[java] What does "Could not find or load main class" mean?

A common problem that new Java developers experience is that their programs fail to run with the error message: Could not find or load main class ...

What does this mean, what causes it, and how should you fix it?

Reason #2 - the application's classpath is incorrectly specified Read the three documents linked above. (Yes ... READ them! It is important that a Java programmer understands at least the basics of how the Java classpath mechanisms works.) I want to add this documentation to this very good post from above JDK Tools and Utilities General General Information (file structure, classpath, how classes are found, changes) Enhancements(enhancements in JDK 7) Standard JDK Tools and Utilities https://docs.oracle.com/javase/7/docs/technotes/tools/index.html https://docs.oracle.com/javase/7/docs/technotes/tools/findingclasses.html https://docs.oracle.com/javase/7/docs/technotes/tools/windows/classpath.html How the Java Launcher Finds Classes Understanding the class path and package names https://docs.oracle.com/javase/7/docs/technotes/tools/solaris/javac.html

ClassLoader in Java The Java ClassLoader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. The Java run time system does not need to know about files and file systems because of classloaders. Java classes aren’t loaded into memory all at once, but when required by an application. At this point, the Java ClassLoader is called by the JRE and these ClassLoaders load classes into memory dynamically. https://en.wikipedia.org/wiki/Java_Classloader https://www.geeksforgeeks.org/classloader-in-java/ https://en.wikipedia.org/wiki/Java_virtual_machine

If your source code name is HelloWorld.java, your compiled code will be HelloWorld.class.

You will get that error if you call it using:

java HelloWorld.class

Instead, use this:

java HelloWorld

In the context of IDE development (Eclipse, NetBeans or whatever) you have to configure your project properties to have a main class, so that your IDE knows where the main class is located to be executed when you hit "Play".

1. Right click your project, then Properties
2. Go to the Run category and select your Main Class
3. Hit the Run button.

This might help you if your case is specifically like mine: as a beginner I also ran into this problem when I tried to run a Java program.

I compiled it like this:

javac HelloWorld.java

And I tried to run also with the same extension:

java Helloworld.java

When I removed the .java and rewrote the command like java HelloWorld, the program ran perfectly. :)

Right click the project.

1. Select "Open module settings"
2. Mark the src folder as "sources"
3. Go to edit configurations, and then select your main class
4. Click OK or the Apply button

This worked for me.

If it's a Maven project:

1. Go to the POM file.
2. Remove all the dependencies.
3. Save the POM file.
4. Again import only the necessary dependencies.
5. Save the POM file.

The issue should go away.

What fixed the problem in my case was:

Right click on the project/class you want to run, then Run As->Run Configurations. Then you should either fix your existing configuration or add new in the following way:

open the Classpath tab, click on the Advanced... button then add bin folder of your project.

After searching for 2 days I found this solution and this works. It is pretty weird but it works for me.

package javaapplication3;
public class JavaApplication3 {

/**
 * @param args the command line arguments
 */
public static void main(String[] args) {
    // TODO code application logic here
    System.out.println("Hello");
}

}

this is my program i want to run that locates at C:\Java Projects\JavaApplication3\src\javaapplication3

Now open cmd on this location and compile program using this command

javac JavaApplication3.java

After compiling navigate one directory down i.e. C:\Java Projects\JavaApplication3\src

now run following command to execute program

java javaapplication3.JavaApplication3

Sometimes what might be causing the issue has nothing to do with the main class, and I had to find this out the hard way. It was a referenced library that I moved, and it gave me the:

Could not find or load main class xxx Linux

I just deleted that reference, added it again, and it worked fine again.

I had same problem and finally found my mistake :) I used this command for compiling and it worked correctly:

javac -cp "/home/omidmohebbi/AAAATest/jars/core-1.7.jar:/home/omidmohebbi/AAAATest/jars/javase-1.7.jar:/home/omidmohebbi/AAAATest/jars/qrgen-1.2.jar" qrcode.java

But this command did not work for me (I could not find or load the main class, qrcode):

java -cp "/home/omidmohebbi/AAAATest/jars/core-1.7.jar:/home/omidmohebbi/AAAATest/jars/javase-1.7.jar:/home/omidmohebbi/AAAATest/jars/qrgen-1.2.jar" qrcode

Finally I just added the ':' character at end of the classpath and the problem was solved:

java -cp "/home/omidmohebbi/AAAATest/jars/core-1.7.jar:/home/omidmohebbi/AAAATest/jars/javase-1.7.jar:/home/omidmohebbi/AAAATest/jars/qrgen-1.2.jar:" qrcode

Class file location: C:\test\com\company

File Name: Main.class

Fully qualified class name: com.company.Main

Command line command:

java  -classpath "C:\test" com.company.Main

Note here that class path does NOT include \com\company

I had a weird one:

Error: Could not find or load main class mypackage.App

It turned out I had a reference to POM (parent) coded up in my project's pom.xml file (my project's pom.xml was pointing to a parent pom.xml) and the relativePath was off/wrong.

