Thread safe logging class, using final and static

Question

Hi,
I'm looking through a logging class (which writes entries to a text file and so on) written by someone else, and I'm trying to determine whether it's thread-safe...
Essentially the class looks like this:
public final class Logger {
private static FileOutputStream fout;
private static PrinterWriter pout;
private static final long MaxLength = 100000;
static {
/* basically checks size of logfile, if bigger than 100k, then copy it as bak, and start again */
public void write(String msg) {
/* write entry into log file */
public void writeWithTimeStamp(string msg) {
/* write entry with time stamp into log file */
Now, I could be wrong, but I don't think that the class is thread-safe is it? in order to be thread-safe, I would have to use synchronized before write() and writeWithTimeStamp(), right?
My confusion arises from the use of the keyword "final" at the top of the class. This class isn't being inherited (or rather, there's no reason for the class not to be inheritable)... so what is it's purpose, if there is one?!
And my other question concerns the static block. But I need to describe the current setup first: The Logger class is being use by a file server. File server basically sits there, accepts socket connections from the outside, and takes text files and output them into a local directory. When a new socket connection is created, a new thread is created. As it stands right now, each thread instantiates its own Logger object which seems weird! Bringing me to my question which was, if each thread instantiates its own Logger object, the static block is only ran ONCE, regardless of how many threads (and therefore Logger objects) are created, right??
And wouldn't it be a better idea to simply create only ONE Logger object and pass it by reference to all newly created threads as they are needed so that all threads access the same and only Logger object? (and therefore utilize sychronization)
Thanks!

Answer 1

In JDK 1.4, there are already classes written that do all of that work. Check out the docs for the java.util.logging package. Specifically, check out FileHandler (which can rotate log files if they are greater than a specified size) and LogRecord (which is a standardized way to store logging entries). I believe the class is threadsafe, it doesn't say it isn't. Try it and see. If not, you can easily make a threadsafe wrapper for it (or any class for that matter).
The online documentation is at:
http://java.sun.com/j2se/1.4/docs/api/index.html
If you are curious, the simplest way to make any class threadsafe, if you don't want to or are not able to modify the class code itself is to define another class that calls methods of the original class, but that is synchronized.
Example:
    // NonThreadSafe.java
    public class NonThreadSafe {
        public NonThreadSafe (String s) {
            // Constructor
        public void SomeMethod (int param) {
            // Do some important stuff here...
        public int AnotherMethod (boolean param) {
            // Do some more important stuff here...
    // ThreadSafeWrapper.java
    public class ThreadSafeWrapper {
        protected NonThreadSafe nts;
        public ThreadSafeWrapper (String s) {
            nts = new NonThreadSafe(s);
        public synchronized void SomeMethod (int param) {
            nts.SomeMethod(param);
        public synchronized int AnotherMethod (boolean param) {
            return nts.AnotherMethod(param);
        public NonThreadSafe GetNonThreadSafe () {
            return nts;
    };Unfortunately, ThreadSafeWrapper isn't derived from NonThreadSafe, so you are somewhat limited in what you can do with it compared to what you could do with NonThreadSafe. AFAIK, you can't override unsynchronized methods with synchronized ones.
Another thing you could do is just write a method that writes to your logging class, and synchronize just that method. For example:
    // ThreadSafeLogger.java
    public class ThreadSafeLogger {
        public static synchronized void LogMessage (MyLogger log, String msg) {
            log.writeString(msg);
    // In another thread, far, far away:
        ThreadSafeLogger.LogMessage(theLog, "Blah");
        ...Hope that helps.
Jason Cipriani
[email protected]
[email protected]