JMH使用说明
一、
性能往往是特定情景下的评价，泛泛地说性能“好”或者“快”，往是具有误导性的。通过引入基准测试，我们可以定义性能对比的明确条件、具体的指标，进而保证得到定量的、可重复的对比数据，这是工程中的实际需要。
不同的基准测试其具体内容和范围也存在很大的不同。如果是专业的性能工程师，更加熟悉的可能是类似SPEC提供的工业标准的系统级测试；而对于大多数 Java 开发者，更熟悉的则是范围相对较小、关注点更加细节的微基准测试（Micro-Benchmark）。
目前应用最为广泛的框架之一就是JMH，OpenJDK 自身也大量地使用 JMH 进行性能对比，如果你是做 Java API 级别的性能对比，JMH 往往是你的首选。
二、如果要在现有Maven项目中使用JMH，只需要把生成出来的两个依赖以及shade插件拷贝到项目的pom中即可：
<dependency> <groupId>org.openjdk.jmh</groupId> <artifactId>jmh-core</artifactId> <!-- https://mvnrepository.com/artifact/org.openjdk.jmh/jmh-core --> <version>1.19</version> </dependency> <dependency> <groupId>org.openjdk.jmh</groupId> <artifactId>jmh-generator-annprocess</artifactId> <version>1.19</version> <scope>provided</scope> </dependency> <build> <plugins> <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-shade-plugin</artifactId> <version>2.0</version> <executions> <execution> <phase>package</phase> <goals> <goal>shade</goal> </goals> <configuration> <finalName>microbenchmarks</finalName> <transformers> <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer"> <mainClass>org.openjdk.jmh.Main</mainClass> </transformer> </transformers> </configuration> </execution> </executions> </plugin> </plugins> </build>
TestJmh.java
package com.jmh; import java.util.concurrent.TimeUnit; import org.openjdk.jmh.annotations.Benchmark; import org.openjdk.jmh.annotations.BenchmarkMode; import org.openjdk.jmh.annotations.Mode; import org.openjdk.jmh.annotations.OutputTimeUnit; import org.openjdk.jmh.annotations.Scope; import org.openjdk.jmh.annotations.State; 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; @BenchmarkMode(Mode.Throughput) // 测试方法平均执行时间 @OutputTimeUnit(TimeUnit.MICROSECONDS) // 输出结果的时间粒度为微秒 @State(Scope.Thread) // 每个测试线程一个实例 public class TestJmh { @Benchmark public String stringConcat() { String a = "a"; String b = "b"; String c = "c"; String s = a + b + c; System.out.println(s); return s; } public static void main(String[] args) throws RunnerException { Options opt = new OptionsBuilder().include(TestJmh.class.getSimpleName()).forks(1).warmupIterations(5) .measurementIterations(5).build(); new Runner(opt).run(); } }
三、详细说明
3.1 基本概念
首先看看JMH的几个基本概念：
Mode
Mode 表示 JMH 进行 Benchmark 时所使用的模式。通常是测量的维度不同，或是测量的方式不同。目前 JMH 共有四种模式：
Throughput: 整体吞吐量，例如“1秒内可以执行多少次调用”。
AverageTime: 调用的平均时间，例如“每次调用平均耗时xxx毫秒”。
SampleTime: 随机取样，最后输出取样结果的分布，例如“99%的调用在xxx毫秒以内，99.99%的调用在xxx毫秒以内”
SingleShotTime: 以上模式都是默认一次 iteration 是 1s，唯有 SingleShotTime 是只运行一次。往往同时把 warmup 次数设为0，用于测试冷启动时的性能。
Iteration
Iteration 是 JMH 进行测试的最小单位。在大部分模式下，一次 iteration 代表的是一秒，JMH 会在这一秒内不断调用需要 benchmark 的方法，然后根据模式对其采样，计算吞吐量，计算平均执行时间等。
Warmup
Warmup 是指在实际进行 benchmark 前先进行预热的行为。为什么需要预热？因为 JVM 的 JIT 机制的存在，如果某个函数被调用多次之后，JVM 会尝试将其编译成为机器码从而提高执行速度。为了让 benchmark 的结果更加接近真实情况就需要进行预热。
3.2 注解与选项
3.2.1 常用注解说明
@BenchmarkMode
对应Mode选项，可用于类或者方法上， 需要注意的是，这个注解的value是一个数组，可以把几种Mode集合在一起执行，还可以设置为Mode.All，即全部执行一遍。
@State
类注解，JMH测试类必须使用@State注解，State定义了一个类实例的生命周期，可以类比Spring Bean的Scope。由于JMH允许多线程同时执行测试，不同的选项含义如下：
Scope.Thread：默认的State，每个测试线程分配一个实例；
Scope.Benchmark：所有测试线程共享一个实例，用于测试有状态实例在多线程共享下的性能；
Scope.Group：每个线程组共享一个实例；
@OutputTimeUnit
benchmark 结果所使用的时间单位，可用于类或者方法注解，使用java.util.concurrent.TimeUnit中的标准时间单位。
@Benchmark
方法注解，表示该方法是需要进行 benchmark 的对象。
@Setup
方法注解，会在执行 benchmark 之前被执行，正如其名，主要用于初始化。
@TearDown
方法注解，与@Setup 相对的，会在所有 benchmark 执行结束以后执行，主要用于资源的回收等。
@Param
