衡量Java中对象创建的性能

Question

I'm trying to compare performance of creation the object of some class with creation of String from byte[] . Here is the benchmark I wrote for this:

public class MyBenchmark {
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public void tsts(Blackhole b) {
        b.consume(new TestClass(i(), str()));
    }

    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public void str(Blackhole b) {
        b.consume(new String(b()));
    }

    @CompilerControl(CompilerControl.Mode.DONT_INLINE)
    public String str(){
        return "asdasfa";
    }

    @CompilerControl(CompilerControl.Mode.DONT_INLINE)
    public int i(){
        return 23;
    }

    @CompilerControl(CompilerControl.Mode.DONT_INLINE)
    public byte[] b(){
        return new byte[]{49, 66, 43, 65, 78, 123, 96, 54};
    }
}

where

private static class TestClass{
    private int i;
    private String s;

    public TestClass(Integer i, String s) {
        this.i = i;
        this.s = s;
    }
}

On my machine I got the following results:

Benchmark         Mode  Cnt   Score   Error  Units
MyBenchmark.str   avgt   20  47.695 ± 1.869  ns/op
MyBenchmark.tsts  avgt   20   6.999 ± 0.191  ns/op

Is it correct way to do this? Or I made some mistake in the benchmark and missed something?

Answer 1

This is because JIT knows the exact address of the literal String isntance ("asdasfa"). So in that case there is no need for creating a new object. You can clearly see it if you decompile str() method:

0x000000010dd1a2b0: sub    rsp,0x18
0x000000010dd1a2b7: mov    QWORD PTR [rsp+0x10],rbp  ;
0x000000010dd1a2bc: movabs rax,0x1bc1f1440    ;   {oop("asdasfa")}
0x000000010dd1a2c6: add    rsp,0x10
0x000000010dd1a2ca: pop    rbp
0x000000010dd1a2cb: test   DWORD PTR [rip+0xfffffffffe642d2f],eax           # 0x000000010c35d000;
0x000000010dd1a2d1: ret    

The method consists of one instruction movabs rax,0x1bc1f1440 which puts an address to rax register, that's why it's a lot more faster. Other instructions are auxiliary.

For the other method str(Blackhole b) you will see a lot more including a compiled body of String constructor with getting a default charset and decoding of byte array through method call.