Class.forName("com.proj.someclass").newInstance()

This works fine if "someclass" is in classpath but doesn't work if "someclass" is in jar file. How to make it work with classes packed into jar?

Hi Mike.
I think you need to write your own customized ClassLoader. Haven't done it myself, but I know the question has been up before, and ansered, and problem solved... Do some search. And maybee you coud be more specific about what you want to achive. Is this an applet or an application? And why do you need to use Class.forName? And why do you want the class to be in a .jar file?
Ragnvald Barth
Software enigneer

Similar Messages

Error in Class.forName("com.mysql.jdbc.driver")

Hi forum,
Please help me to solve the issue.
im using the following jsp code for genrating the reports using JASPER REPORTS
the JSP FILE
<%@ page contentType="text/html;charset=windows-1252"%>
<%@ page import="java.io.*"%>
<%@ page import="java.util.*"%>
<%@ page import="java.sql.*"%>
<%@ page import="javax.sql.DataSource"%>
<%@ page import="javax.naming.InitialContext"%>
<%@ page import="net.sf.jasperreports.engine.*"%>
<%@ page import="net.sf.jasperreports.engine.design.JasperDesign"%>
<%@ page import="net.sf.jasperreports.engine.xml.JRXmlLoader"%>
<%@ page import="net.sf.jasperreports.engine.export.*" %>
<%@ page import ="net.sf.jasperreports.engine.*"%>
<%@ page import ="net.sf.jasperreports.engine.JasperFillManager"%>
<%@ page import ="net.sf.jasperreports.engine.JRException"%>
<%@ page import="net.sf.jasperreports.engine.JasperReport"%>
<%@ page import="net.sf.jasperreports.engine.JasperPrint"%>
<html>
<body bgcolor="00ffcc">
<%
try{
Connection con = null;
String url="jdbc:mysql://localhost/customer";
String username = "root";
String password = "cmsadmin";
InputStream input=new FileInputStream(new File("C:/Documents and Settings/user/My Documents/NetBeansProjects/jasperreports/web/helloworld.xml"));
JasperDesign design = JRXmlLoader.load(input);
JasperReport report = JasperCompileManager.compileReport(design);
Map params = new HashMap();
params.put("reportTitle", "helloworld");
params.put("author", "Muthu Kumar");
params.put("startDate", (new java.util.Date()).toString());
params.put("ReportTitle", "PDF JasperReport");
<img class="emoticon" src="images/emoticons/confused.gif" border="0" alt="" />Class.forName("com.mysql.jdbc.Driver");<img class="emoticon" src="images/emoticons/confused.gif" border="0" alt="" /><img src="images/emoticons/confused.gif" border="0" alt="" />
con = DriverManager.getConnection(url,username,password);
JasperPrint print = JasperFillManager.fillReport(report, params, con);
OutputStream output=new FileOutputStream(new File("C:/Documents and Settings/user/My Documents/NetBeansProjects/jasperreports/helloreportworld.pdf"));
JasperExportManager.exportReportToPdfStream(print, output);
// JasperViewer.viewReport(print);
catch(SQLException es) {
out.println(es);
catch(JRException ex){
//ex.printStackTrace();
out.println(ex);
%>
</body>
</html>The error it is saying is in the line Class.forName(....) ;
Please look for the emoctions with question mark
i DOn know what to do.
Please help
Im comparin the below JRXML file as with the above code
<?xml version="1.0"?>
<!DOCTYPE jasperReport
PUBLIC "-//JasperReports//DTD Report Design//EN"
"http://jasperreports.sourceforge.net/dtds/jasperreport.dtd">
<jasperReport name="helloworld">
<parameter name="reportTitle" class="java.lang.String"/>
<parameter name="author" class="java.lang.String"/>
<parameter name="startDate" class="java.lang.String"/>
<queryString>
<![CDATA[SELECT * FROM customer order by UserID ]]>
</queryString>
<field name="UserID" class="java.lang.String"/>
<field name="UserName" class="java.lang.String"/>
<field name="City" class="java.lang.String"/>
<field name="State" class="java.lang.String"/>
<title>
<band height="60">
<textField>
<reportElement x="0" y="10" width="500" height="40"/>
<textElement textAlignment="Center">
<font size="24"/>
</textElement>
<textFieldExpression class="java.lang.String">
<![CDATA[$P{reportTitle}]]>
</textFieldExpression>
</textField>
<textField>
<reportElement x="0" y="40" width="500" height="20"/>
<textElement textAlignment="Center"/>
<textFieldExpression class="java.lang.String">
<![CDATA["Run by: " + $P{author}
+ " on " + $P{startDate}]]>
</textFieldExpression>
</textField>
</band>
</title>
<columnHeader>
<band height="30">
<rectangle>
<reportElement x="0" y="0" width="500" height="25"/>
<graphicElement/>
</rectangle>
<staticText>
<reportElement x="5" y="5" width="50" height="15"/>
<textElement/>
<text><![CDATA[UserID]]></text>
</staticText>
<staticText>
<reportElement x="55" y="5" width="150" height="15"/>
<text><![CDATA[UserName]]></text>
</staticText>
<staticText>
<reportElement x="205" y="5" width="255" height="15"/>
<text><![CDATA[City, State]]></text>
</staticText>
</band>
</columnHeader>
<detail>
<band height="20">
<textField>
<reportElement x="5" y="0" width="50" height="15"/>
<textElement/>
<textFieldExpression class="java.lang.String">
<![CDATA[$F{UserID}]]>
</textFieldExpression>
</textField>
<textField>
<reportElement x="55" y="0" width="150" height="15"/>
<textElement/>
<textFieldExpression class="java.lang.String">
<![CDATA[$F{UserName}]]>
</textFieldExpression>
</textField>
<textField>
<reportElement x="205" y="0" width="255" height="15"/>
<textElement/>
<textFieldExpression class="java.lang.String">
<![CDATA[$F{City} + ", " + $F{State}]]>
</textFieldExpression>
</textField>
</band>
</detail>
</jasperReport>