Below is a partial of my project's pom.xml file:

<parent>
    <groupId>myGroupId</groupId>
    <artifactId>pom-parent</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <relativePath>../badPathHere/pom.xml</relativePath>
</parent>

Once I resolved the POM relativePath, the error went away.

Go figure.

In my case, error appeared because I had supplied the source file name instead of the class name.

We need to supply the class name containing the main method to the interpreter.

All answers here are directed towards Windows users it seems. For Mac, the classpath separator is :, not ;. As an error setting the classpath using ; is not thrown then this can be a difficult to discover if coming from Windows to Mac.

Here is corresponding Mac command:

java -classpath ".:./lib/*" com.test.MyClass

Where in this example the package is com.test and a lib folder is also to be included on classpath.

Solving "Could not Load main class error"

After reading all the answers, I noticed most didn't work for me. So I did some research and here is what I got. Only try this if step 1 doesn't work.

1. Try to install JRE 32 or 64. If it doesn't work,

2. Open go to C:\Program Files (x86)\Java or C:\Program Files\Java

   • i. open the jdk folder and then the bin folder.

   • ii. Copy the path and add it to environment variables. Make sure you separate variables with a semi-colon, ;. For example, "C:\Yargato\bin;C:\java\bin;". If you don't, it will cause more errors.

   • iii. Go to the jre folder and open its bin folder.

   • iv. Here search for rt.jar file. Mine is:

     C:\Program Files (x86)\Java\jre1.8.0_73\lib\rt.jar Copy and under environment variable and search for the classpath variable and paste it there.

   • v. Now restart cmd and try running again. The error will disappear.
   • vi. I will post a link to my YouTube video tutorial.

What helped me was specifying the classpath on the command line, for example:

1. Create a new folder, C:\temp

2. Create file Temp.java in C:\temp, with the following class in it:

   public class Temp {
       public static void main(String args[]) {
           System.out.println(args[0]);
       }
   }
   

3. Open a command line in folder C:\temp, and write the following command to compile the Temp class:

   javac Temp.java
   

4. Run the compiled Java class, adding the -classpath option to let JRE know where to find the class:

   java -classpath C:\temp Temp Hello!
   

I spent a decent amount of time trying to solve this problem. I thought that I was somehow setting my classpath incorrectly but the problem was that I typed:

java -cp C:/java/MyClasses C:/java/MyClasses/utilities/myapp/Cool  

instead of:

java -cp C:/java/MyClasses utilities/myapp/Cool   

I thought the meaning of fully qualified meant to include the full path name instead of the full package name.

When the same code works on one PC, but it shows the error in another, the best solution I have ever found is compiling like the following:

javac HelloWorld.java
java -cp . HelloWorld

I had such an error in this case:

java -cp lib.jar com.mypackage.Main

It works with ; for Windows and : for Unix:

java -cp lib.jar; com.mypackage.Main

You really need to do this from the src folder. There you type the following command line:

[name of the package].[Class Name] [arguments]

Let's say your class is called CommandLine.class, and the code looks like this:

package com.tutorialspoint.java;

    /**
     * Created by mda21185 on 15-6-2016.
     */

    public class CommandLine {
        public static void main(String args[]){
            for(int i=0; i<args.length; i++){
                System.out.println("args[" + i + "]: " + args[i]);
            }
        }
    }

Then you should cd to the src folder and the command you need to run would look like this:

java com.tutorialspoint.java.CommandLine this is a command line 200 -100

And the output on the command line would be:

args[0]: this
args[1]: is
args[2]: a
args[3]: command
args[4]: line
args[5]: 200
args[6]: -100

I got this issue for my demo program created in IntelliJ.

There are two key points to solve it:

1. the package name of program
2. the current working dir of the terminal/cmd prompt

my demo program:

package io.rlx.tij.c2;

public class Ex10 {
    public static void main(String[] args) {
        // do something
    }
}

the path of source code:

../projectRoot/src/main/java/io/rlx/tij/c2/Ex10.java

1. go to java dir: cd ../projectRoot/src/main/java
2. compile to class: javac ./io/rlx/tij/c2/Ex10.java
3. run program: java io.rlx.tij.c2.Ex10

if I run program in ../projectRoot/src/main/java/io/rlx/tij/c2 or I run it without package name, I will get this error: Error: Could not find or load main class.

