This
article is part 2 of our java.lang.NoClassDefFoundError troubleshooting series.
It will focus on the more simple type of NoClassDefFoundError problem. This
article is ideal for Java beginners and I highly recommend that you compile,
run and study the sample Java program.
If not done already, I suggest that you first review the java.lang.NoClassDefFoundError - Part 1.
The following writing format will be used going forward and will provide you with:
- Description of the problem case and type of NoClassDefFoundError.
- Sample Java program “simulating” the problem case.
- ClassLoader chain view.
- Recommendations and resolution strategies.
NoClassDefFoundError problem case 1 – missing JAR file
The first problem case we will cover is related to a Java program packaging and / or classpath problem. A typical Java program can include one or many JAR files created at compile time. NoClassDefFoundError can often be observed when you forget to add JAR file(s) containing Java classes referenced by your Java or Java EE application.
This type of problem is normally not hard to resolve once you analyze the Java Exception and missing Java class name.
Sample Java program
The following simple Java program is split as per below:
- The main Java program NoClassDefFoundErrorSimulator
- The caller Java class CallerClassA
- The referencing Java class ReferencingClassA
- A util class for ClassLoader and logging related facilities JavaEETrainingUtil
This program is simple attempting to create a new instance and execute a method of the Java class CallerClassA which is referencing the class ReferencingClassA.It will demonstrate how a simple classpath problem can trigger NoClassDefFoundError. The program is also displaying detail on the current class loader chain at class loading time in order to help you keep track of this process. This will be especially useful for future and more complex problem cases when dealing with larger class loader chains.
#### NoClassDefFoundErrorSimulator.java
package org.ph.javaee.training1;
import org.ph.javaee.training.util.JavaEETrainingUtil;
/**
* NoClassDefFoundErrorTraining1
* @author Pierre-Hugues Charbonneau
*
*/
public class NoClassDefFoundErrorSimulator {
/**
* @param args
*/
public static void main(String[] args) {
System.out.println("java.lang.NoClassDefFoundError Simulator - Training 1");
System.out.println("Author: Pierre-Hugues Charbonneau");
System.out.println("http://javaeesupportpatterns.blogspot.com");
// Print current Classloader context
System.out.println("\nCurrent ClassLoader chain: "+JavaEETrainingUtil.getCurrentClassloaderDetail());
// 1. Create a new instance of CallerClassA
CallerClassA caller = new CallerClassA();
// 2. Execute method of the caller
caller.doSomething();
System.out.println("done!");
}
}
#### CallerClassA.java
package org.ph.javaee.training1;
import org.ph.javaee.training.util.JavaEETrainingUtil;
/**
* CallerClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class CallerClassA {
private final static String CLAZZ = CallerClassA.class.getName();
static {
System.out.println("Classloading of "+CLAZZ+" in progress..."+JavaEETrainingUtil.getCurrentClassloaderDetail());
}
public CallerClassA() {
System.out.println("Creating a new instance of "+CallerClassA.class.getName()+"...");
}
public void doSomething() {
// Create a new instance of ReferencingClassA
ReferencingClassA referencingClass = new ReferencingClassA();
}
}
#### ReferencingClassA.java
package org.ph.javaee.training1;
import org.ph.javaee.training.util.JavaEETrainingUtil;
/**
* ReferencingClassA
* @author Pierre-Hugues Charbonneau
*
*/
public class ReferencingClassA {
private final static String CLAZZ = ReferencingClassA.class.getName();
static {
System.out.println("Classloading of "+CLAZZ+" in progress..."+JavaEETrainingUtil.getCurrentClassloaderDetail());
}
public ReferencingClassA() {
System.out.println("Creating a new instance of "+ReferencingClassA.class.getName()+"...");
}
public void doSomething() {
//nothing to do...
}
}
#### JavaEETrainingUtil.java
package org.ph.javaee.training.util;
import java.util.Stack;
import java.lang.ClassLoader;
/**
* JavaEETrainingUtil
* @author Pierre-Hugues Charbonneau
*
*/
public class JavaEETrainingUtil {
/**
* getCurrentClassloaderDetail
* @return
*/
public static String getCurrentClassloaderDetail() {
StringBuffer classLoaderDetail = new StringBuffer();
Stack<ClassLoader> classLoaderStack = new Stack<ClassLoader>();
ClassLoader currentClassLoader = Thread.currentThread().getContextClassLoader();
classLoaderDetail.append("\n-----------------------------------------------------------------\n");
// Build a Stack of the current ClassLoader chain
while (currentClassLoader != null) {
classLoaderStack.push(currentClassLoader);
currentClassLoader = currentClassLoader.getParent();
}
// Print ClassLoader parent chain
while(classLoaderStack.size() > 0) {
ClassLoader classLoader = classLoaderStack.pop();
// Print current
classLoaderDetail.append(classLoader);
if (classLoaderStack.size() > 0) {
classLoaderDetail.append("\n--- delegation ---\n");
} else {
classLoaderDetail.append(" **Current ClassLoader**");
}
}
classLoaderDetail.append("\n-----------------------------------------------------------------\n");
return classLoaderDetail.toString();
}
}
Problem reproduction
In order to replicate the problem, we will simply “voluntary” omit one of the JAR files from the classpath that contains the referencing Java class ReferencingClassA.
