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It's been a while since I do Java programming and I am surprised to come back to it with the entire landscape being foreign to me post project jig-saw.

I have trouble using Eclipse (2018-09, 4.9.0) standard Java project with mixed modular and non-modular environment. Specifically, I am attempting to combine JavaFX 11 (modularized) and Apache POI 4.1 (non-modularized) using Eclipse platform (base Java project without Gradle or Maven).

In my module-info.java I have the following,

module myapp {
        requires javafx.base;
        requires javafx.graphics;
        requires javafx.fxml;
        requires javafx.controls;
        requires javafx.web;

        exports myapp.gui;

        opens myapp.gui to javafx.fxml;
}

I find that wherever I have the Apache POI code I get the following error in Eclipse

The import cannot be resolved

Adding the following in the module-info.java using the automatic module created for Apache POI like so, 

        requires poi;

Produces a warning in Eclipse indicating automatic module is not stable which seems to be recognized but continue to produce the cannot be resolved error.

I have also tried putting the main POI jar file in class path as oppose to module-path with no avail.

The code involving Apache POI in separation from the UI works. I simply have to remove the use of module-info.java which I assume puts the project in some sort of legacy development mode sans modularization?

Can someone give me a pointer as to what I am doing wrong and guide me to setup a project with mixed modularized and non-modularized libraries?

Thanks in advance.

adizen
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2 Answers2

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Your code is encapsulated in a module if you have a module-info.java file in the default package. To use something of a JAR inside a module,

  • the JAR must be on the Modulepath and
  • the module must be explicitly declared as required in the module-info.java: requires <module_name>;.

Otherwise, import statements can not be resolved. Nothing of the items on the Classpath is accessible from a module; and nothing on the Modulepath without a requires ... in module-info.java can be used in a module. Therefore, in your case, it wouldn't work without requires poi; (and without the POI library on the Modulepath).

The "Name of automatic module '...' is unstable" warning is only a hint, that the POI JAR has not built as a modul, but used as a module. Actually this means that if you rename the JAR file, the module name can also change and you have to adapt the module-info.java file. Technically, the POI JAR contains neither a module-info.class file nor a META-INF/MANIFEST.MF file with the line Automatic-Module-Name: .... So the module name poi is derived from the file name of the JAR. The "Name of automatic module '...' is unstable" warning can be turned off in Project > Properties: Java Compiler > Errors/Warnings in the Module section.

To sum it up, you do not have mixed modular and non-modular dependencies. Instead, legacy JARs automatically become modules by deriving the module name from the file name. In this case, Eclipse warns you by default that the application can be broken by simply changing the file name of that JAR.

howlger
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  • Thank you for your reply, so if even then Eclipse still fail to compile the code does that mean all signs points to problem in Eclipse? – adizen Apr 11 '19 at 19:32
  • @adizen Nope. I've rewritten my answer. I hope it's clearer now. – howlger Apr 11 '19 at 21:21
  • @howlger I had the same error here with the Java Mail activation.jar file. Thanks for complete answer. – Adalberto José Brasaca Aug 04 '20 at 11:55
  • Your second paragraph after the bullets above comports with [the docs](https://docs.oracle.com/javase/specs/jls/se11/html/jls-7.html#jls-7.7.1). I understand the warnings, but how can I reference via `requires` a non-module package from module-info? Error is _com.package cannot be resolved to a module_. – dudeNumber4 Jan 12 '23 at 17:43
  • @dudeNumber4 When the JAR does not contain `module-info.java` and no `META-INF/MANIFEST.MF` with `Automatic-Module-Name: ...`, it depends on the file name of the JAR. To be used as module, the JAR must not contain classes in the default package or packages that are also used in the system library or other modules. – howlger Jan 13 '23 at 08:40
  • In the end, I got the project to build (with warning) just referenced the non-modular jar by package name, not jar name. I thought I cleaned the previous time, but in any case I no longer saw the error I mentioned above. – dudeNumber4 Jan 13 '23 at 19:26
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So, as it turned out. I did goofed. There are 5 inter dependent jar file from the POI library which has caused the error "The import cannot be resolved"

Once I included all 5 files in the module-info.java the project compiled.

        requires poi.excelant;
        requires poi.ooxml;
        requires poi.ooxml.schemas;
        requires poi.scratchpad;
        requires poi;

I am sure not all of them is necessary, but I included all of them just in case anyways.

Thanks everyone for reading and offering affirmation that I have not done anything to break Eclipse.

adizen
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