Fixed Java file extension spelling error and add a test (#2604)

Co-authored-by: nfl <fuli.niu@zealcomm.cn>

kotlin-analysis/src/main/kotlin/org/jetbrains/dokka/analysis/AnalysisEnvironment.kt

@@ -571,7 +571,7 @@ class AnalysisEnvironment(val messageCollector: MessageCollector, val analysisPl
     fun addSources(sourceDirectories: Iterable<File>) {
         sourceDirectories.forEach { directory ->
             configuration.addKotlinSourceRoot(directory.path)
-            if (directory.isDirectory || directory.extension == ".java") {
+            if (directory.isDirectory || directory.extension == "java") {
                 configuration.addJavaSourceRoot(directory)
             }
         }

plugins/base/src/test/kotlin/filter/JavaFileFilterTest.kt

@@ -0,0 +1,37 @@
+package filter
+
+import org.jetbrains.dokka.base.testApi.testRunner.BaseAbstractTest
+import org.junit.jupiter.api.Assertions
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+
+class JavaFileFilterTest : BaseAbstractTest() {
+    @Test
+    fun `java file should be included`() {
+        val configuration = dokkaConfiguration {
+            sourceSets {
+                sourceSet {
+                    skipEmptyPackages = false
+                    sourceRoots = listOf("src/main/java/basic/Test.java")
+                }
+            }
+        }
+
+        testInline(
+            """
+            |/src/main/java/basic/Test.java
+            |package example;
+            |
+            |
+            |
+        """.trimMargin(),
+            configuration
+        ) {
+            preMergeDocumentablesTransformationStage = {
+                Assertions.assertTrue(
+                    it.first().packages.isNotEmpty()
+                )
+            }
+        }
+    }
+}