Java代码中字符串拼接方式分析

本文研讨的字符串拼接方式为以下4种:“+”号、StringBuilder、StringJoiner、String#join,对比分析及探讨最佳实践。

结论

后面内容比较枯燥,所以先说结论:

  1. 本文研讨的字符串拼接方式为以下4种:“+”号、StringBuilder、StringJoiner、String#join
  2. 在简单的字符串拼接场景中「如:"a" + "b" + "c"」,以上四种方式性能无明显差异。
  3. 在循环字符串拼接的场景下,使用“+”号性能最低,其他三种方式性能也无明显差异,但是根据验证结果可粗浅发现,指定初始容量的StringBuilder效率最高。当然不光考虑性能,也要考虑垃圾回收效率的问题,避免OOM。
  4. 本文最后补充对比了StringBuffer,在无争抢共享资源的场景下,StringBuffer性能并未明显变差。

最佳实践

  1. 阿里巴巴Java开发手册-日志规约「5」可进行优化:使用占位符的形式可读性、便捷性不佳,可考虑使用Lambda,延迟字符串的拼接,且使用更加便利。
  2. 阿里巴巴Java开发手册-OOP 规约「23」可进行优化:循环拼接时须使用StringBuilder;在拼接大量的大容量字符串时,使用StringBuilder尽量指定初始容量。
  3. 简单的字符串拼接可用任意方式,推荐直接使用“+”号拼接,可读性最优。
  4. 尽量使用JDK等直接提供的特性「如“+”号拼接字符串,Synchronized关键词等」,因为编译器+JVM会持续对此进行优化,JDK升级即可获得更大的收益。除非有明确的理由可以自行实现类似的功能。
  5. 在需要考虑线程安全的场景可以考虑使用StringBuffer进行字符串拼接,不过一般来说没有这种需求,故不应该使用StringBuffer,避免增加复杂性。

分析过程

环境

  1. 系统: windows 10 21H1
  2. JDK: OpenJDK 1.8.0_302
  3. 分析用示例代码:
@Slf4j
public class StringConcat {

    @SneakyThrows
    public static void main(String[] args) {
        log.info("java虚拟机预热开始");
        String[] strs = new String[6000000];
        for (int i = 0; i < strs.length; i++) {
            strs[i] = id();
        }
        loopStringJoiner(strs);
        loopStringJoin(strs);
        loopStringBuilder(strs);
        log.info("java虚拟机预热结束");
        Thread.sleep(1000);
        log.info("开始测试:");

        Thread.sleep(1000);
        Stopwatch stopwatchLoopPlus = Stopwatch.createStarted();
//        loopPlus(strs);
        log.info("loop-plus: " + stopwatchLoopPlus.elapsed(TimeUnit.MILLISECONDS));

        Thread.sleep(1000);
        Stopwatch stopwatchLoopStringBuilderCapacity = Stopwatch.createStarted();
        loopStringBuilderCapacity(strs);
        log.info("loop-stringBuilderCapacity: " + stopwatchLoopStringBuilderCapacity.elapsed(TimeUnit.MILLISECONDS));

        Thread.sleep(1000);
        Stopwatch stopwatchLoopStringBuilder = Stopwatch.createStarted();
        loopStringBuilder(strs);
        log.info("loop-stringBuilder: " + stopwatchLoopStringBuilder.elapsed(TimeUnit.MILLISECONDS));

        Thread.sleep(1000);
        Stopwatch stopwatchLoopJoin = Stopwatch.createStarted();
        loopStringJoin(strs);
        log.info("loop-String.join: " + stopwatchLoopJoin.elapsed(TimeUnit.MILLISECONDS));

        Thread.sleep(1000);
        Stopwatch stopwatchLoopStringJoiner = Stopwatch.createStarted();
        loopStringJoiner(strs);
        log.info("loop-stringJoiner: " + stopwatchLoopStringJoiner.elapsed(TimeUnit.MILLISECONDS));

        Thread.sleep(1000);
        Stopwatch stopwatchSimplePlus = Stopwatch.createStarted();
        for (int i = 0; i < 500000; i++) {
            simplePlus(id(), id(), id());
        }
        log.info("simple-Plus: " + stopwatchSimplePlus.elapsed(TimeUnit.MILLISECONDS));

        Thread.sleep(1000);
        Stopwatch stopwatchSimpleStringBuilder = Stopwatch.createStarted();
        for (int i = 0; i < 500000; i++) {
            simpleStringBuilder(id(), id(), id());
        }
        log.info("simple-StringBuilder: " + stopwatchSimpleStringBuilder.elapsed(TimeUnit.MILLISECONDS));

        Thread.sleep(1000);
        Stopwatch stopwatchSimpleStringBuffer = Stopwatch.createStarted();
        for (int i = 0; i < 500000; i++) {
            simpleStringBuffer(id(), id(), id());
        }
        log.info("simple-StringBuffer: " + stopwatchSimpleStringBuffer.elapsed(TimeUnit.MILLISECONDS));

