Examples of Class Loading.

Are there any good tutorials or examples available on Dynamic Class Loading?
peace
neil

Have you tried the Reflection tutorial at:
http://java.sun.com/docs/books/tutorial/reflect/index.html
Grant

Similar Messages

How can I get a dynamic list of Classes Loaded

I assume this may need to use some sort of reflection. Does
anyone have code or ideas where I can:
a) I can get a list of all Classes loaded and their
properties. This would probably be all instances of Class
b) A list of all
global instance variables. (This may not even be possible. I
know it can be done in other languages)
I assume this may need to use some sort of reflection.
Thank you

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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.

Problem in Class Loading

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 & regards

Hi,
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.

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 Renholm

Hi,
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 in J2ME

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 and Stubs

How Dynamic Class Loading is used on Java RMI, since stubs are generated on clients using Reflection API?
I was reading Dynamic code downloading using JavaTM RMI (http://java.sun.com/javase/6/docs/technotes/guides/rmi/codebase.html), it seems to be out of date.
For example, "The client requests the class definition from the codebase. The codebase the client uses is the URL that was annotated to the stub instance when the stub class was loaded by the registry. Back in step 1, the annotated stub for the exported object was then registered with the Java RMI registry bound to a name."

"Enhancements in J2SETM 5.0
Dynamic Generation of Stub Classes
This release adds support for the dynamic generation of stub classes at runtime, obviating the need to use the Java(tm) Remote Method Invocation (Java RMI) stub compiler, rmic, to pregenerate stub classes for remote objects. *Note that rmic must still be used to pregenerate stub classes for remote objects that need to support clients running on _earlier versions_.*
When an application exports a remote object (using the constructors or static exportObject methods(1) of the classes java.rmi.server.UnicastRemoteObject or java.rmi.activation.Activatable) and a pregenerated stub class for the remote object's class cannot be loaded, the remote object's stub will be a java.lang.reflect.Proxy instance (whose class is dynamically generated) with a java.rmi.server.RemoteObjectInvocationHandler as its invocation handler.
An existing application can be deployed to use dynamically generated stub classes unconditionally (that is, whether or not pregenerated stub classes exist) by setting the system property java.rmi.server.ignoreStubClasses to "true". If this property is set to "true", pregenerated stub classes are never used.
Notes:
* If a remote object has pre-5.0 clients, that remote object should use a stub class pregenerated with rmic or the client will get an ClassNotFoundException deserializing the remote object's stub. Pre-5.0 clients will not be able to load an instance of a dynamically generated stub class, because such a class contains an instance of RemoteObjectInvocationHandler, which was not available prior to this release.
* Prior to the J2SE 5.0 release, exporting a remote object would throw a java.rmi.StubNotFoundException if the pregenerated stub class for the remote object's class could not be loaded. With the added support for dynamically generated stub classes, exporting a remote object that has no pregenerated stub class will silently succeed instead. A user deploying a server application to support pre-5.0 clients must still make sure to pregenerate stub classes for the server's remote object classes, even though missing stub classes are no longer reported at export time. Such errors will instead be reported to a pre-5.0 client when it deserializes a dynamically generated stub class.
(1) The static method UnicastRemoteObject.exportObject(Remote) is declared to return java.rmi.server.RemoteStub and therefore cannot be used to export a remote object to use a dynamically generated stub class for its stub. An instance of a dynamically generated stub class is a java.lang.reflect.Proxy instance which is not assignable to RemoteStub."
http://java.sun.com/j2se/1.5.0/docs/guide/rmi/relnotes.html

Dynamic class loading from String name

Hello all,
I have a question for dynamic class loading:
If I have a String variable that represents a class name, how can i use it to make dynamically an object of the class tha the string variable represents?
For example i have a string like this
String classname="String";
Using this i want to make a new String object .
Can anyone tell me how can i do this(example plz)?

I have worked out the code and this works fine:
import java.lang.*;
class dynamictestname{
     public dynamictestname() {
    public static Class test(String objname){
         try{
         Class theClass = Class.forName(objname);
             return theClass;
        } catch (ClassNotFoundException e)
            throw new NoClassDefFoundError (e.getMessage());
     public static void main (String [] args){
          String classname="java.lang.String";
          Class thisClass=dynamictestname.test(classname);
          try{
          Object instance = thisClass.newInstance();
          }catch(InstantiationException e){
          }catch(IllegalAccessException ie){
}The new problem i 've faced is that i can't instatiate a new object by the class i have created.I saw some examples with use of the java.lang.reflect but this class i used with user created classes which have methods that the user wants to run dynamically.
I just want to make an object from the class and give it a value.
For example if the object i have created with the
Object instance = thisClass.newInstance();is an object of java.lang.String class
how can i do something like this below dynamically from instance object :
String x="TEST ";
Thanks for the help..

