Gradle plugin warning after migrating to Gradle 4.0

[czyzby]

Flyway 4.2.0, Gradle 4.0, Postgres 9.6.

All Flyway tasks work as expected from what I can tell, but they generate the following warning on each run:

> Task :db:flywayMigrate
Gradle now uses separate output directories for each JVM language, but this build assumes a single directory for all classes from a source set. This behaviour has been deprecated and is scheduled to be removed in Gradle 5.0

This seems to be an issue with current Gradle plugin implementation.

[sgrimm-sg]

I believe the issue is that Gradle 4 wants plugins to call SourceSetOutput.getClassesDirs() instead of getClassesDir(), since a source set can now have multiple classes dirs.

A naive "fix" is probably as simple as

--- a/flyway-gradle-plugin/src/main/java/org/flywaydb/gradle/task/AbstractFlywayTask.java
+++ b/flyway-gradle-plugin/src/main/java/org/flywaydb/gradle/task/AbstractFlywayTask.java
@@ -303,9 +303,11 @@ public abstract class AbstractFlywayTask extends DefaultTask {
                 JavaPluginConvention plugin = getProject().getConvention().getPlugin(JavaPluginConvention.class);
 
                 for (SourceSet sourceSet : plugin.getSourceSets()) {
-                    URL classesUrl = sourceSet.getOutput().getClassesDir().toURI().toURL();
-                    getLogger().debug("Adding directory to Classpath: " + classesUrl);
-                    extraURLs.add(classesUrl);
+                    for (File classesDir : sourceSet.getOutput().getClassesDirs().getFiles()) {
+                        URL classesUrl = classesDir.toURI().toURL();
+                        getLogger().debug("Adding directory to Classpath: " + classesUrl);
+                        extraURLs.add(classesUrl);
+                    }
 
                     URL resourcesUrl = sourceSet.getOutput().getResourcesDir().toURI().toURL();
                     getLogger().debug("Adding directory to Classpath: " + resourcesUrl);

but this would cause the plugin to stop working with pre-4.0 Gradle versions, and would require the plugin to be built against Gradle 4.x libraries. You could get around that by trying to dynamically invoke the method via reflection; I'm not sure if that's the approach Flyway's maintainers would want to take here.

[type11]

I have gone in and modified things to be ready for gradle 5 but as mentioned then this breaks everything on pre gradle 4.x. Unfortunately we still need to use gradle 3.x for some things. I get I can use different wrappers for different projects but individual developers are also a consideration not just the build system. Is there no good way to change this for gradle 5 and have it work in gradle 3.x?

[axelfontaine]

With Flyway 5.0.0 the new minimum supported Gradle version is 3.0. We can't move to 4.0 yet as it is still too recent. If someone wants to contribute a pull request (probably using some reflection) to bridge the gap between Gradle 3.0 and 4.0+, I would be more than glad to merge it.

[smoothreggae]

@axelfontaine, I believe I have something that might work for both Gradle 3.x and Gradle 4.x. I was able to test these changes locally with simple projects using Gradle 3.x/4.x based on https://flywaydb.org/getstarted/firststeps/gradle.