ava – What does “Could not find or load main class” mean?

The Question :

1464 people think this question is useful

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?

The Question Comments :
  • Please note that this is a “self-answer” question that is intended to be a generic reference Q&A for new Java users. I could not find an existing Q&A that covers this adequately (IMO).

The Answer 1

1309 people think this answer is useful

The java <class-name> command syntax

First of all, you need to understand the correct way to launch a program using the java (or javaw) command.

The normal syntax1 is this:

    java [ <options> ] <class-name> [<arg> ...]

where <option> is a command line option (starting with a “-” character), <class-name> is a fully qualified Java class name, and <arg> is an arbitrary command line argument that gets passed to your application.


1 – There are some other syntaxes which are described near the end of this answer.

The fully qualified name (FQN) for the class is conventionally written as you would in Java source code; e.g.

    packagename.packagename2.packagename3.ClassName

However some versions of the java command allow you to use slashes instead of periods; e.g.

    packagename/packagename2/packagename3/ClassName

which (confusingly) looks like a file pathname, but isn’t one. Note that the term fully qualified name is standard Java terminology … not something I just made up to confuse you ๐Ÿ™‚

Here is an example of what a java command should look like:

    java -Xmx100m com.acme.example.ListUsers fred joe bert

The above is going to cause the java command to do the following:

  1. Search for the compiled version of the com.acme.example.ListUsers class.
  2. Load the class.
  3. Check that the class has a main method with signature, return type and modifiers given by public static void main(String[]). (Note, the method argument’s name is NOT part of the signature.)
  4. Call that method passing it the command line arguments (“fred”, “joe”, “bert”) as a String[].

Reasons why Java cannot find the class

When you get the message “Could not find or load main class …”, that means that the first step has failed. The java command was not able to find the class. And indeed, the “…” in the message will be the fully qualified class name that java is looking for.

So why might it be unable to find the class?

Reason #1 – you made a mistake with the classname argument

The first likely cause is that you may have provided the wrong class name. (Or … the right class name, but in the wrong form.) Considering the example above, here are a variety of wrong ways to specify the class name:

  • Example #1 – a simple class name:

    java ListUser
    
    

    When the class is declared in a package such as com.acme.example, then you must use the full classname including the package name in the java command; e.g.

    java com.acme.example.ListUser
    
    
  • Example #2 – a filename or pathname rather than a class name:

    java ListUser.class
    java com/acme/example/ListUser.class
    
    
  • Example #3 – a class name with the casing incorrect:

    java com.acme.example.listuser
    
    
  • Example #4 – a typo

    java com.acme.example.mistuser
    
    
  • Example #5 – a source filename (except for Java 11 or later; see below)

    java ListUser.java
    
    
  • Example #6 – you forgot the class name entirely

    java lots of arguments
    
    

Reason #2 – the application’s classpath is incorrectly specified

The second likely cause is that the class name is correct, but that the java command cannot find the class. To understand this, you need to understand the concept of the “classpath”. This is explained well by the Oracle documentation:

So … if you have specified the class name correctly, the next thing to check is that you have specified the classpath correctly:

  1. 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.)
  2. Look at command line and / or the CLASSPATH environment variable that is in effect when you run the java command. Check that the directory names and JAR file names are correct.
  3. If there are relative pathnames in the classpath, check that they resolve correctly … from the current directory that is in effect when you run the java command.
  4. Check that the class (mentioned in the error message) can be located on the effective classpath.
  5. Note that the classpath syntax is different for Windows versus Linux and Mac OS. (The classpath separator is ; on Windows and : on the others. If you use the wrong separator for your platform, you won’t get an explicit error message. Instead, you will get a nonexistent file or directory on the path that will be silently ignored.)

Reason #2a – the wrong directory is on the classpath

When you put a directory on the classpath, it notionally corresponds to the root of the qualified name space. Classes are located in the directory structure beneath that root, by mapping the fully qualified name to a pathname. So for example, if “/usr/local/acme/classes” is on the class path, then when the JVM looks for a class called com.acme.example.Foon, it will look for a “.class” file with this pathname:

  /usr/local/acme/classes/com/acme/example/Foon.class

If you had put “/usr/local/acme/classes/com/acme/example” on the classpath, then the JVM wouldn’t be able to find the class.

Reason #2b – the subdirectory path doesn’t match the FQN

If your classes FQN is com.acme.example.Foon, then the JVM is going to look for “Foon.class” in the directory “com/acme/example”:

  • If your directory structure doesn’t match the package naming as per the pattern above, the JVM won’t find your class.

  • If you attempt rename a class by moving it, that will fail as well … but the exception stacktrace will be different. It is liable to say something like this:

    Caused by: java.lang.NoClassDefFoundError: <path> (wrong name: <name>)
    
    

    because the FQN in the class file doesn’t match what the class loader is expecting to find.

To give a concrete example, supposing that:

  • you want to run com.acme.example.Foon class,
  • the full file path is /usr/local/acme/classes/com/acme/example/Foon.class,
  • your current working directory is /usr/local/acme/classes/com/acme/example/,

then:

# wrong, FQN is needed
java Foon

# wrong, there is no `com/acme/example` folder in the current working directory
java com.acme.example.Foon

# wrong, similar to above
java -classpath . com.acme.example.Foon

# fine; relative classpath set
java -classpath ../../.. com.acme.example.Foon

# fine; absolute classpath set
java -classpath /usr/local/acme/classes com.acme.example.Foon

Notes:

  • The -classpath option can be shortened to -cp in most Java releases. Check the respective manual entries for java, javac and so on.
  • Think carefully when choosing between absolute and relative pathnames in classpaths. Remember that a relative pathname may “break” if the current directory changes.