Glass_Fish wrote:
I have set the classpath in the environment variables in the my computer properties.The web container has it's own properties. The "system" classpath means absolutely nothing to it. Read your server's documentation.

Help! JSP w/ struts can't find Class.forName("com.mysql.jdbc.Driver")

I think this is just a directory structure issue but I can't figure it out. I am writing a JSP / Struts / MYSQL web application which uses the mysql JDBC connector. The connector (mysql-connector-java-5.1.6-bin.jar) is in the referenced libraries. If I write the line of code:
Class.forName("com.mysql.jdbc.Driver");
To load up the driver, it works in JSP files in the folder /projectname/webroot/web-inf just fine and I can go on to execute MySQL queries. However, I need to load up the driver in an action (.java action) with the directory structure /projectname/src/action and it throws the exception "class not found", I believe, it can no longer seem to get to the driver. Can anyone tell me how to resolve this problem?
Thanks in advance, I hope this is the correct place (I have done my best to find the right one).
Edited by: Arkanin on Apr 24, 2008 4:39 PM

Arkanin wrote:
I think this is just a directory structure issue but I can't figure it out. I am writing a JSP / Struts / MYSQL web application which uses the mysql JDBC connector. The connector (mysql-connector-java-5.1.6-bin.jar) is in the referenced libraries. Nope, it's not. I don't know what "referenced libraries" means. It has to be in CLASSPATH, and that isn't an environment variable.
If I write the line of code:
Class.forName("com.mysql.jdbc.Driver"); No, wrong.
The JAR file for the Driver class might go in the WEB-INF/lib of your web app. If you're using Tomcat 5.5.26, put it in common/lib.
%

Problems with Class.forName("com.sun.sql.jdbc.db2.DB2Driver")

In sun java studio enterprise 8
I'm able to connect to my local DB2 sample database by creating a database schema.
But when I try to execute a java class (main) using
Class.forName("com.sun.sql.jdbc.db2.DB2Driver"). It returns a not found condition.
what am I doing wrong? do I have to point the classpath to some directory?

try to add jar containing driver to
Your project -> Properties -> Libraries -> Run list.

How to create an object of our own class by using Class.forName()??

how to create an object of our own class by using Class.forName()??
plzz anser my qustion soon..

Class.forName does not create an object. It returns a reference to the Class object that describes the metadata for the class in question--what methods and fields it has, etc.
To create an object--regardless of whether it's your class or some other class--you could call newInstance on the Class object returned from Class.forName, BUT only if that class has a no-arg constructor that you want to call.
Class<MyClass> clazz = Class.forName("com.mycompany.MyClass");
MyClass mine = clazz.newInstance();If you want to use a constructor that takes parameters, you'll have to use java.lang.reflect.Constructor.
Google for java reflection tutorial for more details.
BUT reflection is often abused, and often employe when not needed. Why is it that you think you need this?

