Как исправить ошибку 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 

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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This article is intended for Java beginners currently facing java.lang.ClassNotFoundException challenges. It will provide you with an overview of this common Java exception, a sample Java program to support your learning process and resolution strategies.

 # Class.forName()
private static final int PROBLEM_SCENARIO = 1;

# ClassLoader.loadClass()
private static final int PROBLEM_SCENARIO = 2;

 package org.ph.javaee.training5;

/**
 * ClassNotFoundExceptionSimulator
 * @author Pierre-Hugues Charbonneau
 *
 */
public class ClassNotFoundExceptionSimulator {

       private static final String CLASS_TO_LOAD = "org.ph.javaee.training5.ClassA";
       private static final int PROBLEM_SCENARIO = 1;

       /**
        * @param args
        */
       public static void main(String[] args) {

             System.out.println("java.lang.ClassNotFoundException Simulator - Training 5");
             System.out.println("Author: Pierre-Hugues Charbonneau");
             System.out.println("http://javaeesupportpatterns.blogspot.com");

             switch(PROBLEM_SCENARIO) {

                    // Scenario #1 - Class.forName()
                    case 1:

                           System.out.println("n** Problem scenario #1: Class.forName() **n");
                           try {
                                 Class<?> newClass = Class.forName(CLASS_TO_LOAD);

                                 System.out.println("Class "+newClass+" found successfully!");

                           } catch (ClassNotFoundException ex) {

                                 ex.printStackTrace();

                                 System.out.println("Class "+CLASS_TO_LOAD+" not found!");

                           } catch (Throwable any) {                           
                                 System.out.println("Unexpected error! "+any);
                           }

                           break;

                    // Scenario #2 - ClassLoader.loadClass()
                    case 2:

                           System.out.println("n** Problem scenario #2: ClassLoader.loadClass() **n");                     
                           try {
                                 ClassLoader classLoader = Thread.currentThread().getContextClassLoader();            
                                 Class<?> callerClass = classLoader.loadClass(CLASS_TO_LOAD);

                                 Object newClassAInstance = callerClass.newInstance();

                                 System.out.println("SUCCESS!: "+newClassAInstance);
                           } catch (ClassNotFoundException ex) {

                                 ex.printStackTrace();

                                 System.out.println("Class "+CLASS_TO_LOAD+" not found!");

                           } catch (Throwable any) {                           
                                 System.out.println("Unexpected error! "+any);
                           }

                           break;
             }

             System.out.println("nSimulator done!");
       }
}

 package org.ph.javaee.training5;

/**
 * ClassA
 * @author Pierre-Hugues Charbonneau
 *
 */
public class ClassA {

private final static Class<ClassA> CLAZZ = ClassA.class;

       static {
             System.out.println("Class loading of "+CLAZZ+" from ClassLoader '"+CLAZZ.getClassLoader()+"' in progress...");
       }

       public ClassA() {
             System.out.println("Creating a new instance of "+ClassA.class.getName()+"...");

             doSomething();
       }

       private void doSomething() {           
             // Nothing to do...
       }
}

java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com

** Problem scenario #1: Class.forName() **

Class loading of class org.ph.javaee.training5.ClassA from ClassLoader 'sun.misc.Launcher$AppClassLoader@bfbdb0' in progress...
Class class org.ph.javaee.training5.ClassA found successfully!

Simulator done!

java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com

** Problem scenario #2: ClassLoader.loadClass() **

Class loading of class org.ph.javaee.training5.ClassA from ClassLoader 'sun.misc.Launcher$AppClassLoader@2a340e' in progress...
Creating a new instance of org.ph.javaee.training5.ClassA...
SUCCESS!: org.ph.javaee.training5.ClassA@6eb38a

Simulator done!

 private static final String CLASS_TO_LOAD = "org.ph.javaee.training5.ClassB";

java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com

** Problem scenario #1: Class.forName() **

java.lang.ClassNotFoundException: org.ph.javaee.training5.ClassB
       at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
       at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
       at java.security.AccessController.doPrivileged(Native Method)
       at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
       at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
       at java.lang.Class.forName0(Native Method)
       at java.lang.Class.forName(Class.java:186)
       at org.ph.javaee.training5.ClassNotFoundExceptionSimulator.main(ClassNotFoundExceptionSimulator.java:29)
Class org.ph.javaee.training5.ClassB not found!

Simulator done!

java.lang.ClassNotFoundException Simulator - Training 5
Author: Pierre-Hugues Charbonneau
http://javaeesupportpatterns.blogspot.com

** Problem scenario #2: ClassLoader.loadClass() **

java.lang.ClassNotFoundException: org.ph.javaee.training5.ClassB
       at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
       at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
       at java.security.AccessController.doPrivileged(Native Method)
       at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:423)
       at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
       at java.lang.ClassLoader.loadClass(ClassLoader.java:356)
       at org.ph.javaee.training5.ClassNotFoundExceptionSimulator.main(ClassNotFoundExceptionSimulator.java:51)
Class org.ph.javaee.training5.ClassB not found!

Simulator done!