for循环中的字符串连接。 Java 9

问题描述

如果我错了，请纠正我。在Java 8中，出于性能原因，当通过+运算符连接多个字符串时，调用了StringBuffer。创建一堆中间字符串对象并污染字符串池的问题已解决。

Please correct me if i'm wrong. In Java 8, for performance reasons, when concatenating several strings by the "+" operator StringBuffer was invoked. And the problem of creating a bunch of intermediate string objects and polluting the string pool was "resolved".

Java 9怎么样？
有一个新功能添加为Invokedynamic。还有一个新的类可以更好地解决问题，StringConcatFactory。

What about Java 9?There'a new feature added as Invokedynamic. And a new class that resolves the problem even better, StringConcatFactory.

String result = "";
List<String> list = Arrays.asList("a", "b", "c");
for (String n : list) {
 result+=n;
}

我的问题是：在这个循环中创建了多少个对象？有中介对象吗？我该如何验证？

My question are: How many objects are created in this loop? Are there any intermedier objects? And how can i verify that?

推荐答案

记录中，这是一个 JMH 测试......

For the record, here is a JMH test...

@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.MILLISECONDS)
@Warmup(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
@Measurement(iterations = 5, time = 5, timeUnit = TimeUnit.SECONDS)
@State(Scope.Thread)
public class LoopTest {

    public static void main(String[] args) throws RunnerException {
        Options opt = new OptionsBuilder().include(LoopTest.class.getSimpleName())
                .jvmArgs("-ea", "-Xms10000m", "-Xmx10000m")
                .shouldFailOnError(true)
                .build();
        new Runner(opt).run();
    }

    @Param(value = {"1000", "10000", "100000"})
    int howmany;

    @Fork(1)
    @Benchmark
    public String concatBuilder(){
        StringBuilder sb = new StringBuilder();
        for(int i=0;i<howmany;++i){
            sb.append(i);
        }
        return sb.toString();
    }

    @Fork(1)
    @Benchmark
    public String concatPlain(){
        String result = "";
        for(int i=0;i<howmany;++i){
            result +=i;
        }
        return result;
    }
}

生成结果（仅适用于这里显示的是100000 ，我没想到：

Produces result (only for 100000 shown here) that I did not really expect:

LoopTest.concatPlain       100000  avgt    5  3902.711 ± 67.215  ms/op
LoopTest.concatBuilder     100000  avgt    5     1.850 ±  0.574  ms/op

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