Re: Safety Of Non-Synchronized Collections

Eric Sosman <esosman@comcast-dot-net.invalid>
Thu, 10 Jan 2013 09:27:39 -0500
On 1/10/2013 6:37 AM, Jukka Lahtinen wrote:

Daniel Pitts <> writes:

On 1/9/13 2:51 PM, Lew wrote:

'StringBuffer' is no more thread safe than any other class with
synchronized methods.

Which is more safe than other classes without synchronized methods.
They are thread-safe to the point that each method call is atomic. What
else could you ask for? They didn't lie.

Whenever thread safety is needed, you mostly need to synchronize not
only the single method call to an instance of StringBuffer or some other
class of the jdk, but also some context around it.
And when you add a synchronized block around some code containing the
StringBuffer call, the synchronization of the StringBuffer method is
most likely not needed any more, because of your own synchronized
block. And then you can just use StringBuilder.

     This is probably not the case, because another thread might
call an (unsynchronized) StringBuilder method while you're in
the middle of your synchronized block:

    StringBuilder sb = ...;

    // Thread T1:
    synchronized(sb) {
        if (sb.charAt(sb.length() - 1) == '\n') {
            sb.deleteCharAt(sb.length() - 1);

    // Thread T2:

The synchronization in T1's code is no protection against
interference from T2. If `sb' were changed from a StringBuilder
to a StringBuffer, the race condition would disappear.

Eric Sosman

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