What causes and what are the differences between NoClassDefFoundError and ClassNotFoundException

Question

What is the difference between NoClassDefFoundError and ClassNotFoundException   What causes them to be thrown  How can they be resolved   I often encounter these throwables when modifying existing code to include new jar files  I have hit them on both the client side and the server side for a java app distributed through webstart   Possible reasons I have come across    packages not included in build xml for the client side of code runtime classpath missing for the new jars we are using version conflicts with previous jar   When I encounter these today I take a trail-and-error approach to get things working  I need more clarity and understanding

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A ClassNotFoundException is thrown when the reported class is not found by the ClassLoader. This typically means that the class is missing from the CLASSPATH. It could also mean that the class in question is trying to be loaded from another class which was loaded in a parent classloader and hence the class from the child classloader is not visible. This is sometimes the case when working in more complex environments like an App Server (WebSphere is infamous for such classloader issues).

People often tend to confuse java.lang.NoClassDefFoundError with java.lang.ClassNotFoundException however there's an important distinction. For example an exception (an error really since java.lang.NoClassDefFoundError is a subclass of java.lang.Error) like

java.lang.NoClassDefFoundError:
org/apache/activemq/ActiveMQConnectionFactory

does not mean that the ActiveMQConnectionFactory class is not in the CLASSPATH. Infact its quite the opposite. It means that the class ActiveMQConnectionFactory was found by the ClassLoader however when trying to load the class, it ran into an error reading the class definition. This typically happens when the class in question has static blocks or members which use a Class that's not found by the ClassLoader. So to find the culprit, view the source of the class in question (ActiveMQConnectionFactory in this case) and look for code using static blocks or static members. If you don't have access the the source, then simply decompile it using JAD.

On examining the code, say you find a line of code like below, make sure that the class SomeClass in in your CLASSPATH.

private static SomeClass foo = new SomeClass();

Tip : To find out which jar a class belongs to, you can use the web site jarFinder . This allows you to specify a class name using wildcards and it searches for the class in its database of jars. jarhoo allows you to do the same thing but its no longer free to use.

If you would like to locate the which jar a class belongs to in a local path, you can use a utility like jarscan ( http://www.inetfeedback.com/jarscan/ ). You just specify the class you'd like to locate and the root directory path where you'd like it to start searching for the class in jars and zip files.

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I remind myself the following again and again when I need to refresh  ClassNotFoundException  Class Hierarchy  ClassNotFoundException extends ReflectiveOperationException extends Exception extends Throwable   While debugging   Required jar  class is missing from the classpath   Verify all the required jars are in classpath of jvm    NoClassDefFoundError  Class Hierarchy  NoClassDefFoundError extends LinkageError  extends Error extends Throwable   While debugging   Problem with loading a class dynamically  which was compiled properly Problem with static blocks  constructors  init   methods of dependent class and the actual error is wrapped by multiple layers  especially when you use spring  hibernate the actual exception is wrapped and you will get NoClassDefError  When you face  ClassNotFoundException  under a static block of dependent class Problem with versions of class   This happens when you have two versions v1  v2  of same class under different jar packages  which was compiled successfully using v1 and v2 is loaded at the runtime which doesn t has the relevant methods vars amp  you will see this exception   I once resolved this issue by removing the duplicate of log4j related class under multiple jars that appeared in the classpath

