add some basic testing

Author: SylvainJuge

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/JmxTelemetry.java

@@ -35,5 +35,4 @@ public void startRemote(Supplier<List<? extends MBeanServerConnection>> connecti
   public void startLocal() {
     service.startLocal(metricConfiguration);
   }
-
 }

instrumentation/jmx-metrics/library/src/main/java/io/opentelemetry/instrumentation/jmx/JmxTelemetryBuilder.java

@@ -39,6 +39,9 @@ public class JmxTelemetryBuilder {
    */
   @CanIgnoreReturnValue
   public JmxTelemetryBuilder beanDiscoveryDelay(long delayMs) {
+    if (delayMs < 0) {
+      throw new IllegalArgumentException("delay must be greater than 0");
+    }
     this.discoveryDelayMs = delayMs;
     return this;
   }
@@ -62,7 +65,8 @@ public JmxTelemetryBuilder addClasspathRules(String target) {
       RuleParser parserInstance = RuleParser.get();
       parserInstance.addMetricDefsTo(metricConfiguration, inputStream, target);
     } catch (Exception e) {
-      throw new IllegalArgumentException("Unable to load JMX rules from classpath: " + yamlResource, e);
+      throw new IllegalArgumentException(
+          "Unable to load JMX rules from classpath: " + yamlResource, e);
     }
     return this;
   }

instrumentation/jmx-metrics/library/src/test/java/io/opentelemetry/instrumentation/jmx/JmxTelemetryTest.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.jmx;
+
+import static io.opentelemetry.sdk.testing.assertj.OpenTelemetryAssertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import io.opentelemetry.api.OpenTelemetry;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+
+public class JmxTelemetryTest {
+
+  @Test
+  void createDefault() {
+    JmxTelemetryBuilder builder = JmxTelemetry.builder(OpenTelemetry.noop());
+    assertThat(builder.build()).isNotNull();
+  }
+
+  @Test
+  void missingClasspathTarget() {
+    JmxTelemetryBuilder builder = JmxTelemetry.builder(OpenTelemetry.noop());
+    assertThatThrownBy(() -> builder.addClasspathRules("should-not-exist"))
+        .isInstanceOf(IllegalArgumentException.class);
+  }
+
+  @Test
+  void invalidExternalYaml(@TempDir Path dir) throws Exception {
+    Path invalid = Files.createTempFile(dir, "invalid", ".yaml");
+    JmxTelemetryBuilder builder = JmxTelemetry.builder(OpenTelemetry.noop());
+    assertThatThrownBy(() -> builder.addCustomRules(invalid))
+        .isInstanceOf(IllegalArgumentException.class);
+  }
+
+  @Test
+  void invalidStartDelay() {
+    JmxTelemetryBuilder builder = JmxTelemetry.builder(OpenTelemetry.noop());
+    assertThatThrownBy(() -> builder.beanDiscoveryDelay(-1))
+        .isInstanceOf(IllegalArgumentException.class);
+  }
+}