Jabbering Giraffe

Java Platform Encoding

This came up at $WORK recently. We had a java program that was given input through command line arguments. Unfortunately, it went wrong when being passed UTF-8 characters (U+00A9 COPYRIGHT SIGN [©]). Printing out the command line arguments from inside Java showed that we had double encoded Unicode.

Initially, we just slapped -Dfile.encoding=UTF-8 on the command line. But that failed when the site that called this code went through an automatic restart. So we investigated the issue further.

We quickly found that the presence of absence of the LANG environment variable had a bearing on the matter.

NB: ShowSystemProperties.jar is very simple and just lists all system properties in sorted order.

$ java -version
java version "1.6.0_16"
Java(TM) SE Runtime Environment (build 1.6.0_16-b01)
Java HotSpot(TM) Server VM (build 14.2-b01, mixed mode)
$ echo $LANG
en_GB.UTF-8
$ java -jar ShowSystemProperties.jar | grep encoding
file.encoding=UTF-8
file.encoding.pkg=sun.io
sun.io.unicode.encoding=UnicodeLittle
sun.jnu.encoding=UTF-8
$ LANG= java -jar ShowSystemProperties.jar | grep encoding
file.encoding=ANSI_X3.4-1968
file.encoding.pkg=sun.io
sun.io.unicode.encoding=UnicodeLittle
sun.jnu.encoding=ANSI_X3.4-1968

So, setting file.encoding works, but there’s an internal property, sun.jnu.encoding as well.

Next, see what happens when we add the explicit override.

$ LANG= java -Dfile.encoding=UTF-8 -jar ShowSystemProperties.jar | grep encoding
file.encoding=UTF-8
file.encoding.pkg=sun.io
sun.io.unicode.encoding=UnicodeLittle
sun.jnu.encoding=ANSI_X3.4-1968

Hey! sun.jnu.encoding isn’t changing!

Now, as far as I can see, sun.jnu.encoding isn’t actually documented anywhere. So you have to go into the source code for Java (openjdk’s jdk6-b16 in this case) to figure out what’s up.

Let’s start in main(), which is in java.c. Actually, it’s JavaMain() that we’re really interested in. In there you can see:

int JNICALL
JavaMain(void * _args)
{
  …
  jobjectArray mainArgs;

  …
  /* Build argument array */
  mainArgs = NewPlatformStringArray(env, argv, argc);
  if (mainArgs == NULL) {
      ReportExceptionDescription(env);
      goto leave;
  }
  …
}

NewPlatformStringArray() is defined in java.c and calls NewPlatformString() repeatedly with each command line argument. In turn, that calls new String(byte[], encoding). It gets the encoding from getPlatformEncoding(). That essentially calls System.getProperty("sun.jnu.encoding").

So where does that property get set? If you look in System.c, Java_java_lang_System_initProperties() calls:

    PUTPROP(props, "sun.jnu.encoding", sprops->sun_jnu_encoding);

sprops appears to get set in GetJavaProperties() in java_props_md.c. This interprets various environment variables including the one that control the locale. It appears to pull out everything after the period in the LANG environment variable as the encoding in order to get sun_jnu_encoding.

Phew. So we now know that there is a special property which gets used for interpreting “platform” strings like:

* Command line arguments
* Main class name
* Environment variables

And it can be overridden:

$ LANG= java -Dsun.jnu.encoding=UTF-8 -Dfile.encoding=UTF-8 -jar ShowSystemProperties.jar | grep encoding
file.encoding=UTF-8
file.encoding.pkg=sun.io
sun.io.unicode.encoding=UnicodeLittle
sun.jnu.encoding=UTF-8