add JMH sample

spring-boot-example/pom.xml

@@ -344,6 +344,20 @@
         <groupId>org.springframework.boot</groupId>
         <artifactId>spring-boot-maven-plugin</artifactId>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-compiler-plugin</artifactId>
+        <version>3.13.0</version>
+        <configuration>
+          <annotationProcessorPaths>
+            <path>
+              <groupId>org.openjdk.jmh</groupId>
+              <artifactId>jmh-generator-annprocess</artifactId>
+              <version>${jmh-core.version}</version>
+            </path>
+          </annotationProcessorPaths>
+        </configuration>
+      </plugin>
       <plugin>
         <artifactId>maven-surefire-plugin</artifactId>
         <version>3.5.2</version>

spring-boot-example/src/test/java/de/rieckpil/blog/jmh/StringConcatenationBenchmark.java

@@ -0,0 +1,52 @@
+package de.rieckpil.blog.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.Param;
+import org.openjdk.jmh.annotations.Scope;
+import org.openjdk.jmh.annotations.State;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+@State(Scope.Thread)
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.NANOSECONDS)
+public class StringConcatenationBenchmark {
+
+  @Param({"5", "10", "15"})
+  private int iterations;
+
+  public static void main(String[] args) throws Exception {
+    Options opt = new OptionsBuilder()
+      .include(StringConcatenationBenchmark.class.getSimpleName())
+      .forks(1)
+      .warmupIterations(3)
+      .measurementIterations(5)
+      .build();
+
+    new Runner(opt).run();
+  }
+
+  @Benchmark
+  public String testStringBuilder() {
+    StringBuilder sb = new StringBuilder();
+    for (int i = 0; i < iterations; i++) {
+      sb.append("test");
+    }
+    return sb.toString();
+  }
+
+  @Benchmark
+  public String testStringConcatenation() {
+    String result = "";
+    for (int i = 0; i < iterations; i++) {
+      result += "test";
+    }
+    return result;
+  }
+}