Ошибка java lang classnotfoundexception

A classpath is a list of locations to load classes from.

These ‘locations’ can either be directories, or jar files.

For directories, the JVM will follow an expected pattern for loading a class. If I have the directory C:/myproject/classes in my classpath, and I attempt to load a class com.mycompany.Foo, it will look under the classes directory for a directory called com, then under that a directory called mycompany, and finally it will look for a file called Foo.class in that directory.

In the second instance, for jar files, it will search the jar file for that class. A jar file is in reality just a zipped collection of directories like the above. If you unzip a jar file, you’ll get a bunch of directories and class files following the pattern above.

So the JVM traverses a classpath from start to finish looking for the definition of the class when it attempts to load the class definition. For example, in the classpath :

C:/myproject/classes;C:/myproject/lib/stuff.jar;C:/myproject/lib/otherstuff.jar

The JVM will attempt to look in the directory classes first, then in stuff.jar and finally in otherstuff.jar.

When you get a ClassNotFoundException, it means the JVM has traversed the entire classpath and not found the class you’ve attempted to reference. The solution, as so often in the Java world, is to check your classpath.

You define a classpath on the command line by saying java -cp and then your classpath. In an IDE such as Eclipse, you’ll have a menu option to specify your classpath.

ClassNotFoundException is a checked exception and occurs when the Java Virtual Machine (JVM) tries to load a particular class and the specified class cannot be found in the classpath.

In older days, there are no editors like Eclipse are available. Even in Notepad, people have done java coding and by using the “javac” command to compile the java files, and they will create a ‘.class’ file. Sometimes accidentally the generated class files might be lost or set in different locations and hence there are a lot of chances of “ClassNotFoundException” occurs. After the existence of editors like Eclipse, Netbeans, etc., IDE creates a “ClassPath” file kind of entries.

From the above image, we can see that many jar files are present. They are absolutely necessary if the java code wants to interact with MySQL, MongoDB, etc., kind of databases, and also few functionalities need these jar files to be present in the build path. If they are not added, first editors show the errors themselves and provide the option of corrections too.

Implementation: Sample program of connecting to MySQL database and get the contents

Example

Output:

Case 1: In the above code, we are using com.mysql.cj.jdbc.Driver and in that case if we are not having mysql-connector-java-8.0.22.jar, then we will be getting ClassNotFoundException.

Case 2: So, keep the jar in the build path as shown below.

Note: Similarly for any database connectivity, we need to have the respective jars for connecting to that database. The list of database driver jars required by java to overcome ClassNotFoundException is given below in a tabular format 

Database	Command Line
MySQL	mysql-connector-java-8.0.22.jar
MongoDB	mongo-java-driver-3.12.7.jar
SQL Server	sqljdbc4.jar
MYSQL	sqljdbc.jar
Oracle	oracle.jdbc.driver.oracledriver

Note: 

• When we are developing Web based applications, the jars must be present in ‘WEB-INF/lib directory’.
• In Maven projects, jar dependency should be present in pom.xml
• Sample snippet of pom.xml for spring boot

Example 1 With Spring boot

Example 2 Without spring boot

Example 3 Gradle based dependencies (MongoDB) 

Similarly, other DB drivers can be specified in this way. It depends upon the project nature, the dependencies has to be fixed. For ordinary class level projects, all the classes i.e parent class, child class, etc should be available in the classpath. If there are errors, then also .class file will not be created which leads to ClassNotFoundException, and hence in order to get the whole code working, one should correct the errors first by fixing the dependencies. IDE is much helpful to overcome such sort scenarios such as when the program throws ClassNotFoundException, it will provide suggestions to users about the necessity of inclusion of jar files(which contains necessary functionalities like connecting to database.

Last Updated :
30 Sep, 2021

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What is ClassNotFoundException in Java
ClassNotFoundException is one of Java nightmare every Java developer face in there day to day life. java.lang.NoClassDefFoundError and java.lang.ClassNotFoundException are two errors  which occurs by and now and chew up of your precious time while finding and fixing root cause. From the name java.lang.ClassNotFoundException looks quite simple but underlying cause of it is always different and which classifies it as an environmental issue. In this java tutorial, we will see what is ClassNotFoundException in java, what is real cause of it, and how to fix it along with some more frequent and infamous examples of java.lang.ClassNotFoundException in Java or J2EE, Don’t mistake this exception with NoClassDefFoundError in Java which is also due to incorrect classpath in Java. 


Though both of them are related to missing class file when Java tries to load class in Java they are completely different to each other.  Correct understanding of  When class is loaded in Java and How Classpath works  is must to troubleshoot and fix this error quickly.

What is java.lang.classNotFoundException in Java

1) When we try to load a class by using Class.forName() method and .class file or binary of class is not available in classpath.
2) When Classloader try to load a class by using findSystemClass () method.
3) While using loadClass() method of class ClassLoader in Java.

These statements are completely true in terms of theory of ClassNotFoundExcepiton in Java but as per my experience the concept is «ClassNotFoundException will come only when JVM tries to load a class at run-time, nothing related to compile time unlike NoClassDefFoundError«. 


Also since till run time JVM doesn’t know about this Class, it can only be done by above-specified method or by employing Reflection to read the name of class from some configuration file like in case of struts its struts-config.xml file and then load the class specified on those configuration files. Reflection is the great power of Java but you need to be aware of java.lang.classNotFoundException while using it or loading class in Java. See Understanding the Java Virtual Machine: Class Loading and Reflection to learn more about how exactly class loading works inside JVM.

Examples of classnotfoundexception in java

Though java.lang.classNotFoundException is very common and it can come for any classes, I usually see it while doing JDBC connectivity like when I was writing Java program to connect Oracle database. I am going to list some of the most common scenario where you will get classnotfoundexception in java.

