StringBuilder vs. .concat vs. “+"eclipse中的操作员相对性能与命令行不同?

2022-01-16 00:00:00 jvm java eclipse

我正在阅读有关 java 编译器如何在可能的情况下将与+"运算符连接的字符串编译为 StringBuilder 的实例，以及如何更好地使用简单的+"运算符，因为它们 编译成相同的代码.(除非您在 while 循环中构建字符串，在这种情况下显然最好使用 StringBuilder.)

I was reading about how when possible the java compiler will compile strings concatenated with the "+" operator into instances of StringBuilder, and how this makes it better to use the simple "+" operator since they compile to the same code. (Except when you are building the string in a while loop, in which case it is clearly best to use a StringBuilder.)

我还了解到，字符串上的 .concat 方法一直是最糟糕的选择(以至于它被 Findbugs 变成了一个错误！).

I've also read that the .concat method on strings is the worst choice all the time (so much so that it was made into a bug by Findbugs!).

所以我决定自己在eclipse中编写一个小的java类来测试它.我的结果让我有点吃惊.我发现，如果我在 Eclipse 和命令行中编译并运行它们，不同的方法相对更快或更慢.

So I decided to test it myself writing a little java class in eclipse. My results surprised me a bit. What I found was that different methods were relatively faster or slower if I complied and ran them in eclipse vs. on the command line.

首先我的日食结果是:

the total millis to concatenate with + was: 12154
the total millis to concatenate with .concat was: 8840
the total millis to concatenate with StringBuilder was: 11350
the total millis to concatenate with StringBuilder with a specified size was: 5611

所以在eclipse中指定大小的StringBuilder最快，其次是.concat(奇怪)，然后StringBuilder和+"连接几乎相同.

So in eclipse StringBuilder with the size specified was fastest, followed by .concat (weird), then StringBuilder and "+" concatenation were pretty much the same.

然而，我在命令行上的结果是:

My results on the command line, however, were:

the total millis to concatenate with + was: 4139
the total millis to concatenate with .concat was: 8590
the total millis to concatenate with StringBuilder was: 10888
the total millis to concatenate with StringBuilder with a specified size was: 6033

所以当我从命令行编译并运行时，+"运算符显然是最快的，其次是带有大小的 String builder，然后是 concat，最后是普通 StringBuilder！

So when I compiled and ran from the commnad line the "+" operator was clearly the fastest, followed by String builder with size, then concat, and last was normal StringBuilder!

这对我来说没有意义.显然，我读到的所有 stackoverflow 答案都说 + 运算符编译成普通的旧 StringBuilder 实例必须是过时的.

This doesn't make sense to me. Obviously all the stackoverflow answers I read saying that + operators compile into normal old StringBuilder instances must be outdated.

有人知道这里到底发生了什么吗?

Does anyone know what's really going on here?

我正在使用 jdk1.7.0_07，据我所知，eclipse 和我的命令行都引用了完全相同的一个.我知道的唯一区别是 eclipse 是使用javaw"，但从我读过的内容来看，这应该没什么区别.

I'm using jdk1.7.0_07, and so far as I can tell both eclipse and my command line are referencing the exact same one. The only difference I know of is eclipse is using "javaw", but from what I've read, that shouldn't make a difference.

如果你想验证我没有做错什么，这是我的测试课程，但我很确定它是可靠的.

Here's my test class if you want to verify I'm not doing anything wrong, but I'm pretty sure it's solid.

public class Test {

    static final int LOOPS = 100000000;
    static final String FIRST_STRING = "This is such";
    static final String SECOND_STRING = " an awesomely cool ";
    static final String THIRD_STRING = "to write string.";

    /**
     * @param args
     */
    public static void main(String[] args) {

        Test.plusOperator();
        Test.dotConcat();
        Test.stringBuilder();
        Test.stringBuilderSizeSpecified();

    }

    public static void plusOperator() {
        String localOne = FIRST_STRING;
        String localTwo = SECOND_STRING;
        String localThree = THIRD_STRING;

