问题
I have gone through this thread What causes and what are the differences between NoClassDefFoundError and ClassNotFoundException? This is what one of the ans,which has max ups, in thread is : NoClassDefFoundError :"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."
ClassNotFoundException : 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.
I did a small experiment . I created a main class, class A and tried to call other class, class B from it , compiled successfully.
Then i deleted the class B which is being called in class A. I got the java.lang.ClassNotFoundException but as per the answer in the tread, i should have got NoClassDefFoundError (source was compiled succesfully but at runtime class files were not found) Could anyone explain what am i missing in the interpretation of the ans in the thread ?
package com.random;
public class A {
public static void main(String[] args) {
B b= new B();
}
}
package com.random;
public class B {
}
回答1:
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.
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.
You have to understand that the JVM can't realize the definition of the class you deleted can't be found, as the class itself can't be found which automatically throw the ClassNotFoundException.
This exception happen at runtime so it does not matter if it compiled first or not, you deleted the file, therefore it can't be found and throw the exception.
Note that NoClassDefFoundError is not actually an exception, it is an Error derived from LinkageError while ClassNotFoundException derive directly from java.lang.Exception.
To resume, the NoClassDefFoundError globally simply mean that the JVM tried to access at runtime something that according to the compiled code should exists, but does not actually exist (or is not in the classpath).
Example to reproduce ClassNotFoundException
public class ClassNotFoundExceptionExample {
private static final String CLASS_TO_LOAD = "main.java.Utils";
public static void main(String[] args) {
try {
Class loadedClass = Class.forName(CLASS_TO_LOAD);
System.out.println("Class " + loadedClass + " found successfully!");
}
catch (ClassNotFoundException ex) {
System.err.println("A ClassNotFoundException was caught: " + ex.getMessage());
ex.printStackTrace();
}
}
}
Example to reproduce NoClassDefFoundError
Create a simple class Test
public class Test {
public Test() {
System.out.println("A new instance of the Test class was created!");
}
}
And a class NoClassDefFoundErrorExample
public class NoClassDefFoundErrorExample {
private static Test test = new Test();
public static void main(String[] args) {
System.out.println("The definition of Test was found!");
}
}
Now create a n executable .jar which execute the main method. You can specify it in the Manifest.txt file inside the .jar
Main-Class: NoClassDefFoundErrorExample
Now run the following commands
javac Test.java
javac NoClassDefFoundErrorExample.java
jar cfm NoClassDefFoundErrorExample.jar Manifest.txt NoClassDefFoundErrorExample.class
java -jar NoClassDefFoundErrorExample.jar
Notice the NoClassDefFoundError
Exception in thread "main" java.lang.NoClassDefFoundError: TestClass
at NoClassDefFoundErrorExample.(NoClassDefFoundErrorExample.java:2)
Caused by: java.lang.ClassNotFoundException: TestClass
at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 1 more
回答2:
Well... ClassNotFoundException occurs when the runtime is trying to find the class named by some String for example Class.forName(java.lang.String) method take a string argument and tries to find the class with this name. In this case the class-name is a sting and can only be checked at runtime. here the exception clearly says... this "class" is not found. So... it can happen for two reasons :
Reason 1. Class-name is not a valid java-class ( example - "java.bang.kiting").
// Example
Class cdef = Class.forName( "java.bang.kiting" );
Reason 2. Class-name is was a valid class... but somehow it was not packaged with the jar or is not resolved in class-path. So as far as the runtime knows... it can be a wrong class name... similar to case 1.
// Example
Class cdef =Class.forName( "apache.some.SomeLegitClass" );
Where as NoClassDefFoundError for cases where the actual class reference was used,
// example
import apache.some.SomeLegitClass
SomeLegitClass i = (SomeLegitClass) instanceOfSomeLegitClass;
So basically the everything was correct but somehow the class is not packaged with the jar ( or more generally - is not resolved in the class-path ). In this case we get NoClassDefFoundError.
Here runtime knows that the class is valid since it compiled successfully... but it can not find the "class definition".
回答3:
The difference depends on who asked the class to be loaded:
ClassNotFoundExceptionis thrown when the code is directly trying to load a class, passing theStringargument representing a Fully Qualified Name of the class.- e.g.
Class.forName(String), orClassLoader.loadClass(String).
- e.g.
NoClassDefFoundErroris thrown when the JVM is asked to load a class indirectly.- e.g. when class A is using class B and class B is not on classpath,
NoClassDefFoundErrorwill be thrown.
- e.g. when class A is using class B and class B is not on classpath,
回答4:
NoClassDefFoundError is usually called when you are using a library(for example, Guava, Gson, CommonsIO). You put the library in classpath of your project, but you didn't export it together, you will get a NoClassDefFoundError when the application is running.
How to get NoClassDefFoundError:
Create a new project, with this class.
public class A
{
public void do()
{
System.out.println("Do!");
}
}
Export it as a .jarfile.
