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Table of contents
1.
Introduction
2.
What is Java CLASSPATH?
3.
Steps to Set Java CLASSPATH in Windows
4.
How to Fix Errors Related to CLASSPATH in Java?
5.
How to Set CLASSPATH in Windows Using CMD (Command Prompt)
6.
Difference between PATH and CLASSPATH
7.
How Does Ext Folder Work in Java?
8.
Difference between the Extension Mechanism and the CLASSPATH
9.
Frequently Asked Questions
9.1.
What is class path in Java with example?
9.2.
How to set Java in classpath?
9.3.
What is Java boot classpath?
9.4.
When to use classpath?
10.
Conclusion
Last Updated: Mar 27, 2024
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Java Classpath

Author Sourabh
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Introduction

Welcome to our blog on "CLASSPATH in Java." Navigating the intricacies of CLASSPATH is crucial for Java developers aiming to seamlessly manage dependencies and execute applications. The CLASSPATH is an environmental variable that specifies the location where the Java compiler and Java Virtual Machine (JVM) should look for classes to be loaded. 

Classpath in Java

This article provides an in-depth analysis of Java classpath. And it also how to set classpath in Windows. And we will also explore the differences between path and classpath.

What is Java CLASSPATH?

It is an environment variable that is used by the Application ClassLoader to find and load the .class files. It is used to define the path that finds third-party and user-defined classes which are not part or extension of the Java platform. It includes all the directories which have JAR files and .class files when setting up the CLASSPATH.

You need to set the CLASSPATH if:

  • You have to load a class that is not present in our current directory or any sub-directory.
     
  • You have to load a class that is not in the location specified by the system extensions mechanism.
     

At the end of the CLASSPATH, there is a name of either a directory or a file. The following points explain what should be the end of the CLASSPATH.

  • When the CLASSPATH is configured, if any JAR or zip file has class files, then it will conclude with the name of that specific zip or JAR file.
     
  • If the class files are situated in a package that is nameless, then the end of CLASSPATH is with the directory which includes those class files.
     
  • The placement of class files in a named package determines how the CLASSPATH will end. Specifically, it ends with the directory containing the root package - or, put another way, the very first package listed in its complete name.
     

The value of CLASSPATH default is a dot(.). This value changes if you set the variable CLASSPATH or use the command "-cp" for shorthand. To include the current directory in your search path, add a dot (.) to your new settings.

Once a class file is discovered in the present directory, CLASSPATH will load and use it, no matter if the identically named class exists in any other unused directories.

If you want to establish several classpaths in Java, then using a semicolon (;) between every CLASSPATH is mandatory.

The JVM can be used by third-party applications like MySQL and Oracle. These applications can modify the CLASSPATH environment variable, adding the libraries they use to it. Directories or archive files can store the classes these applications use. For example, classes from rt.jar are used by the Java platform.

We can set CLASSPATH by two methods: one is through Command Prompt, and the other is by setting Environment Variable.

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Steps to Set Java CLASSPATH in Windows

Step 1: Search for the environment variable in the window search bar.
 

environment variable


Step 2: Click on open.
 

environment variable open bar

Step 3: A dialog box will open. Click on Environment Variables.
 

System properties


Step 4: Click on New under System Variables to create a new variable.
 

System Variables to create a new variable

Step 5: Provided there is already an existing CLASSPATH in the System Variables, one can easily edit it by clicking on the Edit button, which lets them put a semicolon at the end of its value. Paste the path of the MySQL Connector Java.jar file into the input field.
If there is no classpath in System Variables, then click on the New button and enter the Variable name as CLASSPATH and then enter the Variable value as 

C:\Program Files\Javafiles\jre1.8\MySQL-Connector Java.jar;.;

Remember: We will Put ;.; at the end of the CLASSPATH.

CLASSPATH in the System Variables

Step 6: Select OK.

How to Fix Errors Related to CLASSPATH in Java?

To fix errors related to the CLASSPATH in Java, consider the following steps:

1. Check CLASSPATH Setting: Ensure that the CLASSPATH is correctly set. Verify that it includes the directories and JAR files required by your Java application.

2. Use -cp Option (for Compilation and Execution): When compiling and running Java programs, you can use the -cp option to specify the CLASSPATH directly. For example:

javac -cp .:path/to/your/lib/* YourJavaFile.java
java -cp .:path/to/your/lib/* YourJavaClass

 

Adjust the paths accordingly.

3. Update Environment Variables: Update the CLASSPATH environment variable. Ensure it includes the necessary directories and JAR files. You can set it using commands like:

export CLASSPATH=.:/path/to/your/lib/*

 

4. Use Manifest File (for JAR Files): If you're working with JAR files, ensure the JAR file contains a manifest file with a Class-Path entry listing the required dependencies. For example:

Class-Path: lib/dependency1.jar lib/dependency2.jar

 

5. Use Build Tools (Maven, Gradle): If you're using build tools like Maven or Gradle, update the dependencies in your project configuration file (pom.xml for Maven, build.gradle for Gradle).