        Calendar startTime = Calendar.getInstance();
        for (int x = 0; x < LOOPS; x++) {
            String toPrint = localOne + localTwo + localThree;
        }
        Calendar endTime = Calendar.getInstance();
        System.out.println("the total millis to concatenate with + was: " + 
                (endTime.getTimeInMillis() - startTime.getTimeInMillis()));
    }

    public static void stringBuilder() {
        String localOne = FIRST_STRING;
        String localTwo = SECOND_STRING;
        String localThree = THIRD_STRING;

        Calendar startTime = Calendar.getInstance();
        for (int x = 0; x < LOOPS; x++) {
            StringBuilder toBuild = new StringBuilder()
                .append(localOne)
                .append(localTwo)
                .append(localThree);
        }
        Calendar endTime = Calendar.getInstance();
        System.out.println("the total millis to concatenate with StringBuilder was: " + 
                (endTime.getTimeInMillis() - startTime.getTimeInMillis()));
    }

    public static void stringBuilderSizeSpecified() {
        String localOne = FIRST_STRING;
        String localTwo = SECOND_STRING;
        String localThree = THIRD_STRING;

        Calendar startTime = Calendar.getInstance();
        for (int x = 0; x < LOOPS; x++) {
            StringBuilder toBuild = new StringBuilder(50)
                .append(localOne)
                .append(localTwo)
                .append(localThree);
        }
        Calendar endTime = Calendar.getInstance();
        System.out.println("the total millis to concatenate with StringBuilder with a specified size was: " + 
                (endTime.getTimeInMillis() - startTime.getTimeInMillis()));
    }

    public static void dotConcat() {
        String localOne = FIRST_STRING;
        String localTwo = SECOND_STRING;
        String localThree = THIRD_STRING;

        Calendar startTime = Calendar.getInstance();
        for (int x = 0; x < LOOPS; x++) {
            String toPrint = localOne.concat(localTwo).concat(localThree);
        }
        Calendar endTime = Calendar.getInstance();
        System.out.println("the total millis to concatenate with .concat was: " + 
                (endTime.getTimeInMillis() - startTime.getTimeInMillis()));
    }

}

推荐答案

在 Oracle JDK 1.7 (javac 1.7.0_17) 上，+"运算符仍然使用 StringBuilder 实现，如运行所示javap -c 在类上获取字节码(这里只显示循环):

On Oracle JDK 1.7 (javac 1.7.0_17), the "+" operator is still implemented using StringBuilder, as shown by running javap -c on the class to get the bytecode (only showing the loops here):

public static void plusOperator();
Code:

  16: iload         4
  18: ldc           #10                 // int 100000000
  20: if_icmpge     53
  23: new           #11                 // class java/lang/StringBuilder
  26: dup           
  27: invokespecial #12                 // Method java/lang/StringBuilder."<init>":()V
  30: aload_0       
  31: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  34: aload_1       
  35: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  38: aload_2       
  39: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  42: invokevirtual #14                 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
  45: astore        5
  47: iinc          4, 1
  50: goto          16


public static void stringBuilder();
Code:

  16: iload         4
  18: ldc           #10                 // int 100000000
  20: if_icmpge     50
  23: new           #11                 // class java/lang/StringBuilder
  26: dup           
  27: invokespecial #12                 // Method java/lang/StringBuilder."<init>":()V
  30: aload_0       
  31: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  34: aload_1       
  35: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  38: aload_2       
  39: invokevirtual #13                 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
  42: astore        5
  44: iinc          4, 1
  47: goto          16

这两者之间唯一的区别是带+"的版本在循环中将StringBuilder转换为String.

The only difference between these two is that the version with "+" converts the StringBuilder to a String within the loop.

所以问题变成了:为什么您的测试会针对相同的代码显示如此不同的结果.或者更完整地说，为什么这不是一个有效的微基准测试.以下是一些可能的原因:

So the question becomes: why does your test show such different results for the same code. Or more completely, why is this not a valid micro-benchmark. Here are some possible reasons:

  • 您正在计算挂钟时间.这意味着您实际上是在测量 JVM 在运行测试时所做的一切.其中包括垃圾收集(这很重要，因为您正在创建大量垃圾).您可以通过获取线程 CPU 时间来缓解这种情况.
  • 您无法验证 HotSpot 何时或是否正在编译方法.这就是为什么您应该在任何微基准测试之前进行预热阶段:基本上，在运行实际测试之前多次运行 main().

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