Class loader Exception

I am getting the following error even though the corresponding file is present in the app.jar.
This happens when hibernates are getting loaded , when i start the oracle app server.
Does anyone knows why its so.
Caused by: org.hibernate.MappingException: entity class not found: com.iflex.fcr.app.deposit.td.us.dto.DepositRateHistoryDTOCheckNew
     at org.hibernate.mapping.PersistentClass.getMappedClass(PersistentClass.java:99)
     at org.hibernate.tuple.PropertyFactory.getGetter(PropertyFactory.java:166)
     at org.hibernate.tuple.PropertyFactory.buildIdentifierProperty(PropertyFactory.java:44)
     at org.hibernate.tuple.EntityMetamodel.<init>(EntityMetamodel.java:115)
     at org.hibernate.persister.entity.AbstractEntityPersister.<init>(AbstractEntityPersister.java:412)
     at org.hibernate.persister.entity.SingleTableEntityPersister.<init>(SingleTableEntityPersister.java:108)
     at org.hibernate.persister.PersisterFactory.createClassPersister(PersisterFactory.java:55)
     at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:216)
     at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1176)
     at com.iflex.fcr.infra.das.orm.hibernate.SessionFactoryLoader.<clinit>(Unknown Source)
     ... 5 more
Caused by: oracle.classloader.util.AnnotatedClassNotFoundException:
     Missing class: com.iflex.fcr.app.deposit.td.us.dto.DepositRateHistoryDTOCheckNew
     Dependent class: org.hibernate.util.ReflectHelper
     Loader: global.libraries:1.0
     Code-Source: /D:/product/10.1.3.1/OracleAS_3/j2ee/FSI_RETAIL/applib/hibernate313.jar
     Configuration: <code-source> in /D:/product/10.1.3.1/OracleAS_3/j2ee/FSI_RETAIL/config/server.xml
This load was initiated at global.libraries:1.0 using the Class.forName() method.
The missing class is not available from any code-source or loader in the system.
     at oracle.classloader.PolicyClassLoader.handleClassNotFound(PolicyClassLoader.java:2078)
     at oracle.classloader.PolicyClassLoader.internalLoadClass(PolicyClassLoader.java:1679)
     at oracle.classloader.PolicyClassLoader.loadClass(PolicyClassLoader.java:1635)
     at oracle.classloader.PolicyClassLoader.loadClass(PolicyClassLoader.java:1620)
     at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:319)
     at java.lang.Class.forName0(Native Method)
     at java.lang.Class.forName(Class.java:164)
     at org.hibernate.util.ReflectHelper.classForName(ReflectHelper.java:108)
     at org.hibernate.mapping.PersistentClass.getMappedClass(PersistentClass.java:96)
     ... 14 more

Neither, not using Oracle HTTP Server nor using JDK 1.4 incluence the behaviour of OC4J. It is your application deployment. As it should be with every Java EE server you provide an EAR file structure with modules like WAR files or EJB jar files. Each of these modules has more or less its own class loading environment.
OC4J however supports application and global shared libraries. Application shared libraries can be packaged in the EAR file using the Java EE 5 library mechanism. In essence your libraries must be known by the class loader of the Java EE module otherwise your application doesn't work.
What I need to know is what type of application (WAR, EJB, combo of both) you're using and how the files are laid out in the directory structure.
To have an example, see this blog entry: http://blogs.oracle.com/olaf/2007/07/25
--olaf

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.
Thanks

If 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.

Custom applet class loader, how?

Fellow Java programmers;
I would like to load 'secure' class files (ie encrypted) into an applet at startup time.
I have code that does this for a normal java program.
How do you go about using a custom class loader in an applet?
Would something like this work? (using say *.Xclass local encrypted class files)
..somewhere in theapplet: init().. runClass()
.public void runClass(name)
{ aCL = new AppletClassLoader(); // extends ClassLoader
Class c = aCL.loadClass(name);
Method m= c.getMethod("main"..)
m.invoke(...);
Your comments or pehaps a code snippet would be appreciated.
Thanks, John