成员注解，可以用来指定某项参数的多种情况。特别适合用来测试一个函数在不同的参数输入的情况下的性能。@Param注解接收一个String数组，在@setup方法执行前转化为为对应的数据类型。多个@Param注解的成员之间是乘积关系，譬如有两个用@Param注解的字段，第一个有5个值，第二个字段有2个值，那么每个测试方法会跑5*2=10次。
原文：https://blog.csdn.net/lxbjkben/article/details/79410740
JMH使用说明一、概述JMH，即Java Microbenchmark Harness，是专门用于代码微基准测试的工具套件。何谓Micro Benchmark呢？简单的来说就是基于方法层面的基准测试，精度可以达到微秒级。当你定位到热点方法，希望进一步优化方法性能的时候，就可以使用JMH对优化的结果进行量化的分析。和其他竞品相比——如果有的话，JMH最有特色的地方就是，它是由Oracle内部实现JIT的那拨人开发的，对于JIT以及JVM所谓的“profile guided optimization”对基准测试准确性的影响可谓心知肚明(smile)
JMH比较典型的应用场景有：
想准确的知道某个方法需要执行多长时间，以及执行时间和输入之间的相关性；对比接口不同实现在给定条件下的吞吐量；查看多少百分比的请求在多长时间内完成；二、第一个例子接下来，我们看看如何使用JMH。
要使用JMH，首先需要准备好Maven环境，JMH的源代码以及官方提供的Sample就是使用Maven进行项目管理的，github上也有使用gradle的例子可自行搜索参考。使用mvn命令行创建一个JMH工程：
mvn archetype:generate \ -DinteractiveMode=false \ -DarchetypeGroupId=org.openjdk.jmh \ -DarchetypeArtifactId=jmh-java-benchmark-archetype \ -DgroupId=co.speedar.infra \ -DartifactId=jmh-test \ -Dversion=1.01234567如果要在现有Maven项目中使用JMH，只需要把生成出来的两个依赖以及shade插件拷贝到项目的pom中即可：
<dependency> <groupId>org.openjdk.jmh</groupId> <artifactId>jmh-core</artifactId> <version>0.7.1</version> </dependency> <dependency> <groupId>org.openjdk.jmh</groupId> <artifactId>jmh-generator-annprocess</artifactId> <version>0.7.1</version> <scope>provided</scope> </dependency>... <plugin> <groupId>org.apache.maven.plugins</groupId> <artifactId>maven-shade-plugin</artifactId> <version>2.0</version> <executions> <execution> <phase>package</phase> <goals> <goal>shade</goal> </goals> <configuration> <finalName>microbenchmarks</finalName> <transformers> <transformer implementation="org.apache.maven.plugins.shade.resource.ManifestResourceTransformer"> <mainClass>org.openjdk.jmh.Main</mainClass> </transformer> </transformers> </configuration> </execution> </executions> </plugin>123456789101112131415161718192021222324252627282930313233然后，就可以着手写第一个JMH例子了：
package co.speedar.infra.test;import java.util.concurrent.TimeUnit;import org.openjdk.jmh.annotations.Benchmark;import org.openjdk.jmh.annotations.BenchmarkMode;import org.openjdk.jmh.annotations.Mode;import org.openjdk.jmh.annotations.OutputTimeUnit;import org.openjdk.jmh.annotations.Scope;import org.openjdk.jmh.annotations.State;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;import org.slf4j.Logger;import org.slf4j.LoggerFactory;@BenchmarkMode(Mode.AverageTime) // 测试方法平均执行时间@OutputTimeUnit(TimeUnit.MICROSECONDS) // 输出结果的时间粒度为微秒@State(Scope.Thread) // 每个测试线程一个实例public class FirstBenchMark { private static Logger log = LoggerFactory.getLogger(FirstBenchMark.class); @Benchmark public String stringConcat() { String a = "a"; String b = "b"; String c = "c"; String s = a + b + c; log.debug(s); return s; } public static void main(String[] args) throws RunnerException { // 使用一个单独进程执行测试，执行5遍warmup，然后执行5遍测试 Options opt = new OptionsBuilder().include(FirstBenchMark.class.getSimpleName()).forks(1).warmupIterations(5) .measurementIterations(5).build(); new Runner(opt).run(); }}1234567891011121314151617181920212223242526272829303132333435在上面的测试代码中，加了几个类注解以及一个方法注解，在main方法中指明了测试的一些选项，然后使用JMH提供的Runner执行测试。在注释中提供了大致的讲解，具体的选项说明后边再详述。接下来我们直接跑起来这个测试看看结果如何。执行测试，可能会遇到报错： Exception in thread "main" java.lang.RuntimeException: ERROR: Unable to find the resource: /META-INF/BenchmarkList 解决方法：
先执行mvn clean install然后再在ide中执行main方法；或者在eclipse中安装m2e-apt插件，然后启用Automatically configure JDT APT选项；
然后，就可以愉快地看到测试结果如下：
