(Class ? extends Set String ) Class.forName("example.SetImpl").asSubclass

Is there a way to do the following without an Unchecked cast?
Class<? extends Set<String>> clazz;
clazz = (Class<? extends Set<String>>) Class.forName("example.SetImpl").asSubclass(Set.class);
Set<String> mySet = clazz.newInstance();
cheers,
reto

No. The last line will always generate that warning making the second line pointless. There is no way to create a new instance of a generic class because there is (for most intents and purposes) no such thing as a generic class at runtime.

Similar Messages

Class.forName() with dynamic String

I'm trying to create objects of a class whose name depends on a runtime database lookup. So my code looks like this:
package com.mycompany.ourproduct.ourpackage;
String className = rs.getString(1);
Class c = Class.forName(className);
When I run the program, I get a ClassNotFoundException. But everything runs fine if I replace the code above with this:
package com.mycompany.ourproduct.ourpackage;
// String className = rs.getString(1);
String className = "com.mycompany.ourproduct.ourpackage.oursubpackage.MyClass"
Class c = Class.forName(className);
The problem seems to be in the dynamic linking. Although the class I'm trying to load is in a different package, it is a public class (and I can load it with a static string). Neither does moving it to the same package fix the problem.
I've noticed that if I supply a mis-named class with a static string, the exception reads:
java.lang.ClassNotFoundException: com.mycompany.ourproduct.misspelledpackage.SomeClass
but the same exception prints as follows when the string comes from the database:
java.lang.ClassNotFoundException: com/mycompany/ourproduct/misspelledpackage/SomeClass
I thought the difference was one of compile-time linking and run-time linking, but oddly enough, I still get the dots for misspelled classes that I (1) supply on the command line, (2) read from a file, or (3) concatenate from command line arguments plus info from a file. It's just when I pull the class name from the database that I get the slashes. Does anyone know what's going on?
Here's what I get from java -version:
java version "1.2.2"
Solaris VM (build Solaris_JDK_1.2.2_05a, native threads, sunwjit)
Any help would be greatly appreicated!

Hmm. Weird. Have you looked at the String you're getting back from the database? Does it have slashes or dots? (Should be dots.) Have you made sure any leading/trailing spaces have been trimmed? Try doing classname.equals("com.mycompany... etc.") to see if it really is the same string as the constant that does work.
If it turns out the String is correct, and the only difference is whether that String came from a databaes or a literal, then I don't know what's going on. The first step, though, is to verify whether that's really the case, or if you're not getting the String you think you are.

EXTENDING the string class

ok, i know extending the string class is illegal because it's final, but i want to make an advanced string class that basically "extends" the string class and i've seen online this can be done through wrapper classes and/or composition, but i'm lost
here is my sample code that is coming up with numerous compile time errors due to the fact that when i declare a new AdvString object, it doesn't inherit the basic string features (note: Add is a method that can add a character to a specified location in a string)
class AdvString
     private String s;
     public AdvString(String s)
          this.s = s;
     public void Add(int pos, char ch)
          int this_len = (this.length()) + 1;
          int i;
          for(i=0;i<(this_len);i++)
               if(pos == i)
                    this = this + ch;
               else if(pos < i)
                    this = this + this.charAt(i-1);
               else
                    this = this + this.charAt(i);
     public static void main(String[] args)
          AdvString s1;
          s1 = new AdvString("hello");
          char c = 'x';
          int i = 3;
          s1.Add(i,c);
          //s2 = Add(s1,i,c);
          //String s2_reversed = Reverse(s2);
          System.out.println("s1 is: " + s1);
any tips?

see REString at,
http://www.geocities.com/rmlchan/mt.html
you will have to replicate all the String methods you are interested in, and just forward it to the String instance stored in REString or the like. it is like a conduit class and just passes most processing to the 'real' string. maybe a facade pattern.

Extend the String Class

Is there a way to extend the string class? I know it's final, if i make my class final too would that work? I want to add some functionality to the string class, but I wanna be able to do concatination with the + operator and stuff like that. That's special to the String class.
Any help would be great.
Thanks.
Joe

Well, put your mind at easy with the fact that being
able to use the '+' operator on Strings is a design
flaw in Java. At least from a purist point of view...And from a pragmatist's point of view, it's a reasonable compromise, the benefit of which outweighs the downside. This is consistent with Java's goal as a good general-purpose language. It's not intended to be pure OO or pure anything else.

