Java cannot find main class even though main class is present

I've been trying to run a jar but I keep getting the error:

Error: Could not find or load main class com.myclass
Caused by: java.lang.ClassNotFoundException: com.myclass

This is very confusing because I extracted the jar and the class was present in the proper location. This is how I compiled the jar using gradle:

jar {
  manifest {
    attributes 'Main-Class': 'com.myclass'
  }

  from {
    configurations.compile.collect { it.isDirectory() ? it : zipTree(it) }
  }
}

I believe it is a problem with java since the jar file seems to be correct. Does anyone know how I could fix this?

Your question does not include any details that would directly indicate what you've done wrong.

But I can tell you the general principle:

Unpack that jar (it's a zip file, so any unzipper will do. Alternatively, jar xvf myjar.jar in a freshly created directory will also do the job).

In it, there should be at least 2 files, at these exact locations:

• com/foo/MyClass.class
• META-INF/MANIFEST.MF

Then, open the manifest file with notepad or cat it from the terminal. It should contain this line, again, exactly:

• Main-Class: com.foo.MyClass

If these things are all true, and yet the error you reported in this question is occurring, we've arrived at truly exotic stuff, such as corrupt VMs or corrupt class files. More likely you're misreading something, double check the names.

NB: If you forget to add a properly signatured 'main' method to your class, the error is different, hence, that isn't it.

For what it is worth, this section:

   from {
     configurations.compile.collect { it.isDirectory() ? it : zipTree(it) }
   }

looks non-standard. Is there any particular thing you're trying to make your gradle build file do that isn't already built into the defaults? In which case, you may be doing that wrong; you may want to ask another SO question specifically about that, and include this part (in particular, your findings about what's in that manifest and where the class file is actually inside your jar file) as a footnote to that question.

Packages are mandatory. Things in the unnamed package (which is what happens when classes don't have a package statement) effectively are still in a package, but it is a package you can't type. The only exception is when running java -cp classpath;here myclass which by way of exception does work. The point is, it doesn't work in any manifest attribute, you can't import them at all, and they don't work in module-info files (i.e., everywhere else class names come up, classes in the unnamed package simply cannot be referred to).

So, add a package statement and move things in subdirs that match this.

Usually compiler throws this error with the package name as well, when you are using Eclipse IDE.
Try packing the class with main method inside the specific package.