Note Book : OpenCV v4.5.0-dev On Ubuntu 20.04 But Didn't work cmake build for Java

####Conclusion
1. It's not OpenCV's fault, It's Ubuntu fault.
2. If OpenCV with cuda, It's needed gcc and g++ version 8 under.
 
I found it. But I can't found for Apache Ant.
https://bugs.launchpad.net/ubuntu/+source/python3-defaults/+bug/1876957
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1633839

I put it.
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1902570

####Reason
1. Because the Apache Ant symbolic is broken, link installed by apt-get.
2. It' working. I use export CC and CXX.
  ・export CC=gcc-8
  ・export CXX=g++-8


####Contents
I wanna set up something like this against OpenCV 4.5.0-dev, but I can't build it on ubuntu 20.04 LTS.　Because the symbolic link in Ant is broken.
I'm using the apt-get command to get Apache Ant. But the /bin/ant command does not work. It's a throw ClassNotFoundException.

```shell
$ cat /etc/lsb-release
DISTRIB_ID=Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.1 LTS"
```

It's not work, '/bin/ant'.
![image.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/732297/d07715ba-4839-0b86-4d74-2e046d024186.png)

The symbolic link is a ...?? What's this? 
/usr/bin/ant -> ../share/ant/bin/ant
/bin/ant -> ../share/ant/bin/ant

![image.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/732297/d8891c35-d985-1fbd-79d9-6146f35a7c63.png)
This '/bin/ant -> ../share/ant/bin/ant' is a '/bin/share/ant/bin/ant'. It's nothing the Path.

I delete /bin/ant, And create new symbolic link. But I think upside down, /bin/share/ant/bin/ant is true. Because /bin/share has a higher priority more than /usr/bin/share.


```shell
sudo rm -fr /bin/ant
sudo ln -s /usr/share/ant/bin/ant /bin/ant
```

####What I want to do.