Trouble w/ multiple applets dynamically starting and Class.forName()

I'm trying to start applets dynamically and I see strange behavior on some systems it works fine and gets the applet class from the jar file. On other systems it makes an http request back to the server to try and get the class. I don't know where in the Class or ClassLoader it's deciding whether to get this class from the jarfile or from the server and all of these classes are contained in the jarfile? Any suggestions?
Here's the method to start new applets
public Component startApplet( String className ) {
System.out.println("Starting applet "+className);
Class appletClass = null;
JApplet japplet = null;
try {
appletClass = Class.forName("com.actsolar.ui.applet."+className);
System.out.println(appletClass);
japplet = (JApplet)appletClass.newInstance();
japplet.init();
} catch (Exception e) {
System.out.println("startapplet Exception: "+e.getMessage());
return japplet;
thanks,
andrew

Hi,
I have seen the same problem when I mix up small letters and BIG LETTERS.
Ex :
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<applet code="Function3.class"width="100" height="100">
</applet>
</html>
will work with Function3.class
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<applet code="function3.class"width="100" height="100">
</applet>
</html>
Will NOT work.
Maybe this will help you , I am also new on JAVA so maybe you make the same mistake like I did ?
Best Regards,
Didier.

URLClassLoader vs Class.forName

what is the difference between using these two.
(i have never used the former and am curious)
Class c = new URLClassLoader(somePath).loadClass("com.foo");
Class c = Class.forName("com.foo");is there some (possibly gc) advantage to creating another class loader or is the only advantage that you can specify the path?
any comments or suggestions on when/why to use either technique?

Well one really significant difference is that when
you run those two lines of code you end up with two
different classes. If you use two different 'c'
variables and then compare them (using '==') youwill
find that they are not the same.
Sheesh - that's real helpful (not!) So you'd say that
if I had these two lines:
MyObject o1 = new MyObject();
MyObject o2 = new MyObject();
o1 != o2, therefore the MyObject constructor is
significantly different than itself?
What a bone-headed argument!No not really. For starters 'Class' is not a regular object in java. Everything in java depends on it.
For example from the above the following would be true.
o1.getClass() == o2.getClass();
While if you create two different instances with different class loaders (same class) the above would not be true.
However it is more likely to be seen in the following type of code.
    class MyClass implements MyInterface
    Class c = new URLClassLoader(somePath).loadClass("MyClass");
    // The following line will often fail with a class cast exception
    MyInterface c = (MyInterface)c.newInstance();

Class.forName() throws null exception in servlet

Hi, just wondering if anyone having this similar problem:
when i try to load a class using Class.forName() method inside a servlet, it throws null exception.
1) The exception thrown is neither ClassNotFoundException nor any other Error, it's "null" exception.
2) There's nothing wrong with the code, in fact, the same code has been testing in swing before, works perfectly.
3) I have include all necessary jars/classes into the path, even if i haven't, it should throw ClassNotFoundException instead, not "null" exception.

I have tried to detect any possible nullable variable, and it is able to run until line 15. The exception thrown is actually null only... not NullPointerException... which is why i have confused...
the message i received is "PlugInException: null".
The code is at follow:
* Load plugin
* @return ArrayList of plugins
* @exception PlugInException PlugInException
01 public ArrayList loadPlugin()
02 throws PlugInException
03 {
04 PlugIn plugin;
05 ArrayList plugins = new ArrayList();
06
07 for (int i = 0; i < configLoader.getPluginTotal(); i++)
08 {
09 try
10 {
11 if (debugger > 0)
12 {
13 System.out.print("Loading " configLoader.getPluginClass(i) "...");
14 }
15 if (Class.forName(configLoader.getPluginClass(i)) == null)
16 {
17 if (debugger > 0)
18 {
19 System.out.print(" not found");
20 }
21 }
22 else
23 {
24 if (debugger > 0)
25 {
26 System.out.println(" done");
27 }
28 plugin = (PlugIn)(Class.forName(configLoader.getPluginClass(i)).newInstance());
29 plugin.setContainer(container);
30 plugins.add(plugin);
31 }
32 }
33 catch (Exception e)
34 {
35 throw new PlugInException("PlugIn Exception: " + e.toString());
36 }
37 }
38
39 return plugins;
40 }

