Implementation of Class Loader
i use resource bundle to load values in the properties object instead i have to use class loader ,how can i use the same
The JVM source code are available but i do no where "sun.boot.class.path" is defined. I mean in which part of the source code. There are many folders in hotspot source code. As this is a part of bootstrap classloader code, i want to know where is this bootstrap source code.
Similar Messages
-
Implementing a secure class loader..
Hello,
We have a custom class loader which loads in custom packages from the database or from the file system. We want to ensure that these custom classes do not read or write into the file system. I want to restrict access to these classes when creating them. I read that by setting the proper permissions when defining the class, this could be possible. Here is my implementation of the class loader :
import java.io.File;
import java.io.FileInputStream;
import java.io.FilePermission;
import java.io.IOException;
import java.net.SocketPermission;
import java.net.URL;
import java.security.AccessController;
import java.security.CodeSource;
import java.security.PermissionCollection;
import java.security.Permissions;
import java.security.PrivilegedExceptionAction;
import java.security.SecureClassLoader;
public class TestClassLoader extends SecureClassLoader {
JarParser jp;
public TestClassLoader(JarParser jp) {
super();
this.jp = jp;
public Class findClass(final String name) throws ClassNotFoundException {
try {
return(Class)
AccessController.doPrivileged (
new PrivilegedExceptionAction() {
public Object run() throws ClassNotFoundException {
byte[] buf = null;
try {
if(jp == null)
throw new ClassNotFoundException("Jar file not found");
String className = name.replace( '.', '/' ) + ".class";
buf = jp.getResource(className);
if(buf == null || buf.length == 0)
throw new ClassNotFoundException("Class not found");
CodeSource cs = getCodeSource(name);
return defineClass(name, buf, 0, buf.length, cs);
catch(Exception e) {
throw new ClassNotFoundException(name, e);
catch(Exception e) {
throw new ClassNotFoundException(name, e);
* @param name
protected CodeSource getCodeSource(String name) {
try {
return new CodeSource(new URL("file", "localhost", name), null);
catch(Exception e){
e.printStackTrace();
return null;
protected PermissionCollection getPermissions(CodeSource cs) {
PermissionCollection pc = new Permissions();
pc.add(new RuntimePermission("exitVM"));
pc.add(new FilePermission("${user.home}${/}*", "read"));
pc.add(new FilePermission("${user.dir}${/}*", "read"));
pc.add(new SocketPermission("localhost", "resolve"));
return pc;
public static void main(String[] args) {
try {
byte[] bytes = new byte[8192];
long ttlBytesRead = 0;
int noOfBytesToRead = 0;
File readFile = new File("test.jar");
// "length" here indicates the total no of bytes to read.
// This is equal to the file size sans the offset value.
long length = readFile.length();
Long temp;
FileInputStream readFis = null;
// Create the RandomAccessFile
try{
readFis = new FileInputStream(readFile);
catch(Exception e) {
throw new IOException(e.getMessage());
long tempLong;
StringBuffer buffer = new StringBuffer();
do {
if ((length - ttlBytesRead) > 8192)
tempLong = 8192;
else
tempLong = (length - ttlBytesRead);
temp = new Long((length - ttlBytesRead) > 8192 ? 8192 : (length - ttlBytesRead));
noOfBytesToRead = temp.intValue();
if(noOfBytesToRead == 0)
break;
// Read the specified no of bytes into the byte
// array from the file.
try {
readFis.read(bytes,0,noOfBytesToRead);
catch(Exception e1) {
throw new IOException(e1.getMessage());
String str = new String(bytes, 0, noOfBytesToRead);
buffer.append(str);
ttlBytesRead += noOfBytesToRead;
} while(ttlBytesRead < length);
try {
readFis.close();
catch(Exception e1) {}
JarParser jp = new JarParser("test.jar", buffer.toString().getBytes());
TestClassLoader tcl = new TestClassLoader(jp);
Class class1 = tcl.findClass("com.test.TestFileWrite");
Object obj = class1.newInstance();
catch(Exception e) {
e.printStackTrace();
}JarParser is the class which parses through the jar file and caches the contents as bytes.
The test.jar contains a single class which writes into the user's home directory. As you can see, I have granted only read access on the home dir.
When I run this however, the newly loaded class does manage to write into the file system with no problems. Is there anything further that I need to do to enable restriction on the file system ?
Thanks.If I've been reading correctly, they don't want you
subclassing SecurityManager for security any more.
Everything should be handled by the AccessController
using Permission objects. The easiest way to configure
the AccessController is through policy files.erg. where did you read this? I checked the 1.5.0 beta API just now, and nothing is officially deprecated.
