简单字符串连接示例在Java 7中速度快，在Java 8中速度慢

Question

我有一个小类，它演示了连接字符串的不同方法的一些时序特性（StringBuilder与纯字符串连接）。不管每种策略的相对时间如何，我都注意到了Java 6、后来的Java 7和现在的Java 8的特定性能行为。

我最初是用Java6编写的，Java7的巨大改进给我留下了深刻的印象。我曾以为Java8会显示出类似的性能。然而，当我在Win7/64位笔记本电脑上运行此程序时，我发现Java 8数字甚至比我的Java 6数字还要慢。

我完全有可能看到Java 6和7的问题，JVM正在删除我在运行时测试的代码（微基准测试的常见障碍），但我只是不知道。

我将显示三次运行的性能数字，然后包括源代码。请注意，该测试有一个巨大的预热过程，希望它能在计时运行之前完成所有类加载和JIT工作。

还要注意，Java 6和Java 8中的相对时序是"一致的"。例如，我希望测试2和3是最快的两个，2有时比3快，反之亦然。Java 7的计时都比这低得多，而且基本相同，所以我可能对Java 7 JIT裁剪太多代码有问题。

Java 6:
stringBuilder0:499
stringBuilder1:260
stringBuilder2:262
stringBuilder3:180
stringBuilder4:184
stringBuilder5:210
stringBuilder6:286
stringBuilder7:287
stringBuilder8:203
stringBuilder9:248
Java 7:
stringBuilder0:66
stringBuilder1:269
stringBuilder2:57
stringBuilder3:57
stringBuilder4:57
stringBuilder5:57
stringBuilder6:299
stringBuilder7:57
stringBuilder8:57
stringBuilder9:57
Java 8:
stringBuilder0:714
stringBuilder1:367
stringBuilder2:368
stringBuilder3:279
stringBuilder4:271
stringBuilder5:318
stringBuilder6:415
stringBuilder7:406
stringBuilder8:308
stringBuilder9:349

以下是生成此数据的两个类。

StringBuilderTests:

package timings;
public class StringBuilderTests {
public static void main(String[] args) {
    StringBuilderTests  stringBuilderTests  = new StringBuilderTests(args);
    stringBuilderTests.go();
}
public StringBuilderTests(String[] args) {
}
private void go() {
    final String  str1    = "01234567890123456789012345678901234567890123456789012345678901234567890123456789";
    final String  str2    = "abcdefghiabcdefghiabcdefghiabcdefghiabcdefghi";
    final String  str3    = "xxxxxxxxxxxxxxxx";
    final String  str4    = "yyyyyyyyyyyyyyyy";
    final String  str5    = "zzzzzzzzzzzzzzzz";
    System.out.println("Running warmup pass.");
    for (int ctr = 0; ctr < 20000; ++ ctr) {
    testStringBuilder0(str1, str2, str3, str4, str5);
    testStringBuilder1(str1, str2, str3, str4, str5);
    testStringBuilder2(str1, str2, str3, str4, str5);
    testStringBuilder3(str1, str2, str3, str4, str5);
    testStringBuilder4(str1, str2, str3, str4, str5);
    testStringBuilder5(str1, str2, str3, str4, str5);
    testStringBuilder6(str1, str2, str3, str4, str5);
    testStringBuilder7(str1, str2, str3, str4, str5);
    testStringBuilder8(str1, str2, str3, str4, str5);
    testStringBuilder9(str1, str2, str3, str4, str5);
    }
    int     iters   = 10000000;
    System.out.println("Starting timing runs.");
    new TimingContainer(iters, "stringBuilder0", new TimingTest() {
    public void run() {
        testStringBuilder0(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder1", new TimingTest() {
    public void run() {
        testStringBuilder1(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder2", new TimingTest() {
    public void run() {
        testStringBuilder2(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder3", new TimingTest() {
    public void run() {
        testStringBuilder3(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder4", new TimingTest() {
    public void run() {
        testStringBuilder4(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder5", new TimingTest() {
    public void run() {
        testStringBuilder5(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder6", new TimingTest() {
    public void run() {
        testStringBuilder6(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder7", new TimingTest() {
    public void run() {
        testStringBuilder7(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder8", new TimingTest() {
    public void run() {
        testStringBuilder8(str1, str2, str3, str4, str5);
    }
    }).run();
    new TimingContainer(iters, "stringBuilder9", new TimingTest() {
    public void run() {
        testStringBuilder9(str1, str2, str3, str4, str5);
    }
    }).run();
}
private void useit(String str) {
    System.out.print(str.charAt(0));
}
public static String testStringBuilder0(String str1, String str2, String str3, String str4, String str5) {
    String  result  = null;
    result  = result + str1;
    result  = result + str2;
    result  = result + str3;
    result  = result + str4;
    result  = result + str5;
    return result;
}
public static String testStringBuilder1(String str1, String str2, String str3, String str4, String str5) {
    StringBuilder   sb  = new StringBuilder();
    sb.append(str1);
    sb.append("-");
    sb.append(str2);
    sb.append("-");
    sb.append(str3);
    sb.append("-");
    sb.append(str4);
    sb.append("-");
    sb.append(str5);
    return sb.toString();
}
public static String testStringBuilder2(String str1, String str2, String str3, String str4, String str5) {
    return new StringBuilder().
        append(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).
        toString();
}
public static String testStringBuilder3(String str1, String str2, String str3, String str4, String str5) {
    return new StringBuilder(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).toString();
}
public static String testStringBuilder4(String str1, String str2, String str3, String str4, String str5) {
    return str1 + "-" + str2 + "-" + str3 + "-" + str4 + "-" + str5;
}
public static String testStringBuilder5(String str1, String str2, String str3, String str4, String str5) {
    return new StringBuilder(160).
        append(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).
        toString();
}
public static String testStringBuilder6(String str1, String str2, String str3, String str4, String str5) {
    StringBuffer   sb  = new StringBuffer();
    sb.append(str1);
    sb.append("-");
    sb.append(str2);
    sb.append("-");
    sb.append(str3);
    sb.append("-");
    sb.append(str4);
    sb.append("-");
    sb.append(str5);
    return sb.toString();
}
public static String testStringBuilder7(String str1, String str2, String str3, String str4, String str5) {
    return new StringBuffer().
        append(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).
        toString();
}
public static String testStringBuilder8(String str1, String str2, String str3, String str4, String str5) {
    return new StringBuffer(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).toString();
}
public static String testStringBuilder9(String str1, String str2, String str3, String str4, String str5) {
    return new StringBuffer(160).
        append(str1).append("-").append(str2).append("-").append(str3).append("-").append(str4).append("-").append(str5).
        toString();
}
}

定时容器：

package timings;
public class TimingContainer {
private int         iterations;
private String      label;
private TimingTest  timingTest;
public TimingContainer(int iterations, String label, TimingTest timingTest) {
    this.iterations = iterations;
    this.label      = label;
    this.timingTest = timingTest;
}
public void run() {
    long startTime  = System.nanoTime();
    for (int ctr = 0; ctr < iterations; ++ ctr) {
    timingTest.randomize();
    timingTest.run();
    }
    long    endTime = System.nanoTime();
    long    totalns = (endTime - startTime);
    System.out.println(label + ":" + (totalns / iterations));
}
}

更新：

定时测试：

package timings;
public abstract class TimingTest {
public abstract void run();
public void randomize() {}
}

Answer 1

差异在于我将64位Java 7与32位Java 8进行了比较（Spring Tool Suite 3.5.0中的Eclipse显然无法与64位Java 8一起使用，这就是我使用32位Java发行版的原因）。

当我用64位Java 8运行测试时，我得到了更合理的结果。

此外，我还了解了JMH，它对于编写和运行需要精确的微基准测试非常方便。