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JMH 是什么？

JMH（Java Microbenchmark Harness）是一款专用于代码微基准测试的工具集，其主要聚焦于方法层面的基准测试，精度可达纳秒级别。此工具由 Oracle 内部负责实现 JIT 的杰出人士编写，他们对 JIT 及 JVM 在基准测试方面的影响有着深刻的理解。JMH 不仅能够对 Java 语言进行基准测试，还能对运行在 JVM 上的其他语言开展基准测试。
当热点方法被确定，且希望进一步提升方法性能时，可借助 JMH 对优化后的结果进行量化分析。
JMH 具有一些典型的应用场景，如：

• 精准了解某个方法的执行时间，以及执行时间与输入的相关性；
• 对比接口不同实现方式在特定条件下的吞吐量；
• 查看在特定时间段内完成的请求所占比例等。

JMH 在JDK9 中是自带的 在JDK9 之前 我们需要主动的引入下maven包

<dependency><groupId>org.openjdk.jmh</groupId><artifactId>jmh-core</artifactId><version>1.23</version>
</dependency>
<dependency><groupId>org.openjdk.jmh</groupId><artifactId>jmh-generator-annprocess</artifactId><version>1.23</version>
</dependency>

让我们来些第一个 JMH 测试代码吧！

package cn.ideamake.im.auth.service;import org.openjdk.jmh.annotations.Benchmark;
import org.openjdk.jmh.annotations.BenchmarkMode;
import org.openjdk.jmh.annotations.Fork;
import org.openjdk.jmh.annotations.Level;
import org.openjdk.jmh.annotations.Measurement;
import org.openjdk.jmh.annotations.Mode;
import org.openjdk.jmh.annotations.OutputTimeUnit;
import org.openjdk.jmh.annotations.Param;
import org.openjdk.jmh.annotations.Scope;
import org.openjdk.jmh.annotations.Setup;
import org.openjdk.jmh.annotations.State;
import org.openjdk.jmh.annotations.Threads;
import org.openjdk.jmh.annotations.Warmup;
import org.openjdk.jmh.results.format.ResultFormatType;
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 com.google.common.collect.Lists;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.TimeUnit;/*** @author Barcke* @version 1.0* @projectName im-auth* @className Test* @date 2024/4/30 16:30* @slogan: 源于生活 高于生活* @description:**/// 用来配置 Mode 选项，可用于类或者方法上，这个注解的 value 是一个数组，可以把几种 Mode 集合在一起执行 比如 @BenchmarkMode({Mode.AverageTime, Mode.SampleTime})
@BenchmarkMode(// 每一次方法执行用的平均时间，每次操作的平均时间，单位为 time/op
//        Mode.AverageTime// 随机取样，最后输出取样结果的分布
//        Mode.SampleTime// 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
//        Mode.SingleShotTime// 吞吐量，每秒执行了多少次调用，单位为 ops/timeMode.Throughput// 上面的所有模式都执行一次
//        Mode.All
)
// 预热所需要配置的一些基本测试参数，可用于类或者方法上。一般前几次进行程序测试的时候都会比较慢，所以要让程序进行几轮预热，保证测试的准确性。
@Warmup(// 预热次数iterations = 3// 每次预热的时间, time = 1// 时间单位，默认秒, timeUnit = TimeUnit.SECONDS// 批处理大小，每次操作调用几次方法, batchSize = 100
)
@Measurement(iterations = 5, time = 5)
// 每个测试线程数量，可用于类或者方法上。
@Threads(5)
// 进行 fork 的次数，可用于类或者方法上。如果 fork 数是 3 的话，则 JMH 会 fork 出三个进程来进行测试。
@Fork(3)
// 指定一个对象的作用范围，JMH 根据 scope 来进行实例化和共享操作。@State 可以被继承使用，如果父类定义了该注解，子类则无需定义。由于 JMH 允许多线程同时执行测试
@State(// 默认是 Scope.Benchmark 所有测试线程共享一个实例，测试有状态实例在多线程共享下的性能
//        value = Scope.Benchmark// 线程作用域，每个线程都有自己的实例，每个线程的实例相互独立，不会相互影响value = Scope.Thread// 线程组作用域，线程组内的线程共享一个实例，线程组外的线程不会共享实例，每个线程的实例相互独立，不会相互影响
//        value = Scope.Group
)
// 结果输出时间单位 可用于类或者方法注解
@OutputTimeUnit(TimeUnit.NANOSECONDS)
public class Test {private final String KEY = "barcke";private List<String> arrayList;private List<String> linkedList;// 测试参数@Param(value = {"10", "20", "30"})private int length;// @Setup 测试初始化方法 对应在执行测试方法之后调用的可以使用注解 @TearDown 声明@Setup(// 由于我们设置Warmup和Measurement的不同，因此每一个基准测试方法都会被执行若干个批次，如果想要在每一个基准测试批次执行的前后调用方法，则可以将Level设置为Iteration。
//            Level.Iteration// 意味着在每一个批次的度量过程中，每一次对基准方法的调用前都会执行套件方法。
//            Level.Invocation// Setup和TearDown默认的配置，该套件方法会在每一个基准测试方法的所有批次执行的前后被执行。Level.Trial)public void setup() {arrayList = Lists.newCopyOnWriteArrayList();linkedList = new LinkedList<>();}// Benchmark 用于 测试方法声明@Benchmarkpublic List<String> arrayListAdd() {for (int i = 0; i < length; i++) {arrayList.add(KEY);}return arrayList;}// Benchmark 用于 测试方法声明@Benchmarkpublic List<String> linkedListAdd() {for (int i = 0; i < length; i++) {linkedList.add(KEY);}return linkedList;}public static void main(String[] args) throws RunnerException {Options opt = new OptionsBuilder()// 要导入的测试类.include(Test.class.getSimpleName())// 输出测试结果的文件.result("result.json").resultFormat(ResultFormatType.JSON).build();new Runner(opt).run();}}

需要进行测试的方法会用@Benchmark 注解来标识，关于这些注解的详细含义，将在后续内容中进行具体阐述。
在 main()函数中，首先会对测试用例进行相应的配置。通过采用 Builder 模式来配置测试，将配置的各项参数存入 Options 对象中，然后再利用该 Options 对象来构建 Runner 并启动测试。

使用Main方法执行测试

需注意！！！
idea执行基准测试过程中可能会出现内存泄露的报错：java.lang.OutOfMemoryError: Java heap space
加大JVM的内存参数值即可
如：

做好准备工作后，开始运行代码，静待片刻，测试结果就会出炉，下面对结果做一个简单的分析：

最终结果：

可视化 JMH 工具

JMH Visual Chart：http://deepoove.com/jmh-visual-chart/
JMH Visualizer：https://jmh.morethan.io/

将json文件导入到网站中则可以得到可视化图表数据

我跑出来的json数据 不过吞吐量没有啥区别～

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]

本文到此结束，希望对你有帮助～