How to Find the Default Charset/Encoding in Java?

Question

The obvious answer is to use Charset.defaultCharset() but we recently found out that this might not be the right answer. I was told that the result is different from real default charset used by java.io classes in several occasions. Looks like Java keeps 2 sets of default charset. Does anyone have any insights on this issue?

We were able to reproduce one fail case. It's kind of user error but it may still expose the root cause of all other problems. Here is the code,

public class CharSetTest {

    public static void main(String[] args) {
        System.out.println("Default Charset=" + Charset.defaultCharset());
        System.setProperty("file.encoding", "Latin-1");
        System.out.println("file.encoding=" + System.getProperty("file.encoding"));
        System.out.println("Default Charset=" + Charset.defaultCharset());
        System.out.println("Default Charset in Use=" + getDefaultCharSet());
    }

    private static String getDefaultCharSet() {
        OutputStreamWriter writer = new OutputStreamWriter(new ByteArrayOutputStream());
        String enc = writer.getEncoding();
        return enc;
    }
}

Our server requires default charset in Latin-1 to deal with some mixed encoding (ANSI/Latin-1/UTF-8) in a legacy protocol. So all our servers run with this JVM parameter,

-Dfile.encoding=ISO-8859-1

Here is the result on Java 5,

Default Charset=ISO-8859-1
file.encoding=Latin-1
Default Charset=UTF-8
Default Charset in Use=ISO8859_1

Someone tries to change the encoding runtime by setting the file.encoding in the code. We all know that doesn't work. However, this apparently throws off defaultCharset() but it doesn't affect the real default charset used by OutputStreamWriter.

Is this a bug or feature?

EDIT: The accepted answer shows the root cause of the issue. Basically, you can't trust defaultCharset() in Java 5, which is not the default encoding used by I/O classes. Looks like Java 6 corrects this issue.

Answer 1

This is really strange... Once set, the default Charset is cached and it isn't changed while the class is in memory. Setting the "file.encoding" property with System.setProperty("file.encoding", "Latin-1"); does nothing. Every time Charset.defaultCharset() is called it returns the cached charset.

Here are my results:

Default Charset=ISO-8859-1
file.encoding=Latin-1
Default Charset=ISO-8859-1
Default Charset in Use=ISO8859_1

I'm using JVM 1.6 though.

(update)

Ok. I did reproduce your bug with JVM 1.5.

Looking at the source code of 1.5, the cached default charset isn't being set. I don't know if this is a bug or not but 1.6 changes this implementation and uses the cached charset:

JVM 1.5:

public static Charset defaultCharset() {
    synchronized (Charset.class) {
        if (defaultCharset == null) {
            java.security.PrivilegedAction pa =
                    new GetPropertyAction("file.encoding");
            String csn = (String) AccessController.doPrivileged(pa);
            Charset cs = lookup(csn);
            if (cs != null)
                return cs;
            return forName("UTF-8");
        }
        return defaultCharset;
    }
}

JVM 1.6:

public static Charset defaultCharset() {
    if (defaultCharset == null) {
        synchronized (Charset.class) {
            java.security.PrivilegedAction pa =
                    new GetPropertyAction("file.encoding");
            String csn = (String) AccessController.doPrivileged(pa);
            Charset cs = lookup(csn);
            if (cs != null)
                defaultCharset = cs;
            else
                defaultCharset = forName("UTF-8");
        }
    }
    return defaultCharset;
}

When you set the file encoding to file.encoding=Latin-1 the next time you call Charset.defaultCharset(), what happens is, because the cached default charset isn't set, it will try to find the appropriate charset for the name Latin-1. This name isn't found, because it's incorrect, and returns the default UTF-8.

As for why the IO classes such as OutputStreamWriter return an unexpected result,
the implementation of sun.nio.cs.StreamEncoder (witch is used by these IO classes) is different as well for JVM 1.5 and JVM 1.6. The JVM 1.6 implementation is based in the Charset.defaultCharset() method to get the default encoding, if one is not provided to IO classes. The JVM 1.5 implementation uses a different method Converters.getDefaultEncodingName(); to get the default charset. This method uses its own cache of the default charset that is set upon JVM initialization:

JVM 1.6:

public static StreamEncoder forOutputStreamWriter(OutputStream out,
        Object lock,
        String charsetName)
        throws UnsupportedEncodingException
{
    String csn = charsetName;
    if (csn == null)
        csn = Charset.defaultCharset().name();
    try {
        if (Charset.isSupported(csn))
            return new StreamEncoder(out, lock, Charset.forName(csn));
    } catch (IllegalCharsetNameException x) { }
    throw new UnsupportedEncodingException (csn);
}

JVM 1.5:

public static StreamEncoder forOutputStreamWriter(OutputStream out,
        Object lock,
        String charsetName)
        throws UnsupportedEncodingException
{
    String csn = charsetName;
    if (csn == null)
        csn = Converters.getDefaultEncodingName();
    if (!Converters.isCached(Converters.CHAR_TO_BYTE, csn)) {
        try {
            if (Charset.isSupported(csn))
                return new CharsetSE(out, lock, Charset.forName(csn));
        } catch (IllegalCharsetNameException x) { }
    }
    return new ConverterSE(out, lock, csn);
}

But I agree with the comments. You shouldn't rely on this property. It's an implementation detail.