* questionnaires get opened
         // and their methods get called
         if(o instanceof com.cpm.common.base.CBaseQuestionnaireFile)
            // Create the object
CSystem.PrintDebugMessage("Test Is instance of CBaseQuestionnaireFile");
CSystem.PrintDebugMessage("CPM Test - Casting to Base Class");
            questBase = (com.cpm.common.base.CBaseQuestionnaireFile)o;
CSystem.PrintDebugMessage("CPM Test - Getting languages from questionnaire");
            GetMainFrame().GetLanguagesFromQuestionnaire(questBase);
CSystem.PrintDebugMessage("CPM Test - Setting locale from Questionnaire selection");
            questBase.SetQuestionnaireLocale(GetMainFrame().m_locale);
CSystem.PrintDebugMessage("CPM Test - Initializing Questionnaire");
            questBase.Initialize();
            questBase.InitializeCards();
          else
            Object[] optionsConfirm = { "Ok" };
            JOptionPane.showOptionDialog(GetMainFrame(),"Questionnaire file is either damaged or has been tampered with.", "Questionnaire File Error",
            JOptionPane.DEFAULT_OPTION, JOptionPane.WARNING_MESSAGE, null, optionsConfirm, optionsConfirm[0]);
            GetMainFrame().m_strQuestionnaireFileName = "";
            // Clear the wait cursor
            GetMainFrame().ClearWaitCursor();
            return null;
      catch (Exception e)
          exDialog.showForThrowable(e.toString(),e);
     // Clear the wait cursor
     GetMainFrame().ClearWaitCursor();
     // Set MouseListener Pointer
//     questBase.SetMouseListenerPointer(GetMainFrame().m_mouseListener);
     return questBase;
   }