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# PROJ bridge to Java
This is the third release of JNI wrappers for the main PROJ functions.
The first release of JNI wrappers were created by http://www.hydrologis.com.
The second release of JNI wrappers were created by http://www.geoapi.org.
## What is "PROJ bridge to Java"
_Proj bridge to Java_ is a small library of Java classes that wrap a few PROJ functions
by using the Java Native Interface (JNI). The main Java class is `org.proj4.PJ`.
### Compilation
To compile the native part, `configure` has to be run in the PROJ directory like this:
CFLAGS=-Iinclude2 ./configure --with-jni=include1
where
* `include1` = folder in which the header file `jni.h` resides (usually `$JAVA_HOME/include`)
* `include2` = folder in which the header file `jni_md.h` resides (usually `$JAVA_HOME/include/linux` or whatever)
On MacOS, those two folders are `/Library/Java/JavaVirtualMachines/.../Contents/Home/include/`.
The java part is compiled by running Ant inside the `jniwrap` folder.
This will compile the classes and archive them in a JAR file.
It applies to Linux, MacOS and Windows (and virtually to every system supporting java).
### Requirements
Beyond the ones already put by PROJ, you need:
* For compilation:
* Java 9+, the Java standard development kit version 9 or above
* Ant 1.9.8+, to run the build.
* For execution:
* If a Java version less than the current version on the local machine is desired,
add a `release` attribute in the `javac` task of `build.xml` before to compile.
### Documentation
The documentation is held inside the code and can be retrieved by running
`ant javadoc` inside the folder `jniwrap`. This will create the HTML format
documentation inside of `jniwrap/out/apidocs`
### License
GPL for the first release.
Proj.4 license for the second release.
### Authors
* Andrea Antonello (andrea.antonello@hydrologis.com)
* Martin Desruisseaux (martin.desruisseaux@geomatys.com)
## Usage & a fast example:
The `proj.jar` is all is needed to implement PROJ support in java applications.
The whole job is done by the PROJ library, so there are just a couple of functions that be used.
The best way is to see everything through an example.
In the following example we create two Coordinate Reference System and transform 3 points.
The Coordinate Reference Systems and the points are hard-coded for simplicity.
Of course, real applications would read them from a file or other data source.
import org.proj4.*;
import java.util.Arrays;
/**
* Converts coordinates from EPSG:32632 (WGS 84 / UTM zone 32N) to WGS84,
* then prints the result to the standard output stream.
*/
public class Main {
public static void main(String[] args) throws PJException {
PJ sourcePJ = new PJ("+init=epsg:32632"); // (x,y) axis order
PJ targetPJ = new PJ("+proj=latlong +datum=WGS84"); // (λ,φ) axis order
double[] coordinates = {
500000, 0, // First coordinate
400000, 100000, // Second coordinate
600000, -100000 // Third coordinate
};
sourcePJ.transform(targetPJ, 2, coordinates, 0, 3);
System.out.println(Arrays.toString(coordinates));
}
}
### Compile the Main code
We assume that PROJ was compiled with the right flag to support the bridge to Java.
Therefore we have a library called `proj.jar`.
Thus we compile the `Main.java` with the command:
javac --class-path <path to the jar library>/proj.jar Main.java
and execute the created test case with:
java --class-path .:<path to the jar library>/proj.jar -Djava.library.path=<path to the libproj, if needed> Main
### Troubleshooting
If an `java.lang.UnsatisfiedLinkError` is thrown at execution time, add the following line in the Java code:
System.out.println(System.getProperty("java.library.path"));
Then verify that the `libproj.so` (Linux), `libproj.dylib` (MacOS) or `libproj.dll` (Windows) file is located
in one of the directories listed by above code. If this is not the case, then try configuring the
`LD_LIBRARY_PATH` (Linux), `DYLD_LIBRARY_PATH` (MacOS) or `PATH` (Windows) environment variable.
If the problem persist, adding the `-verbose:jni` option to the `java` command may help more advanced diagnostics.
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