Java MessageFormat类线程是否安全？ （与SimpleDateFormat相对）

Question

I know that SimpleDateFormat and NumberFormat are NOT thread safe.
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4101500

But what about the other Format classes like MessageFormat?

Fortify 360 is flagging the use of "MessageFormat.format(String, Object...)" static method as a "Race Condition - Format Flaw" issue, but when I analyze the the source code of MessageFormat, I saw that in that method, it creates a new local instance of MessageFormat itself.

Is the Java MessageFormat Class thread safe?

Answer 1

The javadoc for MessageFormat says:

Message formats are not synchronized. It is recommended to create separate format instances for each thread. If multiple threads access a format concurrently, it must be synchronized externally.

So officially, no - it's not thread-safe.

The docs for SimpleDateFormat say much the same thing.

Now, the docs may just be being conservative, and in practice it'll work just fine in multiple threads, but it's not worth the risk.

Answer 2

If you are referrring to the method

public static String format(String pattern, Object... arguments)

this is thread-safe since as described in the javadoc it creates a new MessageFormat to do the formatting.

BTW, thats a funny typo in your title 'SimpleThreadFormat' :)

Answer 3

Per the javadoc, MessageFormat objects are not thread-safe. You can use a ThreadLocal to create a separate object for each thread that needs one.

ThreadLocal<MessageFormat> threadLocalMessageFormat =
    new ThreadLocal<MessageFormat>() {
        @Override
        protected MessageFormat initialValue() {
            return new MessageFormat(pattern);
        }
    };

You can then use threadLocalMessageFormat.get() to obtain a MessageFormat for the current thread.