:{ I don't have time to figure out the AccessController API right now.
I have yet to find a good tutorial on the matter of
security and classloaders. If anyone has a good
reference, it would be much appreciated.I second that.
And as a temporary solution (because we're already behind schedule), I went ahead and wrote my own SecurityManager, using the ideas I mentioned above.
I'm posting it at the site below in case anyone can offer any feedback (such as pointing out fatal weaknesses). We tried yesterday for an hour or so to break it, and it withstood all our tests; but security is not our specialty, so there's probably room for improvement.
(I'd post it in this message, but it's a wee bit large.)
www.personal.utulsa.edu/~jeremy-wood/software/ThreadBasedSecurityManager.java
(And if it passes your scrutiny and looks useful, feel free to use it. But note the disclaimers.) -
How to load a class dynamically in the current/system class loader
I need to dynamically load a new jdbc driver jar to the current/system class loader... Please note that creating a new classloader will not help since the DriverManager refers to the systemclassloader itself.
Restarting the application by appending the jar to its classpath will solve the problem but I want to avoid doing this.Did you then create a ClassLoader to load the JDBC
driver and then install it into the system as
directed by the JDBC specification (ie
Class.forName(someClassName))?
And then try to use it from a class loaded fromsome
other ClassLoader (i.e. the system class loader)?
If you did not try this please explain why not.O.K. I just looked at the source to
java.sql.DriverManager. I did not know what I was
talking about, as what I suggested above will not
work.
This is my new Idea:
Create a URLClassLoader to load the JDBC driver also
in this ClassLoader you need to place a helper class
that does the following:
public class Helper {
public Driver getJDBCDriver(String driverClassName,
String url) {
try {
Class.forName(driverClassName);
Driver d = DriverManager.getDriver(url);
return d;
catch(Exception ex) {
ex.printStackTrace();
return null;
}Now create an instance of the Helper class in the new
ClassLoader, and call its getJDBCDriver method to get
an instance of the driver (you will probably have to
create an interface in the root class loader that the
Helper implements so that you can easily call it).
Now from the root classloader you can make calls
directly to the returned Driver and bypass the
DriverManager and its restrictions on cross
ClassLoader access.
The only catch here is that you would have to call to
the returned Driver directly and not use the Driver
Manager.This sounds like will work but I did not want to load DriverManager in a new classloader.. I did a hack
I unzip the jar dynamically in a previously known location (which I included in my classpath when launching the app). The classLoader finds the class now though it did not exist when the app was launched !
A hack of-course but works eh .. -
Dinamyc class loading and jar files
I'm new to java. I would like to load some plugins in my application (it's going to be packaged as a jar at the end). I managed to find some dinamyc class loading examples but they search for the classes to load in a directory.
This is the code:
public static void main(String[] args) {
File pluginDirectory = new File("src/pluginsreloaded/plugins");
if (!pluginDirectory.exists()) { // the plugin directory does not exist
System.out.println("The plugins directory does not exist!");
return;
FilenameFilter filter = new FilenameFilter() {
public boolean accept(File dir, String name) {
return name.endsWith(".class");
String[] pluginFiles = pluginDirectory.list(filter);
for (int i = 0; i < pluginFiles.length; i++) {
if (pluginFiles.indexOf("$") == -1) {
System.out.println("Loading: " + pluginFiles[i].substring(0, pluginFiles[i].length() - 6));
IPlugin plugin = pm.loadPlugin(pluginFiles[i].substring(0, pluginFiles[i].length() - 6));
System.out.println(plugin.description());
protected static IPlugin loadPlugin(String name) {
// Query the plugin list for the plugin
PluginFactory _plugin = (PluginFactory) pluginList.get(name);
if (_plugin == null) { // the plugin is not loaded
try {
Class.forName("pluginsReloaded.plugins." + name);
// The plugin makes an entry in the plugin list
// when loaded
_plugin = (PluginFactory) pluginList.get(name);
if (_plugin == null) {
return null;
} catch (ClassNotFoundException e) {
System.out.println("Plugin " + name + " not found!");
return _plugin.create();
}IPlugin is an interface. I am using netbeans 5.0. The error I get is this:
Exception in thread "main" java.lang.NoClassDefFoundError: pluginsReloaded/plugins/plugin1 (wrong name: pluginsreloaded/plugins/plugin1)
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:620)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:124)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:260)
at java.net.URLClassLoader.access$100(URLClassLoader.java:56)
at java.net.URLClassLoader$1.run(URLClassLoader.java:195)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:188)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:268)
at java.lang.ClassLoader.loadClass(ClassLoader.java:251)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
at java.lang.Class.forName0(Native Method)
at java.lang.Class.forName(Class.java:164)
at pluginsreloaded.PluginManager.loadPlugin(PluginManager.java:30)
at pluginsreloaded.Main.main(Main.java:44)
Java Result: 1As far as I can see it can't find the class. My question is how can I load the class/where can I find it?
