Class loader question
Hi,
I got NoClassDefFoundError and when I use ClasspathDebug jsp it shows that the class is loaded by "java.net.URLClassLoader". I put the jar file containing the class in <domain>/lib directory. According to docs the jar under <domain>/lib should be loaded by system class loader. what is "java.net.URLClassLoader"? why isn't system loader loading my class? I am using wls91. Thanks
weblogic.servlet.jsp.TagFileClassLoader
weblogic.utils.classloaders.ChangeAwareClassLoader
weblogic.utils.classloaders.GenericClassLoader
weblogic.utils.classloaders.GenericClassLoader
java.net.URLClassLoader
sun.misc.Launcher$AppClassLoader
sun.misc.Launcher$ExtClassLoader
Bootstrap classloader
I'm not sure from reply 2 whether you figured out what's going on or not. It sounds like you did, but just in case...
The null/primordial/bootstrap class loader loads classes in the core API--stuff it finds in the ext dir(s?) (e.g. rt.jar)--Object, String, java.util.*, java.sql.*, etc. It has no parent. It is the root of the normal delegating classloader tree.
The system classloader is the one that uses classpath to load your classes and 3rd party classes. Everything not in the core API, as long as neither you nor those other classes explicitly specify a different classloader.
The way the normal delegation mechanism works is that a request to load a class is first passed to the loader that loaded the current class. This loader first gives its parent a chance to load it, and so on up the tree to the primordial loader.
Once it hits the root (null/primordial/bootstrap), that classloader tries to load it. If it can't (and the primordial loader can't, in fact, load your class, since it's not in the ext dir), then it hands it back to its child. This continues down the line, with the parent getting first shot at loading, then the child trying if the parent fails.
The primordial loader (which loaded the JdbcOdbcDriver) can't load your class. It hands it back to its child--the system classloader--which can and does load it, using classpath.
I hope that answers any lingering questions, if there were in fact such questions.
Similar Messages
-
Classload.load and Class.forName question
I know there are several threads about this, but I have a doubt.
I'm getting a ClassNotFoundException running the JUnitSampler of JMeter.
This test is run by maven (using a specific plugin for it). This plugin creates a new Thread to call JMeter with all the parameters that needs.
The class that the JUnitSampler is trying to run is in the classpath (is not jar).
I know this because I run maven with -X parameter and it shows this:
[DEBUG] Test Classpath :
[DEBUG] /path/to/project/target/classes
[DEBUG] /path/to/project/target/test-classesLooking at JUnitSampler code I can see this
theclazz = Thread.currentThread().getContextClassLoader().loadClass(className.trim());Is there any reason that the class couldn't be found?No idea - it is your code.It's JMeter code, not mine.You said it couldn't find your test classes.
>
My original question was about if there were any
difference in using
Class.forName
or
Thread.currentThread().getContextClassLoader().loadClass
No idea. You can print the class loader though.
That was because the JUnitSampler is using the second
option to instantiate the test classes, and
I was receiving a ClassNotFoundException. That
problem is now solved, because it can find the class,
but is unable to cast it to TestCase.
I want to know if there is a way to put my test
classes in the ClassLoader that JMeter is using.You have an instance of some class.
To create that class it was loaded by a class loader.
You can get that class loader via the methods of java.lang.Class.
You can use that class loader to load your class.
That might or might not solve your problem.
Because there are any number of ways it can fail to solve you problem. Below is one, one of many, that might do it.
It won't solve it if you have already loaded your class via the system loaders which will happen if you use absolutely any reference to it. -
Say, I hava a class A which has inner classes, are those inner classes loaded when I load class A into JVM? Thanks
Hmm ... depends on the way you use the inner classes.
For example if there's a way to gather instances of
"A.aa" by calling a (factory) method on class "A",
you'd only have to get an instance of "A" via the
URLClassLoader. The ClassLoader of "A" would do the
job of loading "A.aa" in this case. If you want to
instantiate inner classes directly through e.g. a
call to Class.newInstance(), you'd have to do as you
wrote. I guess.My situation is that there is one instance of each of those inner classes. And those instances are created when they are declared in "A" as member variables. -
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. -
Class Loader Hierarchy in Weblogic 7.0
I have read the BEA documentation on the class loader hierarchy in Weblogic Server,
and I have some questions regarding some behavior I am seeing.
I am running Weblogic Server 7.0.
I have an ear file that contains 3 web apps (wars) and several utility jars. The
web apps' manifests contain the Class-Path entry for the utility jars. My understanding
of this is that each web app SHOULD have its own class loader. Also, the utility
jars will be scoped in a separate class loader and WILL NOT have visibility to
the web app classes. The web app classes should have visibility to the utility
jars.
