Java library class not found

How to Fix ClassNotFoundException in Java

The java.lang.ClassNotFoundException is a checked exception in Java that occurs when the JVM tries to load a particular class but does not find it in the classpath.

Since the ClassNotFoundException is a checked exception, it must be explicitly handled in methods which can throw this exception — either by using a try-catch block or by throwing it using the throws clause.

What Causes ClassNotFoundException in Java

Common causes of the java.lang.ClassNotFoundException are using the following methods to load a class:

In all the above cases, the class attempted to be loaded is not found on the classpath, leading to the ClassNotFoundException in Java.

ClassNotFoundException in Java Example

A very common example of ClassNotFoundException is when a JDBC driver is attempted to be loaded using Class.forName() and the driver’s JAR file is not present in the classpath:

public class ClassNotFoundExceptionExample < private static final String DRIVER_CLASS = "com.mysql.jdbc.Driver"; public static void main(String[] args) throws Exception < System.out.println("Loading MySQL JDBC driver"); Class.forName(DRIVER_CLASS); >> 

Since the MySQL JDBC driver JAR file is not present in the classpath, running the above code leads to a java.lang.ClassNotFoundException :

Loading MySQL JDBC driver Exception in thread "main" java.lang.ClassNotFoundException: com.mysql.jdbc.Driver at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:602) at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178) at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522) at java.base/java.lang.Class.forName0(Native Method) at java.base/java.lang.Class.forName(Class.java:340) at ClassNotFoundExceptionExample.main(ClassNotFoundExceptionExample.java:6)

To fix the Java exception, the mysql-connector JAR should be included in the application classpath.

How to Resolve ClassNotFoundException in Java

The following steps should be followed to resolve a ClassNotFoundException in Java:

1. Find out which JAR file contains the problematic Java class. For example, in the case of com.mysql.jdbc.driver, the JAR file that contains it is mysql-connector-java.jar.
2. Check whether this JAR is present in the application classpath. If not, the JAR should be added to the classpath in Java and the application should be recompiled.
3. If that JAR is already present in the classpath, make sure the classpath is not overridden (e.g. by a start-up script). After finding out the exact Java classpath used by the application, the JAR file should be added to it.

To fix the Java exception, the mysql-connector JAR should be included in the application classpath.

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How to Fix ClassNotFoundException in Java

The java.lang.ClassNotFoundException is a checked exception in Java that occurs when the JVM tries to load a particular class but does not find it in the classpath.

Since the ClassNotFoundException is a checked exception, it must be explicitly handled in methods which can throw this exception — either by using a try-catch block or by throwing it using the throws clause.

What Causes ClassNotFoundException in Java

Common causes of the java.lang.ClassNotFoundException are using the following methods to load a class:

In all the above cases, the class attempted to be loaded is not found on the classpath, leading to the ClassNotFoundException in Java.

ClassNotFoundException in Java Example

A very common example of ClassNotFoundException is when a JDBC driver is attempted to be loaded using Class.forName() and the driver’s JAR file is not present in the classpath:

public class ClassNotFoundExceptionExample < private static final String DRIVER_CLASS = "com.mysql.jdbc.Driver"; public static void main(String[] args) throws Exception < System.out.println("Loading MySQL JDBC driver"); Class.forName(DRIVER_CLASS); >> 

Since the MySQL JDBC driver JAR file is not present in the classpath, running the above code leads to a java.lang.ClassNotFoundException :

Loading MySQL JDBC driver Exception in thread "main" java.lang.ClassNotFoundException: com.mysql.jdbc.Driver at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:602) at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178) at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522) at java.base/java.lang.Class.forName0(Native Method) at java.base/java.lang.Class.forName(Class.java:340) at ClassNotFoundExceptionExample.main(ClassNotFoundExceptionExample.java:6)

To fix the Java exception, the mysql-connector JAR should be included in the application classpath.

How to Resolve ClassNotFoundException in Java

The following steps should be followed to resolve a ClassNotFoundException in Java:

1. Find out which JAR file contains the problematic Java class. For example, in the case of com.mysql.jdbc.driver, the JAR file that contains it is mysql-connector-java.jar.
2. Check whether this JAR is present in the application classpath. If not, the JAR should be added to the classpath in Java and the application should be recompiled.
3. If that JAR is already present in the classpath, make sure the classpath is not overridden (e.g. by a start-up script). After finding out the exact Java classpath used by the application, the JAR file should be added to it.

To fix the Java exception, the mysql-connector JAR should be included in the application classpath.

Track, Analyze and Manage Java Errors With Rollbar

Managing errors and exceptions in your code is challenging. It can make deploying production code an unnerving experience. Being able to track, analyze, and manage errors in real-time can help you to proceed with more confidence. Rollbar automates Java error monitoring and triaging, making fixing errors easier than ever. Try it today.