Thanks.You can use the java.util.jar.JarFile class to enumerate the contents of a jar. It's not good practice to search for plugins in this way though. A plugin may well involve serveral classes, which don't all have to be internal. Furthermore its wise to avoid any comitment about the mechaism by which class files are to be fetched. You might want to do it remotely, some time, or load them from a database.
What seesms to be the standard approach is to put a list of plugin classes into the jar as a text file.
Plugins for a given purpose usually implement some specified interface, or extend some abstract class to which their objects can be cast once loaded (without this they aren't really much use).
Say your plugins implment org.myorg.WidgetFactory then you put a list of them, one fully qualified name to a line, in a file or jar entry called META-INF/services/org.myorg.WidgetFactory. You framework picks up all such files from the classpath using ClassLoader.getResources() and loads all the classes whose names if finds, hence you can add new sets of plugins just by adding a new jar or class directory to the class path. -
I am using Sun implementation of JAXB(jaxb-api.jar) for java-to-xml binding in my web application deployed in the latest version of oracle app server(10g release 3). The web server is loading Oracle implementation of JAXB from the shared archive xml.jar. To direct the web server to load application specific JAXB classes, I have used the property(<web-app-class-loader search-local-classes-first="true" include-war-manifest-class-path="false" />) in the deployment description - orion.xml file. But it does not solve the problem!
Thanks & regardsHi,
Refer to this link on OC4J's Classloading Framework... http://download-east.oracle.com/docs/cd/B25221_04/web.1013/b14433/classload.htm#sthref58 there are a couple of options you may be able to employ, including overriding the shared library - there is an example of doing this with the Oracle XML parser and Xerces.
I'm guessing you're on the right track, but you may want to try include-war-manifest-class-path="true". Also, are you sure that you've got your deployment descriptor file defined correctly? Its normally called orion-application.xml, and that particular element isn't defined in the documentation (http://download-east.oracle.com/docs/cd/B25221_04/web.1013/b14433/descriptors.htm#sthref337). You could always try configuring it through the administration console. -
Application Class Loader problem calling virtual function
Hello everyone
I have run in to very strange behavior of JVM
I have created a class loader which allows my to load classes from
jar file, regardless that URLClassLoader supplies this functionality.
I am using this class loader to load some classes and call their virtual functions. The class diagram looks like that:
public interface I {
public void init();
public class AAA implements
public void init(){
public class BBB extends AAA{
public void init(){
//here comes implementation
}Interface I and class AAA are loaded with System Class loader and Class BBB is loaded using my class loader from jar file.
ByteStreamClassLoader classLoader = new ByteStreamClassLoader ();
Class cl = classLoader. loadClass(�com.product.BBB�,true);
I myInterface = cl.newInstance();
myInterface.init();The problem is that from some unknown reason java class AAA.init() instead of BBB.init().
Can somebody help me what am I doing wrong?
Class Loader code attached below
public class ByteStreamClassLoader extends ClassLoader {
protected HashMap m_cache = new HashMap();
public void clearCashe() {
m_cache = new HashMap();
private String definePackage(String className) {
StringBuilder strB = new StringBuilder();
//Class name must be removed from the URI in order to define a package
String[] packageArray = className.split("\\.");
for (int i = 0; i < packageArray.length - 1; i++) {
strB.append(packageArray).append(".");
String packageName = strB.toString();
packageName = packageName.substring(0, packageName.length() - 1);
if (getPackage(packageName) == null) {
m_logger.log(Level.FINEST, "Defining package '" + packageName + "'");
definePackage(packageName, null, null, null, null, null, null, null);
return packageName;
public synchronized Class loadClass(String name, boolean resolve) throws
ClassNotFoundException {
name = name.replaceAll("/", ".").replaceAll(".class", "");
//Try to locate the Class in cashe
Class c = (Class) m_cache.get(name);
//Try to locate the Class in the System Class Loader
if (c == null) {
try {
c = ClassLoader.getSystemClassLoader().loadClass(name);
catch (Exception ex) {}
else {
m_logger.log(Level.FINEST, "Class '" + name + "' found in cache");
//Load the class from byte array
if (c == null) {
String resourceName = name;
if (!resourceName.endsWith(".class")) {
resourceName = resourceName.concat(".class");
//Retrieve class byte representation
if (resourceName.indexOf(".") != -1) {
resourceName =
resourceName.replaceAll("\\.", "/").replaceAll("/class", ".class");
//Use the ByteStreamClassLoader to load the class from byte array
byte[] classByteArray = null;
try {
classByteArray = getResourceBytes(resourceName);
catch (IOException ex1) {
throw new ClassNotFoundException(
"Could not load class data." + ex1.getMessage());
m_logger.log(
Level.FINEST, "Loading class '" +
name + "' Byte Length: " + classByteArray.length);
String p = definePackage(name);
c = defineClass(
name,
classByteArray,
0,
classByteArray.length,
ByteStreamClassLoader.class.getProtectionDomain());
m_cache.put(name, c);
if (resolve) {
resolveClass(c);
return c;Hello everyone
I have run in to very strange behavior of JVM
I have created a class loader which allows my to load classes from
jar file, regardless that URLClassLoader supplies this functionality.