John,
this does not work without modifying the runtime client environment.
Security constraints will prevent you from setting up a new class loader, so you will have to at least grant this permission in a policy file deployed to your clients as you will have to deplay the encryption keys.
As bringing in a new class loader from an applet is highly security critical, this should be backed by a rather strict policy and by signing the applet.
Though the example you provided might work, the class loader implementation itself is not so easy-
If you would like to imitate the findClass() and loadClass() methods from SUN you will see that those are complex and not suitable for "hooking" in an encrypted stream.
You might try an easier way:
Load the class data as an encrypted byte array, decrypt it and use SUNs class loader methods to define a JAVA class out of this byte array:
ClassLoader.defineClass(String�name, byte[]�b, int off, int�len)
This seems to be easier than implmenting your own loader.
Oliver

Custom class loader and local class accessing local variable

I have written my own class loader to solve a specific problem. It
seemed to work very well, but then I started noticing strange errors in
the log output. Here is an example. Some of the names are in Norwegian,
but they are not important to this discussion. JavaNotis.Oppstart is the
name of my class loader class.
java.lang.ClassFormatError: JavaNotis/SendMeldingDialog$1 (Illegal
variable name " val$indeks")
at java.lang.ClassLoader.defineClass0(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:502)
at java.lang.ClassLoader.defineClass(ClassLoader.java:431)
at JavaNotis.Oppstart.findClass(Oppstart.java:193)
at java.lang.ClassLoader.loadClass(ClassLoader.java:299)
at java.lang.ClassLoader.loadClass(ClassLoader.java:255)
at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:315)
at JavaNotis.SendMeldingDialog.init(SendMeldingDialog.java:78)
at JavaNotis.SendMeldingDialog.<init>(SendMeldingDialog.java:54)
at JavaNotis.Notistavle.sendMelding(Notistavle.java:542)
at JavaNotis.Notistavle.access$900(Notistavle.java:59)
at JavaNotis.Notistavle$27.actionPerformed(Notistavle.java:427)
JavaNotis/SendMeldingDialog$1 is a local class in the method
JavaNotis.SendMeldingDialog.init, and it's accessing a final local
variable named indeks. The compiler automatically turns this into a
variable in the inner class called val$indeks. But look at the error
message, there is an extra space in front of the variable name.
This error doesn't occur when I don't use my custom class loader and
instead load the classes through the default class loader in the JVM.
Here is my class loading code. Is there something wrong with it?
Again some Norwegian words, but it should still be understandable I hope.
     protected Class findClass(String name) throws ClassNotFoundException
         byte[] b = loadClassData(name);
         return defineClass(name, b, 0, b.length);
     private byte[] loadClassData(String name) throws ClassNotFoundException
         ByteArrayOutputStream ut = null;
         InputStream inn = null;
         try
             JarEntry klasse = arkiv.getJarEntry(name.replace('.', '/')
+ ".class");
             if (klasse == null)
                throw new ClassNotFoundException("Finner ikke klassen "
+ NOTISKLASSE);
             inn = arkiv.getInputStream(klasse);
             ut = new ByteArrayOutputStream(inn.available());
             byte[] kode = new byte[4096];
             int antall = inn.read(kode);
             while (antall > 0)
                 ut.write(kode, 0, antall);
                 antall = inn.read(kode);
             return ut.toByteArray();
         catch (IOException ioe)
             throw new RuntimeException(ioe.getMessage());
         finally
             try
                if (inn != null)
                   inn.close();
                if (ut != null)
                   ut.close();
             catch (IOException ioe)
     }I hope somebody can help. :-)
Regards,
Knut St�re

I'm not quite sure how Java handles local classes defined within a method, but from this example it seems as if the local class isn't loaded until it is actually needed, that is when the method is called, which seems like a good thing to me.
The parent class is already loaded as you can see. It is the loading of the inner class that fails.
But maybe there is something I've forgotten in my loading code? I know in the "early days" you had to do a lot more to load a class, but I think all that is taken care of by the superclass of my classloader now. All I have to do is provide the raw data of the class. Isn't it so?

RMI Dynamic Class Loading

Hello,
I have a standalone Java Client which is connecting to an RMI based svr component on WLS 6.0sp1. However, before connecting to the RMI Svr, I am trying to boot strap couple of other CLASS files from the App Svr. using the RMIClassLoader. Now here's the problem...
I have placed my classes( one which I want to download and instantiate before I start making conventional calls to RMI Svr remote methods) in the folder WL_HOME/config/examples/rmi/*.class
which is the code_base url I use. However, the error I get when trying to download the classs using RMIClass loader is:
No Security Manager: RMI Class Loader disabled"...
due to which I can not even get connected to my remote object after that. Can somebody please tell me how can I set whatever option it is so that I can get this thing working.
I tried using the "Grant all" security policy too on the client side & server side in vain.
Any help is truly appreciated. Thanks a lot!
cl I am trying to