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Java ClassNotFoundException

As it goes by the name, ClassNotFoundException occurs in Java when a specific class is tried to load by a Java Virtual Machine(JVM). The requested class is not found in the path of the class specified by you, meaning that the path of the class specified by you is broken, which problem is really common in the world of Java. Hence, ClassNotFoundException is also common in Java. This problem is very confusing for those who are beginners in Java, and ClassNotFoundException must be catch or thrown to the caller ClassNotFoundException is a checked exception.

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The syntax ofClassNotFoundException in Java is as follows:

java.lang.ClassNotFoundException:Class_name at location

Working of ClassNotFoundException in Java

• When a class loader is not able to find the class in the path of the class specified while the application is trying to load the class, that is when the ClassNotFoundExceptionoccurs in Java.
• Making use of Class.forName and ClassLoader.loadClass for loading the class while passing string name of the class as the parameter which is not found in the path of the class that is specified is one of the common causes for java.lang.ClassNotFoundException in Java.
• ClassNotFoundException in Java must be catch or thrown to the caller because it is a checked exception.
• The class is indirectly loading using the Classloader. As a result, ClassNotFoundException in Java occurs at runtime. There is no way that a Java compiler can know if a class is present in the path of the class specified or not at runtime.
• Trying to load the JDBC drivers using Class.forName and not adding the jar file in the path of the class is one of the common examples of ClassNotFoundExceptionin Java.
• Consider the below program to demonstrate ClassNotFoundException in Java:

//a class called program is defined public class Program < //main method is called public static void main(String args[]) < //class not found exception is defined using try and catch block try < // the forname method in class class looks for the mentioned class Class.forName("The Class do not Exist"); >catch (ClassNotFoundException e) < e.printStackTrace(); >> >

The output of the above program is as shown in the snapshot below:

In the above program, a class called Program is defined. Then the main method is called. Then class not found exception is defined using try and catch block. There is noThe class does not exist.java class which the class loader is trying to find andtheforname method in class looks for the mentioned class, which it fails to find; hence the ClassNotFoundException is thrown. The output of the program is as shown in the snapshot above.

Constructors

There are several constructors for ClassNotFoundException in Java. They are:

• ClassNotFoundException(): A new ClassNotFoundException is constructed which includes the current trace of the stack.

• ClassNotFoundException(String): A new ClassNotFoundException is constructed, which includes the current trace of the stack and the message in detail, specified.

• ClassNotFoundException(IntPtr, JniHandleOwnership): During the creation of JNI objects, while managing their representations, this constructor is used, and the runtime calls it.

• ClassNotFoundException(String, Throwable): A new ClassNotFoundException is constructed, which includes the current trace of the stack along with the message in detail that is specified and the exception that occurs when trying to load the class.

Examples of Java ClassNotFoundException

Here are the following examples mention below:

Example #1

Java program to demonstrate ClassNotFoundException:

//a class called exceptiondemo is defined public class Exceptiondemo < //a string variable is defined private static final String DRIVE_CLASS = "com.mysql.jdbc.Driver"; //main method is called including the exception public static void main(String[] args) throws Exception < System.out.println("MySQL JDBC driver loading attempt"); //the forname method in class class looks for the mentioned class Class.forName(DRIVE_CLASS); >>

The output of the above program is as shown in the snapshot below:

In the above program, a class called Exception demo is defined. Then the main method is called. Then a string variable is defined to which the JDBC driver path is assigned. Then the main method is called, which throws the exception. The class loader is trying to find the JDBC driver path of the class specified and the forname method in class looks for the mentioned class, which it fails to find; hence the ClassNotFoundException is thrown. The output of the program is as shown in the snapshot above.

Example #2

Java program to demonstrate ClassNotFoundException(String)

//a class called check is defined public class Check < //main method is called public static void main(String args[]) < //class not found exception is defined using try catch block try < //the forname method in class class looks for the mentioned class Class.forName("Demonstrating class not found exception"); >catch(ClassNotFoundException e) < //the string specified along with the class not found exception is displayed. System.out.println("There is no class as specified in the path " + e); >> >

The output of the above program is as shown in the snapshot below:

In the above program, a class called check is defined. Then the main method is called. Then the main method is called. Then class not found exception is defined by using try and catch block. Then the forename method in class looks for the mentioned class, which it fails to find; hence the ClassNotFoundException is thrown and the string specified as a detailed message along with the class not found exception is displayed. The output of the program is as shown in the snapshot above.

How to Avoid ClassNotFoundException?

Steps to avoid ClassNotFoundException:

• The file of the class causing the problem on which the jar file is present must be found out.
• We should check if the path of the class or classpath consists of that jar. If the jar is not present in the classpath or path of the class, the class must be added in the class or classpath path.
• If the class is present in the class or classpath path, then the chances are that there is an overriding of classpath or path of the class or the path specified in the jar file or the script used for starting up is being used by the application. To solve this problem, we need to find the exact path of the class that is being used by the application.

Conclusion

In this tutorial, we understand the concept of Class Not Found Exception in Java through definition, the syntax of Class Not Found Exception in Java, working of Class Not Found Exception in Java, and their constructors through examples and their outputs.

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