Is StringBuilder threadsafe (using it with parallelStream)?

Question

My code:

StringBuilder sb = new StringBuilder();

events.parallelStream().forEach(event -> {
    sb.append(event.toString());
    sb.append("\n");
});

I don't care about the order of the events.toString() in the final result. But I care that the events.toString() will correctly appear one line after another, without mixing / messing up of course.

Is parallelStream (instead of stream) safe in this regard?

Answer 1

No, it is not thread-safe.

This is the main difference between the old StringBuffer and the new StringBuilder - the former's methods are synchronized, while the latter's are not.

It's not very useful to do it this way, even if you'd use StringBuffer instead - the threads would have to wait on each other to write to the StringBuffer.

Answer 2

No, it is not. As noted in its javadoc:

A mutable sequence of characters. This class provides an API compatible with StringBuffer, but with no guarantee of synchronization.

Use StringBuffer instead.