Necessity of synchronized on local variable

Question

In the JSON-java library (org.json.JSONArray) I have found this code snippet with a synchronized block around a method-local variable

public String toString(int indentFactor) throws JSONException {
    StringWriter sw = new StringWriter();
    synchronized (sw.getBuffer()) {
        return this.write(sw, indentFactor, 0).toString();
    }
}

I do not understand the necessity for the synchronization here, as the StringWriter is only local to the given method (and, why the synchronization is on the Buffer). Is the synchronization really necessary here, and if, why?

Answer 1

This may be a performance optimization. In the oracle jvm, re-acquiring an already held lock is very fast. Presumably, the write call is making numerous calls into the StringBuffer. By locking before calling write, the lock will be held through all of those calls instead of being released and re-acquired for each call.

Answer 2

The empty constructor for StringWriter is

/**
 * Create a new string writer using the default initial string-buffer
 * size.
 */
public StringWriter() {
    buf = new StringBuffer();
    lock = buf;
}

Nothing is shared, therefore the synchronized block is unnecessary.

Unless... write delegates to another thread, but I seriously doubt that.