Class.forName() debunked

This issue is about: Dynamic loading and Inheritance.
Debugging an odd ClassNotFoundException in a J2EE Application, I was involved investigating deeply Class.ForName(). This method - namely the version with only the class name as parameter - is used inside xsdbeans.jar, an optiopnal library, shipped with IBM WebSphere 5.x. Moving the jar from the EAR classloader to the server runtime classloader caused weird behavior on my app.
I try to simplify the core question:
Suppose class E extends B. Base class B declares an instance method, dynLoad(String name) that uses internally Class.forName(String name).
Say 'e' an instance of E. Invoking e.dynLoad("Foo"), Class.forName("Foo") should use the ClassLoader of current class. But which is the 'current class': E or B? Is it the class of current object (E) or the declaring superclass (B)?
My instinctive answer was: E. But my J2EE app was behaving as the current class was B. Indeed E and B may be defined by different classloaders, and this makes a big difference.
A rapid lookup at Sun's docs confirmed my first hypothesis:
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String)
> Returns the Class object associated with the class or interface with the given string name.
Invoking this method is equivalent to:
Class.forName(className, true, currentLoader)
where currentLoader denotes the defining class loader of the current class.
ok, this is a little misleading, but going on:
http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Class.html#forName(java.lang.String,%20boolean,%20java.lang.ClassLoader)
> For example, in an instance method the expression:
>
Class.forName("Foo")
is equivalent to:
Class.forName("Foo", true, this.getClass().getClassLoader())
Not equivocal. The current class is the class of current object, no matter (apparently) if the instance method is declared in a superclass.
Right? Wrong. Here a minimal counter-example:
//-- B.java
import java.net.*;
public class B {
    public void dynLoad(String name) throws Exception {
        System.out.println(getClass().getSuperclass()
            + " defining classloader is: "
            + getClass().getSuperclass().getClassLoader());
        System.out.println(getClass()
            + " defining classloader is: "
            + getClass().getClassLoader());
        System.out.println("\n1) Class.forName(\""+name+"\");");
        // Loading with 'currentLoader'
        Class.forName(name)
            .newInstance();
        System.out.println("\n2) Class.forName(\""+name
            +"\", true, this.getClass().getClassLoader());");
        // This should be equivalent (by Sun J2SE API spec)
        Class.forName(name, true, this.getClass().getClassLoader())
            .newInstance();
    public static void main(String[] args) {
        ClassLoader syscl = ClassLoader.getSystemClassLoader();
        if (!URLClassLoader.class.isInstance(syscl)) {
            System.err.println("Cannot continue the trick.");
            System.exit(1);
        URL[] urls = ((URLClassLoader)syscl).getURLs();
        // Yet Another ClassLoader
        URLClassLoader yacl = new URLClassLoader(urls) {
            // Force Parent-last delegation policy to assure
            // demanded classes are defined by _this_ classloader
            public Class loadClass(String name)
                throws ClassNotFoundException {
                Class c = findLoadedClass(name);
                if (c == null) {
                    try {
                        // Simulate no visibility to base class to assure
                        // it is _not_ defined by this classloader too
                        if (name.equalsIgnoreCase("B")) {
                            throw new ClassNotFoundException();
                        c = findClass(name);
                    } catch (ClassNotFoundException e) {
                        c = getParent().loadClass(name);
                return c;
        try {
            // Loading class E with a different classloader
            Class c = yacl.loadClass("E");
            c.getMethod("dynLoad", new Class[]{String.class})
                .invoke(c.newInstance(), new Object[]{"Foo"});
        } catch (Exception ex) {
            ex.printStackTrace();
            System.exit(1);
//-- E.java
public class E extends B {
//-- Foo.java
public class Foo {
public Foo() {
    System.out.println(getClass()
      + " defining classloader is: "
      + getClass().getClassLoader());
}Running above B.main() produce the following output on Sun JDK 1.3, 1.4.2, 1.5:
> class B defining classloader is: sun.misc.Launcher$AppClassLoader@3d200480
class E defining classloader is: B$1@3d2c0480
1) Class.forName("Foo");
class Foo defining classloader is: sun.misc.Launcher$AppClassLoader@3d200480
2) Class.forName("Foo", true, this.getClass().getClassLoader());
class Foo defining classloader is: B$1@3d2c0480
The evidence is that classloader used is not the same. in the case 1) it uses the classloader of superclass B. Hence the two form of Class.forName() invocation are not equivalent, as stated in Sun documentation.
Disclosure
The evidence is confirmed and explained looking at Sun's implementation of java.lang.Class.java and java.lang.ClassLoader.java: finally the Class.forName(String name) relies on sun.reflect.Reflection.getCallerClass(int) to find the famous 'current class' mentioned in the javadocs. This undocumented method returns the i-th caller class of current stack frames; each stack frame represents a method invocation coupled with its declaring class.
I wonder why in Sun docs this poorly explained issue was remaining immutable from 1.3 to date. Furthermore: which was the intended behavior? Another reason to the long said "Class.forName() is evil..." or am I missing something?
regards, Lorenzo