I'll try having a crack at this, see inline comments
Chirag Shah wrote:
Hello,
I have a standalone Java Client which is connecting to an RMI based svr component on WLS 6.0sp1. However, before connecting to the RMI Svr, I am trying to boot strap couple of other CLASS files from the App Svr. using the RMIClassLoader. Now here's the problem...
I have placed my classes( one which I want to download and instantiate before I start making conventional calls to RMI Svr remote methods) in the folder WL_HOME/config/examples/rmi/*.class
which is the code_base url I use. However, the error I get when trying to download the classs using RMIClass loader is:I'll assume that you're using a proper URL (file://) to instanciate your ClassLoader.
>
No Security Manager: RMI Class Loader disabled"...
Does your client explicitly set a SecurityManager using the following command?
if (System.getSecurityManager() == null) {
System.setSecurityManager(new java.rmi.RMISecurityManager());
Also,I believe that it should only be your client changing its policy file (for the network classload).
>
due to which I can not even get connected to my remote object after that. Can somebody please tell me how can I set whatever option it is so that I can get this thing working.
I tried using the "Grant all" security policy too on the client side & server side in vain.
Any help is truly appreciated. Thanks a lot!
cl I am trying toHope it helps,
Stephane Vaucher
CIRANO
Research Professional

Using system resource to redirect class loading

It seems to be an open secret that the JVM can be manipulated to load classes (or packages?) from alternate locations.
Take, for example, this excerpt from a SAX user guide:
     In case your Java environment did not arrange for a compiled-in default (or to use
     the META-INF/services/org.xl.sax.driver system resource), you'll probably need to
     set the org.xml.sax.driver Java system property to the full classname of the SAX
     driver, as in
          java -Dorg.xml.sax.driver=com.example.xml.SAXDriver MySAXApp sample.xml
So what is the "META-INF/services/org.xml.sax.driver" system resource? Clearly this resource is tied (historically) to the Manifest file in the jar archive format. Is the META-INF/services prefix special for remapping class names? What other services are there? What other resources can be placed in the META-INF directory? Does this work only for Manifest files in jar archives, or does it also work for conventional file systems?
Does the -D option set class locations or package locations? How is the replacement class located (using the same class path)? Is the actual package/class name available for every (potentially) loaded package/class?
How does this work? Where is this documented? What is this called?

Thanks for the reply, but this doesn't really answer
the questions that I intented. Let me try again.
The original posting shows two ways that the JVM can
be coerced to load a implementation from an alternate
location. This coeretion process seems to be an
undocumented, ad hoc extension to the JVM. For the
purposes of these question, I call this process "class
substitution".No, you are misinterpreting what you are seeing there.
The example command that you gave does nothing but define a property which is available via System.getProperty().
That is all it does in terms of java (actually Sun's implementation of java.)
>
A) Where in the Java documentation is this type of
class substitution discussed? I have searched the
Java language definition, Sun's Java site, and "Java
In a Nutshell" for references to "system resources",
"services", and various alternatives to no avail. The
documentation has some reference to Properties, but
the connection to class substitution is opaque to me.
There is no class substitution.
There is however, class loading. And that can be done in several ways.
1. For one you can write your own class loader - see java.lang.ClassLoader.
2. Or you can also load your own classes, which is likely to be what the command line you see is doing (when you run other code associated with that in your own application.) That is called reflection. You could start with the java.lang.Class - and practically any java book will cover it.
3. And it is possible to to load your own class instead of a core class. For example you could replace java.lang.String. Your command line does not do that. But if you look that the documentation for java (Sun) you will find references to the Xbootclasspath option.
For 1 and 2 above the Sun JVM will do nothing with the command example you gave unless you write some code or use some code (outside the sun jvm) that uses it. For 3 it doesn't have anything to do with example you gave.
B) Can I achieve this result (class substitution) If I
put a META_INF directory in a normal file system class
directory, or does this only work for JAR files. I'm
sure a simple experiment would suffice, but I'd like
to know WHY?