This happened to me too. In my case, it only happened when a HttpServlet class was present in source code (IntelliJ IDEA didn't give a compile time error; the servlet package got imported just fine, however at run time there was this main class error).

I managed to solve it. I went to menu File → Project Structure...:

Then to Modules:

There was a Provided scope near the servlet module. I changed it to Compile:

And it worked!

When running the java with the -cp option as advertised in Windows PowerShell you may get an error that looks something like:

The term `ClassName` is not recognized as the name of a cmdlet, function, script ...

In order to for PowerShell to accept the command, the arguments of the -cp option must be contained in quotes as in:

java -cp 'someDependency.jar;.' ClassName

Forming the command this way should allow Java process the classpath arguments correctly.

If you use IntelliJ and get the error while running the main method from the IDE, just make sure your class is located in java package, not in kotlin

If this issue is Eclipse-related:

Try adding the project to your class path.

See the below image:

This method worked for me.

I ran into this error while trying to execute this code written in intellij idea on onlinegdb compiler.

For me commenting out this line helped me prevent this error.

//package com.company;

By default, Java uses ., the current working directory, as the default CLASSPATH. What this means is that when you type a command at the prompt e.g. java MyClass, the command is interpreted as if you had type java -cp . MyClass. Did you see that dot between -cp and MyClass? (cp is short for the longer classpath option)

This is sufficient for most cases and things seems to work just fine until at some time you try to add a directory to your CLASSPATH. In most cases when programmers need to do this, they just run a command like set CLASSPATH=path\to\some\dir. This command creates a new environment variable called CLASSPATH having the value path\to\some\dir or replaces its value with path\to\some\dir if CLASSPATH was already set before.

When this is done, you now have a CLASSPATH environment variable and Java no longer uses its default classpath (.) but the one you've set. So the next day you open your editor, write some java program, cd to the directory where you saved it, compile it, and try to run it with the command java MyClass, and you are greeted with a nice output: Could not find or load main class ... (If your commands were working well before and you are now getting this output, then this might be the case for you).

What happens is that when you run the command java MyClass, Java searches for the class file named MyClass in the directory or directories that you have set in your CLASSPATH and not your current working directory so it doesn't find your class file there and hence complains.

What you need to do is add . to your class path again which can be done with the command set CLASSPATH=%CLASSPATH%;. (notice the dot after the semicolon). In plain english this command says "Pick what was initially the value of CLASSPATH (%CLASSPATH%), add . to it (;.) and assign the result back to CLASSPATH".

And voila, you are once again able to use your command java MyClass as usual.

Seems like when I had this problem, it was unique.

Once I removed the package declaration at the top of the file, it worked perfectly.

Aside from doing that, there didn't seem to be any way to run a simple HelloWorld.java on my machine, regardless of the folder the compilation happened in, the CLASSPATH or PATH, parameters or folder called from.

Answering with respect to an external library -

Compile:

javac -cp ./<external lib jar>: <program-name>.java

Execute:

java -cp ./<external lib jar>: <program-name>

The above scheme works well in OS X and Linux systems. Notice the : in the classpath.

In Java, when you sometimes run the JVM from the command line using the java executable and are trying to start a program from a class file with public static void main (PSVM), you might run into the below error even though the classpath parameter to the JVM is accurate and the class file is present on the classpath:

Error: main class not found or loaded

This happens if the class file with PSVM could not be loaded. One possible reason for that is that the class may be implementing an interface or extending another class that is not on the classpath. Normally if a class is not on the classpath, the error thrown indicates as such. But, if the class in use is extended or implemented, java is unable to load the class itself.

Reference: https://www.computingnotes.net/java/error-main-class-not-found-or-loaded/

If you use Maven to build the JAR file, please make sure to specify the main class in the pom.xml file:

<build>
    <plugins>
        <plugin>
            <artifactId>maven-jar-plugin</artifactId>
            <configuration>
                <archive>
                    <manifest>
                        <mainClass>class name us.com.test.abc.MyMainClass</mainClass>
                    </manifest>
                </archive>
            </configuration>
        </plugin>
    </plugins>
</build>

One more scenario that got me scratch my head, and I found no reference to it herein, is:

package com.me
Public class Awesome extends AwesomeLibObject {
    ....
    public static void main(String[] argv) {
         System.out.println("YESS0");
    }
}

Where AwesomeLibObject is a class defined in an external lib. I got the same confusing error message for it:

Error: Could not find or load main class com.Awesome

The resolution is simple: the external lib must be in classpath as well!

This is how I solved my issue.

