Add assertion for no re-registering Kafka metrics

This commit also copies OutputCaptureExtension to assert it.

See gh-6068

Signed-off-by: Johnny Lim <[email protected]>

Author: izeye

Diff for: micrometer-core/src/test/java/io/micrometer/core/instrument/binder/kafka/KafkaMetricsTest.java

@@ -21,6 +21,8 @@
 import io.micrometer.core.instrument.MeterRegistry;
 import io.micrometer.core.instrument.Tag;
 import io.micrometer.core.instrument.simple.SimpleMeterRegistry;
+import io.micrometer.core.testsupport.system.CapturedOutput;
+import io.micrometer.core.testsupport.system.OutputCaptureExtension;
 import org.apache.kafka.common.Metric;
 import org.apache.kafka.common.MetricName;
 import org.apache.kafka.common.metrics.KafkaMetric;
@@ -29,6 +31,7 @@
 import org.apache.kafka.common.utils.Time;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
 
 import java.util.Collections;
 import java.util.HashMap;
@@ -39,6 +42,7 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 
+@ExtendWith(OutputCaptureExtension.class)
 class KafkaMetricsTest {
 
     private KafkaMetrics kafkaMetrics;
@@ -257,7 +261,7 @@ void shouldRemoveOlderMeterWithLessTagsWhenCommonTagsConfigured() {
 
     @Issue("#2212")
     @Test
-    void shouldRemoveMeterWithLessTagsWithMultipleClients() {
+    void shouldRemoveMeterWithLessTagsWithMultipleClients(CapturedOutput output) {
         // Given
         AtomicReference<Map<MetricName, KafkaMetric>> metrics = new AtomicReference<>(new LinkedHashMap<>());
         Supplier<Map<MetricName, ? extends Metric>> supplier = () -> metrics.updateAndGet(map -> {
@@ -320,6 +324,8 @@ void shouldRemoveMeterWithLessTagsWithMultipleClients() {
         registry.getMeters()
             .forEach(meter -> assertThat(meter.getId().getTags()).extracting(Tag::getKey)
                 .containsOnly("key0", "key1", "client.id", "kafka.version"));
+
+        assertThat(output).doesNotContain("This Gauge has been already registered");
     }
 
     @Issue("#2726")

Diff for: micrometer-core/src/test/java/io/micrometer/core/testsupport/system/CapturedOutput.java

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2012-2019 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.micrometer.core.testsupport.system;
+
+/**
+ * Provides access to {@link System#out System.out} and {@link System#err System.err}
+ * output that has been captured by the {@link OutputCaptureExtension}. Can be used to
+ * apply assertions either using AssertJ or standard JUnit assertions. For example:
+ * <pre class="code">
+ * assertThat(output).contains("started"); // Checks all output
+ * assertThat(output.getErr()).contains("failed"); // Only checks System.err
+ * assertThat(output.getOut()).contains("ok"); // Only checks System.out
+ * </pre>
+ *
+ * @author Madhura Bhave
+ * @author Phillip Webb
+ * @author Andy Wilkinson
+ * @see OutputCaptureExtension
+ */
+public interface CapturedOutput extends CharSequence {
+
+    @Override
+    default int length() {
+        return toString().length();
+    }
+
+    @Override
+    default char charAt(int index) {
+        return toString().charAt(index);
+    }
+
+    @Override
+    default CharSequence subSequence(int start, int end) {
+        return toString().subSequence(start, end);
+    }
+
+    /**
+     * Return all content (both {@link System#out System.out} and {@link System#err
+     * System.err}) in the order that it was captured.
+     * @return all captured output
+     */
+    String getAll();
+
+    /**
+     * Return {@link System#out System.out} content in the order that it was captured.
+     * @return {@link System#out System.out} captured output
+     */
+    String getOut();
+
+    /**
+     * Return {@link System#err System.err} content in the order that it was captured.
+     * @return {@link System#err System.err} captured output
+     */
+    String getErr();
+
+}