I wanna doing samething liks like this configuration.

```text:build
-- General configuration for OpenCV 4.5.0-dev =====================================
--   Version control:               4.5.0-76-g691c3d1e3c
--
--   Extra modules:
--     Location (extra):            /usr/local/src/opencv_contrib/modules
--     Version control (extra):     4.5.0-13-gb65b3cc3
--
--   Platform:
--     Timestamp:                   2020-11-01T15:56:15Z
--     Host:                        Linux 5.4.0-52-generic x86_64
--     CMake:                       3.16.3
--     CMake generator:             Unix Makefiles
--     CMake build tool:            /usr/bin/make
--     Configuration:               RELEASE
--
--   CPU/HW features:
--     Baseline:                    SSE SSE2 SSE3
--       requested:                 SSE3
--     Dispatched code generation:  SSE4_1 SSE4_2 FP16 AVX AVX2 AVX512_SKX
--       requested:                 SSE4_1 SSE4_2 AVX FP16 AVX2 AVX512_SKX
--       SSE4_1 (17 files):         + SSSE3 SSE4_1
--       SSE4_2 (2 files):          + SSSE3 SSE4_1 POPCNT SSE4_2
--       FP16 (1 files):            + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 AVX
--       AVX (5 files):             + SSSE3 SSE4_1 POPCNT SSE4_2 AVX
--       AVX2 (31 files):           + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 FMA3 AVX AVX2
--       AVX512_SKX (7 files):      + SSSE3 SSE4_1 POPCNT SSE4_2 FP16 FMA3 AVX AVX2 AVX_512F AVX512_COMMON AVX512_SKX
--
--   C/C++:
--     Built as dynamic libs?:      YES
--     C++ standard:                11
--     C++ Compiler:                /usr/bin/g++-8  (ver 8.4.0)
--     C++ flags (Release):         -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wsuggest-override -Wno-delete-non-virtual-dtor -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -fvisibility-inlines-hidden -O3 -DNDEBUG  -DNDEBUG
--     C++ flags (Debug):           -fsigned-char -W -Wall -Werror=return-type -Werror=non-virtual-dtor -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wundef -Winit-self -Wpointer-arith -Wshadow -Wsign-promo -Wuninitialized -Winit-self -Wsuggest-override -Wno-delete-non-virtual-dtor -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -fvisibility-inlines-hidden -g  -O0 -DDEBUG -D_DEBUG
--     C Compiler:                  /usr/bin/gcc-8
--     C flags (Release):           -fsigned-char -W -Wall -Werror=return-type -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -O3 -DNDEBUG  -DNDEBUG
--     C flags (Debug):             -fsigned-char -W -Wall -Werror=return-type -Werror=address -Werror=sequence-point -Wformat -Werror=format-security -Wmissing-declarations -Wmissing-prototypes -Wstrict-prototypes -Wundef -Winit-self -Wpointer-arith -Wshadow -Wuninitialized -Winit-self -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -Wno-long-long -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections  -msse -msse2 -msse3 -fvisibility=hidden -g  -O0 -DDEBUG -D_DEBUG
--     Linker flags (Release):      -Wl,--exclude-libs,libippicv.a -Wl,--exclude-libs,libippiw.a   -Wl,--gc-sections -Wl,--as-needed
--     Linker flags (Debug):        -Wl,--exclude-libs,libippicv.a -Wl,--exclude-libs,libippiw.a   -Wl,--gc-sections -Wl,--as-needed
--     ccache:                      NO
--     Precompiled headers:         NO
--     Extra dependencies:          m pthread cudart_static -lpthread dl rt nppc nppial nppicc nppicom nppidei nppif nppig nppim nppist nppisu nppitc npps cublas cudnn cufft -L/usr/lib/x86_64-linux-gnu
--     3rdparty dependencies:
--
--   OpenCV modules:
--     To be built:                 alphamat aruco bgsegm bioinspired calib3d ccalib core cudaarithm cudabgsegm cudacodec cudafeatures2d cudafilters cudaimgproc cudalegacy cudaobjdetect cudaoptflow cudastereo cudawarping cudev datasets dnn dnn_objdetect dnn_superres dpm face features2d flann freetype fuzzy gapi hdf hfs highgui img_hash imgcodecs imgproc intensity_transform java line_descriptor mcc ml objdetect optflow phase_unwrapping photo plot python3 quality rapid reg rgbd saliency shape stereo stitching structured_light superres surface_matching text tracking ts video videoio videostab viz xfeatures2d ximgproc xobjdetect xphoto
--     Disabled:                    world
--     Disabled by dependency:      -
--     Unavailable:                 cnn_3dobj cvv js julia matlab ovis python2 sfm
--     Applications:                tests perf_tests apps
--     Documentation:               NO
--     Non-free algorithms:         NO
--
--   GUI:
--     GTK+:                        YES (ver 3.24.20)
--       GThread :                  YES (ver 2.64.3)
--       GtkGlExt:                  NO
--     VTK support:                 YES (ver 7.1.1)
--
--   Media I/O:
--     ZLib:                        /usr/lib/x86_64-linux-gnu/libz.so (ver 1.2.11)
--     JPEG:                        /usr/lib/x86_64-linux-gnu/libjpeg.so (ver 80)
--     WEBP:                        /usr/lib/x86_64-linux-gnu/libwebp.so (ver encoder: 0x020e)
--     PNG:                         /usr/lib/x86_64-linux-gnu/libpng.so (ver 1.6.37)
--     TIFF:                        /usr/lib/x86_64-linux-gnu/libtiff.so (ver 42 / 4.1.0)
--     JPEG 2000:                   OpenJPEG (ver 2.3.1)
--     OpenEXR:                     /usr/lib/x86_64-linux-gnu/libImath.so /usr/lib/x86_64-linux-gnu/libIlmImf.so /usr/lib/x86_64-linux-gnu/libIex.so /usr/lib/x86_64-linux-gnu/libHalf.so /usr/lib/x86_64-linux-gnu/libIlmThread.so (ver 2_3)
--     HDR:                         YES
--     SUNRASTER:                   YES
--     PXM:                         YES
--     PFM:                         YES
--
--   Video I/O:
--     DC1394:                      NO
--     FFMPEG:                      YES
--       avcodec:                   YES (58.54.100)
--       avformat:                  YES (58.29.100)
--       avutil:                    YES (56.31.100)
--       swscale:                   YES (5.5.100)
--       avresample:                YES (4.0.0)
--     GStreamer:                   NO
--     v4l/v4l2:                    YES (linux/videodev2.h)
--
--   Parallel framework:            pthreads
--
--   Trace:                         YES (with Intel ITT)
--
--   Other third-party libraries:
--     Intel IPP:                   2020.0.0 Gold [2020.0.0]
--            at:                   /usr/local/src/opencv/build/3rdparty/ippicv/ippicv_lnx/icv
--     Intel IPP IW:                sources (2020.0.0)
--               at:                /usr/local/src/opencv/build/3rdparty/ippicv/ippicv_lnx/iw
--     Lapack:                      YES (/usr/lib/x86_64-linux-gnu/liblapack.so /usr/lib/x86_64-linux-gnu/libcblas.so /usr/lib/x86_64-linux-gnu/libatlas.so)
--     Eigen:                       YES (ver 3.3.7)
--     Custom HAL:                  NO
--     Protobuf:                    build (3.5.1)
--
--   NVIDIA CUDA:                   YES (ver 10.1, CUFFT CUBLAS FAST_MATH)
--     NVIDIA GPU arch:             30 35 37 50 52 60 61 70 75
--     NVIDIA PTX archs:
--
--   cuDNN:                         YES (ver 7.6.5)
--
--   OpenCL:                        YES (no extra features)
--     Include path:                /usr/local/src/opencv/3rdparty/include/opencl/1.2
--     Link libraries:              Dynamic load
--
--   Python 3:
--     Interpreter:                 /usr/bin/python3 (ver 3.8.5)
--     Libraries:                   /usr/lib/x86_64-linux-gnu/libpython3.8.so (ver 3.8.5)
--     numpy:                       /usr/lib/python3/dist-packages/numpy/core/include (ver 1.17.4)
--     install path:                lib/python3.8/dist-packages/cv2/python-3.8
--
--   Python (for build):            python3
--
--   Java:
--     ant:                         /bin/ant (ver 1.10.7)
--     JNI:                         /usr/lib/jvm/java-14-openjdk-amd64/include /usr/lib/jvm/java-14-openjdk-amd64/include/linux /usr/lib/jvm/java-14-openjdk-amd64/include
--     Java wrappers:               YES
--     Java tests:                  YES
--
--   Install to:                    /usr/local/lib/opencv
-- -----------------------------------------------------------------
--
-- Configuring done
-- Generating done
-- Build files have been written to: /usr/local/src/opencv/build
```