I noticed if you are including jar files with your compilation, adding the current directory (./) to the classpath helps.

javac -cp "abc.jar;efg.jar" MyClass.java
java -cp "abc.jar;efg.jar" MyClass

VS

javac -cp "./;abc.jar;efg.jar" MyClass.java
java -cp "./;abc.jar;efg.jar" MyClass

In this instance you have:

Could not find or load main class ?classpath

It's because you are using "-classpath", but the dash is not the same dash used by java on the command prompt. I had this issue copying and pasting from Notepad to cmd.

[Java Version: 11]

If you are using Java 11 then you don't need to compile and run your java file.

Just run like

Java ClassName.java

Example:

class abc{ 
    public static void main(String[] args){
        System.out.println("hello Jarvis ");    
    }
}

Now Run the command

java abc.java

Scenario: using command prompt(CMD in Windows) for compile and run a simple 'java' program which have only 'Main.java' file, with specified 'package main'.

Source file path :

some-project-name-folder\src\main\Main.java

Destination folder :

some-project-name-folder\dest

Destination file path (folder '\main' and file '\Main.class' will be produced by 'javac') :

some-project-name-folder\dest\main\Main.class

Main.java is as follow :

package main;

public class Main {
    public static void main(String[] args) {
        System.out.println("Hello world");
    }
}

Compilation :

// 'javac' compiler will produce 'Main.class' in the 'dest\main' folder.
// 'main' folder is created because in the source file(in our case: 'Main.java') is
// specified 'package main'.

javac -d ./dest ./src/main/Main.java

Run compiled file (in our case: 'Main.class') :

// '-cp'(is the same as '-classpath')
// './dest'(means destination folder, where resides compiled 'Main.class').
// 'main.Main'(means 'package' 'main', which contains class 'Main'('Main.class'))
// WARNING: when run 'java' class, MUST NOT type extension '.class'
//          after 'class name
//          (in our case: 'main.Main'(<package>.<class-name>) WITHOUT extension 
//           '.class').

java -cp ./dest main.Main

// Hello world

• in Intellij Check your Global libraries and local libraries
• check in pom.xml the libraries version , maybe it is an old library
• they are so many possibilities mention above that need to be tried too

Here's another issue that took me a bit of time: The command line class path param doesn't behave as you'd expect. I'm on MacOS calling the CLI directly, and I'm including two jars in the call.

For example, both of these were confusing the tool about the name of the main class:

This one because the asterisk was causing it to parse the args incorrectly:

java -cp path/to/jars/* com.mypackage.Main

And this one because -- I'm not sure why:

java -cp "*.jar" com.mypackage.Main

This worked:

java -cp "path/to/jars/*" com.mypackage.Main

Listing the two jars explicitly also worked:

java -cp path/to/jars/jar1.jar:path/to/jars/jar2.jar com.mypackage.Main

All right, there are many answers already, but no one mentioned the case where file permissions can be the culprit.

When running, a user may not have access to the JAR file or one of the directories of the path. For example, consider:

Jar file in /dir1/dir2/dir3/myjar.jar

User1 who owns the JAR file may do:

# Running as User1
cd /dir1/dir2/dir3/
chmod +r myjar.jar

But it still doesn't work:

# Running as User2
java -cp "/dir1/dir2/dir3:/dir1/dir2/javalibs" MyProgram
Error: Could not find or load main class MyProgram

This is because the running user (User2) does not have access to dir1, dir2, or javalibs or dir3. It may drive someone nuts when User1 can see the files, and can access to them, but the error still happens for User2.

In my case I just change the JRE in Eclipse.

Please find the attached screen shot:

For a database connection I was getting this one. I just added the following in the class path:

export CLASSPATH=<path>/db2jcc4.jar:**./**

Here I appended ./ in last so that my class also get identified while loading.

Now just run:

java ConnectionExample <args>

It worked perfectly fine.

This is a specific case, but since I came to this page looking for a solution and didn't find it, I'll add it here.

Windows (tested with 7) doesn't accept special characters (like á) in class and package names. Linux does, though.

I found this out when I built a .jar in NetBeans and tried to run it in command line. It ran in NetBeans but not in command line.

According to the error message ("Could not find or load main class"), there are two categories of problems:

1. Main class could not be found
2. Main class could not be loaded (this case is not fully discussed in the accepted answer)

Main class could not be found when there is typo or wrong syntax in the fully qualified class name or it does not exist in the provided classpath.

Main class could not be loaded when the class cannot be initiated, typically the main class extends another class and that class does not exist in the provided classpath.

For example:

public class YourMain extends org.apache.camel.spring.Main

If camel-spring is not included, this error will be reported.