Strange use of Class.forName() in JDBC

The following is the classical code to retreive data from a database via JDBC
        Connection con;
        Statement stmt;
        String querystring;
        String parametervalue = "";
        try {
            Class.forName("com.mysql.jdbc.Driver");
        } catch(java.lang.ClassNotFoundException e) {
            System.err.print("ClassNotFoundException: ");
            System.err.println(e.getMessage());
        try {
            con = DriverManager.getConnection(dburl);
            stmt = con.createStatement();
            querystring = "select ParameterValue from Profile ";
            ResultSet rs = stmt.executeQuery(querystring);
            if(rs.next()) parametervalue = rs.getString("ParameterValue");
            rs.close();
            stmt.close();
            con.close();
        } catch(SQLException ex) {
            System.err.println("-----SQLException-----");
            System.err.println("SQLState: " + ex.getSQLState());
            System.err.println("Message: " + ex.getMessage());
            System.err.println("Vendor: " + ex.getErrorCode());
        return parametervalue;         but the use of Class.forName("com.mysql.jdbc.Driver") is strange. it doesn't need to get the returned Class
like: Class t = Class.forName("com.mysql.jdbc.Driver"),and the DriverManager knows which driver to use!
Any one can give an explanation?
thanks

* It's more natural from an OO perspective for me to
tell the manager about the classes it manages thanfor
the classes to know about the manager and tell it
about themselves.
No.Hmmm. Care to elaborate? I don't have anything to really back this up, but it seems to me that a class shouldn't have to know too much about the context in which it is used. Drivers shouldn't have to know that there is a DriverManager that will be managing them. IMHO.
I thought of another reason: Since you're doing either the Class.forName or the DriverManger.register in the client, no work is saved there either way (as you said, register is just as easy to call as forName), but using Class.forName adds extra complexity in the Drivers. I guess this is probably the same as, or very close to "protected users from themselves" though.
If the forName() doesn't happen somewhere then the
class doesn't get loaded.I'm thinking more along the lines of classes getting loaded in a different area of the code than the DB client. Some kind of plugin-type thing maybe, where the plugin manager automatically loads all the classes in a certain classpath-like set of diretories and jars, without knowing or caring what each one is (DB plugin, graphics plugin, encryption plugin, messaging plugin, etc.) or how it's used. The DB client then wouldn't know which specific driver(s) it has available, only that whatever was visible to the plugin manager will be loaded, and some subset of that could be DB drivers.
In this situation, it's not appropriate for the plugin manager to register the drivers, since it doesn't know anything about JDBC, and it's not appropriate for the DB client to register them, since it doesn't know what classes the plugin manager loaded.
I suppose the DB client could query the plugin manager for all its loaded plugins, then check if each one implements Driver, and if it does, then register it. Or maybe the plugin manager could even store a map whose keys are interface Class objects or interface names and whose values are lists of the plugins that implement that interface.
I don't know if this scenario is used, or is even good design, but it doesn't seem too farfetched. This is the kind of situation where it seems to me the driver is the one in the best position to know both a) he has been loaded and b) he needs to be registered.

Can't load class with Class.forName()

Hi, can somebody help me with this problem. I have clas copiled with jdk 1.3.1 (I also test it with jdk 1.4.1). If I try to load a Class instance with Class.forName("package.app.MyClass)") I have a runtime exception:
java.lang.ClassNotFoundException: com.unisys.ebs.all.ispecs.GD130IspecModel
     at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
     at java.security.AccessController.doPrivileged(Native Method)
     at java.net.URLClassLoader.findClass(URLClassLoader.java:183)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:294)
     at java.lang.ClassLoader.loadClass(ClassLoader.java:250)
     at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:310)
     at java.lang.Class.forName0(Native Method)
     at java.lang.Class.forName(Class.java:115)
     at com.unisys.util.ObjectLoader.load(ObjectLoader.java:60)
     at com.unisys.jellybeans.IspecFactory.getIspec(IspecFactory.java:74)
     at com.unisys.jellybeans.LINCEnvironment.getIspec(LINCEnvironment.java:1012)
     at com.unisys.ebs.middleware.TestLinc.initConnection(TestLinc.java:52)
     at com.unisys.ebs.middleware.TestLinc.main(TestLinc.java:106)
but if instead of this I instantiate the class calling the constructor and writing an import sentence, it works fine. I really need to instantiate this class via Class.forName because it extends a superclass and know the class to instantiate only at runtime. I will appreciate any help. Thanks.
Pablo Antonioletti

I removed the import in the file header and wrote this code:
try{
ispecModel = new com.unisys.ebs.all.ispecs.GD130IspecModel();
Class ispecClass = Class.forName("com.unisys.ebs.all.ispecs.GD130IspecModel.class");
catch(Exception e)
e.printStackTrace();
the new sentence work fine, if write mor code I can access methods and members. When the runtime execute the Class.forName sentence the exception is thrown. I put all in the same file and class to avoid mistakes typing class path.
This is very rare I never seen it before and I developed java application since five years ago.