I am using this class loader to load some classes and call their virtual functions. The class diagram looks like that:
public interface I {
public void init();
public class AAA implements
public void init(){
public class BBB extends AAA{
public void init(){
//here comes implementation
}Interface I and class AAA are loaded with System Class loader and Class BBB is loaded using my class loader from jar file.
ByteStreamClassLoader classLoader = new ByteStreamClassLoader ();
Class cl = classLoader. loadClass(�com.product.BBB�,true);
I myInterface = cl.newInstance();
myInterface.init();The problem is that from some unknown reason java class AAA.init() instead of BBB.init().
Can somebody help me what am I doing wrong?
Class Loader code attached below
public class ByteStreamClassLoader extends ClassLoader {
protected HashMap m_cache = new HashMap();
public void clearCashe() {
m_cache = new HashMap();
private String definePackage(String className) {
StringBuilder strB = new StringBuilder();
//Class name must be removed from the URI in order to define a package
String[] packageArray = className.split("\\.");
for (int i = 0; i < packageArray.length - 1; i++) {
strB.append(packageArray).append(".");
String packageName = strB.toString();
packageName = packageName.substring(0, packageName.length() - 1);
if (getPackage(packageName) == null) {
m_logger.log(Level.FINEST, "Defining package '" + packageName + "'");
definePackage(packageName, null, null, null, null, null, null, null);
return packageName;
public synchronized Class loadClass(String name, boolean resolve) throws
ClassNotFoundException {
name = name.replaceAll("/", ".").replaceAll(".class", "");
//Try to locate the Class in cashe
Class c = (Class) m_cache.get(name);
//Try to locate the Class in the System Class Loader
if (c == null) {
try {
c = ClassLoader.getSystemClassLoader().loadClass(name);
catch (Exception ex) {}
else {
m_logger.log(Level.FINEST, "Class '" + name + "' found in cache");
//Load the class from byte array
if (c == null) {
String resourceName = name;
if (!resourceName.endsWith(".class")) {
resourceName = resourceName.concat(".class");
//Retrieve class byte representation
if (resourceName.indexOf(".") != -1) {
resourceName =
resourceName.replaceAll("\\.", "/").replaceAll("/class", ".class");
//Use the ByteStreamClassLoader to load the class from byte array
byte[] classByteArray = null;
try {
classByteArray = getResourceBytes(resourceName);
catch (IOException ex1) {
throw new ClassNotFoundException(
"Could not load class data." + ex1.getMessage());
m_logger.log(
Level.FINEST, "Loading class '" +
name + "' Byte Length: " + classByteArray.length);
String p = definePackage(name);
c = defineClass(
name,
classByteArray,
0,
classByteArray.length,
ByteStreamClassLoader.class.getProtectionDomain());
m_cache.put(name, c);
if (resolve) {
resolveClass(c);
return c; -
Dynamic class loading from JARs in web application
Hello,
I'm working on a web project in which we would like to dynamically load plugins without server restart.
We have developed our own ClassLoader in order to load the plugins from a path or with a user interface upload function.
The class loader hierarchy should be something like this:
Bootstrap
|
System
|
Common
Catalina Shared
Webapp1 OurSystem
PluginClassLoaderThe all works fine within the classes loaded in the PluginClassLoader, but classes loaded in OurSystems class loader cannot access classes loaded in PluginClassLoader. For example when Hibernate tries to load classes definied in mapping files we got a java.lang.ClassNotFoundException.
Is there a way to load classes dynamically to OurSystems class loader or notify it about PluginClassLoaders classes?
Or is this a bad way to do it?
