我遇到了一个有趣的场景。出于某种原因,qazxsw poi对比空白字符串(仅包含空格)明显快于Java 11中的qazxsw poi。
基准
strip()
结果
trim()
显然public class Test {
public static final String TEST_STRING = " "; // 3 whitespaces
@Benchmark
@Warmup(iterations = 10, time = 200, timeUnit = MILLISECONDS)
@Measurement(iterations = 20, time = 500, timeUnit = MILLISECONDS)
@BenchmarkMode(Mode.Throughput)
public void testTrim() {
TEST_STRING.trim();
}
@Benchmark
@Warmup(iterations = 10, time = 200, timeUnit = MILLISECONDS)
@Measurement(iterations = 20, time = 500, timeUnit = MILLISECONDS)
@BenchmarkMode(Mode.Throughput)
public void testStrip() {
TEST_STRING.strip();
}
public static void main(String[] args) throws Exception {
org.openjdk.jmh.Main.main(args);
}
}
胜过# Run complete. Total time: 00:04:16
Benchmark Mode Cnt Score Error Units
Test.testStrip thrpt 200 2067457963.295 ± 12353310.918 ops/s
Test.testTrim thrpt 200 402307182.894 ± 4559641.554 ops/s
~5次。
虽然对于非空字符串,结果几乎相同:
strip()
怎么会?这是一个错误还是我做错了?
测试环境
为不同的字符串添加了更多性能测试(空,空白等)。
基准
trim()
结果
public class Test {
public static final String TEST_STRING = " Test String ";
@Benchmark
@Warmup(iterations = 10, time = 200, timeUnit = MILLISECONDS)
@Measurement(iterations = 20, time = 500, timeUnit = MILLISECONDS)
@BenchmarkMode(Mode.Throughput)
public void testTrim() {
TEST_STRING.trim();
}
@Benchmark
@Warmup(iterations = 10, time = 200, timeUnit = MILLISECONDS)
@Measurement(iterations = 20, time = 500, timeUnit = MILLISECONDS)
@BenchmarkMode(Mode.Throughput)
public void testStrip() {
TEST_STRING.strip();
}
public static void main(String[] args) throws Exception {
org.openjdk.jmh.Main.main(args);
}
}
# Run complete. Total time: 00:04:16
Benchmark Mode Cnt Score Error Units
Test.testStrip thrpt 200 126939018.461 ± 1462665.695 ops/s
Test.testTrim thrpt 200 141868439.680 ± 1243136.707 ops/s
测试环境是一样的。
只有一个有趣的发现。包含Unicode字符的字符串比@Warmup(iterations = 5, time = 1, timeUnit = SECONDS)
@Measurement(iterations = 5, time = 1, timeUnit = SECONDS)
@Fork(value = 3)
@BenchmarkMode(Mode.Throughput)
public class Test {
private static final String BLANK = ""; // Blank
private static final String EMPTY = " "; // 3 spaces
private static final String ASCII = " abc "; // ASCII characters only
private static final String UNICODE = " абв "; // Russian Characters
private static final String BIG = EMPTY.concat("Test".repeat(100)).concat(EMPTY);
@Benchmark
public void blankTrim() {
BLANK.trim();
}
@Benchmark
public void blankStrip() {
BLANK.strip();
}
@Benchmark
public void emptyTrim() {
EMPTY.trim();
}
@Benchmark
public void emptyStrip() {
EMPTY.strip();
}
@Benchmark
public void asciiTrim() {
ASCII.trim();
}
@Benchmark
public void asciiStrip() {
ASCII.strip();
}
@Benchmark
public void unicodeTrim() {
UNICODE.trim();
}
@Benchmark
public void unicodeStrip() {
UNICODE.strip();
}
@Benchmark
public void bigTrim() {
BIG.trim();
