False positive when making a package-private class public that already has a public static method's different return type

[garydgregory]

I don't see how this breaks binary compatibility:

• Define a package private class C with a public static method M that returns interface F, release that as version N
• For version N+1:
  • Change the return type of M to a concrete implementation of F (the kind return type change should not matter)
  • Run JApiCmp and all is well, obviously, since this C is a package-private class
  • Change the visibility of C from package-private to public
  • JApiCmp now fails.

The failure does not make sense since no one outside the package could call C in version N.
For example:

• git clone -n https://gitbox.apache.org/repos/asf/commons-io.git
• git checkout 877b9e3f5e553f8ad988b89b8adb6b93fa97a964
• Make the class org.apache.commons.io.StreamIterator public
• Run mvn clean install -DskipTests japicmp:cmp -Dcommons.japicmp.version=0.18.1

[ERROR] Failed to execute goal com.github.siom79.japicmp:japicmp-maven-plugin:0.18.1:cmp (default-cli) on project commons-io: There is at least one incompatibility: org.apache.commons.io.filefilter.PathMatcherFileFilter:METHOD_ABSTRACT_ADDED_IN_IMPLEMENTED_INTERFACE,org.apache.commons.io.function.Uncheck.get(org.apache.commons.io.function.IOSupplier,java.util.function.Supplier):CLASS_GENERIC_TEMPLATE_CHANGED,org.apache.commons.io.StreamIterator.iterator(java.util.stream.Stream):METHOD_RETURN_TYPE_CHANGED -> [Help 1]

[siom79]

Changing the return type from an interface to a concrete implementation causes the incompatibility METHOD_RETURN_TYPE_CHANGED as now source code assigning the return value to another implementation of the interface won't work any longer.
This is already detected in the first run of japicmp, but the class is package protected and therefore filtered.
If you change the configuration of japicmp to package_protected, you will see the change in the first run:

<plugin>
        <groupId>com.github.siom79.japicmp</groupId>
        <artifactId>japicmp-maven-plugin</artifactId>
        <configuration>
          <parameter>
            <accessModifier>package_protected</accessModifier>
          </parameter>
        </configuration>
      </plugin>

PS: The documentation is currently wrong, as the option is named there protected and not package_protected.

[garydgregory]

Uh? Please help me understand: If a released version of a library contains a package private class, then that class is NOT accessible to the outside world. So how can there be a break in compatibility for clients in the next version when that class is made public, no matter what changes?

[siom79]

You are right. When the class changes to public, all incompatibilities should be ignored.

[garydgregory]

You are right. When the class changes to public, all incompatibilities should be ignored.

Ah, thank you 😄 😄

[siom79]

Fixed with this commit.