Best regards,
Kristoffer RenholmHi,
Sounds like a classpath problem that the folks in the workshop newsgroup
could help with. Try asking your question in:
http://newsgroups.bea.com/cgi-bin/dnewsweb?cmd=xover&group=weblogic.developer.interest.workshop&utag=
Bruce
Graeme Dougal wrote:
>
Hi, I am developing a web service with weblogic workshop. The JWS file references
other classes one of which is a factory for distributing various implementations
of an interface. I am trying to dynamically load the relevant class to be distributed
from the factory via its name, e.g. Class c = Class.forName(className)
However I keep getting a classNotFoundException.
Any ideas ?? -
Dynamic Class Loading and Unloading
I am trying to create a system where the class name and method name is
picked up from a meta-data database and executed.
This was accompanied using Dynamic Class loading. I tried to extend this to
support versioning of meta-data. Here depending on the version of meta-data
different libraries can be loaded and different implementations of object
with the same name can be executed. This does not seem to work with Forte
3.0.
When the second Library is loaded the method execution does not work.
(Even though the unload flag on the LoadLibrary is set)
If the application is stopped and restarted pointing to the second library
there is no problem. In a running application a dynamically loaded library
does not seem to unload and reload a new library for the same class names.
Has anyone tried sometime similar, is there a workaround for this type of
problem.
- VivekI am trying to create a system where the class name and method name is
picked up from a meta-data database and executed.
This was accompanied using Dynamic Class loading. I tried to extend this to
support versioning of meta-data. Here depending on the version of meta-data
different libraries can be loaded and different implementations of object
with the same name can be executed. This does not seem to work with Forte
3.0.
When the second Library is loaded the method execution does not work.
(Even though the unload flag on the LoadLibrary is set)
If the application is stopped and restarted pointing to the second library
there is no problem. In a running application a dynamically loaded library
does not seem to unload and reload a new library for the same class names.
Has anyone tried sometime similar, is there a workaround for this type of
problem.
- Vivek -
Hi all,
Couple of questions. Is dynamic class loading using classloaders supported in any, if not all versions of J2ME? I guess I should ask first, what exactly does J2ME cover? I see KVM, but do watches and PDA's, set top boxes, refrigerators and so forth all run the same J2ME JVM? Or are their "less feature full" versions? I was hoping the J2ME spec would be the "lowest common denominator" to program for, but I thought I read somewhere that small devices like watches may even have a "smaller" J2ME JVM or something, less capable. So can I write code for J2ME and it will run on all small devices like cell phones, pda's, and so forth? Or is there another J2ME version, perhaps small than J2ME.
So, from what I have found so far, it appears dynamic class loading is done at startup from a DB (of sorts) as opposed to being able to dynamically find/load classes. If this is so, is there any way to support downloading and reloading new classes like you can with J2SE, such as the hot-swap feature of web servers? Does Class.forName() at least work in that you can "replace" a class with a new version, even if it is not able to have a separate classloader instance? My guess is that J2ME supports only a single classloader space, but I thought I read somewhere that Class.forName() is available. J2ME API shows it I believe, but I could be wrong.
Any help on this topic would be appreciated.
Thanks.Dynamic class loading is not available in most (if not all) J2ME profiles and configurations. Class.forName() is available (at least in the MID profile), but not to be used for dynamic class loading. It can be used when using device-specific APIs where you can try to load a class containing device specific methods (for example a class that works only on certain Nokia phones, and has methods playSound() and vibrate(), which are not available in MIDP), and if you catch a ClassNotFound exception you can load a class that doesn't use the device specific APIs and contains stubs of the methods, or you can disable certain features in you application that need those features. For an example implementation you can have a look at Nokia's Block Game example (available from their developer site - http://www.forum.nokia.com/main.html).
As for your other more general questions about different JVM's and such, you should read all about the different configurations and profiles available in J2ME (plenty of information using in the J2ME link on this site). -
Dynamic class loading when CODEBASE is unreachable. A bug?
Let us suppose that we have a large-scale distributed application with ca. 1000 participants communicating via RMI and utilizing dynamic class loading. As we all know, a HTTP code server must be available for this purpose in order to provide dynamically downloaded code, usually the communication proxy code of remote objects. In a real-world scenario, the HTTP server will never be 100% available, so that we will have cases that a Java process will not be able to download the necessary Java classes, causing the RMI communication to fail with a ClassNotFoundException or similar exception. In such a case, a robust application would perform some recovery activities and retry the remote call. Eventually, the HTTP server becomes available again and the distributed system recovers automatically. This seems to work fine with J2SE 1.4.2_10, but not with 1.4.2_11 and newer versions. Considering Java 5, the Update 9 exhibits the same problem.