####It's finaly it.

I install this library.

```shell:install
#/usr/bin/sh

sudo apt update
sudo apt -yV upgrade

sudo apt-get -yV install build-essential
Sudo apt-get -yV install gcc-8.0 g++-8.0
sudo apt-get -yV install cmake

sudo apt-get -yV install libeigen3-dev

sudo apt-get -yV install libgtk-3-dev
sudo apt-get -yV install qt5-default
sudo apt-get -yV install libvtk7-qt-dev
sudo apt-get -yV install freeglut3-dev

sudo apt-get -yV install libtbb-dev

sudo apt-get -yV install libjpeg-dev
sudo apt-get -yV install libopenjp2-7-dev
sudo apt-get -yV install libpng++-dev
sudo apt-get -yV install libtiff-dev
sudo apt-get -yV install libopenexr-dev
sudo apt-get -yV install libwebp-dev

sudo apt-get -yV install libavresample-dev

sudo apt-get -yV install libhdf5-dev

sudo apt-get -yV install libpython3-dev
sudo apt-get -yV install python3-numpy python3-scipy python3-matplotlib

sudo apt-get -yV install libatlas-base-dev
sudo apt-get -yV install liblapacke-dev

sudo apt-get -yV install ubuntu-drivers-common
sudo ubuntu-drivers autoinstall

# dpkg -l | grep cuda
# sudo apt-get --purge remove cuda*
# sudo apt-get autoremove

sudo apt-get -yV install linux-headers-$(uname -r)

sudo apt-get -yV install wget
sudo apt-get -yV install nvidia-cuda-dev nvidia-cuda-toolkit nvidia-cuda-toolkit-gcc

sudo apt-get -yV install openjdk-14-jdk

cat /usr/lib/cuda/version.txt

cd /tmp

echo "deb https://developer.download.nvidia.com/compute/machine-learning/repos/ubuntu1804/x86_64 /" | sudo tee /etc/apt/sources.list.d/nvidia-ml.list

sudo apt update

sudo apt-get -yV install libcudnn7-dev
sudo apt-get -yV install cuda
sudo apt-get -yV install cuda-drivers

sudo apt-get -yV install openjdk-14-jdk
sudo apt-get -yV install ant
sudo rm -rf /bin/ant
sudo ln -s /usr/share/ant/bin/ant /bin/ant
```