Is this correct????
I added a static segment of code in each web app and printed the class loader
for each servlet when it was loaded. I also printed the class loader from a class
that is DEFINITELY contained in one of the utility jars. Here is the result:
Utility Class ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04 Utility Class
Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
Servlet 1 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04 Servlet 1 Parent
ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
Servlet 2 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04 Servlet 2 Parent
ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
I'm a little confused.... I expected to see 3 different class loaders (i.e. one
for each class above). I believe the above printout says that all 3 classes are
being loaded by the SAME class loader instance (Launcher$AppClassLoader@b9d04).
Am I interpreting this correctly? If so, what's going on?I rechecked the classpath for the user that starts Weblogic, and in the classpath
I found the project "src" directory (must have missed it earlier). When we did
our build, the classes are placed in the src structure then copied to the build
area. That's the reason I was not seeing the appropriate class loader hierarchy.
Thanks for all of the comments.
"Sanjeev Chopra" <[email protected]> wrote:
>
"Mark Cotherman" <[email protected]> wrote in message
news:[email protected]...
Thanks for your comments again Mark. I'm just trying to get a goodhandle
on how
this is working.
I'll assume that somehow my web app classes are being loaded into theroot
classloader.
The next question is... why??Just to be sure - is there any way these classes are sneaking into the
system classpath ?
My ear file contains the following:
a.war
b.war
c.war
lib/util1.jar
lib/util2.jar
lib/util3.jar
lib/util4.jar
The manifest in all three wars reference all util jars. This ear deployssuccessfully
on Weblogic with no errors.
How could these separate servlets (one in each war) not have seperateclass loaders
seperate from the root where the utils should reside. I don't thinkthat
I have
any control over where Weblogic loads the war classes, or do I?
See comments below....
Mark Spotswood <[email protected]> wrote:
Mark Cotherman wrote:
Thanks for the follow-up.
I want to make sure I follow what you are saying. All classes in
the
manifest
Class-Path of WARs are exported to the parent classloader.That's right.
To me this is the correct behavior since the manifest Class-Path
is
meant to provide
a way to share common utility classes among several web apps. AllEJB jars and
manifest Class-Path entries should be loaded by the same class loader.Its a way to share class definitions, but not necessarilly class
instances. I don't think that a web application should be extending
the classpath of its parent's classloader. This leads to namespace
problems as well as reloadability issues.
Ok, you lost me here. Shouldn't delegation handle the namespacecollisions??
If the web app class loader has a class definition (webapp:com.xyz.ClassA) with
exactly the same name in the same package as the root class loader(rootloader:
com.xyz.ClassA), I thought the web app would use (delegate loading)the
class
definition from the root class loader when PreferWebInf is set to false.
Isn't this why the PreferWebInf attribute, when set to true, can causeClassCastExceptions??
The web app when creating an instance of (webapp: com.xyz.ClassA)from
the web
app class loader can potentially pass a reference to this instanceto a
class
instance loaded from the root loader. The root class loader has adifferent class
definition for ClassA.
REALLY what makes since is that all common jar files be defined
in
the manifest
Class-Path OF THE ear FILE (if the WAR(s) are in an ear). These
jar
files should
then be loaded by the same class loader as the EJB jars. There shouldbe no need
for the WARs to have any reference to the utility jars since the
EJB
class loader
is the parent of the WAR class loaders.The ear file doesn't have a manifest classpath, but what you are getting
at makes sense. If you add a manifest to any EJBs in your app, theall
webapps (as well as all other EJBs) will be able to see it, sincewith
our structure, EJBs are loaded into the application's root classloader.
My problem is that the ACTUAL SERVLET classes are NOT being loadedby a separate
class loader from the EJB and common jar class loader. This is
completely
against
what is being said in the Weblogic documentation. The Manifest
Class-Path
should
have nothing to do with where the classes that reside in
WEB-INF/classes
of my
servlet are loaded.Classloaders will ask their parent for the class first before loading
it
themselves. So if the parent classloader somehow has visibility to
classes that your webpapp references, then it will get loaded by the
parent classloader.
I am in the middle of migrating an app from an older version of
Weblogic,
and
it would be helpful to have the ACTUAL class loading hierarchy welldocumented.
The basic hierarchy is all EJBs are in a root shared classloader and
each web application is loaded by a classloader that is a child of
that root.
