Question

I’ve got a Java web application (using Spring), deployed with Jetty. If I try to run it on a Windows machine everything works as expected, but if I try to run the same code on my Linux machine, it fails like this:

[normal startup output] 11:16:39.657 INFO   [main] org.mortbay.jetty.servlet.ServletHandler$Context.log>(ServletHandler.java:1145) >16> Set web app root system property: 'webapp.root' = [/path/to/working/dir] java.lang.reflect.InvocationTargetException         at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)         at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)         at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)         at java.lang.reflect.Method.invoke(Method.java:597)         at org.mortbay.start.Main.invokeMain(Main.java:151)         at org.mortbay.start.Main.start(Main.java:476)         at org.mortbay.start.Main.main(Main.java:94) Caused by: java.lang.ExceptionInInitializerError         at org.springframework.web.util.Log4jWebConfigurer.initLogging(Log4jWebConfigurer.java:129)         at org.springframework.web.util.Log4jConfigListener.contextInitialized(Log4jConfigListener.java:51)         at org.mortbay.jetty.servlet.WebApplicationContext.doStart(WebApplicationContext.java:495)         at org.mortbay.util.Container.start(Container.java:72)         at org.mortbay.http.HttpServer.doStart(HttpServer.java:708)         at org.mortbay.util.Container.start(Container.java:72)         at org.mortbay.jetty.Server.main(Server.java:460)         ... 7 more Caused by: org.apache.commons.logging.LogConfigurationException: org.apache.commons.logging.LogConfigurationException: No suitable Log constructor [Ljava.lang.Class;@15311bd for org.apache.commons.logging.impl.Log4JLogger (Caused by java.lang.NoClassDefFoundError: org/apache/log4j/Category) (Caused by org.apache.commons.logging.LogConfigurationException: No suitable Log constructor [Ljava.lang.Class;@15311bd for org.apache.commons.logging.impl.Log4JLogger (Caused by java.lang.NoClassDefFoundError: org/apache/log4j/Category))         at org.apache.commons.logging.impl.LogFactoryImpl.newInstance(LogFactoryImpl.java:543)         at org.apache.commons.logging.impl.LogFactoryImpl.getInstance(LogFactoryImpl.java:235)         at org.apache.commons.logging.impl.LogFactoryImpl.getInstance(LogFactoryImpl.java:209)         at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:351)         at org.springframework.util.SystemPropertyUtils.(SystemPropertyUtils.java:42)         ... 14 more Caused by: org.apache.commons.logging.LogConfigurationException: No suitable Log constructor [Ljava.lang.Class;@15311bd for org.apache.commons.logging.impl.Log4JLogger (Caused by java.lang.NoClassDefFoundError: org/apache/log4j/Category)         at org.apache.commons.logging.impl.LogFactoryImpl.getLogConstructor(LogFactoryImpl.java:413)         at org.apache.commons.logging.impl.LogFactoryImpl.newInstance(LogFactoryImpl.java:529)         ... 18 more Caused by: java.lang.NoClassDefFoundError: org/apache/log4j/Category         at java.lang.Class.getDeclaredConstructors0(Native Method)         at java.lang.Class.privateGetDeclaredConstructors(Class.java:2389)         at java.lang.Class.getConstructor0(Class.java:2699)         at java.lang.Class.getConstructor(Class.java:1657)         at org.apache.commons.logging.impl.LogFactoryImpl.getLogConstructor(LogFactoryImpl.java:410)         ... 19 more Caused by: java.lang.ClassNotFoundException: org.apache.log4j.Category         at java.net.URLClassLoader$1.run(URLClassLoader.java:200)         at java.security.AccessController.doPrivileged(Native Method)         at java.net.URLClassLoader.findClass(URLClassLoader.java:188)         at java.lang.ClassLoader.loadClass(ClassLoader.java:307)         at java.lang.ClassLoader.loadClass(ClassLoader.java:252)         at java.lang.ClassLoader.loadClassInternal(ClassLoader.java:320)         ... 24 more [shutdown output]

I’ve run the app with java -verbose:class, and according to that output, org.apache.log4j.Category is loaded from the log4j JAR in my /WEB-INF/lib, just before the first exception is thrown.

Now, the Java versions on the two machines are slightly different. Both the machines have Sun’s java, the Linux machine has 1.6.0_10, while the Windows machine has 1.6.0_08, or maybe 07 or 06, I can’t remember the exact number right now, and don’t have the machine at hand. But even though the minor versions of the Javas are slightly different, the code shouldn’t break like this. Does anyone understand what’s wrong here?

Answer 1

You must understand that a classloader can’t see everything; they can only see what a parent classloader has loaded or what they have loaded themselves. So if you have two classloaders, say one for Jetty and another for your webapp, your webapp can see log4j (since the JAR is the WEB-INF/lib) but Jetty’s classloader can’t.

If you manage to make a class available to Jetty (for example something in the DB layer) which uses log4j but which ends up running in the context (and classloader) of Jetty, you will get an error.

To debug this, set a breakpoint in org.springframework.web.util.Log4jWebConfigurer.initLogging(). If you can, copy the source of this class into your project (don’t forget to delete it afterwards) and add this line:

ClassLoader cl = Thread.currentThread().getContextClassLoader(); 

Have a look at the cl object in your debugger. That should give you some information who created it. My guess is that this is the classloader from Jetty.

[EDIT] Note that you get in a different mess if you have log4j in both classloaders: In this case, you will have two classes with the same name which create objects which are not assignment compatible! So make sure there is only a single instance of this jar or that instances of log4j will never be passed between the two contexts (which is usually not possible).