UseJava: clean class files from previous builds

If java_class_filelist exists, remove previous class files before building the
new ones. If java_sources content has changed, clean and rebuild the class
files of the target.

Fixes: #20377

Modules/UseJava/ClearClassFiles.cmake

@@ -0,0 +1,17 @@
+# Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+# This script deletes compiled Java class files.
+
+if(CMAKE_JAVA_CLASS_OUTPUT_PATH)
+  if(EXISTS "${CMAKE_JAVA_CLASS_OUTPUT_PATH}/java_class_filelist")
+    file(STRINGS "${CMAKE_JAVA_CLASS_OUTPUT_PATH}/java_class_filelist" classes)
+    list(TRANSFORM classes PREPEND "${CMAKE_JAVA_CLASS_OUTPUT_PATH}/")
+    if(classes)
+      file(REMOVE ${classes})
+      message(STATUS "Clean class files from previous build")
+    endif()
+  endif()
+else()
+  message(FATAL_ERROR "Can't find CMAKE_JAVA_CLASS_OUTPUT_PATH")
+endif()