cmake: Improved portability of OpenCV Java binding search (#2348)

The CMake option OPENCV_JAVA_INSTALL_DIR can be set to the location
to search for the jar file. Note that the file name cannot be
changed, as, after checking several Linux package repositories,
that seems to be constant across all of them.

Fixes #2346.

CMakeLists.txt

@@ -55,6 +55,7 @@ option(USE_VCPKG_LIBUV "Use vcpkg libuv" OFF)
 option(USE_VCPKG_EIGEN "Use vcpkg eigen" OFF)
 option(FLAT_INSTALL_WPILIB "Use a flat install directory" OFF)
 option(WITH_SIMULATION_MODULES "build simulation modules" OFF)
+set(OPENCV_JAVA_INSTALL_DIR "" CACHE PATH "Location to search for the OpenCV jar file")
 
 if (NOT WITHOUT_JAVA AND NOT BUILD_SHARED_LIBS)
     message(FATAL_ERROR "
@@ -64,6 +65,15 @@ FATAL: Cannot build static libs with Java enabled.
 ")
 endif()
 
+if (WITHOUT_JAVA OR WITHOUT_CSCORE)
+    if(NOT "${OPENCV_JAVA_INSTALL_DIR}" STREQUAL "")
+        message(WARNING "
+WARNING: OpenCV Java dir set but java is not enabled!
+It will be ignored.
+")
+    endif()
+endif()
+
 set( wpilib_dest wpilib)
 set( include_dest wpilib/include )
 set( main_lib_dest wpilib/lib )

README-CMAKE.md

@@ -34,6 +34,8 @@ The following build options are available:
   * TODO
 * EXTERNAL_HAL_FILE
   * TODO
+* OPENCV_JAVA_INSTALL_DIR
+  * Set this option to the location of the archive of the OpenCV Java bindings (it should be called opencv-xxx.jar, with the x'es being version numbers). NOTE: set it to the LOCATION of the file, not the file itself!
 
 ## Build Setup
 

cscore/CMakeLists.txt

@@ -69,7 +69,9 @@ if (NOT WITHOUT_JAVA)
 
     #find java files, copy them locally
 
-    set(OPENCV_JAVA_INSTALL_DIR ${OpenCV_INSTALL_PATH}/share/OpenCV/java/)
+    if("${OPENCV_JAVA_INSTALL_DIR}" STREQUAL "")
+        set(OPENCV_JAVA_INSTALL_DIR ${OpenCV_INSTALL_PATH}/share/OpenCV/java/)
+    endif()
 
     find_file(OPENCV_JAR_FILE NAMES opencv-${OpenCV_VERSION_MAJOR}${OpenCV_VERSION_MINOR}${OpenCV_VERSION_PATCH}.jar PATHS ${OPENCV_JAVA_INSTALL_DIR} ${OpenCV_INSTALL_PATH}/bin NO_DEFAULT_PATH)
     find_file(OPENCV_JNI_FILE NAMES libopencv_java${OpenCV_VERSION_MAJOR}${OpenCV_VERSION_MINOR}${OpenCV_VERSION_PATCH}.so