For tracking down the problem, I've written a simple distributed test application, consisting of a client and a server. A server listens on a port, and sends a MarshalledObject to the client. The code of the MarshalledObject is annotated with the value of the "java.rmi.server.codebase" system property. The annotation contains an URL of the JAR file containing the code of the original object. The client connects to the server, reads data form the socket and unmarshalls the original object. This is basically the same procedure as when objects are accross the wire as arguments/return values/exceptions by the RMI/JRMP engine. This procedure is repeated forever in the loop. Due to the fact that the client's CLASSPATH doesn't contain the code of the original object, this code should dynamically be loaded from the HTTP server using the appropriate annotation provided by the server.
If we start the client while the HTTP server is down, the client will keep generating the ClassNotFoundException over and over again, as expected. So far, so good. If we now start the HTTP server while the client is still running, we will observe different behaviors, depending on the version of the client's JVM:
1. In J2SE 1.4.2_10, the client will download the code from the HTTP server and successfully unmarshal the original object sent by the server. ClassNotFoundExceptions will not be generated again.
2. In J2SE 1.4.2_11, 1.4.2_12 and 1.4.2_13 as well as in J2SE 5.0 Update 9, the client will continue generating ClassNotFoundExceptions. Analysis of the HTTP server's access log shows that there were no attempts to download the JAR file required for unmarshaling the object sent by the server.
It seems that in the newer JVM versions the RMI engine remembers URLs which have failed and does not attempt to access them anymore. Althogh this may have some advantages considering the overall network load, the dynamical class loading becomes practically useless in productive large distributed systems. The very first attempt to load the codebase of the communication peer must succeed, otherwise the whole process must be restarted for the communication to work, which is a very expensive (and for most customers unacceptable) operation in terms of preformance and resources usage.
Should this be seen as a bug or a feature of the JVM? What do you think?
Regards,
Miran
Here is the code to reproduce:
Server code
package server;
import java.net.*;
import java.rmi.*;
import java.io.*;
public class Server implements Serializable {
private int value = 42;
public Server() {
public String toString() {
return "The Answer is " + value;
public static void main( String[] args ) {
if( args.length!=1 ) {
System.out.println( "Usage: server.Server <port>" );
System.exit( 1 );
try {
MarshalledObject data = new MarshalledObject( new Server() );
int port = Integer.parseInt( args[0] );
ServerSocket serverSocket = new ServerSocket( port );
System.out.println( "Accepting connections..." );
while( true ) {
Socket s = serverSocket.accept();
new Thread( new SocketHandler( s, data ) ).start();
} catch( Exception ex ) {
ex.printStackTrace();
System.exit( 0 );
public static class SocketHandler implements Runnable {
private Socket s;
private Serializable data;
public SocketHandler( Socket s, Serializable data ) {
this.s = s;
this.data = data;
public void run() {
try {
OutputStream os = s.getOutputStream();
ObjectOutputStream oos = new ObjectOutputStream( os );
oos.writeObject( data );
oos.close();
os.close();
s.close();
System.out.println( "Serving socket succeeded" );
} catch( Exception ex ) {
System.out.println( "Serving socket failed" );
ex.printStackTrace();
Client code
package client;
import java.rmi.*;
import java.net.*;
import java.io.*;
public class Client {
public static void main( String[] args ) {
if( args.length!=1 ) {
System.out.println( "Usage: client.Client <port>" );
System.exit( 1 );
try {
if( System.getSecurityManager()==null ) {
System.setSecurityManager( new RMISecurityManager() );
int port = Integer.parseInt( args[0] );
for( int i = 1; true; ++i ) {
try {
Socket s = new Socket( "localhost", port );
InputStream is = s.getInputStream();
ObjectInputStream ois = new ObjectInputStream( is );
Object o = ois.readObject();
ois.close();
is.close();
s.close();
Object umo = ((MarshalledObject) o).get();
System.out.println( i + ". Retreiving MarshalledObject succeeded: "
+ umo );
} catch( Exception ex ) {
System.out.println( i + ". Retreiving MarshalledObject failed" );
ex.printStackTrace();
System.out.println( i + ". Waiting for 10 sec" );
Thread.sleep( 10000 );
} catch( Exception ex ) {
ex.printStackTrace();
System.exit( 0 );
Start command for the server
java -cp server.jar -Djava.rmi.server.codebase="http://localhost/playground/server.jar" server.Server 33933
Start command for the client
java -cp client.jar -Djava.security.policy=all.policy client.Client 33933
The policy.all file should look as follows
// All permissions
grant {
permission java.security.AllPermission;
};The server.jar file should only contain the classes from the server package. This file should also be made accessible via HTTP (e.g. by using the Apache HTTP server).
The client.jar file should only contain the classes from the client package.no body know about this??
-
Dynamic Class Loading with interface
Hello
I would appreciate any help on the following problem.