# JMH 1.14.1 (released 525 days ago, please consider updating!)# VM version: JDK 1.8.0_91, VM 25.91-b14# VM invoker: /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/bin/java# VM options: -Dfile.encoding=UTF-8# Warmup: 5 iterations, 1 s each# Measurement: 5 iterations, 1 s each# Timeout: 10 min per iteration# Threads: 1 thread, will synchronize iterations# Benchmark mode: Average time, time/op# Benchmark: co.speedar.infra.test.FirstBenchMark.stringConcat# Run progress: 0.00% complete, ETA 00:00:10# Fork: 1 of 1# Warmup Iteration 1: 0.009 us/op# Warmup Iteration 2: 0.011 us/op# Warmup Iteration 3: 0.007 us/op# Warmup Iteration 4: 0.006 us/op# Warmup Iteration 5: 0.006 us/opIteration 1: 0.006 us/opIteration 2: 0.005 us/opIteration 3: 0.005 us/opIteration 4: 0.006 us/opIteration 5: 0.006 us/op
Result "stringConcat": 0.006 ±(99.9%) 0.001 us/op [Average] (min, avg, max) = (0.005, 0.006, 0.006), stdev = 0.001 CI (99.9%): [0.005, 0.006] (assumes normal distribution)
# Run complete. Total time: 00:00:10Benchmark Mode Cnt Score Error UnitsFirstBenchMark.stringConcat avgt 5 0.006 ± 0.001 us/op12345678910111213141516171819202122232425262728293031测试结果表明，被测试方法平均耗时为0.006微秒，误差为±0.001微秒。
三、详细说明3.1 基本概念首先看看JMH的几个基本概念：
Mode Mode 表示 JMH 进行 Benchmark 时所使用的模式。通常是测量的维度不同，或是测量的方式不同。目前 JMH 共有四种模式：
Throughput: 整体吞吐量，例如“1秒内可以执行多少次调用”。
AverageTime: 调用的平均时间，例如“每次调用平均耗时xxx毫秒”。
SampleTime: 随机取样，最后输出取样结果的分布，例如“99%的调用在xxx毫秒以内，99.99%的调用在xxx毫秒以内”
SingleShotTime: 以上模式都是默认一次 iteration 是 1s，唯有 SingleShotTime 是只运行一次。往往同时把 warmup 次数设为0，用于测试冷启动时的性能。
Iteration Iteration 是 JMH 进行测试的最小单位。在大部分模式下，一次 iteration 代表的是一秒，JMH 会在这一秒内不断调用需要 benchmark 的方法，然后根据模式对其采样，计算吞吐量，计算平均执行时间等。
Warmup
Warmup 是指在实际进行 benchmark 前先进行预热的行为。为什么需要预热？因为 JVM 的 JIT 机制的存在，如果某个函数被调用多次之后，JVM 会尝试将其编译成为机器码从而提高执行速度。为了让 benchmark 的结果更加接近真实情况就需要进行预热。
3.2 注解与选项3.2.1 常用注解说明@BenchmarkMode 对应Mode选项，可用于类或者方法上， 需要注意的是，这个注解的value是一个数组，可以把几种Mode集合在一起执行，还可以设置为Mode.All，即全部执行一遍。
@State 类注解，JMH测试类必须使用@State注解，State定义了一个类实例的生命周期，可以类比Spring Bean的Scope。由于JMH允许多线程同时执行测试，不同的选项含义如下：
Scope.Thread：默认的State，每个测试线程分配一个实例；
Scope.Benchmark：所有测试线程共享一个实例，用于测试有状态实例在多线程共享下的性能；
Scope.Group：每个线程组共享一个实例；
@OutputTimeUnit benchmark 结果所使用的时间单位，可用于类或者方法注解，使用java.util.concurrent.TimeUnit中的标准时间单位。
@Benchmark 方法注解，表示该方法是需要进行 benchmark 的对象。
@Setup 方法注解，会在执行 benchmark 之前被执行，正如其名，主要用于初始化。
@TearDown 方法注解，与@Setup 相对的，会在所有 benchmark 执行结束以后执行，主要用于资源的回收等。
@Param 成员注解，可以用来指定某项参数的多种情况。特别适合用来测试一个函数在不同的参数输入的情况下的性能。@Param注解接收一个String数组，在@setup方法执行前转化为为对应的数据类型。多个@Param注解的成员之间是乘积关系，譬如有两个用@Param注解的字段，第一个有5个值，第二个字段有2个值，那么每个测试方法会跑5*2=10次。
3.2.2 注解使用例子以下示例代码来自JMH官方例子，为了节省篇幅删除了头部的license声明和重复的注释。
@BenchmarkMode和@OutputTimeUnitpublic class JMHSample_02_BenchmarkModes { @Benchmark @BenchmarkMode(Mode.Throughput) @OutputTimeUnit(TimeUnit.SECONDS) public void measureThroughput() throws InterruptedException { TimeUnit.MILLISECONDS.sleep(100); } /* * Mode.AverageTime measures the average execution time, and it does it * in the way similar to Mode.Throughput. * * Some might say it is the reciprocal throughput, and it really is. * There are workloads where measuring times is more convenient though. */ @Benchmark @BenchmarkMode(Mode.AverageTime) @OutputTimeUnit(TimeUnit.MICROSECONDS) public void measureAvgTime() throws InterruptedException { TimeUnit.MILLISECONDS.sleep(100); } /* * Mode.SampleTime samples the execution time. With this mode, we are * still running the method in a time-bound iteration, but instead of * measuring the total time, we measure the