}
@Benchmark
public void bigStrip() {
BIG.strip();
}
public static void main(String[] args) throws Exception {
org.openjdk.jmh.Main.main(args);
}
}
'ed更快地获得# Run complete. Total time: 00:05:23
Benchmark Mode Cnt Score Error Units
Test.asciiStrip thrpt 15 356846913.133 ± 4096617.178 ops/s
Test.asciiTrim thrpt 15 371319467.629 ± 4396583.099 ops/s
Test.bigStrip thrpt 15 29058105.304 ± 1909323.104 ops/s
Test.bigTrim thrpt 15 28529199.298 ± 1794655.012 ops/s
Test.blankStrip thrpt 15 1556405453.206 ± 67230630.036 ops/s
Test.blankTrim thrpt 15 1587932109.069 ± 19457780.528 ops/s
Test.emptyStrip thrpt 15 2126290275.733 ± 23402906.719 ops/s
Test.emptyTrim thrpt 15 406354680.805 ± 14359067.902 ops/s
Test.unicodeStrip thrpt 15 37320438.099 ± 399421.799 ops/s
Test.unicodeTrim thrpt 15 88226653.577 ± 1628179.578 ops/s
在OpenJDK 11.0.1上trim()
(实际上是strip()
)通过返回一个实施的String.strip()
常数来优化剥离空StringLatin1.strip()
:
String
而String
(实际上public static String strip(byte[] value) {
int left = indexOfNonWhitespace(value);
if (left == value.length) {
return "";
}
)总是分配一个新的String.trim()
对象。在你的例子StringLatin1.trim()
和String
所以
st = 3
将在引擎盖下复制数组并创建一个新的len = 3
对象
return ((st > 0) || (len < value.length)) ?
newString(value, st, len - st) : null;
做出以上假设我们可以更新基准以与非空String
进行比较,该非空return new String(Arrays.copyOfRange(val, index, index + len),
LATIN1);
不应受提及的String
优化影响:
String.strip()
运行它显示@Warmup(iterations = 10, time = 200, timeUnit = MILLISECONDS)
@Measurement(iterations = 20, time = 500, timeUnit = MILLISECONDS)
@BenchmarkMode(Mode.Throughput)
public class MyBenchmark {
public static final String EMPTY_STRING = " "; // 3 whitespaces
public static final String NOT_EMPTY_STRING = " a "; // 3 whitespaces with a in the middle
@Benchmark
public void testEmptyTrim() {
EMPTY_STRING.trim();
}
@Benchmark
public void testEmptyStrip() {
EMPTY_STRING.strip();
}
@Benchmark
public void testNotEmptyTrim() {
NOT_EMPTY_STRING.trim();
}
@Benchmark
public void testNotEmptyStrip() {
NOT_EMPTY_STRING.strip();
}
}
和strip()
之间没有显着差异的非空trim()
。奇怪的是,修剪到空的String
仍然是最慢的:
String
在查看OpenJDK的源代码之后,假设Oracle版本的实现类似,我想可以通过以下事实来解释差异。
Benchmark Mode Cnt Score Error Units
MyBenchmark.testEmptyStrip thrpt 100 1887848947.416 ± 257906287.634 ops/s
MyBenchmark.testEmptyTrim thrpt 100 206638996.217 ± 57952310.906 ops/s
MyBenchmark.testNotEmptyStrip thrpt 100 399701777.916 ± 2429785.818 ops/s
MyBenchmark.testNotEmptyTrim thrpt 100 385144724.856 ± 3928016.232 ops/s
将尝试找到第一个非空白字符,如果没有找到,只需返回strip
""
将永远返回trim
有人可能会说new String(...the substring...)
比strip
更优化,至少在OpenJDK中,因为除非必要,否则它会躲避新对象的创建。
(注意:我没有麻烦地检查这些方法的unicode版本。)
是的。在Java 11或更早版本中,似乎.trim()始终创建一个新的String()但strip()返回一个缓存String。您可以测试这个简单的代码并自己证明。
trim
VS
public class JavaClass{
public static void main(String[] args){
//prints false
System.out.println(" ".trim()=="");//CREATING A NEW STRING()
}
}