我正在研究 String.equals() 是否真的很糟糕，并且在尝试对其进行一些基准测试时遇到了一些令人惊讶的结果。
使用 jmh ，我写了一个简单的测试(最后是代码和 pom)，看看该函数可以在 1 秒内运行多少次。
Benchmark Mode Samples Score Score error Units c.s.SimpleBenchmark.testEqualsIntern thrpt 5 698910949.710 47115846.650 ops/s c.s.SimpleBenchmark.testEqualsNew thrpt 5 529118.774 21164.872 ops/s c.s.SimpleBenchmark.testIsEmpty thrpt 5 470846539.546 19922172.099 ops/s
The this is a 1300x factor between testEqualsIntern and testEqualsNew which is frankly quite surprising to me.
The code for String.equals() does have a test for the same object, which would kick the identical (interned in this case) string objects out quite quickly. I just have have significant difficulty believing that the additional code which appears to amount to walking over an array of size 1 for the two tests and comparing elements is that much of a performance hit.
I've also put in a test with another simple method call in the String to make sure I wasn't seeing something that is too crazy.
package com.shagie;
import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.runner.Runner;
import org.openjdk.jmh.runner.RunnerException;
import org.openjdk.jmh.runner.options.Options;
import org.openjdk.jmh.runner.options.OptionsBuilder;
public class SimpleBenchmark {
public final static int ITERATIONS = 1000;
public final static String EMPTY = "";
public final static String NEW_EMPTY = new String("");
@Benchmark
public int testEqualsIntern() {
int count = 0;
String str = EMPTY;
for(int i = 0; i < ITERATIONS; i++) {
if(str.equals(EMPTY)) {
count++;
}
}
return count;
}
@Benchmark
public int testEqualsNew() {
int count = 0;
String str = NEW_EMPTY;
for(int i = 0; i < ITERATIONS; i++) {
if(str.equals(EMPTY)) {
count++;
}
}
return count;
}
@Benchmark
public int testIsEmpty() {
int count = 0;
String str = NEW_EMPTY;
for(int i = 0; i < ITERATIONS; i++) {
if(str.isEmpty()) {
count++;
}
}
return count;
}
public static void main(String[] args) throws RunnerException {
Options opt = new OptionsBuilder()
.include(".*" + SimpleBenchmark.class.getSimpleName() + ".*")
.warmupIterations(5)
.measurementIterations(5)
.forks(1)
.build();
new Runner(opt).run();
}
}
maven 的 .pom(如果您想重现它，可以自己快速设置它):
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.shagie</groupId>
<artifactId>bench</artifactId>
<version>1.0</version>
<packaging>jar</packaging>
<name>String Benchmarks with JMH</name>
<prerequisites>
<maven>3.0</maven>
</prerequisites>
<dependencies>
<dependency>
<groupId>org.openjdk.jmh</groupId>
<artifactId>jmh-core</artifactId>
<version>${jmh.version}</version>
</dependency>
<dependency>
<groupId>org.openjdk.jmh</groupId>
<artifactId>jmh-generator-annprocess</artifactId>
<version>${jmh.version}</version>
<scope>provided</scope>
</dependency>
</dependencies>
<properties>
<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
<jmh.version>0.9.5</jmh.version>
<javac.target>1.6</javac.target>
<uberjar.name>benchmarks</uberjar.name>
</properties>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<version>3.1</version>
<configuration>
<compilerVersion>${javac.target}</compilerVersion>
<source>${javac.target}</source>
<target>${javac.target}</target>
</configuration>
</plugin>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-shade-plugin</artifactId>
<version>2.2</version>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>shade</goal>
</goals>
<configuration>
<finalName>${uberjar.name}</finalName>
<transformers>
<transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer">
<mainClass>org.openjdk.jmh.Main</mainClass>
</transformer>
</transformers>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
<pluginManagement>
<plugins>
<plugin>
<artifactId>maven-clean-plugin</artifactId>
<version>2.5</version>
</plugin>
<plugin>
<artifactId>maven-deploy-plugin</artifactId>
<version>2.8.1</version>
</plugin>
<plugin>
<artifactId>maven-install-plugin</artifactId>
<version>2.5.1</version>
</plugin>
<plugin>
<artifactId>maven-jar-plugin</artifactId>
<version>2.4</version>
</plugin>
<plugin>
<artifactId>maven-javadoc-plugin</artifactId>
<version>2.9.1</version>
</plugin>
<plugin>
<artifactId>maven-resources-plugin</artifactId>
<version>2.6</version>
</plugin>
<plugin>
<artifactId>maven-site-plugin</artifactId>
<version>3.3</version>
</plugin>
<plugin>
<artifactId>maven-source-plugin</artifactId>
<version>2.2.1</version>
</plugin>
<plugin>
<artifactId>maven-surefire-plugin</artifactId>
<version>2.17</version>
</plugin>
</plugins>
</pluginManagement>
</build>
</project>
这是自动生成的(对组和工件进行了适当的调整):
$ mvn archetype:generate \
-DinteractiveMode=false \
-DarchetypeGroupId=org.openjdk.jmh \
-DarchetypeArtifactId=jmh-java-benchmark-archetype \
-DgroupId=org.sample \
-DartifactId=test \
-Dversion=1.0
运行测试:
$ mvn clean install
$ java -jar target/benchmarks.jar ".*SimpleBenchmark.*" -wi 5 -i 5 -f 1
这将是一个问题，它运行的 Java 版本是:
$ java -version
java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-462-11M4609)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-462, mixed mode)
硬件(可能会有问题)是 OS X，Intel Xeon 处理器上的 10.9.4。
最佳答案
编写有缺陷的微基准测试非常容易……而您会陷入困境。
了解发生了什么的唯一方法是查看汇编代码。您必须自己检查生成的代码是否符合您的预期，或者是否发生了一些不需要的魔法。让我们一起尝试吧。您必须使用 addProfile(LinuxPerfAsmProfiler.class) 才能查看汇编代码。
testEqualsIntern 的汇编代码是什么:
....[Hottest Region 1]..............................................................................