5. Check File and Directory Permissions: Ensure that the files and directories specified in the CLASSPATH have the necessary read permissions.

6. Verify File Paths: Double-check that the paths specified in the CLASSPATH are accurate and point to the correct locations.

7. Debugging with -verbose:class Option: Use the -verbose:class option when running your Java program to get information about which classes are being loaded. This can help identify issues with class resolution.

8. Avoid Spaces in Path Names: If your paths contain spaces, enclose them in quotes or consider using alternative paths without spaces.

How to Set CLASSPATH in Windows Using CMD (Command Prompt)

Type the below command in your Command Prompt and then press enter.

set CLASSPATH=%CLASSPATH%;C:\Program Files\Javafiles\jre1.8\rt.jar;  

The above-written command set command is used, and we know that “set” is an internal DOS command that will allow the user to change the value of the variable. CLASSPATH is the name of the variable. Using a percentage sign (%), one can enclose a variable that already exists. To separate elements, semicolons are used, and in this case, the path for the rt.jar file is located after the semicolon (;).

Also see, Duck Number in Java 

Difference between PATH and CLASSPATH

PATH CLASSPATH
It is used by the OS to find the executable files (.exe). The .class file is found by the Application ClassLoader when it needs to use.
PATH is an environment variable. CLASSPATH is also an environment variable.
All directories that possess .exe files are to be included. All directories that possess .class and JAR files are to be included.
PATH environment variables cannot be overridden after they are set. By using the commands Javac and Java, the CLASSPATH environmental variable may be changed by using a command line option, either -cp or -CLASSPATH.

How Does Ext Folder Work in Java?

Similar to the CLASSPATH, the ext directory also operates. A component of the class loading method is the ext directory. Java applications can access the classes included within JARs in the ext directory.

Difference between the Extension Mechanism and the CLASSPATH

Characteristics CLASSPATH Extension Mechanism
Class loading order After bootstrap and extension loading, CLASSPATH will load. ext would load after bootstrap loading but also before CLASSPATH loading
Scope It is product-specific. The environment variable CLASSPATH contains all JREs on the host. All JVMs run in specific JRE java.ext.dirs.
Specification When specifying a file, it is essential to include the extension.jar and the directory that contains .class files. All JVMs that we are using are running in specific JRE java.ext.dirs.
Package name To locate where class files are kept, java.class.path is utilized for identifying the folders and JAR archives containing these files. The purpose of Java.ext.dirs is to indicate where classes are loaded by the extension mechanism.

Even if a file doesn't have the .jar extension, the method will still pick it up from the extension directory. This is implemented by allowing users to alter a jar's name and extension to something other than.jar when it is placed in a classpath directory. The asterisk (*) does not recognize it. The extension directory will not support this method.

In order to grasp the execution process, let us consider an example.

A.java

import java.io.*;
public class A {
	public String toString() {
		return "hello world";
	}
} 

B.java

public class B  
{  
   public static void main(final String[] args){ 
      System.out.println(new A());  
    }  
} 

Make the A.java file compile. The compiled A.class file will be archived as an A.jar file. Put the compiled B.class file in a different directory than the JAR file.
We put the A.jar file in the directory C: JavaPrograms to show how to use the classpath. B will then access that JAR by using the wildcard (*) character.

We discovered that even after deleting A.class from the current directory, B could still load it. C: JavaProgram was specifically searched for to find the Java launcher. Without having to be explicitly stated in the classpath or stored in the exact same directory as other files, classes may still be loaded by a system, which showcases its adaptability.

Because all programs utilizing that JRE can see the same classes without explicitly putting them on the classpath, this is frequently regarded as an advantage of adopting the extension technique.

What happens if we rename A.jar to A.backup in the same directory CLASSPATH references? When we try the same, we run into a NoClassDefFoundError error since the CLASSPATH-reference does not have the .jar extension.

Frequently Asked Questions

What is class path in Java with example?

The CLASSPATH in Java specifies where the JVM looks for classes. Example: java -cp .:lib/* MyClass sets the classpath to the current directory and the "lib" directory.

How to set Java in classpath?

We can use the -cp option or update the CLASSPATH environment variable to set the CLASSPATH. Example: export CLASSPATH=.:/path/to/your/lib/*

What is Java boot classpath?

The Java Boot Classpath contains core Java classes required for the JVM to start. It is managed by the JVM and typically includes system libraries.

When to use classpath?

We can use the classpath when compiling or running Java programs to specify the locations where the JVM should look for classes and dependencies.

Conclusion

One of the most important features of running Java applications is understanding classpath correctly. The articles give an extensive view and deep dive into the topic, which assists in comprehending its significance concerning locating both third-party and user-defined classes not part of the standard Java Platform. Furthermore, there will be precise steps illustrating how Windows users can change or create this environment variable by using Command Prompt. The article is a complete tutorial for programmers who want to enhance their knowledge of Java classpath and master its efficient use.

To Learn about Java Development Kit(JDK)  you can refer to this article. 

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