time spent in *some* of * the benchmark method calls. * * This allows us to infer the distributions, percentiles, etc. * * JMH also tries to auto-adjust sampling frequency: if the method * is long enough, you will end up capturing all the samples. */ @Benchmark @BenchmarkMode(Mode.SampleTime) @OutputTimeUnit(TimeUnit.MICROSECONDS) public void measureSamples() throws InterruptedException { TimeUnit.MILLISECONDS.sleep(100); } /* * Mode.SingleShotTime measures the single method invocation time. As the Javadoc * suggests, we do only the single benchmark method invocation. The iteration * time is meaningless in this mode: as soon as benchmark method stops, the * iteration is over. * * This mode is useful to do cold startup tests, when you specifically * do not want to call the benchmark method continuously. */ @Benchmark @BenchmarkMode(Mode.SingleShotTime) @OutputTimeUnit(TimeUnit.MICROSECONDS) public void measureSingleShot() throws InterruptedException { TimeUnit.MILLISECONDS.sleep(100); } /* * We can also ask for multiple benchmark modes at once. All the tests * above can be replaced with just a single test like this: */ @Benchmark @BenchmarkMode({Mode.Throughput, Mode.AverageTime, Mode.SampleTime, Mode.SingleShotTime}) @OutputTimeUnit(TimeUnit.MICROSECONDS) public void measureMultiple() throws InterruptedException { TimeUnit.MILLISECONDS.sleep(100); } /* * Or even... */ @Benchmark @BenchmarkMode(Mode.All) @OutputTimeUnit(TimeUnit.MICROSECONDS) public void measureAll() throws InterruptedException { TimeUnit.MILLISECONDS.sleep(100); } /* * ============================== HOW TO RUN THIS TEST: ==================================== * * You are expected to see the different run modes for the same benchmark. * Note the units are different, scores are consistent with each other. * * You can run this test: * * a) Via the command line: * $ mvn clean install * $ java -jar target/benchmarks.jar JMHSample_02 -wi 5 -i 5 -f 1 * (we requested 5 warmup/measurement iterations, single fork) * * b) Via the Java API: * (see the JMH homepage for possible caveats when running from IDE: * http://openjdk.java.net/projects/code-tools/jmh/) */ public static void main(String[] args) throws RunnerException { Options opt = new OptionsBuilder() .include(JMHSample_02_BenchmarkModes.class.getSimpleName()) .warmupIterations(5) .measurementIterations(5) .forks(1) .build(); new Runner(opt).run(); }}1234567891011121314151617181920212223242526272829303132333435363738394041424344454647484950515253545556575859606162636465666768697071727374757677787980818283848586878889909192939495969798@Statepublic class JMHSample_03_States { @State(Scope.Benchmark) public static class BenchmarkState { volatile double x = Math.PI; } @State(Scope.Thread) public static class ThreadState { volatile double x = Math.PI; } /* * Benchmark methods can reference the states, and JMH will inject the * appropriate states while calling these methods. You can have no states at * all, or have only one state, or have multiple states referenced. This * makes building multi-threaded benchmark a breeze. * * For this exercise, we have two methods. */ @Benchmark public void measureUnshared(ThreadState state) { // All benchmark threads will call in this method. // // However, since ThreadState is the Scope.Thread, each thread // will have it's own copy of the state, and this benchmark // will measure unshared case. state.x++; } @Benchmark public void measureShared(BenchmarkState state) { // All benchmark threads will call in this method. // // Since BenchmarkState is the Scope.Benchmark, all threads // will share the state instance, and we will end up measuring // shared case. state.x++; }
