Can synchronization be optimised away?

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.

Using -XX:BiasedLockingStartupDelay=0 improves the situation where only one thread uses the lock.  This helps in the situation where locking has been used but isn't needed, but at the cost of locks which are used when they are needed.

StringBuffer: took 34 ns per word
StringBuilder: took 31 ns per word
StringBuffer: took 51 ns per word
StringBuilder: took 25 ns per word
StringBuffer: took 28 ns per word
StringBuilder: took 25 ns per word

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.