    }

    private static String loopPlus(String[] strs) {
        String str = "";
        for (String s : strs) {
            str = str + "+" + s;
        }
        return str;
    }

    private static String loopStringBuilder(String[] strs) {
        StringBuilder str = new StringBuilder();
        for (String s : strs) {
            str.append("+");
            str.append(s);
        }
        return str.toString();
    }

    private static String loopStringBuilderCapacity(String[] strs) {
        StringBuilder str = new StringBuilder(strs[0].length() * strs.length);
        for (String s : strs) {
            str.append("+");
            str.append(s);
        }
        return str.toString();
    }

    private static String loopStringJoin(String[] strs) {
        StringJoiner joiner = new StringJoiner("+");
        for (String str : strs) {
            joiner.add(str);
        }
        return joiner.toString();
    }

    private static String loopStringJoiner(String[] strs) {
        return String.join("+", strs);
    }

    private static String simplePlus(String a, String b, String c) {
        return a + "+" + b + "+" + c;
    }

    private static String simpleStringBuilder(String a, String b, String c) {
        StringBuilder builder = new StringBuilder();
        builder.append(a);
        builder.append("+");
        builder.append(b);
        builder.append("+");
        builder.append(c);
        return builder.toString();
    }

    private static String simpleStringBuffer(String a, String b, String c) {
        StringBuffer buffer = new StringBuffer();
        buffer.append(a);
        buffer.append("+");
        buffer.append(b);
        buffer.append("+");
        buffer.append(c);
        return buffer.toString();
    }

    private static String id() {
        return UUID.randomUUID().toString();
    }

}

结果及总结

- java虚拟机预热开始
- java虚拟机预热结束
- 开始测试:
- loop-plus: 执行超时
- loop-stringBuilderCapacity: 285
- loop-stringBuilder: 1968
- loop-String.join: 1313
- loop-stringJoiner: 1238
- simple-Plus: 812
- simple-StringBuilder: 840
- simple-StringBuffer: 857
  1. 多次测试,可发现在字符串循环拼接场景下,直接使用“+”号性能最低,有初始容量的StringBuilder性能最高,其他方式性能均没有太大差异。
  2. 多次测试,可发现在字符串简单拼接场景下,使用“+”号、StringBuilder、StringBuffer性能差距在5%左右,可理解为测试误差,可认为三种方式性能一致。

代码及结果分析

1. StringBuilder与StringBuffer对比

在无争抢共享资源的场景下,JVM会使用偏向锁等方法优化,甚至会进行锁消除,使用Synchronized关键词与否,性能并无明显差异。

2. 字节码分析

对比上述#simplePlus和#simpleStringBuilder两个方法的字节码,可明显看到两方法执行内容基本一致,但是直接使用"+"号时处理流程更短,可见编译器进行了深度优化,使用优化后的字节码理论上会有更高的性能:

  // access flags 0xA
  private static simplePlus(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
    // parameter  a
    // parameter  b
    // parameter  c
   L0
    LINENUMBER 125 L0
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder. ()V
    ALOAD 0
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    LDC "+"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    LDC "+"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    ALOAD 2
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ARETURN
   L1
    LOCALVARIABLE a Ljava/lang/String; L0 L1 0
    LOCALVARIABLE b Ljava/lang/String; L0 L1 1
    LOCALVARIABLE c Ljava/lang/String; L0 L1 2
    MAXSTACK = 2
    MAXLOCALS = 3

  // access flags 0xA
  private static simpleStringBuilder(Ljava/lang/String;Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
    // parameter  a
    // parameter  b
    // parameter  c
   L0
    LINENUMBER 129 L0
    NEW java/lang/StringBuilder
    DUP
    INVOKESPECIAL java/lang/StringBuilder. ()V
    ASTORE 3
   L1
    LINENUMBER 130 L1
    ALOAD 3
    ALOAD 0
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    POP
   L2
    LINENUMBER 131 L2
    ALOAD 3
    LDC "+"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    POP
   L3
    LINENUMBER 132 L3
    ALOAD 3
    ALOAD 1
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    POP
   L4
    LINENUMBER 133 L4
    ALOAD 3
    LDC "+"
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    POP
   L5
    LINENUMBER 134 L5
    ALOAD 3
    ALOAD 2
    INVOKEVIRTUAL java/lang/StringBuilder.append (Ljava/lang/String;)Ljava/lang/StringBuilder;
    POP
   L6
    LINENUMBER 135 L6
    ALOAD 3
    INVOKEVIRTUAL java/lang/StringBuilder.toString ()Ljava/lang/String;
    ARETURN
   L7
    LOCALVARIABLE a Ljava/lang/String; L0 L7 0
    LOCALVARIABLE b Ljava/lang/String; L0 L7 1
    LOCALVARIABLE c Ljava/lang/String; L0 L7 2
    LOCALVARIABLE builder Ljava/lang/StringBuilder; L1 L7 3
    MAXSTACK = 2
    MAXLOCALS = 4

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