Is a Java socket's PrintWriter thread safe?

It's a poor design to have these multiple PrintWriters on the same stream. Really you want at least the object the calls them to be synchronised (or thread confined).

However, assuming for some reason you do want multiple PrintWriters:

First problem: Writers do not use this as the lock. PrintWriter and BufferedWriter by default both use the Writer they are constructed with as the lock. This is obviously completely broken. They should be using the Writer's lock, not the Writer itself. An easy mistake given that having locking a feature of Object removes static type safety. So you'll need to construct a PrintWriter with the socket OutputStream (or some other common object) as the lock.

Secondly, we have buffering within PrintWriter. So come the end of a buffer, half get written and half wait for the next write. To prevent that, either externally lock to combine a print and flush, or use auto-flushing and add a new line character.

So, it isn't meaningfully thread-safe, but you can hack it. Or you can use a better design.

You need a way to use the same PrintWriter between the threads (t1.writer == t2.writer, not just PrintWriters created from the same OutputStream). With the same PrintWriter, all writing operations are synchronized.