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：
- 本文研讨的字符串拼接方式为以下4种：“+”号、StringBuilder、StringJoiner、String#join
- 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。
- 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。
- 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
- 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。
- 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。
- 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。
- 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。
- 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
- 系统: windows 10 21H1
- JDK: OpenJDK 1.8.0_302
- 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- 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。
- 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