Of course, it comes down to the question: What is the current class?
Not something well defined. You are looking for this.getClass(), it's taking the class at compile time. If it went your way, woundn't that create an inconsistency in forName in static vs instance contexts?
Actually I think the static approach is more natural, because it means that forName behaves the way that resolving external references work. Your way would also go against the spirit of Java security policies which requires references down the classloader chain something to be treated with suspision.
The mechanism intended, I think, for these situations is the contextClassLoader property of Thread. When executing plugin type code the contextClassLoader should be set to the ClassLoader that loaded the plugin, and any references to resources or dynamic class loads from the plugin should be made using the contextClassLoader. This enables your shared library code to access classes and resources on behalf of the plugin code with access to classes and resources inside the plugin.
Thus, if you write library code or other container stuff likely to be called from a plugin you should probably always use:
Class cls = Thread.currentThread().getContextClassLoader().loadClass("foo");

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, how to use it in the case?

we have a java application:
c:\app\TestForName.class
inside the app, there is a call:
Class.forName("ClassLib");
ClassLib.class is a simplest class for testing with no package.
ClassLib.class can be in any directory except the app's directory
c:\app
because ClassLib.class stands for our library class for multiple projects and multiple uses, it is not allowed to be in a special app directory.
and, ClassLoader and URLClassLoder are not suitable in our case.
i tried following 3 ideas, none of them is OK, please help.
1. put ClassLib.class in directory
c:\lib\
(that is c:\lib\ClassLib.class)
call application with command -cp
java -cp c:\lib;....; TestForName
(method Class.forName("ClassLib") call is inside TestForName.class)
2. put ClassLib.class in
c:\jdk\bin\
(VM says it is one of "java.library.path")
java -cp ....; TestForName
3. put ClassLib.class in
c:\jdk\JAR\classes\
(VM says if is one of "sun.boot.class.path")
java -cp ....; TestForName
i really wondering how to make Class.forName() call successfuf inside application TestForName.class
1. which directory is right place for ClassLib.class
2. how to change command line or java code inside TestForName.class
thx for any help.

Why don't u use a class loader. Try this code
// loader class
public class Loader extends ClassLoader {
    String path;
    public Loader(String path) {
          this.path = path;
    public Class findClass(String name) {
        byte[] b = loadClassData(path+name+".class");
        return defineClass(name, b, 0, b.length);
    private byte[] loadClassData(String name) {
        File file = new File(name);
        byte[] data = null;
        try {
            InputStream in = new FileInputStream(file);
            data = new byte[ (int) file.length()];
            for (int i = 0; i < data.length; i++) {
                data[i] = (byte) in.read();
        catch (IOException ex) {
        file = null;
        return data;
// in your caller class use
   Loader loader = new Loader(libPath); // lib path may be taken from a command line arg
   Object main = loader.loadClass("ClassLib", true).newInstance();I think this is what u r looking for
Uditha Nagahawatta

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.

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.

