A native implementation of getenv

Implementing a replacement for the non-functional java.lang.System.getenv using JNI is quite straightforward. The constraints of what we're trying to do and how JNI works pretty much determine what the code will look like. The code here is released into the public domain with no licensing restrictions: do with it what you will.

First you need a class to contain the new method. Because I couldn't think of anything better I've called mine uk.co.tigress.System:

package uk.co.tigress ;

/**
 * The System class provides an implementation of
 * getenv that actually works, unlike the one in
 * java.lang.System.  The class cannot be instantiated.
 *
 * @author   R M Yorston
 * @version  1.0
 */
public class System {
    static {
        java.lang.System.loadLibrary("getenv") ;
    }

    private System() {
    }

    /**
     * Gets an environment variable. An environment variable is a
     * system-dependent external variable that has a string value.
     *
     * @param    name    name of the environment variable
     * @return    the value of the variable, or null if the
     * variable is not defined.
     */
    public static native String getenv(String name) ;
}

#include "uk_co_tigress_System.h"
#include 

JNIEXPORT jstring JNICALL Java_uk_co_tigress_System_getenv
  (JNIEnv *env, jclass c, jstring jname)
{
    const char *name, *value ;

    if ( jname == NULL ) {
        return NULL ;
    }

    name = (*env)->GetStringUTFChars(env, jname, (jboolean *)NULL) ;

    value = getenv(name) ;

    (*env)->ReleaseStringUTFChars(env, jname, name) ;

    return value ? (*env)->NewStringUTF(env, value) : NULL ;
}

EXPORT jint JNICALL JNI_OnLoad(JavaVM *vm, void *reserved)
{
    /*
     * getenv.dll doesn't use any features of JNI beyond 1.1, so
     * returning JNI_VERSION_1_1 is fine.
     */
    return JNI_VERSION_1_1 ;
}

JNIEXPORT void JNICALL JNI_OnUnload(JavaVM *vm, void *reserved)
{
    return ;
}

Using the method is straightforward. It's probably best to refer to the class using its full name rather than importing it, as this will only lead to confusion with java.lang.System.

public class test {
    public static void main(String args[]) {
        System.out.println(uk.co.tigress.System.getenv("HOME")) ;
    }
}

To install the files go to the top directory of a JRE installation, or the jre directory of an SDK installation and unpack the archive file.

Source

• gzipped tar file
• compressed tar file
• zip file

Linux/i386

• gzipped tar file
• gzipped tar file for IBM

Solaris/SPARC

• compressed tar file

AIX

• compressed tar file

Microsoft Windows

• zip file

Other platforms

As an alternative approach, Pierre-Charles David has pointed out that on some systems it's possible to access the environment variables for a process through the /proc filesystem. This opens the way to a pure Java implementation of getenv on those systems.

If you have enjoyed this code you might also care to read my rant about why I think Sun are wrong to disable java.lang.System.getenv.