为什么StringBuilder链接模式sb.append（x）.append（y）比常规sb.append（x）更快; sb.append（Y）？

问题描述

我有一个显示非常奇怪结果的微基准测试：

I have a microbenchmark that shows very strange results:

@BenchmarkMode(Mode.Throughput)
@Fork(1)
@State(Scope.Thread)
@Warmup(iterations = 10, time = 1, timeUnit = TimeUnit.SECONDS, batchSize = 1000)
@Measurement(iterations = 40, time = 1, timeUnit = TimeUnit.SECONDS, batchSize = 1000)
public class Chaining {

    private String a1 = "111111111111111111111111";
    private String a2 = "222222222222222222222222";
    private String a3 = "333333333333333333333333";

    @Benchmark
    public String typicalChaining() {
        return new StringBuilder().append(a1).append(a2).append(a3).toString();
    }

    @Benchmark
    public String noChaining() {
        StringBuilder sb = new StringBuilder();
        sb.append(a1);
        sb.append(a2);
        sb.append(a3);
        return sb.toString();
    }
}

我期待两个测试的结果都是相同或至少非常接近。但是，差异几乎是5倍：

I'm expecting the results of both tests to be the same or at least very close. However, the difference is almost 5x:

# Run complete. Total time: 00:01:41

Benchmark                  Mode  Cnt      Score     Error  Units
Chaining.noChaining       thrpt   40   8538.236 ± 209.924  ops/s
Chaining.typicalChaining  thrpt   40  36729.523 ± 988.936  ops/s

有人知道这是怎么回事吗？

Does anybody know how that is possible?

推荐答案

字符串连接 a + b + c 是Java程序中非常频繁的模式，因此HotSpot JVM有一个特殊的优化对于它： -XX：+ OptimizeStringConcat 默认情况下为ON。

String concatenation a + b + c is a very frequent pattern in Java programs, so HotSpot JVM has a special optimization for it: -XX:+OptimizeStringConcat which is ON by default.

HotSpot JVM识别 new StringBuilder（）。append（）... append（）。toString（）字节码中的模式，并将其转换为优化的机器代码，而无需调用实际的Java方法，也无需分配中间对象。即这是一种复合JVM内在函数。

HotSpot JVM recognizes new StringBuilder().append()...append().toString() pattern in the bytecode and translates it to the optimized machine code without calling actual Java methods and without allocating intermediate objects. I.e. this is a kind of compound JVM intrinsic.

这是。

另一方面方， sb.append（）; sb.append（）; ...... 未经特别处理。这个序列的编译就像常规的Java方法调用一样。

On the other side, sb.append(); sb.append(); ... is not handled specially. This sequence is compiled just like a regular Java method calls.

如果你用 -XX重新运行基准测试：-OptimizeStringConcat ，两种变体的性能相同。

If you rerun the benchmark with -XX:-OptimizeStringConcat, the performance will be the same for both variants.

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