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A NoClassDefFoundError  NCDFE  happens when your code runs  new Y    and it can t find the Y class   It may simply be that Y is missing from your class loader like the other comments suggest  but it could be that the Y class isn t signed or has an invalid signature  or that Y is loaded by a different classloader not visible to your code  or even that Y depends on Z which couldn t be loaded for any of the above reasons   If this happens  then the JVM will remember the result of loading X  NCDFE  and it will simply throw a new NCDFE every time you ask for Y without telling you why    class a     static class b      public static void main String args          System out println  First attempt new b           try  new b      catch Throwable t   t printStackTrace         System out println   nSecond attempt new b           try  new b      catch Throwable t   t printStackTrace             save this as a java somewhere  The code simply tries to instantiate a new  b  class twice  other than that  it doesn t have any bugs  and it doesn t do anything   Compile the code with javac a java  Then run a by invoking java -cp   a -- it should just print out two lines of text  and it should run fine without errors   Then delete the  a b class  file  or fill it with garbage  or copy a class over it  to simulate the missing or corrupted class   Here s what happens    First attempt new b    java lang NoClassDefFoundError  a b     at a main a java 5  Caused by  java lang ClassNotFoundException  a b     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 sun misc Launcher AppClassLoader loadClass Launcher java 301      at java lang ClassLoader loadClass ClassLoader java 252      at java lang ClassLoader loadClassInternal ClassLoader java 320          1 more  Second attempt new b    java lang NoClassDefFoundError  a b     at a main a java 7    The first invocation results in a ClassNotFoundException  thrown by the class loader when it can t find the class   which must be wrapped in an unchecked NoClassDefFoundError  since the code in question  new b    should just work   The second attempt will of course fail too  but as you can see the wrapped exception is no more  because the ClassLoader seems to remember failed class loaders   You see only the NCDFE with absolutely no clue as to what really happened   So if you ever see a NCDFE with no root cause  you need to see if you can track back to the very first time the class was loaded to find the cause of the error

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Difference Between ClassNotFoundException Vs NoClassDefFoundError

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The difference from the Java API Specifications is as follows   For ClassNotFoundException      Thrown when an application tries to   load in a class through its string   name using          The forName method in class Class    The findSystemClass method in class ClassLoader    The loadClass method in class ClassLoader           but no definition for the class with   the specified name could be found    For NoClassDefFoundError      Thrown if the Java Virtual Machine or   a ClassLoader instance tries to load   in the definition of a class  as part   of a normal method call or as part of   creating a new instance using the new   expression  and no definition of the   class could be found       The searched-for class definition   existed when the currently executing   class was compiled  but the definition   can no longer be found    So  it appears that the NoClassDefFoundError occurs when the source was successfully compiled  but at runtime  the required class files were not found  This may be something that can happen in the distribution or production of JAR files  where not all the required class files were included   As for ClassNotFoundException  it appears that it may stem from trying to make reflective calls to classes at runtime  but the classes the program is trying to call is does not exist   The difference between the two is that one is an Error and the other is an Exception  With NoClassDefFoundError is an Error and it arises from the Java Virtual Machine having problems finding a class it expected to find  A program that was expected to work at compile-time can t run because of class files not being found  or is not the same as was produced or encountered at compile-time  This is a pretty critical error  as the program cannot be initiated by the JVM   On the other hand  the ClassNotFoundException is an Exception  so it is somewhat expected  and is something that is recoverable  Using reflection is can be error-prone  as there is some expectations that things may not go as expected  There is no compile-time check to see that all the required classes exist  so any problems with finding the desired classes will appear at runtime

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What is the reason for getting each of them and any thought process on how to deal with such errors    They re closely related  A ClassNotFoundException is thrown when Java went looking for a particular class by name and could not successfully load it  A NoClassDefFoundError is thrown when Java went looking for a class that was linked into some existing code  but couldn t find it for one reason or another  e g   wrong classpath  wrong version of Java  wrong version of a library  and is thoroughly fatal as it indicates that something has gone Badly Wrong   If you ve got a C background  a CNFE is like a failure to dlopen   dlsym   and an NCDFE is a problem with the linker  in the second case  the class files concerned should never have been actually compiled in the configuration you re trying to use them

