This is an example of manual "lock coarsening" and may have been done to get a performance boost.

Consider these two snippets:

StringBuffer b = new StringBuffer(); for(int i = 0 ; i < 100; i++){ b.append(i); }

versus:

StringBuffer b = new StringBuffer(); synchronized(b){ for(int i = 0 ; i < 100; i++){ b.append(i); } }

In the first case, the StringBuffer must acquire and release a lock 100 times (because append is a synchronized method), whereas in the second case, the lock is acquired and released only once. This can give you a performance boost and is probably why the author did it. In some cases, the compiler can perform this lock coarsening for you (but not around looping constructs because you could end up holding a lock for long periods of time).

By the way, the compiler can detect that an object is not "escaping" from a method and so remove acquiring and releasing locks on the object altogether (lock elision) since no other thread can access the object anyway. A lot of work has been done on this in JDK7.

Update:

I carried out two quick tests:

1) WITHOUT WARM-UP:

In this test, I did not run the methods a few times to "warm-up" the JVM. This means that the Java Hotspot Server Compiler did not get a chance to optimize code e.g. by eliminating locks for escaping objects.

JDK 1.4.2_19 1.5.0_21 1.6.0_21 1.7.0_06 WITH-SYNC (ms) 3172 1108 3822 2786 WITHOUT-SYNC (ms) 3660 801 509 763 STRINGBUILDER (ms) N/A 450 434 475

With JDK 1.4, the code with the external synchronized block is faster. However, with JDK 5 and above the code without external synchronization wins.

2) WITH WARM-UP:

In this test, the methods were run a few times before the timings were calculated. This was done so that the JVM could optimize code by performing escape analysis.

JDK 1.4.2_19 1.5.0_21 1.6.0_21 1.7.0_06 WITH-SYNC (ms) 3190 614 565 587 WITHOUT-SYNC (ms) 3593 779 563 610 STRINGBUILDER (ms) N/A 450 434 475

Once again, with JDK 1.4, the code with the external synchronized block is faster. However, with JDK 5 and above, both methods perform equally well.

Here is my test class (feel free to improve):