I need to load all classes in a particular directory and its subdirectories (top directory is known but not in the classpath) which implement a predefined interface. At the moment I am using a lot of reflection to accomplish this and believe it can be avoided somehow using the fact that I know these classes have to implement a predefined interface.
At the moment, I am searching through the directory and subdirectory, loading all names of classes into a vector using a custom URLClassloader, and then load all classes in the vector into modulecontainers to populate a defaultmutabletree which is displayed on the gui.
I'm sure that knowing the interface means there's a more straightforward way.
Thanks in advance.Finally i've found out myself, i've read some postings in this forum and put them all together, so that my webstart-application finally works the way i want...
That was the topic:
Downloading a jar-file and starting a class from this jar-file within a webstart application (dynamic class loading).
I'll post my final solution for this problem, may be it would be useful in future for anyone else.
//grant all permissions on the clientside
Policy.setPolicy( new Policy() {
public PermissionCollection getPermissions(CodeSource codesource) {
Permissions perms = new Permissions();
perms.add(new AllPermission());
return(perms);
public void refresh(){
//get the current classloader
ClassLoader cl = MyLoadedClass.class.getClassLoader();
//create a new url-classloader while using the current classloader
URL[] urls = new URL[1];
File f = new File(new String(jarName));
urls[0] = f.toURL();
URLClassLoader ul = new URLClassLoader(urls, cl);
//load a class from jarfile
Class c = ul.loadClass(new String(className));
//get an object from loaded class
Object o = c.newInstance();
/* Are we using a class we specifically know about? */
if (o instanceof KnownInterface){
// Yep, lets call a method we know about. */
KnownInterface client = (KnownInterface) o;
client.doAnything(); -
Dynamic class loading with Webstart
Hello !
//within the jar-file at the webstart-directory
public interface MyFrame{
public void createFrame();
//within the jar-file dynamically downloaded
public class MyExtFrame extends JFrame implements MyFrame{
String className = "MyExtFrame";
ClassLoader cl = ClassLoader.getSystemClassLoader();
JarClassLoader jarLoader = new JarClassLoader (cl, jarFile));
/* Load the class from the jar file and resolve it. */
Class c = jarLoader.loadClass (className, true);
/* Create an instance of the class.
Object o = c.newInstance();
/* Are we using a class we specifically know about? */
if (o instanceof MyFrame){
// Yep, lets call a method we know about. */
MyFrame client=(MyFrame) o;
//call a class-method (here creates the whole gui-object at once)
client.createFrame();
This is the code i'm using and i've encountered following problem, if i put this code into an application without webstart, anything works fine, but with webstart i'll get a: java.lang.NoClassDefFoundError
Then i've put the MyFrame-classes into the downloaded jar-file, but this won't work either, i'll get a ClassCastException.
What do i have to do, to become it working ?
Thanks for any conclusions and help.
MichaelFinally i've found out myself, i've read some postings in this forum and put them all together, so that my webstart-application finally works the way i want...
That was the topic:
Downloading a jar-file and starting a class from this jar-file within a webstart application (dynamic class loading).
I'll post my final solution for this problem, may be it would be useful in future for anyone else.
//grant all permissions on the clientside
Policy.setPolicy( new Policy() {
public PermissionCollection getPermissions(CodeSource codesource) {
Permissions perms = new Permissions();
perms.add(new AllPermission());
return(perms);
public void refresh(){
//get the current classloader
ClassLoader cl = MyLoadedClass.class.getClassLoader();
//create a new url-classloader while using the current classloader
URL[] urls = new URL[1];
File f = new File(new String(jarName));
urls[0] = f.toURL();
URLClassLoader ul = new URLClassLoader(urls, cl);
//load a class from jarfile
Class c = ul.loadClass(new String(className));
//get an object from loaded class
Object o = c.newInstance();
/* Are we using a class we specifically know about? */
if (o instanceof KnownInterface){
// Yep, lets call a method we know about. */
KnownInterface client = (KnownInterface) o;
client.doAnything(); -
I am dynamically creating .class files on the fly in my application. Often I need to rebuild the same class (including client classes) multiple times. The problem is that the system ClassLoader keeps the 1st .class file in memory and will not return to the disk to retrieve the new version after the class file(s) have changed.
Do I need to write a new Classloader and substitute it for the main systems class loader? Is there a better way to instantiate the class other than Class.forName( blah ) that will solve this problem?
Thanks,
-Jeff HarmanLike we told you. If the code for a class changes, Java will not automatically notice and load the new version, until you start up the JVM again. If you do not want to re-start the VM, you can create a new ClassLoader (CL), and use it to load the new code for the class.
