Using locks when you don't need to will slow down your application. It could be too small to measure or it could be surprisingly high.
IMHO Often the best approach is to use lock free code in a single threaded program to make it clear this code is not intended to be shared across thread. This could be more important for maintenance than any performance issues.
public static void main(String... args) throws IOException {
for (int i = 0; i < 3; i++) {
perfTest(new Vector<Integer>());
perfTest(new ArrayList<Integer>());
}
}
private static void perfTest(List<Integer> objects) {
long start = System.nanoTime();
final int runs = 100000000;
for (int i = 0; i < runs; i += 20) {
// add items.
for (int j = 0; j < 20; j+=2)
objects.add(i);
// remove from the end.
while (!objects.isEmpty())
objects.remove(objects.size() - 1);
}
long time = System.nanoTime() - start;
System.out.printf("%s each add/remove took an average of %.1f ns%n", objects.getClass().getSimpleName(), (double) time/runs);
}
prints
Vector each add/remove took an average of 38.9 ns
ArrayList each add/remove took an average of 6.4 ns
Vector each add/remove took an average of 10.5 ns
ArrayList each add/remove took an average of 6.2 ns
Vector each add/remove took an average of 10.4 ns
ArrayList each add/remove took an average of 5.7 ns
From a performance point of view, if 4 ns is important to you, you have to use the non-synchronized version.
For 99% of use cases, the clarity of the code is more important than performance. Clear, simple code often performs reasonably good as well.
BTW: I am using a 4.6 GHz i7 2600 with Oracle Java 7u1.
For comparison if I do the following where perfTest1,2,3 are identical.
perfTest1(new ArrayList<Integer>());
perfTest2(new Vector<Integer>());
perfTest3(Collections.synchronizedList(new ArrayList<Integer>()));
I get
ArrayList each add/remove took an average of 2.6 ns
Vector each add/remove took an average of 7.5 ns
SynchronizedRandomAccessList each add/remove took an average of 8.9 ns
If I use a common perfTest method it cannot inline the code as optimally and they are all slower
ArrayList each add/remove took an average of 9.3 ns
Vector each add/remove took an average of 12.4 ns
SynchronizedRandomAccessList each add/remove took an average of 13.9 ns
Swapping the order of tests
ArrayList each add/remove took an average of 3.0 ns
Vector each add/remove took an average of 39.7 ns
ArrayList each add/remove took an average of 2.0 ns
Vector each add/remove took an average of 4.6 ns
ArrayList each add/remove took an average of 2.3 ns
Vector each add/remove took an average of 4.5 ns
ArrayList each add/remove took an average of 2.3 ns
Vector each add/remove took an average of 4.4 ns
ArrayList each add/remove took an average of 2.4 ns
Vector each add/remove took an average of 4.6 ns
one at a time
ArrayList each add/remove took an average of 3.0 ns
ArrayList each add/remove took an average of 3.0 ns
ArrayList each add/remove took an average of 2.3 ns
ArrayList each add/remove took an average of 2.2 ns
ArrayList each add/remove took an average of 2.4 ns
and
Vector each add/remove took an average of 28.4 ns
Vector each add/remove took an average of 37.4 ns
Vector each add/remove took an average of 7.6 ns
Vector each add/remove took an average of 7.6 ns
Vector each add/remove took an average of 7.6 ns
synchronized (lock)will take a roughly fixed amount of time per call to check that the lock is available and then acquire it, even if there are no other threads running. The amount of time will depend on your installation and hardware. Mine (Linux, Oracle Java 8 JRE, Intel i7) took 27.25 nanoseconds average to acquire a lock. – Dietrich