ClassNotFoundException when using Class.forName(), thx

in a study app, i try to use Class.forName() to load any class then get its properties (fields, methods etc. vs reflect).
the (Frame based) app is in directory:
c:\app\StudyApp.class
there is a "Open class" button on the app, click the button, i use FileChooser to open any class file.
i.e. open a class (assume it is not packaged)
d:\dir\TheClass.class
coding in StudyApp.java is:
Class cls=Class.forName("TheClass");
now a ClassNotFoundException throws when call Class.forName() above.
it is easy to understand why throw the exception because i never tell where the class is. it is in directory
d:\dir
my question is: how to tell VM the directory.
the directory d:\dir can not be applied to -classpath when run java.exe StudyApp, because the directory is random one at run-time.
i tried to change System property (i.e. "java.class.path", 'user.dir" etc. none of them can fix the problem.
thanks in advance for any help

This probably does a lot more than you need:
import java.util.*;
import java.io.*;
import java.util.jar.*;
import java.lang.*;
public class ClassFileFinder extends ClassLoader
     List cpath = new LinkedList();
     void addFile(File f) {
          if(f.isDirectory() || (f.isFile() && f.canRead() &&
                                      f.getName().endsWith(".jar")))
               cpath.add(f);
     public byte[] classData(String className) {
          String cname = className.replace('.', File.separatorChar) + ".class";
          Iterator it = cpath.iterator();
          while(it.hasNext()) {
               File f = (File)it.next();
               try {
                    if(f.isDirectory()) {
                         File cFile = new File(f, cname);
                         if(cFile.isFile()) {
                              byte[] buf = new byte[(int)cFile.length()];
                              InputStream in = new FileInputStream(cFile);
                              int off = 0, l;
                              while((l = in.read(buf, off, buf.length - off)) > 0) {
                                   off += l;
                                   if(off >= buf.length) break;
                              in.read(buf);
                              in.close();
                              return buf;
                    } else if (f.isFile()) {
                         JarFile jar = new JarFile(f);
                         JarEntry ent = jar.getJarEntry(cname);
                         if(ent != null) {
                              byte[] buf = new byte[(int)ent.getSize()];
                              int off = 0, l;
                              InputStream in = jar.getInputStream(ent);
                              while((l = in.read(buf, off, buf.length - off)) > 0) {
                                   off += l;
                                   if(off >= buf.length) break;
                              in.close();
                              return buf;
               } catch (IOException e) {
          return null;
     public Class findClass(String className) throws ClassNotFoundException{
          byte[] data = classData(className);
          if(data == null)
               return getParent().loadClass(className);
          else
               return defineClass(className,data,0, data.length);
}Create an instance, Add directories and/or jar files with addFile then loadClass the class you want.

Create a ADF component by Class.forName (11g)

Hello,
I'm loading a list of components by an XML, and I'd like to create an istance by Class.forName, for example:
UIComponent componet = (UIComponent)Class.forName("oracle.adf.view.rich.component.rich.input.RichInputText").newInstance();
but the system give me this error:
java.lang.ClassNotFoundException: oracle.adf.view.rich.component.rich.input.RichInputText
In the class there is the import RichInputText of and in the same class this works:
oracle.adf.view.rich.component.rich.input.RichInputText title = new RichInputText();
Any idea?
Thank you,
Cristian

Hi,
If I create my component extending Container, clients could think it's a Container (but in fact, I only used the Container feature to layout the component).
For instance, if a API client call myCustom.add(aWidget), it will break the layout of my custom component.
I can tell in the javadoc that this component must not be used as a container.
But there are other situations when a client can inadvertantly think it's a container.
Imagine that you write a method addTreeListener() that recursively register a listener to all components of a container. I don't want the Canvas and Scrollbar of my custom component to call back the listeners, but I want to hide my custom component to hide its internal and call back the listeners.
As I said, it's mainly a design question, but I wanted to improve my custom component...
Pierre M�tras

Query on Class.forName() method

What is the difference between instantiating using Class.forName() method and instantiating using the new operator?

You can do one of them at runtime.
For example:
public static void main(String[] args) {
String className = args[0];
Object o = Class.forName(className).newInstance();
}It's essentially a part of the reflection API which is generally used when you don't know exactly what you want to manipulate at compile time.