Now, the problem is, the second CL may think it has to reload many of other classes, since it has no record of loading them itself. You will end up with duplicates of many classes.
If that doesn't suit you, there is not much alternatives.
You could rename the new version of the class, which would mean you'd have to manually track down all the old versions and upgrade them ( by using the new Ctor). You'd then have to update all the references to make them point to the new object. You can make that particular job easier by making both classes (old and new) extend a common base class, or implements the same interface, and use the interface reference (i.e program to an interface).
Another less-than-perfect possibility would be to use serialization where you serialise version-marked objects and reconstitute them with new code...
Good luck. -
Order of class loading in a Web Application
Hi,
I have been trying to find out if there is a defined order in which classes contained in WEB-INF/classes
and/or WEB-INF/lib should be loaded.
There doesn't appear to be any mention of class loading order in either the J2EE 1.3 or J2EE 1.4
specifications. That does not mean that there isn't a defined class load order, it just means I haven't found one.
Typical behaviour from servers such as Tomcat (reference implementation of the Servlet Engine) show
that classes in WEB-INF/classes are loaded before classes contained in jars under WEB-INF/lib.
Because there doesn't appear to be a mandated class load order I cannot assume that the same
behaviour holds for all servers, unless someone here knows better of course.
The reason for the question is that relying on the exhibited class load order means we can patch web applications by simply putting the patch under the WEB-INF/classes directory.
If this should not be relied upon then patches will have to be applied directly to the affected jar files,
this is not a problem in itself however it is nice to be able to keep the patches out of the jars for
ease of rolling back patches without having to copy jar files all over the place.
Any insights would be welcome.
ThanksIf you say that it is server specific i don't find
that to be the correct answer b'cozI didn't give an answer, I am looking for one. I gave an example of the behaviour that Tomcat 4.1.x
exhibits, one that we currently make use of for patching web applications. My question was about
whether or not this behaviour can be relied upon.
the class files are depended on the jar files which
need to checked in first before getting loaded b'coz
it should check for dependent files before only while
getting deployed.This is why the class load order is important.
Is the Web Application Classloader going to look in WEB-INF/classes first every time followed
by WEB-INF/lib everytime, is the order undefined, or is it reversed?
More over if you just think of class loading mechanism
of classes folder only then it is loaded in the order
specified in the web.xml file.
in this we specify the order of loading.As far as I know web.xml does not allow you to specify where classes should be loaded from and in
which classpath order, you define which classes should be used for application components.
The only load order you can specify is the load order of servlets on startup.
If you know different I would love to know how you would configure web.xml to specify the classpath order
in which classes are loaded. -
Codebase classes loading on their own?
I have been tracking down a performance issue. So I am tracing the classes loaded by my clients VM. I have a class defined like so
public final class GraphicFactory implements IGraphicFactory, Serializable{
private static final long serialVersionUID = 1L;
public IDSComponentInstance createComponentInstance(Point location, Dimension size){
if (location == null || size == null) {
throw new NullPointerException();
return new DBComponentInstance(location,size);
public IDSFunctionConnection createFunctionConnection(Point sourceLocation, Point targetLocation) {
if(sourceLocation == null || targetLocation == null){
throw new NullPointerException();
return new DBFunctionConnection(sourceLocation,targetLocation);
}I want to pass this class to the client. So the client calls a method, and this class is returned. When that happens, the client immediately loads this class via the codebase. The thing I can not see is when the client loads the "DBFunctionConnection" and "DBComponentInstance" classes. These classes are also in jar files which can be found by the codebase. Obviously the client is getting them because its working. And they are definitely coming from the server. But I do not see any log of them being loaded.
Its like they get loaded in secret. I bet they do too. I think classes loaded as a result of another class loading are not logged.
But at the same time, I thought a class would not be loaded until it is needed!? If its loading it right away, then this is a good thing because otherwise the RMI system could throw an exception at a most unusual place when the class gets loaded because a method is called.
I can't quite figure out what is happening here :(I guess RMI classloader is just different.
From testing I can see that when the main class is
marshalled it seems as if the classes it contains are
also marshalled.RMIClassLoader is little more than a URLClassLoader. Classes are neither marshalled nor unmarshalled in RMI, they are loaded via the codebase mechanism, using whatever WEB or other protocol you define in the codebase URL.
The log is not telling me when classes are loaded,So you're logging the wrong thing?
So I am still stuck having a hard time knowing when
the RMI is transferring classes.RMI isn't transferring classes. The RMIClassLoader is transferring classes in response to class-loading requirements. This is completely separate from RMI marshalling. It is more strongly associated with RMI unmarshalling but these are still distinct operations.
Maybe you are looking for
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