public static void main(String[] args) throws RunnerException { Options opt = new OptionsBuilder() .include(JMHSample_03_States.class.getSimpleName()) .warmupIterations(5) .measurementIterations(5) .threads(4) .forks(1) .build(); new Runner(opt).run(); }}1234567891011121314151617181920212223242526272829303132333435363738394041424344454647@Param@BenchmarkMode(Mode.AverageTime)@OutputTimeUnit(TimeUnit.NANOSECONDS)@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)@Fork(1)@State(Scope.Benchmark)public class JMHSample_27_Params { /** * In many cases, the experiments require walking the configuration space * for a benchmark. This is needed for additional control, or investigating * how the workload performance changes with different settings. */ @Param({"1", "31", "65", "101", "103"}) public int arg; @Param({"0", "1", "2", "4", "8", "16", "32"}) public int certainty; @Benchmark public boolean bench() { return BigInteger.valueOf(arg).isProbablePrime(certainty); } public static void main(String[] args) throws RunnerException { Options opt = new OptionsBuilder() .include(JMHSample_27_Params.class.getSimpleName())// .param("arg", "41", "42") // Use this to selectively constrain/override parameters .build(); new Runner(opt).run(); }}12345678910111213141516171819202122232425262728293.2.3 常用选项说明include benchmark 所在的类的名字，这里可以使用正则表达式对所有类进行匹配。
fork JVM因为使用了profile-guided optimization而“臭名昭著”，这对于微基准测试来说十分不友好，因为不同测试方法的profile混杂在一起，“互相伤害”彼此的测试结果。对于每个@Benchmark方法使用一个独立的进程可以解决这个问题，这也是JMH的默认选项。注意不要设置为0，设置为n则会启动n个进程执行测试（似乎也没有太大意义）。fork选项也可以通过方法注解以及启动参数来设置。
warmupIterations 预热的迭代次数，默认1秒。
measurementIterations 实际测量的迭代次数，默认1秒。
CompilerControl 可以在@Benchmark注解中指定编译器行为。
CompilerControl.Mode.DONT_INLINE：This method should not be inlined. Useful to measure the method call cost and to evaluate if it worth to increase the inline threshold for the JVM.CompilerControl.Mode.INLINE：Ask the compiler to inline this method. Usually should be used in conjunction with Mode.DONT_INLINE to check pros and cons of inlining.CompilerControl.Mode.EXCLUDE：Do not compile this method – interpret it instead. Useful in holy wars as an argument how good is the JIT.Group 方法注解，可以把多个 benchmark 定义为同一个 group，则它们会被同时执行，譬如用来模拟生产者－消费者读写速度不一致情况下的表现。可以参考如下例子： CounterBenchmark.java
Level 用于控制 @Setup，@TearDown 的调用时机，默认是 Level.Trial。
Trial：每个benchmark方法前后；
Iteration：每个benchmark方法每次迭代前后；
Invocation：每个benchmark方法每次调用前后，谨慎使用，需留意javadoc注释；
Threads 每个fork进程使用多少条线程去执行你的测试方法，默认值是Runtime.getRuntime().availableProcessors()。
四、一些值得注意的地方4.1 无用代码消除（Dead Code Elimination）现代编译器是十分聪明的，它们会对你的代码进行推导分析，判定哪些代码是无用的然后进行去除，这种行为对微基准测试是致命的，它会使你无法准确测试出你的方法性能。JMH本身已经对这种情况做了处理，你只要记住：1.永远不要写void方法；2.在方法结束返回你的计算结果。有时候如果需要返回多于一个结果，可以考虑自行合并计算结果，或者使用JMH提供的BlackHole对象：
/* * This demonstrates Option A: * * Merge multiple results into one and return it. * This is OK when is computation is relatively heavyweight, and merging * the results does not offset the results much. */@Benchmarkpublic double measureRight_1() { return Math.log(x1) + Math.log(x2);}/* * This demonstrates Option B: * * Use explicit Blackhole objects, and sink the values there. * (Background: Blackhole is just another @State object, bundled with JMH). */@Benchmarkpublic void measureRight_2(Blackhole bh) { bh.consume(Math.log(x1)); bh.consume(Math.log(x2));}123456789101112131415161718192021224.2 常量折叠（Constant Folding）常量折叠是一种现代编译器优化策略，例如，i = 320 * 200 * 32，多数的现代编译器不会真的产生两个乘法的指令再将结果储存下来，取而代之的，他们会辨识出语句的结构，并在编译时期将数值计算出来（i = 2,048,000）。
