Issue with `Parameter` of type array with generics wildcard vs raw type

[NicoKiaru]

Hi,

I noticed a strange behaviour with scijava parameters. It looks like the handling of array parameters are very different if they contain a generics wildcard or not:

Let's imagine a class:

public class MyClass<T> {

}

And a scijava converter from String to MyClass<?>[] (not sure the wildcard changes anything if present in the converter)

This:

@Parameter
MyClass[] myobjects;

behaves correctly with regards to the scijava mechanism. The correct converter is called.

However this:

@Parameter
MyClass<?>[] myobjects;

completely fails : in the test case I have, the scijava mechanism attempts to put a File[] object into myobjects, obviously failing.

I narrowed down the issue to the CommandModuleItem class. The line

scijava-common/src/main/java/org/scijava/command/CommandModuleItem.java

Line 76 in afe795a

final Class<?> type = Types.raw(Types.fieldType(field, getDelegateClass()));

returns the type of the parameter. In case the Parameter has no generics wildcard, then the returned type is [L MyClass, while if it has a generics widlcard, this same line returns [L Object, wrongfully allowing irrelevant converters (thus the one from String to File array).

I have not made a minimal example of this issue, but one test in bigdataviewer-playground can be used to reproduce the issue : https://github.com/bigdataviewer/bigdataviewer-playground/blob/fixes-rawtype/src/test/src/sc/fiji/bdvpg/scijava/ScijavaShowDemo.java.

The demo fails. But if you remove the wildcard in the line https://github.com/bigdataviewer/bigdataviewer-playground/blob/daab0a336831b89c44d13932c243dd41e493dbe5/src/main/java/sc/fiji/bdvpg/scijava/command/bdv/BdvSourcesAdderCommand.java#L49:

SourceAndConverter[] sacs;

Then everything works.

How can this be solved ? I'm also open to suggestions on my code. I tend to fear that modifying scijava common is risky. So if there's a solution on bigdataviewer-playground side I would rather do it. I can think of 3 options:

• keeping raws type in the parameters : this creates annoying issues when converting to List + we are not supposed to use raw types in Java as far as I understood
• making a container class : SourceAndConverterList which would hold a single field List<SourceAndConverter>. This seems the most easy thing to do a provides a lot of control on the scijava side - but it's a bit inelegant
• managing to make a working List<SourceAndConverter<?>> scijava parameter, but I have no idea if this is possible. To me, because the generic types are erased at runtime, this looks impossible, but is it really ?

Here's some maybe related issues:

#172

#118

[frauzufall]

Hi @NicoKiaru, I don't really have an answer, but I tried to make a minimal example. It prints the field type and the output of Types.raw() for arrays and lists, typed and raw, of class fields. The output for lists looks fine so I guess the issue with the typed List parameters is somewhere else, maybe you can describe more precisely what's going wrong with typed List parameters.
Regarding arrays - not sure if they are supposed to work as parameters. @ctrueden @imagejan

import org.scijava.util.Types;
import java.lang.reflect.Type;
import java.util.List;

public class FieldTest {

	public static void main(String...args) throws NoSuchFieldException {
		printFieldTypes("listTyped");
		printFieldTypes("listRaw");
		printFieldTypes("arrayTyped");
		printFieldTypes("arrayRaw");
	}

	private static void printFieldTypes(String varName) throws NoSuchFieldException {
		Type type = Types.fieldType(MyCommand.class.getField(varName), MyCommand.class);
		System.out.println(varName + ": " + type + " " + Types.raw(type));
	}
 
	public class MyClass<T> {}
	public class MyCommand {
		public List<MyClass<?>> listTyped;
		public List<MyClass> listRaw;
		public MyClass<?>[] arrayTyped;
		public MyClass[] arrayRaw;
	}

}

This is the output:

listTyped: java.util.List<FieldTest.MyClass<?>> interface java.util.List
listRaw: java.util.List<FieldTest.MyClass<T>> interface java.util.List
arrayTyped: FieldTest.MyClass<?>[] class [Ljava.lang.Object;
arrayRaw: class [LFieldTest$MyClass; class [LFieldTest$MyClass;

[ctrueden]

I would prefer to fix this on the SJC side, but I won't have time to work on it for at least a few weeks. But I appreciate the detailed bug report and analysis, @NicoKiaru! Thank you. And thank you @frauzufall for attempting to create an MCVE!

[NicoKiaru]

I've added a (currently failing) test which reproduces the error in the PR #416. I'm not sure it can be more minimal. Also I noticed that I can overcome this issue by assigning a higher priority to the converter I'd like to use, but still, scijava should not try to put a File array into another sort of array, so it's still worth investigating.

[imagejan]

I tried to add a really minimal test here: #417

It seems the issue is in the private implementation of GenericTypeReflector (forked from GenTyRef), I wonder whether it might have been fixed upstream (or in GeAnTyRef?) already?

[NicoKiaru]

There are some weird things happening with arrays and generics. I can't find it again, but there was an explanation of the reason why an object of class MyClass<?>[] had to be considered as Object[] at runtime. This link may be a starting point https://www.oreilly.com/library/view/learning-java-4th/9781449372477/ch08s10.html, as well as the article mentioned by Curtis in gitter : https://www.ibm.com/developerworks/java/library/j-jtp01255/index.html

[imagejan]

I improved the test and updated #417 with a fix to Types.raw().

That fix also makes your test pass, so I rebased #416 onto my PR to see if it passes on Travis as well.

[hinerm]

Closed in f6d8749