Servlet outputstream doesn't work after class.forname

hi there,
i'm beginner to servlets and jdbc i was trying a code
the servlet outputstream doesn't work after registerting the mysql driver
here is the code:
import java.io.IOException;
import java.io.PrintWriter;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class JDBCServo extends HttpServlet
     private Connection con;
     private Statement stmt;
     protected void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
          // TODO Auto-generated method stub
          try
               ServletOutputStream out = res.getOutputStream();
               out.println("WELCOME");
               Class.forName("com.mysql.jdbc.Driver");
               con = DriverManager.getConnection("jdbc:mysql://localhost/books" , "root" , null);
               stmt = con.createStatement();
               ResultSet result = stmt.executeQuery("select * from book");
               result.next();
               con.close();
               out.println(result.getObject(1)+" "+result.getObject(2));
               out.close();
          }catch(SQLException e){}
          catch(ClassNotFoundException e){e.printStackTrace();}
}

That would almost certainly be because you didn't put that class in the classpath, and therefore Class.forName() throws an exception.

Need Help Loading Sqlbase Driver Using Class.forName(...

I'm having trouble connecting to a Sqlbase database on my PC. The problem seems to be with loading the driver with the "Class.forName" method. My source code (listed below) is in the "C:\My Documents\java" folder. I installed the Sqlbase driver in "C:\com\centurasoft\java\sqlbase" folder. The driver installation modified my autoexec.bat file to include the line "SET CLASSPATH=C:\com\centurasoft\java\sqlbase".
The epdmo database is in a folder on my D:\ drive.
It seems to find the SqlbaseDriver.class file, but for some reason it can't load it. I would greatly appreciate any suggestions as to how I can fix this.
With the line -- Class.forName("centura.java.sqlbase.SqlbaseDriver");
The SqlbaseEx.java program will compile, but I get the following error
when I try to run it:
Exception in thread "main" java.lang.NoClassDefFoundError: SqlbaseDriver (wrong name: com/centurasoft/java/sqlbase/SqlbaseDriver)
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(Unknown Source)
... [several more lines like these]
at SqlbaseEx.main(SqlbaseEx.java:25)
With the line -- DriverManager.registerDriver(new SqlbaseDriver());
The SqlbaseEx.java program will NOT compile. I get the following error:
SqlbaseEx.java:21: cannot access SqlbaseDriver
bad class file: C:\com\centurasoft\java\sqlbase\SqlbaseDriver.class
class file contains wrong class: com.centurasoft.java.sqlbase.SqlbaseDriver
Please remove or make sure it appears in the correct subdirectory of the classpath.
Also, does the line -- String url = "jdbc:sqlbase:epdmo";
look OK? I've seen numerous examples and they all have some values separated by slashes //. Am I
missing something here that will bite me when I get past this driver loading error?
import java.sql.*;
// Create the ...
public class SqlbaseEx {
public static void main(String args[]) {
String url = "jdbc:sqlbase:epdmo";
Connection con;
String createString;
createString = "create table COFFEES " +
"(COF_NAME varchar(32), " +
"SUP_ID int, " +
"PRICE float, " +
"SALES int, " +
"TOTAL int)";
Statement stmt;
try {
Class.forName("centura.java.sqlbase.SqlbaseDriver");
// DriverManager.registerDriver(new SqlbaseDriver());
} catch(java.lang.ClassNotFoundException e) {
System.err.print("ClassNotFoundException: ");
System.err.println(e.getMessage());
try {
con = DriverManager.getConnection(url, "SYSADM", "SYSADM");
stmt = con.createStatement();
stmt.executeUpdate(createString);
stmt.close();
con.close();
} catch(SQLException ex) {
System.err.println("SQLException: " + ex.getMessage());

