Java – why is cloning arrays so slow?

This test

for (;;) {
            int[] a = new int[10];
            System.gc();
            long t0 = System.currentTimeMillis();
            for (int i = 0; i < 1000000; i++) {
//              int[] b =  a.clone();
                int[] b =  Arrays.copyOf(a,a.length);
            }
            System.out.println(System.currentTimeMillis() - t0);
        }

Display arrays Copyof is about 50ms and clone display is about 160ms Cloning is a special native method to make copies. Why is it so slow?

I'm in hotspot client JVM 1.7 0_ Run the test on 11-b21 Note that as the array size increases, the difference between clone and copyof disappears

Solution

I run your code on your system: there is little difference between them Both clocks are about 30 milliseconds My test was conducted on openjdk 7

To confirm, I also run it through caliper and use a larger array to emphasize the actual replication performance:

public class Performance extends SimpleBenchmark {
  final int[] source = new int[1000];

  public int timeClone(int reps) {
    int sum = 0;
    for (int i = reps; i > 0; i--)
      sum += source.clone().length;
    return sum;
  }

  public int timeCopyOf(int reps) {
    int sum = 0;
    for (int i = reps; i > 0; i--)
      sum += Arrays.copyOf(source,source.length).length;
    return sum;
  }
  public static void main(String... args) {
    Runner.main(Performance.class,args);
  }
}

result:

0% Scenario{vm=java,trial=0,benchmark=Clone} 2141.70 ns; σ=5416.80 ns @ 10 trials
50% Scenario{vm=java,benchmark=CopyOf} 2168.38 ns; σ=1545.85 ns @ 10 trials

benchmark   us linear runtime
    Clone 2.14 =============================
   CopyOf 2.17 ==============================

vm: java
trial: 0

Upon request, the array size here is 10:

0% Scenario{vm=java,benchmark=Clone} 30.07 ns; σ=2.12 ns @ 10 trials
50% Scenario{vm=java,benchmark=CopyOf} 29.34 ns; σ=161.38 ns @ 10 trials

benchmark   ns linear runtime
    Clone 30.1 ==============================
   CopyOf 29.3 =============================