In my case, I got the error because I had mixed UPPER- and lower-case package names on a Windows 7 system. Changing the package names to all lower case resolved the issue. Note also that in this scenario, I got no error compiling the .java file into a .class file; it just wouldn't run from the same (sub-sub-sub-) directory.

I was unable to solve this problem with the solutions stated here (although the answer stated has, no doubt, cleared my concepts). I faced this problem two times and each time I have tried different solutions (in the Eclipse IDE).

• Firstly, I have come across with multiple main methods in different classes of my project. So, I had deleted the main method from subsequent classes.
• Secondly, I tried following solution:
  1. Right click on my main project directory.
  2. Head to source then clean up and stick with the default settings and on Finish. After some background tasks you will be directed to your main project directory.
  3. After that I close my project, reopen it, and boom, I finally solved my problem.

I got this error after doing mvn eclipse:eclipse This messed up my .classpath file a little bit.

Had to change the lines in .classpath from

<classpathentry kind="src" path="src/main/java" including="**/*.java"/>
<classpathentry kind="src" path="src/main/resources" excluding="**/*.java"/>

to

<classpathentry kind="src" path="src/main/java" output="target/classes" />
<classpathentry kind="src" path="src/main/resources" excluding="**"  output="target/classes" />

Excluding the following files solved the problem.

META-INF/*.SF

META-INF/*.DSA

META-INF/*.RSA

Added the following code in build.gradle

jar {
    from {
        configurations.compile.collect { it.isDirectory() ? it : zipTree(it) }
    }
    {
        exclude "META-INF/*.SF"
        exclude "META-INF/*.DSA"
        exclude "META-INF/*.RSA"
    }
    manifest {
        attributes(
                'Main-Class': 'mainclass'
        )
    }
}

If your classes are in packages then you have to cd to the root directory of your project and run using the fully qualified name of the class (packageName.MainClassName).

Example:

My classes are in here:

D:\project\com\cse\

The fully qualified name of my main class is:

com.cse.Main

So I cd back to the root project directory:

D:\project

Then issue the java command:

java com.cse.Main

This answer is for rescuing newbie java programmers from the frustration caused by a common mistake, I recommend you read the accepted answer for more in depth knowledge about the java classpath.

Use this command:

java -cp . [PACKAGE.]CLASSNAME

Example: If your classname is Hello.class created from Hello.java then use the below command:

java -cp . Hello

If your file Hello.java is inside package com.demo then use the below command

java -cp . com.demo.Hello

With JDK 8 many times it happens that the class file is present in the same folder, but the java command expects classpath and for this reason we add -cp . to take the current folder as reference for classpath.

I also faced similar errors while testing a Java MongoDB JDBC connection. I think it's good to summarize my final solution in short so that in the future anybody can directly look into the two commands and are good to proceed further.

Assume you are in the directory where your Java file and external dependencies (JAR files) exist.

Compile:

javac -cp mongo-java-driver-3.4.1.jar JavaMongoDBConnection.java

• -cp - classpath argument; pass all the dependent JAR files one by one
• *.java - This is the Java class file which has main method. sdsd

Run:

java -cp mongo-java-driver-3.4.1.jar: JavaMongoDBConnection

• Please do observe the colon (Unix) / comma (Windows) after all the dependency JAR files end
• At the end, observe the main class name without any extension (no .class or .java)

First set the path using this command;

set path="paste the set path address"

Then you need to load the program. Type "cd (folder name)" in the stored drive and compile it. For Example, if my program stored on the D drive, type "D:" press enter and type " cd (folder name)".

Try -Xdiag.

Steve C's answer covers the possible cases nicely, but sometimes to determine whether the class could not be found or loaded might not be that easy. Use java -Xdiag (since JDK 7). This prints out a nice stacktrace which provides a hint to what the message Could not find or load main class message means.

For instance, it can point you to other classes used by the main class that could not be found and prevented the main class to be loaded.

On Windows put .; at the CLASSPATH value in the beginning.

The . (dot) means "look in the current directory". This is a permanent solution.

Also you can set it "one time" with set CLASSPATH=%CLASSPATH%;.. This will last as long as your cmd window is open.

If your classes are in packages (The main class is defined in a package), you should run it over the hierarchical directory, using the full name of the class (packageName.MainClassName).

Assume there is a source code file (Main.java):

package com.test;

public class Main {

    public static void main(String[] args) {
        System.out.println("salam 2nya\n");
    }
}

For running this code, you should place Main.Class in the package like directory ./com/test/Main.Java. And in the root directory use java com.test.Main.

Sometimes, in some online compilers that you might have tried you will get this error if you don't write public class [Classname] but just class [Classname].