How to Solve java.lang.ClassNotFoundException in Java? - GeeksforGeeks

30 Sep, 2021
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
Java
// Java Program to check for MySQL connectivity Issue // Importing database (SQL) librariesimport java.sql.*; // Main Classpublic class MySQLConnectivityCheck {     public static void main(String[] args)    {         // Display message for better readability        System.out.println(            "---------------------------------------------");         // Initially setting connection object        // and result set to null        Connection con = null;        ResultSet res = null;         // Try block to check for exceptions        try {             // We need to have mysql-connector-java-8.0.22            // or relevant jars in build path of project             // Loading drivers            // This driver is the latest one            Class.forName("com.mysql.cj.jdbc.Driver");             con = DriverManager.getConnection(                "jdbc:mysql://localhost:3306/test?serverTimezone=UTC",                "root", "");             // Try block to check for exceptions            try {                 // Set of statements to be checked            }             // Catch block 1            catch (SQLException s) {                 // Display message when SQLException is                // encountered                System.out.println(                    "SQL statement is not executed!");            }        }         catch (Exception e) {             // In case of general Exception            // print and display the line number where the            // exception occurred            e.printStackTrace();        }        finally {             // Finally for all cases indirectly closing the            // connections & making the resultset and            // connection object to null            res = null;            con = null;        }    }}
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
XML
<!-- Spring boot mongodb dependency --><dependency>            <groupId>org.springframework.boot</groupId>            <artifactId>spring-boot-starter-data-mongodb</artifactId></dependency>
Example 2 Without spring boot
XML
<!-- https://mvnrepository.com/artifact/org.mongodb/mongodb-driver --><dependency>    <groupId>org.mongodb</groupId>    <artifactId>mongodb-driver</artifactId>    <version>3.6.3</version></dependency>
Example 3 Gradle based dependencies (MongoDB) 
XML
dependencies {      compile 'org.mongodb:mongodb-driver:3.2.2'  }
  
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.
 
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