Again, am I missing something here???My suspicion is that somehow these servlets are in the classpath ofthe
root classloader, so when the webapp classloaders delegate to thatone,
it will come up with the class.
mark
Mark Spotswood <[email protected]> wrote:
I believe what you are seeing is a bug in the servlet container.
The classloader organization is what you expect, but each webapp
is exporting the classpath information from its manifest to the
classloader above its classloader (which is common to all
three webapps) rather than to its own classloader. So because
of the delegation that happens with classloading, the common
parent classloader is the one that loads the class.
I believe that this behavior exists as an attempt to avoid
ClassCastExceptions, but I don't think that it is the right
solution to this problem. In our 8.1 release, this behavior
has been changed. That is, web applications no longer export
manifest classpath information to the parent of their classloader.
This change has not been ported back to the 7.x line, but a bug
report has been created (CR099889). You should be able to follow
up with support with this CR number.
mark
Mark Cotherman wrote:
I have read the BEA documentation on the class loader hierarchy
in
Weblogic Server,
and I have some questions regarding some behavior I am seeing.
I am running Weblogic Server 7.0.
I have an ear file that contains 3 web apps (wars) and several
utility
jars. The
web apps' manifests contain the Class-Path entry for the utility
jars.
My understanding
of this is that each web app SHOULD have its own class loader.
Also,
the utility
jars will be scoped in a separate class loader and WILL NOT have
visibility
to
the web app classes. The web app classes should have visibility
to
the utility
jars.
Is this correct????
I added a static segment of code in each web app and printed the
class
loader
for each servlet when it was loaded. I also printed the class loaderfrom a class
that is DEFINITELY contained in one of the utility jars. Here is
the
result:
Utility Class ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04
Utility
Class
Parent ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
Servlet 1 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04 Servlet1 Parent
ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
Servlet 2 ClassLoader: sun.misc.Launcher$AppClassLoader@b9d04 Servlet2 Parent
ClassLoader: sun.misc.Launcher$ExtClassLoader@71732b
I'm a little confused.... I expected to see 3 different class loaders(i.e. one
for each class above). I believe the above printout says that all
3
classes are
being loaded by the SAME class loader instance
(Launcher$AppClassLoader@b9d04).
Am I interpreting this correctly? If so, what's going on? -
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 in an EJB Container
Hello,
We have a need to load classes from a nonstandard place with in an ejb container, ie a database or secure store. In order to do so we have created a custom class loader. Two issues have come up we have not been able to solve, and perhaps some one knows the answer to.
Issue 1.
Given the following code:
public static void main(String args[])
MyClassLoader loader = new MyClassLoader("lic.dat");
System.out.println("Loading class ..." + "TestObject");
Object o = Class.forName("TestObject",true,loader).newInstance();
System.out.println("Object:" + o.toString());
TestObject tstObj = (TestObject) o;
tstObj.doIt();
What happens when executed is as follows. Class.forName calls our loader and does load the class as we hoped, it returns it as well, and the o.toString() properly executes just fine. The cast of the object to the actual TestObject fails with a class not found exception. We know why this happens but don't know what to do about it. It happens because the class that contains main was loaded by the system class loader (the primordial class loader as its sometimes called), That class loader does not know about TestObject, so when we cast even though the class is "loaded" in memory the class we are in has no knowledge of it, and uses its class loader to attempt to find it, which fails.
> So the question is how do you let the main class know to use our class loader instead of
the system class loader dispite the fact is was loaded by the system class loader.
Issue 2:
To make matters worse we want to do this with in an EJB container. So it now begs the question how do you inform an EJB container to use a specific class loader for some classes?
Mike...Holy crap! We are in a similar situation. In creating a plugin engine that dynamically loads classes we use a new class loader for each plugin. The purpose is so that we can easily unload and/or reload a class at runtime. This is a feature supposedly done by J2EE app servers for hot-deploy.
So our plugins are packaged as JAR files. If you put any class in the CLASSPATH, the JVM class loader (or the class loader of the class that is loading the plugin(s)), will load the class. The JavaDoc API states that the parent classloader is first used to see if it can find the class. Even if you create a new URLClassLoader with NULL for parent, it will always use the JVM class loader as its parent. So, ideally, in our couse, plugins will be at web sites, network drives, or outside of the classpath of the application and JVM.
This feature has two benefits. First, at runtime, new updates of plugins can be loaded without having to restart the app. We are even handling versions and dependencies. Second, during development, especially of large apps that have a long startup time, being able to reload only one plugin as opposed to the annoying startup time of Java apps is great! Speeds up development greatly.