Now create another project. Add the exported jar file to classpath.
import ???.A;
public class Main
{
public static void main(String[] args)
{
A a = new A();
a.do();//NoClassDefFoundError thrown at here.
}
}
Export the project, make sure you do not include the jar file(with class A). Run the newly exported jar file, you will see that error!
回答5:
1) ClassNotFoundException
- This occurs, when we try to load a class at run-time using
Class.forName()orClassLoader.loadClass()orClassLoader.findSystemClass()methods and it could not find the required class in the class path. - In this case, we should check the
class pathand add the class in the class path if it is missing. - This is a checked Exception, which is derived from java.lang.Exception class .
- This comes under explicit loading.
2) NoClassDefFoundError
This occurs, when the class was present during the
compile timeand is not available duringrun timefor some reasons. It means the class which is getting loaded ispresentinclasspath, but one of the dependentclasse(s)which are required by this class are either removed or failed to load by the compiler.In this case, we only need to check the
classes which are dependent on this class.- This is an Error, which is derived from java.lang.LinkageError.
- This comes under implicit loading.
回答6:
As mentioned in previous answers, NoClassDefFoundError will occur when the class was present during the compile time and is not available during run time for some reasons.
There is another scenario I wish to add, which could also result in NoClassDefFoundError.
When you are trying to load a class which failed to load due to some exception say failure in static initialization block, system will throw you ExceptionInInitializerError. If you try to load the same class again (which failed to load previously), system will throw NoClassDefFoundError
Lets explore it with an sample
ClassWithStaticBlock.java
public class ClassWithStaticBlock {
static {
int total = 1/0;
}
}
Main.java
public class Main {
public static void main(String[] args) {
ClassWithStaticBlock cs;
try {
cs = new ClassWithStaticBlock();
}catch(Throwable e){
e.printStackTrace();
}
}
}
Result:
java.lang.ExceptionInInitializerError
at Main.main(Main.java:6)
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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.ArithmeticException: / by zero
at ClassWithStaticBlock.<clinit>(ClassWithStaticBlock.java:7)
... 6 more
Lets modify Main.java
public class Main {
public static void main(String[] args) {
ClassWithStaticBlock cs;
try {
cs = new ClassWithStaticBlock();
}catch(Throwable e){
e.printStackTrace();
}
cs = new ClassWithStaticBlock(); //try to use ClassWithStaticBlock again
}
}
Result:
java.lang.ExceptionInInitializerError
at Main.main(Main.java:6)
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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.lang.ArithmeticException: / by zero
at ClassWithStaticBlock.<clinit>(ClassWithStaticBlock.java:7)
... 6 more
Exception in thread "Main Thread" java.lang.NoClassDefFoundError: ClassWithStaticBlock
at Main.main(Main.java:10)
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 com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
When we try to use ClassWithStaticBlock again (which failed to initialize earlier), System is throwing NoClassDefFoundError.
Found the sample from Why am I getting a NoClassDefFoundError in Java?
回答7:
One of cases where NoClassDefFoundError occur is when the Class JVM is trying to access is not found in classpath. But if class is present in classpath, It will result in ClassNotFoundException.
In short NoClassDefFoundError will come if a class was present during compile time but not available in java classpath during runtime.
Just try to run with explicit -classpath option where the classpath does not contain class B.
回答8:
Everything About ClassNotFoundException Vs NoClassDefFoundError article explains the difference between ClassNotFoundException Vs NoClassDefFoundError very clearly with example and according to it.
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 the jar for it.
public class Test {
public static void main(String[] args) throws Exception {
// Provide any class name to Class.forName() which does not exist
// Or compile Test.java and then manually delete Person.class file so Person class will become unavailable
// Run the program using java Test
Class clazz = Class.forName("Person");
Person person = (Person) clazz.newInstance();
person.saySomething();
}
}
class Person {
void saySomething() {
System.out.println("Hello");
}
}
NoClassDefFoundError
Is a subtype of java.lang.Error and Error class indicates an abnormal behavior which really should not happen with an application but and application developers should not try to catch it, it is there for JVM use only.
NoClassDefFoundError 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.
public class Test {
public static void main(String[] args) throws Exception {
// Do javac on Test.java,
// Program will compile successfully because Empoyee class exits
// Manually delete Employee.class file
// Run the program using java Test
Employee emp = new Employee();
emp.saySomething();
}
}
class Employee {
void saySomething() {
System.out.println("Hello");
}
}
回答9:
The other answers in this thread are correct, i just want to add something i spent hours trying to figure out. Even if
Class.forName("apache.some.SomeLegitClass")
works,
Class.forName("apache.some.somelegitclass")
will result in a NoClassDefFoundError. The Class.forName() is case-sensitive. It will result in different exceptions if the classname is spelled wrong, or simply has incorrect casing.
来源：https://stackoverflow.com/questions/28322833/classnotfoundexception-vs-noclassdeffounderror