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ClassNotFoundException and NoClassDefFoundError occur when a particular class is not found at runtime However  they occur at different scenarios   ClassNotFoundException is an exception that occurs when you try to load a class at run time using Class forName   or loadClass   methods and mentioned classes are not found in the classpath       public class MainClass               public static void main String   args                        try                               Class forName  oracle jdbc driver OracleDriver                 catch  ClassNotFoundException e                                e printStackTrace                                         java lang ClassNotFoundException  oracle jdbc driver OracleDriver     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      at java lang Class forName0 Native Method      at java lang Class forName Unknown Source      at pack1 MainClass main MainClass java 17    NoClassDefFoundError is an error that occurs when a particular class is present at compile time  but was missing at run time       class A                some code           public class B               public static void main String   args                        A a   new A                      When you compile the above program  two  class files will be generated  One is A class and another one is B class  If you remove the A class file and run the B class file  Java Runtime System will throw NoClassDefFoundError like below       Exception in thread  main  java lang NoClassDefFoundError  A     at MainClass main MainClass java 10      Caused by  java lang ClassNotFoundException  A     at java net URLClassLoader findClass URLClassLoader java 381      at java lang ClassLoader loadClass ClassLoader java 424      at sun misc Launcher AppClassLoader loadClass Launcher java 331      at java lang ClassLoader loadClass ClassLoader java 357

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ClassNotFoundException is a checked exception that occurs when we tell JVM to load a class by its string name using Class forName   or ClassLoader findSystemClass   or ClassLoader loadClass   methods and mentioned class is not found in the classpath   Most of the time  this exception occurs when you try to run an application without updating the classpath with required JAR files  For Example  You may have seen this exception when doing the JDBC code to connect to your database i e MySQL but your classpath does not have JAR for it   NoClassDefFoundError error occurs when JVM tries to load a particular class that is the part of your code execution  as part of a normal method call or as part of creating an instance using the new keyword  and that class is not present in your classpath but was present at compile time because in order to execute your program you need to compile it and if you are trying use a class which is not present compiler will raise compilation error   Below is the brief description    You can read Everything About ClassNotFoundException Vs NoClassDefFoundError for more details

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Add one possible reason in practise    ClassNotFoundException  as cletus said  you use interface while inherited class of interface is not in the classpath  E g  Service Provider Pattern  or Service Locator  try to locate some non-existing class NoClassDefFoundError  given class is found while the dependency of given class is not found   In practise  Error may be thrown silently  e g  you submit a timer task and in the timer task it throws Error  while in most cases  your program only catches Exception  Then the Timer main loop is ended without any information  A similar Error to NoClassDefFoundError is ExceptionInInitializerError  when your static initializer or the initializer for a static variable throws an exception

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Given the Class loader sussystem actions     This is an article that helped me a lot to understand the difference  http   docs oracle com javase specs jvms se7 html jvms-5 html     If an error occurs during class loading  then an instance of a   subclass of LinkageError must be thrown at a point in the program that    directly or indirectly  uses the class or interface being loaded       If the Java Virtual Machine ever attempts to load a class C during   verification    5 4 1  or resolution    5 4 3   but not initialization      5 5    and the class loader that is used to initiate loading of C   throws an instance of ClassNotFoundException  then the Java Virtual   Machine must throw an instance of NoClassDefFoundError whose cause is   the instance of ClassNotFoundException    So a ClassNotFoundException is a root cause of NoClassDefFoundError  And a NoClassDefFoundError is a special case of type loading error  that occurs at Linking step

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NoClassDefFoundError is a linkage error basically  It occurs when you try and instantiate an object  statically with  new   and it s not found when it was during compilation   ClassNotFoundException is more general and is a runtime exception when you try to use a class that doesn t exist  For example  you have a parameter in a function accepts an interface and someone passes in a class that implements that interface but you don t have access to the class  It also covers case of dynamic class loading  such as using loadClass   or Class forName