It's build by cmake.

```shell

#/usr/bin/sh

export LIB_NAME=opencv
export VERSION=4.5.0-dev
export SRC_DIR=/usr/local/src
export USR_LIB=/usr/local/lib
export INSTALL_PREFIX=${USR_LIB}
export GNU_INCLUDE_DIR=/usr/include/x86_64-linux-gnu
export GNU_LIB_DIR=/usr/lib/x86_64-linux-gnu
export CC=gcc-8
export CXX=g++-8
#export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
#export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
#export JAVA_HOME=/usr/lib/jvm/java-14-openjdk-amd64
#export ANT_HOME=/usr/share/ant
#export ANT_HOME=/usr/local/lib/ant
#export PATH=.:${JAVA_HOME}/bin:${ANT_HOME}/bin:${PATH}
#export PATH=.:${JAVA_HOME}/bin:${PATH}
#export OPENCV_JAVA_SOURCE_VERSION=14
#export OPENCV_JAVA_TARGET_VERSION=14

echo `java -version`
echo `ant -version`

cd ${SRC_DIR}
sudo git clone https://github.com/opencv/opencv.git
sudo chown -R ${USER} ${SRC_DIR}/${LIB_NAME}

sudo git clone https://github.com/opencv/opencv_contrib.git
sudo chown -R ${USER} ${SRC_DIR}/opencv_contrib

cd ${SRC_DIR}/opencv

# sudo rm -rf ${SRC_DIR}/${LIB_NAME}/build
sudo rm -rf ${SRC_DIR}/${LIB_NAME}/build/CMakeCache.txt
sudo mkdir ${SRC_DIR}/${LIB_NAME}/build
sudo chown -R ${USER} ${SRC_DIR}/${LIB_NAME}/build
cd ${SRC_DIR}/${LIB_NAME}/build

cmake -D CMAKE_BUILD_TYPE=RELEASE \
-D CMAKE_INSTALL_PREFIX=${INSTALL_PREFIX}/${LIB_NAME}-${VERSION} \
-D WITH_CUDA=ON \
-D CUDA_FAST_MATH=ON \
-D WITH_CUBLAS=ON \
-D WITH_CUDNN=ON \
-D WITH_NVCUVID=ON \
-D OPENCV_DNN_CUDA=ON \
-D BUILD_opencv_cudaimgproc=ON \
-D INSTALL_C_EXAMPLES=ON \
-D WITH_PYTHON=ON \
-D INSTALL_PYTHON_EXAMPLES=ON \
-D BUILD_opencv_python2=OFF \
-D BUILD_opencv_python3=ON \
-D PYTHON_DEFAULT_EXECUTABLE=python3 \
-D EIGEN_INCLUDE_PATH=/usr/include/eigen3/Eigen/ \
-D OPENCV_EXTRA_MODULES_PATH=${SRC_DIR}/opencv_contrib/modules \
-D OPENCV_DNN_CUDA=ON \
-D BUILD_CUDA_STUBS=OFF \
-D WITH_MKL=ON \
-D BUILD_opencv_python2=OFF \
-D BUILD_opencv_python3=ON \
-D INSTALL_PYTHON_EXAMPLES=ON \
-D WITH_LAPACK=ON \
..

sudo cmake --build ${SRC_DIR}/${LIB_NAME}/build --config release -j 12
sudo cmake --build ${SRC_DIR}/${LIB_NAME}/build --target install
sudo rm -rf /usr/local/lib/opencv
sudo ln -s /usr/local/lib/opencv-4.5.0-dev /usr/local/lib/opencv
```