So, our problem sounds just like yours. Apparently, the Client, which creates an instance of the plugin engine, tries to access a plugin (after the engine loades it via a new classloader instance for each plugin). Apparently, it says NoDefClassFound, because as you say, the client classloader has no idea about the plugin class loaded by another classloader.
My co-developer came up with one solution, which I really dont like, but seems to be the only way. The client MUST have the class (java .class file) in its classpath in order to work with the class loaded by another class loader. Much like how in an EJB container, the web server calling to EJB MUST contain the Home and Remote interface .class files in its classpath, the same thing needs to be done here. For us, this means if a plugin depends on 5 others, it must contain ALL of the .class files of those plugins it depends on, so that the classloader instance that loads the plugin, can also see those classes and work with them.
Have you got any other ideas? I really dislike that this has to be done in this manner. It seems to me that the JVM should be able to allow various class loaders to work with one another in such a way that if a client loaded by one classloader has an import statement in it to use a class, the JVM should be able to fetch the .class out of the other classloader and share it, or something!
This seems to me that there really is no way to actually properly unload and then reload a class so that the JVM will discard and free up the previous class completely, while using the new class.
I would be interested in discussing this topic further. Maybe some others will join in and help out on this topic. I am surprised there isn't a top-level forum topic about things like class loaders, JVM, etc. I wish there was a way to add one! -
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 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.. -
Apparent EJB Class Loader Issue
I am having a problem loading narrowing an EJB. It appears to be a class loader
problem. I am getting a ClassCastException with the following message: Cannot
narrow remote object to com.dte.ejb.facade.AccessoryServiceHome.
Here is my code that looks up the EJB (I've ommitted the 'catch' clauses):
EJBHome ejbHome = (EJBHome) cache.get(homeClass);
try {
Object temp = ctx.lookup("ejb/" + homeClass.getName());
if (ejbHome == null) {
ejbHome = (EJBHome) PortableRemoteObject.narrow(temp, homeClass);
cache.put(homeClass, ejbHome);
Here is the output of some debugging information from the above method. Here I've
displayed the class.getName and the class.getClassLoader for the Class object
passed to this method and the remote object being cast by the narrow method:
homeClass : com.dte.ejb.facade.AccessoryServiceHome
homeClass : weblogic.utils.classloaders.ChangeAwareClassLoader@29164c finder:
weblogic.utils.classloaders.MultiClassFinder@300ec4
From Lookup: com.dte.ejb.facade.AccessoryServiceBean_krx5el_HomeImpl
From Lookup: weblogic.utils.classloaders.GenericClassLoader@1ea02f finder: weblogic.utils.classloaders.MultiClassFinder@6fca08
As you can see, the Class passed into this method has been loaded with the ChangeAwareClassLoader
but the HomeImpl class was loaded with the GenericClassLoader. I think that is
the problem (please correct me if I am wrong).
The application in question is an .ear with 2 .war modules and a ejb.jar file.
It was my understanding that all classes needed by any module in a .ear were all
loaded with the same class loader. Is this true? If so, then why am I having this
problem?
Thank you in advance for your help.I just want to add few comments hoping the iPlanet engineer will be able to help me.
I mange to get one deployment of a war file with the client weblogic EJB to work; only if I add the Installation Directory path of the war file plus “web-inf/classes” to the classpath of the JVM configuration.
This tells me that the class loader looks for the JVM classpath to load the EJB home class at run time. I think; if I can make iPlanet class loader to look for the application classpath instead of the JVM my problem will go away.
Thank you in advance,
Nad -
What is a class loader in java? Is this related to JVM?
Don't double post the same question. I've removed the thread you started in the Java Virtual Machine (JVM) forum.
db -
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. -
Can class loading be affected by concurrency ?
I remember having read that class loading is guaranteed to happen on a single thread, ie. that static initializer blocks do not need to worry about being invoked concurrently.
I can't find any longer the source that stated that; so is anyone able to tell me if that's true (and point me to a resource about that) ?
I could find that java.lang.ClassLoader
defines a :
protected synchronized Class loadClass(String name, boolean resolve)
but I imagine a case where a classloader overrides that method without keeping the synchronization AND a class to be loaded is referenced from many parts of a multithreaded system, so it may happen that multiple thread attempt to load it.
is this a real scenario ?
TIA,
EdoHello,
Based on the JVM specifications, when a class or interface is initialized (execution of the static initializers) the "Java Virtual Machine is responsible for taking care of synchronization". The very first step of the procedure for initializing a class or interface is to "synchronize on the Class object that represents the class or interface to be initialized".
