From 66368f4d2e60f68161c0df2e69b0af905143d414 Mon Sep 17 00:00:00 2001
From: Tommy Ludwig <8924140+shakuzen@users.noreply.github.com>
Date: Wed, 5 Feb 2025 13:15:25 +0900
Subject: [PATCH] Log warning and do nothing on double binding to same registry

---
 .../binder/logging/Log4j2Metrics.java         |  8 ++++++
 .../binder/logging/Log4j2MetricsTest.java     | 27 +++++++++++++++++++
 2 files changed, 35 insertions(+)

diff --git a/micrometer-core/src/main/java/io/micrometer/core/instrument/binder/logging/Log4j2Metrics.java b/micrometer-core/src/main/java/io/micrometer/core/instrument/binder/logging/Log4j2Metrics.java
index 355c6452e0..0fa8aaa88c 100644
--- a/micrometer-core/src/main/java/io/micrometer/core/instrument/binder/logging/Log4j2Metrics.java
+++ b/micrometer-core/src/main/java/io/micrometer/core/instrument/binder/logging/Log4j2Metrics.java
@@ -17,6 +17,8 @@
 
 import io.micrometer.common.lang.NonNullApi;
 import io.micrometer.common.lang.NonNullFields;
+import io.micrometer.common.util.internal.logging.InternalLogger;
+import io.micrometer.common.util.internal.logging.InternalLoggerFactory;
 import io.micrometer.core.instrument.Counter;
 import io.micrometer.core.instrument.MeterRegistry;
 import io.micrometer.core.instrument.Tag;
@@ -60,6 +62,8 @@ public class Log4j2Metrics implements MeterBinder, AutoCloseable {
 
     private static final String METER_DESCRIPTION = "Number of log events";
 
+    private static final InternalLogger logger = InternalLoggerFactory.getInstance(Log4j2Metrics.class);
+
     private final Iterable<Tag> tags;
 
     private final LoggerContext loggerContext;
@@ -83,6 +87,10 @@ public Log4j2Metrics(Iterable<Tag> tags, LoggerContext loggerContext) {
 
     @Override
     public void bindTo(MeterRegistry registry) {
+        if (metricsFilters.containsKey(registry)) {
+            logger.warn("This Log4j2Metrics instance has already been bound to the registry {}", registry);
+            return;
+        }
         Configuration configuration = loggerContext.getConfiguration();
         registerMetricsFilter(configuration, registry);
         loggerContext.updateLoggers(configuration);
diff --git a/micrometer-core/src/test/java/io/micrometer/core/instrument/binder/logging/Log4j2MetricsTest.java b/micrometer-core/src/test/java/io/micrometer/core/instrument/binder/logging/Log4j2MetricsTest.java
index 69c29f4b7e..e4987fabbb 100644
--- a/micrometer-core/src/test/java/io/micrometer/core/instrument/binder/logging/Log4j2MetricsTest.java
+++ b/micrometer-core/src/test/java/io/micrometer/core/instrument/binder/logging/Log4j2MetricsTest.java
@@ -341,4 +341,31 @@ void shouldNotRebindMetricsIfBinderIsClosed() {
         assertThat(registry.get("log4j2.events").tags("level", "error").counter().count()).isEqualTo(1);
     }
 
+    @Test
+    void bindingTwiceToSameRegistry_doesNotDoubleCount() {
+        LoggerContext context = new LoggerContext("test");
+        Logger logger = context.getLogger("com.test");
+
+        try (Log4j2Metrics metrics = new Log4j2Metrics(emptyList(), context)) {
+            // binding twice
+            metrics.bindTo(registry);
+            metrics.bindTo(registry);
+
+            logger.error("first");
+
+            assertThat(registry.get("log4j2.events").tags("level", "error").counter().count()).isEqualTo(1);
+
+            context.reconfigure();
+
+            logger.error("second");
+            assertThat(registry.get("log4j2.events").tags("level", "error").counter().count()).isEqualTo(2);
+        }
+
+        // no additional events should be counted now
+        context.reconfigure();
+
+        logger.error("third");
+        assertThat(registry.get("log4j2.events").tags("level", "error").counter().count()).isEqualTo(2);
+    }
+
 }