Main class in jar

[Gregg Wonderly]

Will java -jar note that the argument is a module and help the user understand how to invoke it, or will it just complain about a missing main-class: attribute?  From a practical perspective, why does it matter and demand a different command line?  What happens when you double click on a module?

Gregg

Sent from my iPad

> On Mar 5, 2018, at 4:25 AM, Alan Bateman <Alan.Bateman at oracle.com> wrote:
> 
>> On 05/03/2018 09:54, Bernard Amade wrote:
>> Hello all
>> the doc in https://docs.oracle.com/javase/9/docs/specs/jar/jar.html
>> lists for Main-Class entry in jars:
>> "attribute defined for stand-alone applications: This attribute is used by stand-alone applications that are bundled into executable jar files which can be invoked by the java runtime directly by running "java -jar x.jar".
>> apparently it means that you should not create a modular jar with a manifest template containing a Main-Class entry
>> (apparently if you run the app with command "java -module-path dir -m com.thing.app" with the generated com.thing.app.jar the main class will not be found : you get a message saying you've got no Mainclass atrribute)
>> but if you generate the jar with a manifest template with no Main-Class entry and instead use the --main-class option then the main class will be found.
>> funnily the MANIFEST-MF will then have the same Main-Class entry!
>> so what is the difference?
> The error message with JDK 9 and JDK 10 has a typo which may be causing confusion here. The typo has been fixed for JDK 11 via JDK-8193819 to correctly say "ModuleMainClass" attribute. Note that this is a class file attribute, not an attribute that you put in the main manifest of a JAR file. The `jar` tool will add both when you specify the main class with `--main-class` or `-e`.
> 
> To summarize:
> 
> 1. `jar -jar app.jar` is the equivalent of `java -cp app.jar <mainclass>` where <mainclass> is value of the Main-Class attribute in the main manifest of the JAR file.
> 
> 2. `java [--module-path <path>] -m <app>` is the way to run a modular application on the command line. The app module must have a ModuleMainClass class file attribute, something the `jar` tool will do for you (Maven plugins and IDEs should do the same but might not be all there yet).  The alternative is `-m app/<mainclass>` which you specify both the initial/main module and also its main class.
> 
> 3.  There is no support for executing modular applications with `java -jar`.
> 
> -Alan