Now, the questions are:
How to interpret the specifications? Are the JVM implementations compliant?
Personally, when I write some static initializer blocks, I do NOT worry about multi-threading issues. But I may be wrong.
For instance, I write:
class Singleton {
private static Singleton SINGLETON = new Singleton();
public Singleton getInstance() { return SINGLETON };
}and not:
class Singleton {
private static Singleton SINGLETON;
static {
synchronized(Singleton.class) {
SINGLETON = new Singleton();
public Singleton getInstance() { return SINGLETON };
}I hope it helps. -
Detecting when exception was thrown using custom class loader
Hello all,
I would like to implement the solution described here - http://stackoverflow.com/questions/75218/how-can-i-detect-when-an-exceptions-been-thrown-globally-in-java - that uses custom class loader in order to detect when an Exeption thrown somewhere in the JVM hosting my app, please note that exceptions might be thrown from 3rd party jars the app is using. So, thanks to help I got from another post, I've managed to code the custom class loader. My question is how can the class loader wrap the original exception, as the methods in ClassLoader deals with classes, not instances. So where should I set the original exception?
Thanks!
Edited by: user9355666 on Sep 28, 2010 10:48 PMuser9355666 wrote:
I think I'm missing something fundumental, forgive me for being slow...
This is what I did so far. For the exception wrapper I made a simple class extens Exception that recieve Exception in its ctor and store it. I also subclassed ClassLoader and override its loadClass(). I've registered it as the system classloader. My thinking was to check in that point that if the requested class is instance of Exception and if yes, returning my wrapper class wrapping this exception. But, since loadClass() return class, how can I set in the wrapper the original exception?
In addition, let's say 2 different places in the code throws NPE, to my understanding the classloader will load NPE only once, so how throwing the NPE in the second time can be detected?you are missing a key point. you should creating a custom implementation of the NPE class which hooks into your detection code in its constructor. from that point forward, anytime any NPE (which is your custom class) is constructed, you can detect it. -
Hi all!
Let me explain my problem.
I have an EJB which has in its same Java package some support classes
& also a startup class. I created an EJB jar file (containing the home,
remote, bean, the deployment descriptors & the container generated stubs)
for deployment. I also compiled all the support classes & the startup class
into the /weblogic/myserver/serverclasses directory. I also added the
required entries in the weblogic.properties file for the startup class.
Now the problem is that the support classes & the startup class contain
methods & constructors which have package level access (i.e the default
access in Java). At runtime, when my bean tries to access one of these
methods, an IllegalAccessException is thrown.
I understand that this exception is thrown when one tries to access a
method which should not be accessed (like private/protected/cross-package
access). However, both the EJB & these classes are in the same package.
So this should not be thrown.
Now the next thing I thought is that the EJB & the support classes are
being loaded by 2 different classloaders. I guess the classes under
/weblogic/myserver/serverclasses are being loaded by the WebLogic
Server ClassLoader & the EJB bean classes are being loaded by the
specialized EJB ClassLoader.
Now my question is,
1) Is it the case that the WebLogic Server does not recognize that the
EJB Bean class is in the same package as the support classes because
they are loaded by 2 separate class loaders? Please note that the EJB
jar file does not contain any support classes - so there is no
duplicate classes here.
2) Is this a bug?
3) What should I do to prevent this problem?
Some workarounds that I have done are:
1) Copied the Bean.class (implementation class of EJB bean) into the
serverclasses directory. Now the server does not throw any
IllegalAccessException. -- This, however, does not appeal as a solution
since it looks like the Bean class is now being loaded by the WebLogic
Class Loader instead of from the EJB jar file.
2) I changed all the default package access methods to public methods.
Works like a charm. -- Problem is that this is a third party set of classes
which I would like to avoid modifying.
Hope this is not too confusing. Please let me know if you need any further info.
Thanks much in advance,
--DasYou have to tell WLAS where to load your new classes. You can change
workingDir of your JSP configuration in weblogic.properties to point to your
new class directory.
Cheers - Wei
Hyung-Jin Kim <[email protected]> wrote in message
news:[email protected]..
> It seems that if I compile a class that a JSP uses while Weblogic is
> running, Weblogic's class loader has this nasty habit of caching/writing
> the class that already has loaded into memory into:
>
> weblogic/myServer/classfiles
>
> Weblogic then refers to the older class file under the above directory
> when the server is started again. Is there any way to turn this "class
> loader caching" off in Weblogic? Thanks!
>
> -hjk
>
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