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:
build.xml
for the client side of codeWhen I encounter these today I take a trail-and-error approach to get things working. I need more clarity and understanding.
This question is related to
java
classpath
noclassdeffounderror
classnotfoundexception
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.
With the names itself we can easily identify one from
Exception
and other one is fromError
.
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.
Difference:
ClassNotFoundException:
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:
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:
NoClassDefFoundError
and ClassNotFoundException
are related to unavailability of a class at run-time. ClassNotFoundException
and NoClassDefFoundError
are related to Java classpath.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)
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
NoClassDefFoundError
Class Hierarchy
NoClassDefFoundError extends LinkageError extends Error extends Throwable
While debugging
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.
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
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.
Add one possible reason in practise:
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.
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.
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()
.
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.
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.
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.
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.
Source: Stackoverflow.com