Java lang classnotfoundexception как исправить

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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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.

If you are interested on more advanced class loader related problems, I recommended that you review my article series on java.lang.NoClassDefFoundError since these Java exceptions are closely related.

java.lang.ClassNotFoundException: Overview

As per the Oracle documentation, ClassNotFoundException is thrown following the failure of a class loading call, using its string name, as per below:

• The Class.forName method
• The ClassLoader.findSystemClass method
• The ClassLoader.loadClass method

In other words, it means that one particular Java class was not found or could not be loaded at “runtime” from your application current context class loader.

This problem can be particularly confusing for Java beginners. This is why I always recommend to Java developers to learn and refine their knowledge on Java class loaders. Unless you are involved in dynamic class loading and using the Java Reflection API, chances are that the ClassNotFoundException error you are getting is not from your application code but from a referencing API. Another common problem pattern is a wrong packaging of your application code. We will get back to the resolution strategies at the end of the article.

java.lang.ClassNotFoundException : Sample Java program

Now find below a very simple Java program which simulates the 2 most common ClassNotFoundException scenarios via Class.forName() & ClassLoader.loadClass(). Please simply copy/paste and run the program with the IDE of your choice (Eclipse IDE was used for this example).

The Java program allows you to choose between problem scenario #1 or problem scenario #2 as per below. Simply change to 1 or 2 depending of the scenario you want to study.

# Class.forName()

private static final int PROBLEM_SCENARIO = 1;

# ClassLoader.loadClass()

private static final int PROBLEM_SCENARIO = 2;

# ClassNotFoundExceptionSimulator

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!");
       }
}

# ClassA

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...
       }
}

If you run the program as is, you will see the output as per below for each scenario:

#Scenario 1 output (baseline)

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!

#Scenario 2 output (baseline)

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!

For the “baseline” run, the Java program is able to load
ClassA
successfully.

Now let’s voluntary change the full name of
ClassA
and re-run the program for each scenario. The following output can be observed:

#ClassA changed to ClassB

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

#Scenario 1 output (problem replication)

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!

#Scenario 2 output (problem replication)

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!

What happened? Well since we changed the full class name to org.ph.javaee.training5.ClassB, such class was not found at runtime (does not exist), causing both Class.forName() and ClassLoader.loadClass() calls to fail.

You can also replicate this problem by packaging each class of this program to its own JAR file and then omit the jar file containing ClassA.class from the main class path Please try this and see the results for yourself…(hint: NoClassDefFoundError)

Now let’s jump to the resolution strategies.

java.lang.ClassNotFoundException
: Resolution strategies

Now that you understand this problem, it is now time to resolve it. Resolution can be fairly simple or very complex depending of the root cause.

• Don’t jump on complex root causes too quickly, rule out the simplest causes first.
• First review the java.lang.ClassNotFoundException stack trace as per the above and determine which Java class was not loaded properly at runtime e.g. application code, third party API, Java EE container itself etc.
• Identify the caller e.g. Java class you see from the stack trace just before the Class.forName() or ClassLoader.loadClass() calls. This will help you understand if your application code is at fault vs. a third party API.
• Determine if your application code is not packaged properly e.g. missing JAR file(s) from your classpath
• If the missing Java class is not from your application code, then identify if it belongs to a third party API you are using as per of your Java application. Once you identify it, you will need to add the missing JAR file(s) to your runtime classpath or web application WAR/EAR file.
• If still struggling after multiple resolution attempts, this could means a more complex class loader hierarchy problem. In this case, please review my NoClassDefFoundError article series for more examples and resolution strategies

I hope this article has helped you to understand and revisit this common Java exception.

Please feel free to post any comment or question if you are still struggling with your java.lang.ClassNotFoundException problem.
 

Reference: java.lang.ClassNotFoundException: How to resolve from our JCG partner Pierre-Hugues Charbonneau at the Java EE Support Patterns & Java Tutorial blog.