Fix KSP not working with only Java files

Author: tschuchortdev

core/src/main/kotlin/com/tschuchort/compiletesting/AbstractKotlinCompilation.kt

@@ -162,13 +162,17 @@ abstract class AbstractKotlinCompilation<A : CommonCompilerArguments> internal c
             )
         )
 
-        // if no Kotlin sources are available, skip the compileKotlin step
-        if (sources.none(File::hasKotlinFileExtension))
-            return KotlinCompilation.ExitCode.OK
-
         // in this step also include source files generated by kapt in the previous step
         val args = arguments.also { args ->
-            args.freeArgs = sources.map(File::getAbsolutePath).distinct()
+            args.freeArgs =
+                sources.map(File::getAbsolutePath).distinct() + if (sources.none(File::hasKotlinFileExtension)) {
+                    /* __HACK__: The Kotlin compiler expects at least one Kotlin source file or it will crash,
+                       so we trick the compiler by just including an empty .kt-File. We need the compiler to run
+                       even if there are no Kotlin files because some compiler plugins may also process Java files. */
+                    listOf(SourceFile.new("emptyKotlinFile.kt", "").writeIfNeeded(sourcesDir).absolutePath)
+                } else {
+                    emptyList()
+                }
             args.pluginClasspaths = (args.pluginClasspaths ?: emptyArray()) + arrayOf(getResourcesPath())
         }
 

ksp/src/test/kotlin/com/tschuchort/compiletesting/KspTest.kt

@@ -51,6 +51,25 @@ class KspTest {
         }
     }
 
+    @Test
+    fun allProcessorMethodsAreCalledWhenOnlyJavaFilesArePresent() {
+        val instance = mock<SymbolProcessor>()
+        val providerInstance = mock<SymbolProcessorProvider>()
+        `when`(providerInstance.create(any(), any(), any(), any())).thenReturn(instance)
+        val result = KotlinCompilation().apply {
+            sources = listOf(DUMMY_JAVA_SRC)
+            symbolProcessorProviders = listOf(providerInstance)
+        }.compile()
+        assertThat(result.exitCode).isEqualTo(ExitCode.OK)
+        providerInstance.inOrder {
+            verify().create(any(), any(), any(), any())
+        }
+        instance.inOrder {
+            verify().process(any())
+            verify().finish()
+        }
+    }
+
     @Test
     fun processorGeneratedCodeIsVisible() {
         val annotation = SourceFile.kotlin(
@@ -308,5 +327,11 @@ class KspTest {
             class Dummy {}
         """.trimIndent()
         )
+
+        private val DUMMY_JAVA_SRC = SourceFile.java(
+            "foo.bar.DummyJava.java", """
+            class DummyJava {}
+        """.trimIndent()
+        )
     }
 }