在微基准测试中，如果你的计算输入是可预测的，也不是一个@State实例变量，那么很可能会被JIT给优化掉。对此，JMH的建议是：1.永远从@State实例中读取你的方法输入；2.返回你的计算结果；3.或者考虑使用BlackHole对象；
见如下官方例子：
@State(Scope.Thread)@BenchmarkMode(Mode.AverageTime)@OutputTimeUnit(TimeUnit.NANOSECONDS)public class JMHSample_10_ConstantFold { private double x = Math.PI; private final double wrongX = Math.PI; @Benchmark public double baseline() { // simply return the value, this is a baseline return Math.PI; } @Benchmark public double measureWrong_1() { // This is wrong: the source is predictable, and computation is foldable. return Math.log(Math.PI); } @Benchmark public double measureWrong_2() { // This is wrong: the source is predictable, and computation is foldable. return Math.log(wrongX); } @Benchmark public double measureRight() { // This is correct: the source is not predictable. return Math.log(x); } public static void main(String[] args) throws RunnerException { Options opt = new OptionsBuilder() .include(JMHSample_10_ConstantFold.class.getSimpleName()) .warmupIterations(5) .measurementIterations(5) .forks(1) .build(); new Runner(opt).run(); }}1234567891011121314151617181920212223242526272829303132333435364.3 循环展开（Loop Unwinding）循环展开最常用来降低循环开销，为具有多个功能单元的处理器提供指令级并行。也有利于指令流水线的调度。例如：
for (i = 1; i <= 60; i++) a[i] = a[i] * b + c;12可以展开成：
for (i = 1; i <= 60; i+=3){ a[i] = a[i] * b + c; a[i+1] = a[i+1] * b + c; a[i+2] = a[i+2] * b + c;}123456由于编译器可能会对你的代码进行循环展开，因此JMH建议不要在你的测试方法中写任何循环。如果确实需要执行循环计算，可以结合@BenchmarkMode(Mode.SingleShotTime)和@Measurement(batchSize = N)来达到同样的效果。参考如下例子：
/* * Suppose we want to measure how much it takes to sum two integers: */int x = 1;int y = 2;/* * This is what you do with JMH. */@Benchmark@OperationsPerInvocation(100)public int measureRight() { return (x + y);}12345678910111213还有这个例子：
@State(Scope.Thread)@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)@Fork(3)@BenchmarkMode(Mode.AverageTime)@OutputTimeUnit(TimeUnit.NANOSECONDS)public class JMHSample_34_SafeLooping { /* * JMHSample_11_Loops warns about the dangers of using loops in @Benchmark methods. * Sometimes, however, one needs to traverse through several elements in a dataset. * This is hard to do without loops, and therefore we need to devise a scheme for * safe looping. */ /* * Suppose we want to measure how much it takes to execute work() with different * arguments. This mimics a frequent use case when multiple instances with the same * implementation, but different data, is measured. */ static final int BASE = 42; static int work(int x) { return BASE + x; } /* * Every benchmark requires control. We do a trivial control for our benchmarks * by checking the benchmark costs are growing linearly with increased task size. * If it doesn't, then something wrong is happening. */ @Param({"1", "10", "100", "1000"}) int size; int[] xs; @Setup public void setup() { xs = new int[size]; for (int c = 0; c < size; c++) { xs[c] = c; } } /* * First, the obviously wrong way: "saving" the result into a local variable would not * work. A sufficiently smart compiler will inline work(), and figure