The Java program is packaged as per below:
- MainProgram.jar (contains NoClassDefFoundErrorSimulator.class and JavaEETrainingUtil.class)
- CallerClassA.jar (contains CallerClassA.class)
- ReferencingClassA.jar (contains ReferencingClassA.class)
Now, let’s run the program as is:
## Baseline (normal execution)
..\bin>java -classpath CallerClassA.jar;ReferencingClassA.jar;MainProgram.jar org.ph.javaee.training1.NoClassDefFoundErrorSimulator
java.lang.NoClassDefFoundError Simulator - Training 1
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
Current ClassLoader chain:
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
-----------------------------------------------------------------
Classloading of org.ph.javaee.training1.CallerClassA in progress...
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
-----------------------------------------------------------------
Creating a new instance of org.ph.javaee.training1.CallerClassA...
Classloading of org.ph.javaee.training1.ReferencingClassA in progress...
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
-----------------------------------------------------------------
Creating a new instance of org.ph.javaee.training1.ReferencingClassA...
done!
For the initial run (baseline), the main program was able to create a new instance of CallerClassA and execute its method successfully; including successful class loading of the referencing class ReferencingClassA.
## Problem reproduction run (with removal of ReferencingClassA.jar)
../bin>java -classpath CallerClassA.jar;MainProgram.jar org.ph.javaee.training1.NoClassDefFoundErrorSimulator
java.lang.NoClassDefFoundError Simulator - Training 1
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com
Current ClassLoader chain:
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
-----------------------------------------------------------------
Classloading of org.ph.javaee.training1.CallerClassA in progress...
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
-----------------------------------------------------------------
Creating a new instance of org.ph.javaee.training1.CallerClassA...
Exception in thread "main" java.lang.NoClassDefFoundError: org/ph/javaee/training1/ReferencingClassA
at org.ph.javaee.training1.CallerClassA.doSomething(CallerClassA.java:25)
at org.ph.javaee.training1.NoClassDefFoundErrorSimulator.main(NoClassDefFoundErrorSimulator.java:28)
Caused by: java.lang.ClassNotFoundException: org.ph.javaee.training1.ReferencingClassA
at java.net.URLClassLoader$1.run(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 2 more
What happened? The removal of the ReferencingClassA.jar, containing ReferencingClassA, did prevent the current class loader to locate this referencing Java class at runtime leading to ClassNotFoundException and NoClassDefFoundError.
This is the typical Exception that you will get if you omit JAR file(s) from your Java start-up classpath or within an EAR / WAR for Java EE related applications.
ClassLoader view
Now let’s review the ClassLoader chain so you can properly understand this problem case. As you saw from the Java program output logging, the following Java ClassLoaders were found:
Classloading of org.ph.javaee.training1.CallerClassA in progress...
-----------------------------------------------------------------
sun.misc.Launcher$ExtClassLoader@17c1e333
--- delegation ---
sun.misc.Launcher$AppClassLoader@214c4ac9 **Current ClassLoader**
-----------------------------------------------------------------
** Please note that the Java bootstrap class loader is responsible to load the core JDK classes and is written in native code **
## sun.misc.Launcher$AppClassLoader
This is the system class loader responsible to load our application code found from the Java classpath specified at start-up.
##sun.misc.Launcher$ExtClassLoader
This is the extension class loader responsible to load code in the extensions directories (<JAVA_HOME>/lib/ext, or any other directory specified by the java.ext.dirs system property).
As you can see from the Java program logging output, the extension class loader is the actual super parent of the system class loader. Our sample Java program was loaded at the system class loader level. Please note that this class loader chain is very simple for this problem case since we did not create child class loaders at this point. This will be covered in future articles.
Recommendations and resolution strategies
Now find below my recommendations and resolution strategies for NoClassDefFoundError problem case 1:
- Review the java.lang.NoClassDefFoundError error and identify the missing Java class
- Verify and locate the missing Java class from your compile / build environment
- Determine if the missing Java class is from your application code, third part API or even the Java EE container itself. Verify where the missing JAR file(s) is / are expected to be found
- Once found, verify your runtime environment Java classpath for any typo or missing JAR file(s)
- If the problem is triggered from a Java EE application, perform the same above steps but verify the packaging of your EAR / WAR file for missing JAR and other library file dependencies such as MANIFEST
Please feel free to post any question or comment. The part 3 is now available.