Seemingly random errors (Class Loading)

Hi,
I wonder if anyone out there could help me or point me in the right direction to solve this problem im having.
For a Uni project I am writing a Java application that takes three components stored in seperate Jar files (GUI, AI, and Model) dynamically from user defined locations by a central loading class.
eg GUIAddress = "c:/java/guis/gui.jar"
Each has a loading class defined in a MANIFEST.MF file within each Jar file and these are in turn defined by interfaces within my central loading program.
I have set it out like this to allow me to quickly test out different components without changing the main structure of the program.
The problem im having is that I keep getting different ClassFormatErrors depending on what I have changed. Errors include:
Local variable name has bad constant pool index
Extra bytes at the end of the class file
Code segment has wrong length
Illegal constant pool index
Code of a method has length 0All these errors are produced with the same compiler (JRE1.4.2) and with minimal changes to the source code.
For example I get the program to a stage where it works then add the un-used constant to a classprivate int foobar = 10; recompile and reload and i get the error Extra bytes at the end of the class file if I take it out again recompile and rerun and alls better.
Now to me that one line shouldnt make a differene to the program in any significant way. But anyway thats just a small example to show you what my problem is.
These problems started when i made a class Span (http://rex.homeip.net/~matt/java/gui/src/gui/graphs/Span.java) which as you can see does nothing special, but when i use it from another class (http://rex.homeip.net/~matt/java/gui/src/gui/GraphViewer.java) all hell breaks loose. Now i know the class is being loaded and methods can be called from it (line 84) but if i try to call setSpan() then i get the error Local variable name has bad constant pool indexIf anyone has any clues please let me know, im getting sick of going round in circles.
Cheers in advance.
Matt
links
Main loading class: http://rex.homeip.net/~matt/java/loader/src/loader/Loader.java
Class Loader: http://rex.homeip.net/~matt/java/loader/src/loader/setup/SetupManager.java
GUI: http://rex.homeip.net/~matt/java/gui/src/gui/Gui.java
GraphViewer: http://rex.homeip.net/~matt/java/gui/src/gui/GraphViewer.java
Span: http://rex.homeip.net/~matt/java/gui/src/gui/graphs/Span.java

I think I have the solution....
I had the same exact (seemingly random) ClassFormatExceptions being thrown from my custom Class-Loader as well. I would get the same variety of debug prints as you (Invalid constant pool, Code segment has wrong length, etc). At times it seemed to happen randomly to different classes that I loaded, depending on small changes I made to the code.
Here is the background and how I solved it.
I dervied from ClassLoader to make my custom class-loader. I overrode the findClass() method, and NOT the loadClass() method. This is the Java 2 method of making class-loaders, which is simplier than implementing your own loadClass() as in the older Java 1.1way of doing things.
My custom class-loader (called JarFileClassLoader, BTW) already had a list of JAR files that it searched when its findClass() method was called from its parent. I was using a JarFile object to examine the contents of a particular JAR file. Once it found the JarEntry that represented the class I wanted to load, it asked for the InputStream by calling:
JarEntry desiredEntry = // assigned to the JarEntry that represents the class you want to load
InputStream myStream = myJar.getInputStream( desiredEntry );
Then I asked how many bytes were available for reading, I created a byte array that could hold that many bytes, and I read the bytes from the InputStream:
int totalBytes = myStream.available();
byte[] classBytes = new byte[totalBytes];
myStream.read( classBytes );
Finally, I would define and resolve my class by:
Class loadedClass = super.defineClass( className, classBytes, 0, classBytes.length );
resolveClass( loadedClass );
Sometimes, on the call to defineClass(), I would get all the weird ClassFormatExeptions.
After the searching around these forums for a while, I found a lot of discussion about this problem, but I didn't find any concrete explanations or solutions. But I did see some cursory discussion about the InputStream class. This lead me to investigate how this class works exactly.
As it turns out, when you call:
myStream.read( classBytes );
it is NOT guaranteed to read all the available bytes and completely fill the byte array you made for it. It could very easily read LESS than the total available bytes. I don't know why it would do this...maybe something to do with internal buffering of the stream. This happens more often on bigger sized class files that you are trying to read.
The read() call will return an integer that represents the actual number of bytes read during that attempt to read. So, the solution is to check if you got all your bytes during the first call to read(), if not, then read some more.
There is another version of the read() method that takes an 'offset' value into your array and a max bytes to read value. I used this method to modify my code to look like:
int total = myStream.available();
int numNeeded = total;
int numRead = 0;
int offset = 0;
byte[] classBytes = new byte[total];
do
numNeeded -= numRead;
offset = numRead;
numRead += inStream.read( classBytes, offset, numNeeded );
} while( numRead < total );
This will continue looping until all the bytes are read. Then I never got those funky ClassFormatExceptions again. Before my fix, I was getting partial class definitions from calls to read() that didn't return all the bytes. Depending on which part of the class was omitted, I got the varied error prints that you got. Now, after this fix, it seems to work just fine.
Hope this helps!
-Mark

Examples of Class Loading.

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