How to Find the Default Charset/Encoding in Java?

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.