How does one set up a java program that can be run as a module simply by doing java -m mymodule?
I know this is possible because I have seen it in, e.g., the jwebserver documentation; you can run this by doing java -m jdk.httpserver. However there doesn’t seem to be any evidence in the jdk source code about specifying the ‘main’ class of the module at any point, neither in the module-info.java file, nor in any of the main-containing classes of the module (of which there are two, not one).
How does the module know which main-containing class to pick as the main entry point for running the module? Is there some sort of convention that’s not well documented? (e.g. if a package in the module contains a class called Main.java, is this special in any way? and should there only be one Main.java file over all packages in the module?)
In https://dev.java/learn/modules/intro/ there’s an example of a module run without specifying the main class in the commandline, but it just says:
To launch a modular app, run the java command with a module path and a so-called initial module – the module that contains the main method
but without clarifying how this main method (and its respective class/package) is detected or specified for the module as the specific package/class to be used as the main entry point; and there’s no requirement for a module’s packages to not have more than one such main-containing class.
Please note the question is not about .jar files or specifying a main class for a jar file containing a module, but for modules themselves. (https://dev.java/learn/modules/building/ has good information for specifying main classes for jar files with modules, but this is not what I’m asking here).
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Answering own question. I found the explanation here: https://youtu.be/C5yX-elG4w0?feature=shared&t=1857
There are two ways to create something that can be launched as java -m <modulename> without specifying the main class (i.e. without having to do java -m <modulename>/<mainclass> explicitly:
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The first is to create a modular jar file, where the –main-class is specified; this allows you then to run the module as
java -p <jarfile>.jar -m <modulename>syntax (as an alternative tojava -jar <jarfile>.jar).However, if one inspects the default jre, there is no
jdk.httpserver.jarfile or similar to be found, nor are we loading one onto the module path. So this is not what is happening in the example in the question. -
The second is to create your own jre environment (i.e. resulting in a ‘java’ command) by using jlink. This allows you to ‘bake’ the modules right into your runtime environment (i.e.
java --list-moduleswill list<modulename>as one of the modules contained in the jre itself).Modules can be provided to
jlinkin two ways: as normal compiled module root folders in the module path, or as .jmod files. In the latter case, one can additionally have the ‘mainclass’ baked into the module-info.class file contained in the .jmod file, by passing the--main-classoption to thejmodcommand during creation of the.jmodfile.When this is the case, running
java -m <modulename>via your new jre will indeed run the module baked into the jre, without that module necessarily appearing anywhere in the generated jre directory structure (not even as a .jmod file).
Since the jdk.httpserver is baked into the default jre environment (which can be confirmed by doing java --list-modules), this allows java -m jdk.httpserver to run directly. The implication here is that the java command itself was generated via a jdk.httpserver.jmod file, which specified the mainclass. We can confirm this is indeed the case by running jmod describe jdk.httpserver.jmod, which yields (among other lines) the line:
main-class sun.net.httpserver.simpleserver.Main