Thanks for the reply.
Upon further testing, I think my original post was slightly incorrect: if I use either of the following lines --
Class.forName("com.centurasoft.java.sqlbase.SqlbaseDriver");
or
Class.forName("centura.java.sqlbase.SqlbaseDriver");
I get the following error at runtime:
ClassNotFoundException: com.centurasoft.java.sqlbase.SqlbaseDriver
or
ClassNotFoundException: centura.java.sqlbase.SqlbaseDriver
It is when I use the line -- Class.forName("SqlbaseDriver");
that I get the long error message in my original post.
I don't understand why it can't find/load the driver. I feel like I've covered all the bases -- I've tried numerous variations of the driver name in the Class.forName method, my classpath variable seems to be set correctly. Does it matter what folder I compile my program in? Right now, I've just been compiling it in the same folder as the driver file -- c:\com\centurasoft\java\sqlbase.

Class.forName error ??

I got error below. Most time, my code call Class.forName("com.perf.perfectform.ViewController"), it works fine. But some time, it throw error below. I print out a lot message , and I have no idea about what cause this error ?? Who has some idea how to debug this error ? what is possible reason ?
Thanks, gary
P.S. com.perf.perfectform.ViewController ----- it is a java class
java.lang.ClassNotFoundException: com.perf.perfectform.ViewController
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClassInternal(Unknown Source)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Unknown Source)
at com.perf.perfectform.ExecuteUpdate.<init>(ExecuteUpdate.java)
at com.perf.perfectform.StartUp.main(StartUp.java)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
at java.lang.reflect.Method.invoke(Unknown Source)
at com.zerog.lax.LAX.g([DashoPro-V1.2-120198])
at ZPUs.run([DashoPro-V1.2-120198])

Question:
1) are you doing the reading and writing on a separate Thread and then load the class on a separate thread? Solution -> synchronize
2) are you running more than one instance of your application? Solution -> FileLock
3) finally (this looks like what is most probably happening). Are you using a separate ClassLoader when you try to instantiate your ViewController? If you are still on the same ClassLoader and at some point in time (before you overwrote your jar file), you tried to instantiate ViewController which does not exist in the classpath yet (meaning you've got a ClassNotFoundException before overwriting the ViewController's jar file) then no matter how hard you try to instantiate your ViewController class. You'll still get a ClassNotFoundException. Solution -> use a new ClassLoader
Regards,
Noel

Why Class.forName("driver class") ?

Hi,
whay we are using Class.forName("driver class"). why we dont import the driver package and access the required classes and methods?
for eg:
insead of calling
Class.forName("com.microsoft.jdbc.sqlserver.SQLServerDriver"); ,can we use import com.microsoft.jdbc.sqlserver.*; ?
what is the use of DriverManager in this context? any connection between DriverManager and Class.forName() ?
expecting answers.

sure you can import the packages and use the classes directly but that defeats the whole purpose of pluggable drivers.
Class.forName causes the class to be loaded and as stated before that should cause the driver to register itself with the driver manager.
so then if you have a system where you are talking to several different database vendors then you don't have to worry about which is which as the DriverManager handles it for you.
the big pros that i have actually seen though for having this be pluggable and not hard-coded (with the exception of the URL although you can do this dynamically as well) is what happens if you switch databases. if you import the package and use fully named classes and you switch databases at some point (could be just an upgrade of the same vendor's database) you are screwed.
same problem could happen if you switch drivers for some reason.
at any rate if you use things properly and are careful then you should only ever have to change your URL the rest of the code is fine if you use the DriverManager and use the drivers dynamically otherwise...

Class.forName related

this is my code :
public class ClassTest {
public ClassTest() {
try{
System.out.println( "***** : " + System.getProperty( "java.class.path") );
System.setProperty( "java.class.path", System.getProperty( "java.class.path") + ";C:\\injar.jar" );
System.out.println( "NEW PATH : " + System.getProperty( "java.class.path") );
Class c = Class.forName( "com.cu.injar.Searcher" );
}catch( Exception e ){
e.printStackTrace();
public static void main(String[] args) {
ClassTest classTest1 = new ClassTest();
but i still got ClassNotFoundException, i just want to load class at runtime from jar,
note : i want to add selected jar into JVM's classpath at runtime then load

I posted a similar problem a while back, and got two good replies. See http://forum.java.sun.com/thread.jsp?forum=4&thread=276737
Answer provided by Friends of the Water Cooler. Please inform forum admin via the
'Discuss the JDC Web Site' forum that off-topic threads should be supported.

Class.forName("com.proj.someclass").newInstance()

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