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.cmake

@@ -597,7 +597,10 @@ function(add_jar _TARGET_NAME)
             # Create the list of files to compile.
             set(_JAVA_SOURCES_FILE ${CMAKE_JAVA_CLASS_OUTPUT_PATH}/java_sources)
             string(REPLACE ";" "\"\n\"" _JAVA_COMPILE_STRING "\"${_JAVA_COMPILE_FILES}\"")
-            file(WRITE ${_JAVA_SOURCES_FILE} ${_JAVA_COMPILE_STRING})
+            set(CMAKE_CONFIGURABLE_FILE_CONTENT "${_JAVA_COMPILE_STRING}")
+            configure_file("${CMAKE_ROOT}/Modules/CMakeConfigurableFile.in"
+              "${_JAVA_SOURCES_FILE}" @ONLY)
+            unset(CMAKE_CONFIGURABLE_FILE_CONTENT)
             list (APPEND _JAVA_SOURCES_FILELISTS "@${_JAVA_SOURCES_FILE}")
         endif()
         if (_JAVA_COMPILE_FILELISTS)
@@ -610,6 +613,10 @@ function(add_jar _TARGET_NAME)
         add_custom_command(
             # NOTE: this command generates an artificial dependency file
             OUTPUT ${CMAKE_JAVA_CLASS_OUTPUT_PATH}/java_compiled_${_TARGET_NAME}
+            COMMAND ${CMAKE_COMMAND}
+                -DCMAKE_JAVA_CLASS_OUTPUT_PATH=${CMAKE_JAVA_CLASS_OUTPUT_PATH}
+                -DCMAKE_JAR_CLASSES_PREFIX="${CMAKE_JAR_CLASSES_PREFIX}"
+                -P ${CMAKE_CURRENT_FUNCTION_LIST_DIR}/UseJava/ClearClassFiles.cmake
             COMMAND ${Java_JAVAC_EXECUTABLE}
                 ${CMAKE_JAVA_COMPILE_FLAGS}
                 -classpath "${CMAKE_JAVA_INCLUDE_PATH_FINAL}"
@@ -617,7 +624,7 @@ function(add_jar _TARGET_NAME)
                 ${_GENERATE_NATIVE_HEADERS}
                 ${_JAVA_SOURCES_FILELISTS}
             COMMAND ${CMAKE_COMMAND} -E touch ${CMAKE_JAVA_CLASS_OUTPUT_PATH}/java_compiled_${_TARGET_NAME}
-            DEPENDS ${_JAVA_COMPILE_FILES} ${_JAVA_COMPILE_FILELISTS} ${_JAVA_COMPILE_DEPENDS}
+            DEPENDS ${_JAVA_COMPILE_FILES} ${_JAVA_COMPILE_FILELISTS} ${_JAVA_COMPILE_DEPENDS} ${_JAVA_SOURCES_FILE}
             WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
             COMMENT "Building Java objects for ${_TARGET_NAME}.jar"
         )

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()