URLClassLoader Class.forName Very Slow

I'm wondering why the following usage of a URLClassLoader's Class.forName() function is slow for me. Should it really take 20 seconds to execute the following on a modern Windows machine (regardless of whether the files are in a jar or not)?
class ClassLoaderTest
    public static void main(String[] args) throws Exception {
        //URL url1 = new URL("file://c:/JavaClassLoader/testDir/app.jar");
        URL url1 = new URL("file://c:/JavaClassLoader/testDir");
        URLClassLoader cl = new URLClassLoader(new URL[] {url1});
        long start = System.currentTimeMillis();
        Class classA = Class.forName("a.A",true,cl);
        long stop = System.currentTimeMillis();
        double duration = (stop - start) / 1000d;
        System.out.println("Completed [" + duration + "s]");
}

I get 0.33ms/class in the following test.
     public static void main(String[] args) throws Exception {
          URL url1 = new URL("file://c:/j2sdk1.4.2_04/jre/lib/rt.jar");
          URLClassLoader cl = new URLClassLoader(new URL[]{url1});
          double start = System.currentTimeMillis();
          String[] classes = ("AbstractCollection,AbstractList,AbstractMap,AbstractSet,ArrayList,Arrays,"+
          "BitSet,Calendar,Collection,Collections,Comparator,Currency,Date,Dictionary,Enumeration,EventListener,EventObject,"+
          "GregorianCalendar,HashMap,HashSet,Hashtable,IdentityHashMap,Iterator,LinkedHashMap,LinkedHashSet,LinkedList,List,ListIterator,Locale,"+
          "Map,Observable,Properties,Random,Set,SortedMap,SortedSet,Stack,Timer,TimerTask,TimerThread,TimeZone,TreeMap,TreeSet,Vector,WeakHashMap")
                    .split(",");
          for(int i =0;i<classes.length;i++)
               Class.forName("java.util."+classes, true, cl);
          double stop = System.currentTimeMillis();
          double duration = (stop - start) / classes.length;
          System.out.println("Completed [" + duration + "ms/class]");

Why is Class.forName( ) so useful?

I still don't get why Class.forName so useful to java?
If I do:
Class c = Class.forName("dog");
Dog d = c.newInstance();But what's wrong with:
Dog d = new Dog();Why does java need that Class.forName thing? Can anyone give an counter example to demonstrate the case in my second way won't work?
Thanks.

cpthk wrote:
I still don't get why Class.forName so useful to java?
If I do:
Class c = Class.forName("dog");
Dog d = c.newInstance();But what's wrong with:
Dog d = new Dog();Why does java need that Class.forName thing? Can anyone give an counter example to demonstrate the case in my second way won't work?
System.out.println("Class to instantiate?");
String className = scanner.nextLine();
Class c = Class.forName(className);
Object o = c.newInstance();Also, Class has a lot more methods besides just forName().

Class.forname("").newInstance(); = Problem!! ;)

I am distributing the application I am developping as a Jar file...
In my application, there is a tool which interacts with a Database..
As there is more than 1 existing database, and that they all require different drivers, I do not want to have to put every driver into my jar file, since it makes the file too big... I'd like to make it possible to download the drivers separatly, as another jar file... Although when I do that, I am not able to create a new instance of the driver...
Here's what I have:
Class.forName("org.gjt.mm.mysql.Driver").newInstance();
This works fine when the driver has been included in the same jar file as my program.. How could I get it to work if the driver is in a separate Jar file, in the same folder as the jar file of my own application?

I am distributing the application I am developping as
a Jar file...
In my application, there is a tool which interacts
with a Database..
As there is more than 1 existing database, and that
they all require different drivers, I do not want to
have to put every driver into my jar file, since it
makes the file too big... I'd like to make it
possible to download the drivers separatly, as another
jar file... Although when I do that, I am not able to
create a new instance of the driver...
Here's what I have:
Class.forName("org.gjt.mm.mysql.Driver").newInstance();
This works fine when the driver has been included in
the same jar file as my program.. How could I get it
to work if the driver is in a separate Jar file, in
the same folder as the jar file of my own application?I think the problem may be the classpath since it has to include the jar name specifically to find the classes in them. Perhaps you can set a new classpath with the System.setProperties() function so that the Class.forName() will work. I haven't tried that so I don't know if it will work.

(Class ? extends Set String ) Class.forName("example.SetImpl").asSubclass

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