[0x7fb9e11acda0:0x7fb9e11acdc8] in org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@19 (line 103)
0x00007fb9e11acd82: movzbl 0x94(%rdx),%r11d ;*getfield isDone
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@29 (line 105)
0x00007fb9e11acd8a: mov $0x2,%ebp
0x00007fb9e11acd8f: test %r11d,%r11d
0x00007fb9e11acd92: jne 0x00007fb9e11acdcc ;*ifeq
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@32 (line 105)
0x00007fb9e11acd94: nopl 0x0(%rax,%rax,1)
0x00007fb9e11acd9c: xchg %ax,%ax ;*aload
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@13 (line 103)
6.50% 3.37% 0x00007fb9e11acda0: mov 0xb0(%rdi),%r11d ;*getfield i1
; - org.openjdk.jmh.infra.Blackhole::consume@2 (line 350)
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@19 (line 103)
0.06% 0.05% 0x00007fb9e11acda7: mov 0xb4(%rdi),%r10d ;*getfield i2
; - org.openjdk.jmh.infra.Blackhole::consume@15 (line 350)
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@19 (line 103)
0.06% 0.09% 0x00007fb9e11acdae: cmp $0x3e8,%r10d
0.03% 0x00007fb9e11acdb5: je 0x00007fb9e11acdf1 ;*return
; - org.openjdk.jmh.infra.Blackhole::consume@38 (line 354)
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@19 (line 103)
48.85% 44.47% 0x00007fb9e11acdb7: movzbl 0x94(%rdx),%ecx ;*getfield isDone
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@29 (line 105)
0.33% 0.62% 0x00007fb9e11acdbe: add $0x1,%rbp ; OopMap{r9=Oop rbx=Oop rdi=Oop rdx=Oop off=226}
;*ifeq
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@32 (line 105)
0.03% 0.05% 0x00007fb9e11acdc2: test %eax,0x16543238(%rip) # 0x00007fb9f76f0000
; {poll}
42.31% 49.43% 0x00007fb9e11acdc8: test %ecx,%ecx
0x00007fb9e11acdca: je 0x00007fb9e11acda0 ;*aload_2
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@35 (line 106)
0x00007fb9e11acdcc: mov $0x7fb9f706fe40,%r10
0x00007fb9e11acdd6: callq *%r10 ;*invokestatic nanoTime
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@36 (line 106)
0x00007fb9e11acdd9: mov %rbp,0x10(%rbx) ;*putfield operations
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@51 (line 108)
0x00007fb9e11acddd: mov %rax,0x28(%rbx) ;*putfield stopTime
; - org.sample.generated.MyBenchmark_testEqualsIntern::testEqualsIntern_thrpt_jmhLoop@39 (line 106)
....................................................................................................
您可能知道，JMH 获取您的基准测试代码并将其插入到自己的测量循环中。您可以通过查看 target/generated-sources 文件夹轻松查看生成的代码。您必须了解此代码的外观才能将其与程序集进行比较。
有趣的部分在这里:
public void testEqualsIntern_avgt_jmhLoop(InfraControl control, RawResults result, MyBenchmark_1_jmh l_mybenchmark0_0, Blackhole_1_jmh l_blackhole1_1) throws Throwable {
long operations = 0;
long realTime = 0;
result.startTime = System.nanoTime();
do {
l_blackhole1_1.consume(l_mybenchmark0_0.testEqualsIntern());
operations++;
} while(!control.isDone);
result.stopTime = System.nanoTime();
result.realTime = realTime;
result.operations = operations;
}
好吧，你看到这个漂亮的 do/while 循环做了两件事:
- 调用你的函数
- 调用 consume 来防止 Hotspot 不必要的优化？
现在让我们回到装配体。尝试在其中找到这三个操作(循环、使用和您的代码)。你能 ？
可以看到JMH循环，就是0x00007fb9e11acdb7: movzbl 0x94(%rdx),%ecx ;*getfield isDone和下面的跳转。
可以看到黑洞，就是从0x00007fb9e11acda0到0x00007fb9e11acdb5:
但是你的代码在哪里？没了。您没有遵循 JMH 的良好做法，并允许 Hotspot 删除您的代码。您正在对 NOOP 进行基准测试。顺便说一句，你有没有试过对 NOOP 进行基准测试？这是一件好事，当您看到接近这个数字时，您就知道必须非常小心。
您可以对第二个基准进行相同的分析。我没有仔细阅读它的汇编代码，但您将能够发现您的 for 循环和对 equals 的调用。您可以让他们再次阅读 JMH 示例，以尽量避免此类问题。
TL;DR 编写正确的微/纳米基准非常困难，你应该仔细检查你是否知道你测量的是什么。集会是唯一的出路。观看所有演示文稿并阅读 Aleksey 的所有博客文章以了解更多信息。他做得很好。最后，此类测量在现实生活中几乎总是无用的，但却是一种很好的学习工具。
关于java - 为什么 String.equals 对于不相同(但相等)的 String 对象要慢得多？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/25351490/