out only the last * work() call needs to be evaluated. Indeed, if you run it with varying $size, the score * will stay the same! */ @Benchmark public int measureWrong_1() { int acc = 0; for (int x : xs) { acc = work(x); } return acc; } /* * Second, another wrong way: "accumulating" the result into a local variable. While * it would force the computation of each work() method, there are software pipelining * effects in action, that can merge the operations between two otherwise distinct work() * bodies. This will obliterate the benchmark setup. * * In this example, HotSpot does the unrolled loop, merges the $BASE operands into a single * addition to $acc, and then does a bunch of very tight stores of $x-s. The final performance * depends on how much of the loop unrolling happened *and* how much data is available to make * the large strides. */ @Benchmark public int measureWrong_2() { int acc = 0; for (int x : xs) { acc += work(x); } return acc; } /* * Now, let's see how to measure these things properly. A very straight-forward way to * break the merging is to sink each result to Blackhole. This will force runtime to compute * every work() call in full. (We would normally like to care about several concurrent work() * computations at once, but the memory effects from Blackhole.consume() prevent those optimization * on most runtimes). */ @Benchmark public void measureRight_1(Blackhole bh) { for (int x : xs) { bh.consume(work(x)); } } /* * DANGEROUS AREA, PLEASE READ THE DESCRIPTION BELOW. * * Sometimes, the cost of sinking the value into a Blackhole is dominating the nano-benchmark score. * In these cases, one may try to do a make-shift "sinker" with non-inlineable method. This trick is * *very* VM-specific, and can only be used if you are verifying the generated code (that's a good * strategy when dealing with nano-benchmarks anyway). * * You SHOULD NOT use this trick in most cases. Apply only where needed. */ @Benchmark public void measureRight_2() { for (int x : xs) { sink(work(x)); } } @CompilerControl(CompilerControl.Mode.DONT_INLINE) public static void sink(int v) { // IT IS VERY IMPORTANT TO MATCH THE SIGNATURE TO AVOID AUTOBOXING. // The method intentionally does nothing. }
public static void main(String[] args) throws RunnerException { Options opt = new OptionsBuilder() .include(JMHSample_34_SafeLooping.class.getSimpleName()) .warmupIterations(5) .measurementIterations(5) .forks(3) .build(); new Runner(opt).run(); }}123456789101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566676869707172737475767778798081828384858687888990919293949596979899100101102103104105106107108109110111112113114115五、License声明文中大部分例子来自JMH官方的实例工程：jmh-samples，基于节省篇幅考虑去掉了头部的license声明，现补充如下：
/* * Copyright (c) 2014, Oracle America, Inc. * All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions are met: * * * Redistributions of source code must retain the above copyright notice, * this list of conditions and the following disclaimer. * * * Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * * * Neither the name of Oracle nor the names of its contributors may be used * to endorse or promote products derived from this software without * specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF * THE POSSIBILITY OF SUCH DAMAGE. */--------------------- 作者：秦沙 来源：CSDN 原文：https://blog.csdn.net/lxbjkben/article/details/79410740 版权声明：本文为博主原创文章，转载请附上博文链接！
