Building Java package for OpenCV in linux

Question

When I do the cmake command the "To be build" section doesn't appear java, what could be the reason for that? I've java installed in my machinewhic

cmake -D CMAKE_BUILD_TYPE=Release -D BUILD_SHARED_LIBS=OFF -DOPENCV_EXTRA_MODULES_PATH=../../opencv_contrib-master/modules/ ..

To be built:
core flann imgproc ml photo reg surface_matching video dnn fuzzy
imgcodecs shape videoio highgui objdetect plot superres ts xobjdetect
xphoto bgsegm bioinspired dpm face features2d line_descriptor saliency
text calib3d ccalib datasets rgbd stereo structured_light tracking
videostab xfeatures2d ximgproc aruco optflow stitching python2

Java version

java version "1.8.0_74"
Java(TM) SE Runtime Environment (build 1.8.0_74-b02)
Java HotSpot(TM) 64-Bit Server VM (build 25.74-b02, mixed mode)

The environment variable is also rightfully set:

echo $JAVA_HOME
/usr/lib/jvm/java-8-oracle/jre/bin/java

references: http://docs.opencv.org/3.1.0/d7/d9f/tutorial_linux_install.html#gsc.tab=0

Can it find Apache Ant? It seems to be required as well. Just edit the # --- Java Support --- section of the top-level CMakeLists.txt and add a couple of messages to see if the dependency detection finds what's required. — Dan Mašek
– Dan Mašek, Commented Apr 1, 2016 at 1:10
yes it does. Thank you the quick reply. I forgot to add this reference as well stackoverflow.com/questions/17386551/… — Rinaldi Segecin
– Rinaldi Segecin, Commented Apr 1, 2016 at 1:12
Now that I understood your comment, apparently it could not find JNI I'll have to take a look of that tomorrow. I'll post it then — Rinaldi Segecin
– Rinaldi Segecin, Commented Apr 1, 2016 at 1:19
Hmm, the JAVA_HOME variable, where does it point? Seems like the java binary to me. What if you shorten it to only /usr/lib/jvm/java-8-oracle/ ? Looking at the CMake module FindJNI.cmake, that seems to be the issue. — Dan Mašek
– Dan Mašek, Commented Apr 1, 2016 at 1:24
Thank you Dan you were right. I edited the FindJNI.cmake and added the right path to the JDK and JRE libraries, after that I also needed to change the environment path to not to point to their binaries but to point to JDK and JRE directories so I was able to run ant. — Rinaldi Segecin
– Rinaldi Segecin, Commented Apr 1, 2016 at 19:13

Rinaldi Segecin · Accepted Answer · 2016-04-02 17:45:51Z

Solution: Extract JDK to /usr/lib/jvm/jdk1.8.0_77/ set environment variables in /etc/environment and in ~/.profile.

JAVA_HOME="/usr/lib/jvm/jdk1.8.0_77/"
JRE_HOME="/usr/lib/jvm/jdk1.8.0_77/jre/"

add java and javac to /usr/bin

sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jdk1.8.0_77/bin/java" 1000
sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/jdk1.8.0_77/bin/javac" 1000

make it sure that they are currently selected:

sudo update-alternatives --config java
sudo update-alternatives --config javac

In the file /usr/share/cmake-3.2/Modules/FindJNI.cmake in the section JAVA_AWT_LIBRARY_DIRECTORIES include

/usr/lib/jvm/jdk1.8.0_77/jre/lib/{libarch}

and in the section JAVA_AWT_INCLUDE_DIRECTORIES include

/usr/lib/jvm/jdk1.8.0_77/include

Install 'ant' and check if /usr/bin/ant -version doesn't say that JAVA_HOME is in the wrong path or something. Finally cmake now should be able to compile libraries for java as well.

Thank you Dan.

addendum:

To compile OpenCv's extra module in java go to CMakeList.txt contained in the directory of the preferred module and add 'java' to the line:

ocv_define_module(face opencv_core opencv_imgproc opencv_objdetect WRAP python java)

Unset BUILD_SHARED_LIBS, BUILD_EXAMPLES, BUILD_TESTS, BUILD_PERF_TESTS, flags and set OPENCV_EXTRA_MODULES_PATH to path: opencv_contrib/modules/.

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