Posted on

How to compile java. How to build jar

Why did I decide to write this post?

Because of these:

Could not find or load main class

or How to specify main class for console app.

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

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 syntax is this:

java [ <option> ... ] <class-name> [<argument> ...]

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

The fully qualified classname 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 classname is standard Java terminology … not something I just made up to confuse you 🙂

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:
* Search for the compiled version of the com.acme.example.ListUsers class.
* Load the class.
* Check that the class has a main method with signature + modifiers public static void main(String[]).
* 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? Basically, there are two main causes:

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 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

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.

Additional Notes:

  1. 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.

  2. 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.)

The java -jar <jar file> syntax

The alternative syntax used for “executable” JAR files is as follows:

    java [ <option> ... ] -jar <jar-file-name> [<argument> ...]

e.g.

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

In this case 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.

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 to break things. 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 and broken project references or launcher configurations.

Other References

Leave a Reply

Your email address will not be published. Required fields are marked *