1. java.lang.classnotfoundexception com.mysql.jdbc.driver

This is a classical and most infamous example of and also my first encounter with java.lang.ClassNotFoundException and comes when you are writing JDBC connectivity code and trying to load the JDBC driver. In this particular case of ClassNotFoundException looks like MySQL driver jar file is missing from Classpath. 


If you pay attention you will find that we use method Class.forName (“driver”) to load the driver class which resides in a particular jar in case of this its mysql-connector.jar and if that jar is not in the classpath or not accessible to JVM it will throw  java.lang.ClassNotFoundException: com.mysql.jdbc.Driver

Here are few more infamous examples of java.lang.ClassnotFoundException which comes here and there while doing any Java J2EE project. 

java.lang.classnotfoundexception org.hibernate.hql.ast.hqltoken

java.lang.classnotfoundexception org.springframework.web.context.contextloaderlistener

java.lang.classnotfoundexception org.eclipse.core.runtime.adaptor.eclipsestarter

java.lang.classnotfoundexception org.apache.catalina.startup.catalina

java.lang.classnotfoundexception javax.mail.messagingexception

2. More Complicated ClassNotFoundException

With the advent of dynamic library e.g. OSGi and ClassLoader in Java, this exception can be more tricky and hard to find. Thanks to Mr. Anonymous who has summarized this beautifully, here it is what he says“It can become a bit more complicated than that. 

In truth, a class does not have to be just visible by the JVM through its classpath but be visible by the Classloader being used. When you are in a multi-classloader environment (In a EE environment, for example, but not limited to), each classloader may have its own rules to search for the classes, and this behavior might depend on the dynamic hierarchy of the Classloaders.

For example, in a project that uses an EAR packaging with WARs inside it, libraries in the lib folder of the EAR are visible to classes inside a WAR, but any classes packaged in a jar put in the WEB-INF/lib on the WAR cannot be seen by classes in different modules (other WARs, EJB-JARS, etc).

It can get really complicated as its common for different modules depending on different versions of the same libraries as different modules depend on each other. It can be a challenge to manage this. Sometimes the classloader can see multiple versions of the same class; sometimes they can see no version at all. 


Sometimes different dependency paths end in different versions of the same class. And many of these cases end in a ClassNotFoundException.

How to fix java.lang.ClassNotFoundException in Java

As you have seen from above examples its clear problem of classpath, so here is my approach to fix or resolve java.lang.ClassNotFoundException:

1) First find out the jar file on which problematic class file is present for example in case of «

com.mysql.jdbc.driver» its mysql-connector-java.jar. If you don’t know how to find which jar file a particular class you can see eclipse shortcuts to do that or you can simply do «Ctrl+T» in Eclipse and type the name of class, It will list all the jar in the order they appear in eclipse classpath.

2) Check whether your classpath contains that jar, if your classpath doesn’t contain the jar then just add that class in your classpath.

3) If it’s present in your classpath then there is high chance that your classpath is getting overridden or application is using classpath specified in jar file or start-up script and to fix that you need to find the exact classpath used by your application.

Live example of reproducing and Fixing ClassNotFoundException in java

I think if we are able to reproduce and solve certain problem we become more comfortable dealing with that, that’s why here we will reproduce java.lang.ClassNotFoundException and solve it by following the concept we have discussed so far.

1)

Create a Class called StockTrading.java

public class StockTrading{
   public String getDescription(){
   return «StockTrading»;
  }
}

2)

create a Class called OnlineStockTranding.java and load the class StockTrading.java as Class.forName («stocktrading»);

public class OnlineStockTrading {
   public static void main(String args[]) throws ClassNotFoundException{
      Class.forName(«StockTrading»);
      System.out.println(«StockTrading class successfully loaded»);
   }
}

3) Compile both Java source file which will create two class files and run the program should run fine.

javin@trading~/java:

javac *.java

javin@trading ~/java:

ls –lrt
-rw-r—r— 1 javin None  90 Aug 21 09:27 StockTrading.java
-rw-r—r— 1 javin None 208 Aug 21 09:28 OnlineStockTrading.java
-rwxr-xr-x 1 javin None 282 Aug 21 09:28 StockTrading.class
-rwxr-xr-x 1 javin None 638 Aug 21 09:28 OnlineStockTrading.class

javin@trading ~/java:$

java OnlineStockTrading
StockTrading class successfully loaded

4) Now just

remove the .class file for stocktrading.java and run the Java program and it will throw java.lang.ClassNotFoundException in java.

javin@trading ~/java:

rm StockTrading.class

javin@trading ~/java:

java OnlineStockTrading
Exception in thread «main» java.lang.ClassNotFoundException: StockTrading
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)

ClassFoundException vs NoClassDefFoundError vs UnSupportedClassVersionError

There are lots of exceptions in java but these three are the one who most haunted the java developer most mainly because these three are mostly related to environment issues and they all depends upon JVM and Classpath behaviour. Though they look similar there is slight difference between ClassFoundException and NoClassDefFoundError and UnSupportedClassVersionError and we will highlight those differences here for easy understanding and differentiating these three:

1)

ClassNotFoundException comes on Runtime when requested class is not available in classpath and mainly due to call to Class.forName () or Classloader.loadClass () or ClassLoader.findSystemClass ().

2)

NoClassDefFoundError comes when problematic class was present when your compiled your application but they are not in classpath while you running your program.

3)

UnSupportedClassVersionError is easy to differentiate because it’s related to version of classpath and usually comes when you compile your code in higher Java version and try to run on lower java version. Can be resolved simply by using one java version for compiling and running your application.

So that’s all on

ClassNotFoundException in java for now , please let me know if you have any tip or  any personal experience on solving java.lang.ClassNotFoundException in Java which you would like to share.

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