Reason #2c – dependencies missing from the classpath

The classpath needs to include all of the other (non-system) classes that your application depends on. (The system classes are located automatically, and you rarely need to concern yourself with this.) For the main class to load correctly, the JVM needs to find:

(Note: the JLS and JVM specifications allow some scope for a JVM to load classes “lazily”, and this can affect when a classloader exception is thrown.)

Reason #3 – the class has been declared in the wrong package

It occasionally happens that someone puts a source code file into the the wrong folder in their source code tree, or they leave out the package declaration. If you do this in an IDE, the IDE’s compiler will tell you about this immediately. Similarly if you use a decent Java build tool, the tool will run javac in a way that will detect the problem. However, if you build your Java code by hand, you can do it in such a way that the compiler doesn’t notice the problem, and the resulting “.class” file is not in the place that you expect it to be.

Still can’t find the problem?

There lots of things to check, and it is easy to miss something. Try adding the -Xdiag option to the java command line (as the first thing after java). It will output various things about class loading, and this may offer you clues as to what the real problem is.

Also, consider possible problems caused by copying and pasting invisible or non-ASCII characters from websites, documents and so on. And consider “homoglyphs”, were two letters or symbols look the same … but aren’t.

Finally, you can apparently run into this problem if you try to launch from a JAR file with incorrect signatures in (META-INF/*.SF).


Alternative syntaxes for java

There are three alternative syntaxes for the launching Java programs using the java command.

1) The syntax used for launching an “executable” JAR file is as follows:

  java [ <options> ] -jar <jar-file-name> [<arg> ...]

e.g.

  java -Xmx100m -jar /usr/local/acme-example/listuser.jar fred

The name of the entry-point class (i.e. com.acme.example.ListUser) and the classpath are specified in the MANIFEST of the JAR file.

2) The syntax for launching an application from a module (Java 9 and later) is as follows:

  java [ <options> ] --module <module>[/<mainclass>] [<arg> ...]

The name of the entrypoint class is either defined by the <module> itself, or is given by the optional <mainclass>.

3) From Java 11 onwards, you can compile and run a single source code file and run it with the following syntax:

  java [ <options> ] <sourcefile> [<arg> ...]

where is (typically) a file with the suffix “.java”.

For more details, please refer to the official documentation for the java command for the Java release that you are using.


IDEs

A typical Java IDE has support for running Java applications in the IDE JVM itself or in a child JVM. These are generally immune from this particular exception, because the IDE uses its own mechanisms to construct the runtime classpath, identify the main class and create the java command line.

However it is still possible for this exception to occur, if you do things behind the back of the IDE. For example, if you have previously set up an Application Launcher for your Java app in Eclipse, and you then moved the JAR file containing the “main” class to a different place in the file system without telling Eclipse, Eclipse would unwittingly launch the JVM with an incorrect classpath.

In short, if you get this problem in an IDE, check for things like stale IDE state, broken project references or broken launcher configurations.

It is also possible for an IDE to simply get confused. IDE’s are hugely complicated pieces of software comprising many interacting parts. Many of these parts adopt various caching strategies in order to make the IDE as a whole responsive. These can sometimes go wrong, and one possible symptom is problems when launching applications. If you suspect this could be happening, it is worth trying other things like restarting your IDE, rebuilding the project and so on.


Other References

The Answer 2

248 people think this answer is useful

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

The Answer 3

148 people think this answer is useful

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.

The Answer 4

65 people think this answer is useful

If you define the main class and main method 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.

The Answer 5

50 people think this answer is useful

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

The Answer 6

38 people think this answer is useful

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!
    
    

The Answer 7

29 people think this answer is useful

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.

The Answer 8

20 people think this answer is useful

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 JDKย 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.

The Answer 9

18 people think this answer is useful

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.

The Answer 10

17 people think this answer is useful

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

The Answer 11

16 people think this answer is useful

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.

The Answer 12

12 people think this answer is useful

I had same problem and finally found my mistake ๐Ÿ™‚ 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

The Answer 13

11 people think this answer is useful

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.

The Answer 14

8 people think this answer is useful

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.

The Answer 15

8 people think this answer is useful

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>

The Answer 16

8 people think this answer is useful

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. ๐Ÿ™‚

The Answer 17

6 people think this answer is useful

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.

The Answer 18

6 people think this answer is useful

enter image description here

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

The Answer 19

6 people think this answer is useful

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.

The Answer 20

5 people think this answer is useful

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)”.

The Answer 21

5 people think this answer is useful

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 รก) 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.

The Answer 22

5 people think this answer is useful

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.

The Answer 23

5 people think this answer is useful

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.

The Answer 24

4 people think this answer is useful

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/

The Answer 25

4 people think this answer is useful

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

The Answer 26

3 people think this answer is useful

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.

The Answer 27

3 people think this answer is useful

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)

The Answer 28

3 people think this answer is useful

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.

The Answer 29

2 people think this answer is useful

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" />

The Answer 30

2 people think this answer is useful

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.
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