Make java library earlier on classpath (#1797)

* Make java library earlier on classpath

Co-Authored-By: Farid Zakaria <farid.m.zakaria@gmail.com>

* work around strict deps check
* add additional build_tag_filters
* another edit to the filters
* code review feedback

---------

Co-authored-by: Farid Zakaria <farid.m.zakaria@gmail.com>

Author: vinnybod

scala/BUILD

@@ -27,6 +27,11 @@ scala_toolchain(
     name = "minimal_direct_source_deps_impl",
     dependency_mode = "plus-one",
     dependency_tracking_method = "ast",
+    # Exclude the shadowing test package from strict-deps to allow classpath-order coverage without disabling strict-deps globally in CI runs.
+    dependency_tracking_strict_deps_patterns = [
+        "",  # keep default include
+        "-//test/src/main/scala/scalarules/test/java_classpath_order",
+    ],
     strict_deps_mode = "error",
     unused_dependency_checker_mode = "error",
 )

scala/private/phases/phase_compile.bzl

@@ -251,10 +251,10 @@ def _compile_or_empty(
 
         full_jars = [ctx.outputs.jar]
         if java_jar:
-            full_jars.append(java_jar.jar)
+            full_jars.insert(0, java_jar.jar)
 
         if java_jar:
-            merged_provider = java_common.merge([scala_compilation_provider, java_jar.java_compilation_provider])
+            merged_provider = java_common.merge([java_jar.java_compilation_provider, scala_compilation_provider])
         else:
             merged_provider = scala_compilation_provider
 

test/src/main/scala/scalarules/test/java_classpath_order/BUILD

@@ -0,0 +1,27 @@
+load("//scala:scala.bzl", "scala_library", "scala_test")
+
+# Original library with the class to be shadowed
+scala_library(
+    name = "original",
+    srcs = ["Original.scala"],
+)
+
+# Library that shadows the Overridable class with a Java file
+# The Java file (Overridable.java) should take precedence over
+# the Overridable class from the :original dependency
+scala_library(
+    name = "with_java_override",
+    srcs = ["Overridable.java"],
+    deps = [":original"],
+)
+
+# Test that verifies the Java override works correctly
+# Note: This test is incompatible with strict deps checking because the checker
+# doesn't understand class shadowing - it sees Overridable as coming from :original
+# rather than the shadowed version from :with_java_override
+scala_test(
+    name = "verify_override_test",
+    srcs = ["VerifyOverride.scala"],
+    unused_dependency_checker_mode = "off",
+    deps = [":with_java_override"],
+)

test/src/main/scala/scalarules/test/java_classpath_order/Original.scala

@@ -0,0 +1,6 @@
+package scalarules.test.java_classpath_order
+
+class Overridable {
+  def getValue(): String = "original"
+}
+

test/src/main/scala/scalarules/test/java_classpath_order/Overridable.java

@@ -0,0 +1,8 @@
+package scalarules.test.java_classpath_order;
+
+public class Overridable {
+  public String getValue() {
+    return "overridden";
+  }
+}
+

test/src/main/scala/scalarules/test/java_classpath_order/VerifyOverride.scala

@@ -0,0 +1,25 @@
+package scalarules.test.java_classpath_order
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+/**
+ * This test verifies that the Java class (Overridable) shadows
+ * the Scala class from the dependency.
+ *
+ * The expected behavior is:
+ * - The Java file compiled in the scala_library with_java_override should
+ *   appear first on the classpath
+ * - When instantiating Overridable, we should get the Java version
+ *   which returns "overridden"
+ */
+class VerifyOverrideTest extends AnyFlatSpec with Matchers {
+
+  "Java class in scala_library" should "shadow the dependency's class" in {
+    val instance = new Overridable()
+    val value = instance.getValue()
+
+    value shouldBe "overridden"
+  }
+}
+