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.bate...@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
