Overview

There is a common misconception that because the JIT is smart and synchronization can be eliminated for an object which is only local to a method that there is no performance impact.
 
 
 
 
 
 

A test comparing StringBuffer and StringBuilder

These two classes do basically the same thing except one is synchronized (StringBuffer) and the other is not. It is also a class which is often used in one method to build a String.  The following test attempts to determine how much difference using one other the other can make.

static String dontOptimiseAway = null;
static String[] words = new String[100000];

public static void main(String... args) {
 for (int i = 0; i < words.length; i++)
 words[i] = Integer.toString(i);

 for (int i = 0; i < 10; i++) {
 dontOptimiseAway = testStringBuffer();
 dontOptimiseAway = testStringBuilder();
 }
}

private static String testStringBuffer() {
 long start = System.nanoTime();
 StringBuffer sb = new StringBuffer();
 for (String word : words) {
 sb.append(word).append(',');
 }
 String s = sb.substring(0, sb.length() - 1);
 long time = System.nanoTime() - start;
 System.out.printf("StringBuffer: took %d ns per word%n", time / words.length);
 return s;
}

private static String testStringBuilder() {
 long start = System.nanoTime();
 StringBuilder sb = new StringBuilder();
 for (String word : words) {
 sb.append(word).append(',');
 }
 String s = sb.substring(0, sb.length() - 1);
 long time = System.nanoTime() - start;
 System.out.printf("StringBuilder: took %d ns per word%n", time / words.length);
 return s;
}

at the end prints with -XX:+DoEscapeAnalysis using Java 7 update 10

StringBuffer: took 69 ns per word
StringBuilder: took 32 ns per word
StringBuffer: took 88 ns per word
StringBuilder: took 26 ns per word
StringBuffer: took 62 ns per word
StringBuilder: took 25 ns per word

Testing with one million words doesn’t change the results significantly.

Conclusion

• While the cost of using synchronization is small, it is measurable and if you can use StringBuilder it is preferred as it states in the Javadocs for this class.
• In theory, synchronization can be optimised away, but it is yet to be the case even in simple cases.

Reference: Can synchronization be optimised away? from our JCG partner Peter Lawrey at the Vanilla Java blog.