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With the names itself we can easily identify one from Exception and other one is from Error      Exception  Exceptions occurs during the execution of program  A programmer can handle these exception by try catch block  We have two types of exceptions  Checked exception which throws at compile time  Runtime Exceptions which are thrown at run time  these exception usually happen because of bad programming    Error  These are not exceptions at all  it is beyond the scope of programmer  These errors are usually thrown by JVM      image source  Difference   ClassNotFoundException    Class loader fails to verify a class byte code we mention in Link phase of class loading subsystem we get ClassNotFoundException  ClassNotFoundException is a checked Exception derived directly from java lang Exception class and you need to provide explicit handling for it  ClassNotFoundException comes up when there is an explicit loading of class is involved by providing name of class at runtime using ClassLoader loadClass    Class forName    and ClassLoader findSystemClass      NoClassDefFoundError     Class loader fails to resolving references of a class in Link phase of class loading subsystem we get NoClassDefFoundError   NoClassDefFoundError is an Error derived from LinkageError class  which is used to indicate error cases  where a class has a dependency on some other class and that class has incompatibly changed after the compilation    NoClassDefFoundError is a result of implicit loading of class because of a method call from that class or any variable access      Similarities      Both NoClassDefFoundError and ClassNotFoundException are related to unavailability of a class at run-time    Both ClassNotFoundException and NoClassDefFoundError are related to Java classpath

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Example  1   class A   void met       Class forName  com example Class1           If com example Class1 doesn t exist in any of the classpaths  then It throws ClassNotFoundException   Example  2   Class B    void met       com example Class2 c   new com example Class2           If com example Class2 existed while compiling B  but not found while execution  then It throws NoClassDefFoundError   Both are run time exceptions

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From http   www javaroots com 2013 02 classnotfoundexception-vs html   ClassNotFoundException   occurs when class loader could not find the required class in class path  So  basically you should check your class path and add the class in the classpath   NoClassDefFoundError   this is more difficult to debug and find the reason  This is thrown when at compile time the required classes are present  but at run time the classes are changed or removed or class s static initializes threw exceptions  It means the class which is getting loaded is present in classpath  but one of the classes which are required by this class are either removed or failed to load by compiler  So you should see the classes which are dependent on this class   Example   public class Test1       public class Test       public static void main String   args               Test1   new Test1                  Now after compiling both the classes  if you delete Test1 class file and run Test class  it will throw  Exception in thread  main  java lang NoClassDefFoundError  Test     at Test1 main Test1 java 5  Caused by  java lang ClassNotFoundException  Test     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          1 more   ClassNotFoundException  thrown when an application tries to load in a class through its name  but no definition for the class with the specified name could be found   NoClassDefFoundError  thrown if the Java Virtual Machine tries to load in the definition of a class and no definition of the class could be found

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ClassNotFoundException is thrown when there is attempt to load the class by referencing it via a String  For example the parameter to in Class forName   is a String  and this raises the potential of invalid binary names being passed to the classloader   The ClassNotFoundException is thrown when a potentially invalid binary name is encountered  for instance  if the class name has the     character  you are bound to get a ClassNotFoundException  It is also thrown when the directly referenced class is not available on the classpath   On the other hand  NoClassDefFoundError is thrown    when the actual physical representation of the class - the  class file is unavailable  or the class been loaded already in a different classloader  usually a parent classloader would have loaded the class and hence the class cannot be loaded again   or if an incompatible class definition has been found - the name in the class file does not match the requested name  or  most importantly  if a dependent class cannot be located and loaded  In this case  the directly referenced class might have been located and loaded  but the dependent class is not available or cannot be loaded  This is a scenario where the directly referenced class can be loaded via a Class forName or equivalent methods  This indicates a failure in linkage    In short  a NoClassDefFoundError is usually thrown on new   statements or method invocations that load a previously absent class  as opposed to the string-based loading of classes for ClassNotFoundException   when the classloader is unable to find or load the class definition s    Eventually  it is upto the ClassLoader implementation to throw an instance of ClassNotFoundException when it is unable to load a class  Most custom classloader implementations perform this since they extend the URLClassLoader  Usually classloaders do not explicitly throw a NoClassDefFoundError on any of the method implementations - this exception is usually thrown from the JVM in the HotSpot compiler  and not by the classloader itself

[java] What causes and what are the differences between NoClassDefFoundError and ClassNotFoundException?

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