2019独角兽企业重金招聘Python工程师标准>>>
最近由于项目需要,需要比较各种情况下进行字符串拼接的性能。主要的字符串拼接方法有下面四种:
- 字符串加法: “Hello” + "Word";
- StringBuilder: new StringBuilder("Hello").append("World");
- 调用String.format模板方法: String.format("%s%s","Hello","World");
- 笔者自己编写的slf4j风格的PlaceholderFormat: FastStringUtils.placeholderFomat("{}{}","Hello","World")
JMH是openJDK JIT小组研发的微基准测试框架,结果还是比较可靠的。
但是,在编写测试用例的时候,有几点需要注意:
- 不要在测试方法内部创建循环,框架本身会多次调用;
- 不要对常量进行字符串加减操作来测试性能,JIT非常聪明,可能会优化成运行时常量,测试毫无意义;
- 尽量减少不必要的操作,比如动态生成字符串,要提前创建好
以下是测试用例:
@State(Scope.Thread)
@BenchmarkMode(Mode.AverageTime)
@OutputTimeUnit(TimeUnit.NANOSECONDS)
@Warmup(iterations = 3)
@Measurement(iterations = 7)
public class StringFormatTest {
private static final int SIZE = 0x4ff;
private int index = 0;
private final String[] a = new String[SIZE];
private final String[] b = new String[SIZE];
private final String[] c = new String[SIZE];
@Setup
public void setup() {
Random random = new Random();
for (int i = 0; i < SIZE; i++) {
a[i] = random.nextInt() + "";
b[i] = random.nextLong() + "";
c[i] = random.nextDouble() + "";
}
}
@Benchmark
public String testStringFormat() {
if (++index >= SIZE)
index = 0;
return String.format("%s:%s:%s", a[index], b[index], c[index]);
}
@Benchmark
public String testStringAdd() {
if (++index >= SIZE)
index = 0;
return a[index] + ':' + b[index] + ':' + c[index];
}
@Benchmark
public String testStringBuilder() {
if (++index >= SIZE)
index = 0;
return new StringBuilder(a[index])
.append(':')
.append(b[index])
.append(':')
.append(c[index])
.toString();
}
@Benchmark
public String testPlaceholderFormat() {
if (++index >= SIZE)
index = 0;
return FastStringUtils.placeholderFormat("{}:{}:{}", a[index], b[index], c[index]);
}
}
测试结果如下:
Benchmark Mode Cnt Score Error Units
StringFormatTest.testPlaceholderFormat avgt 70 130.732 ± 2.519 ns/op
StringFormatTest.testStringAdd avgt 70 102.576 ± 1.368 ns/op
StringFormatTest.testStringBuilder avgt 70 94.908 ± 1.097 ns/op
StringFormatTest.testStringFormat avgt 70 1156.455 ± 16.516 ns/op
总结一下:
- String.format虽然有着更好的可读性,但性能远远不及其它方式,在性能敏感的场景应该谨慎使用;
- StringBuilder是最快的;
- String加法和StringBuilder性能差别不大,具体可能与编译器的优化有关系;
- placeholderFomat性能不差,且有着不错的可读性
另,贴上placeholderFomat源码:
public static String placeholderFormat(String s, Object... args) {
int len1 = s.length(), len2 = args.length;
StringBuilder builder = new StringBuilder();
int i = 0, j = 0;
while (i < len1) {
char c = s.charAt(i);
if (c == '{') {
if (s.charAt(i + 1) == '}' && j < len2) {
builder.append(args[j++]);
i += 2;
continue;
} else {
builder.append('{');
}
} else {
builder.append(c);
}
i++;
}
return builder.toString();
}