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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 an release attribute in the javac task of build.xml.

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

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 -classpath <path to the jar library>/proj.jar Main.java

and execute the created test case with:

java -cp .:<path to the jar library>/proj.jar -Djava.library.path=<path to the libproj, if needed> Main

That's it, enjoy!