From 93a436f8280a90299cca12ef319790c7d707ee67 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Tue, 15 Apr 2025 16:57:02 +0200
Subject: [PATCH 01/14] wip

---
 .../co/elastic/otel/agent/ElasticAgent.java   |  3 ++
 buildscripts/allowed-licenses.json            |  5 +++
 custom/build.gradle.kts                       |  3 ++
 .../logging/ElasticLoggingCustomizer.java     | 31 ++++++++++++++++
 .../otel/logging/Slf4jInternalLogger.java     | 37 +++++++++++++++++++
 gradle/libs.versions.toml                     |  3 ++
 6 files changed, 82 insertions(+)
 create mode 100644 custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
 create mode 100644 custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java

diff --git a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
index f817cb7d3..5fa769114 100644
--- a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
+++ b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
@@ -31,6 +31,9 @@ public class ElasticAgent {
    * @param inst instrumentation
    */
   public static void premain(String agentArgs, Instrumentation inst) {
+
+    System.setProperty("otel.javaagent.logging", "elastic");
+
     OpenTelemetryAgent.premain(agentArgs, inst);
   }
 
diff --git a/buildscripts/allowed-licenses.json b/buildscripts/allowed-licenses.json
index c9308b6f8..5646c0638 100644
--- a/buildscripts/allowed-licenses.json
+++ b/buildscripts/allowed-licenses.json
@@ -8,6 +8,11 @@
       "moduleLicense": "The 2-Clause BSD License",
       "moduleVersion": ".*",
       "moduleName": "org.hdrhistogram:HdrHistogram"
+    },
+    {
+      "moduleLicense": "MIT License",
+      "moduleVersion": "1.7.36",
+      "moduleName": "org.slf4j:slf4j-api"
     }
   ]
 }
diff --git a/custom/build.gradle.kts b/custom/build.gradle.kts
index cf5d954c3..950dd506b 100644
--- a/custom/build.gradle.kts
+++ b/custom/build.gradle.kts
@@ -20,6 +20,9 @@ dependencies {
   compileOnly("io.opentelemetry:opentelemetry-sdk-extension-autoconfigure-spi")
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-extension-api")
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-tooling")
+  compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-bootstrap")
+  compileOnly(libs.slf4j.api)
+  implementation(libs.log4j2)
   compileOnly(libs.bundles.semconv)
 
   implementation(libs.contribSpanStacktrace) {
diff --git a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
new file mode 100644
index 000000000..3b3195534
--- /dev/null
+++ b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
@@ -0,0 +1,31 @@
+package co.elastic.otel.logging;
+
+import com.google.auto.service.AutoService;
+import io.opentelemetry.javaagent.bootstrap.InternalLogger;
+import io.opentelemetry.javaagent.tooling.LoggingCustomizer;
+import io.opentelemetry.javaagent.tooling.config.EarlyInitAgentConfig;
+
+@AutoService(LoggingCustomizer.class)
+public class ElasticLoggingCustomizer implements LoggingCustomizer {
+
+  @Override
+  public String name() {
+    // must match "otel.javaagent.logging" system property for SPI lookup
+    return "elastic";
+  }
+
+  @Override
+  public void init(EarlyInitAgentConfig earlyInitAgentConfig) {
+    InternalLogger.initialize(Slf4jInternalLogger::create);
+  }
+
+  @Override
+  public void onStartupSuccess() {
+  }
+
+  @SuppressWarnings("CallToPrintStackTrace")
+  @Override
+  public void onStartupFailure(Throwable throwable) {
+    throwable.printStackTrace();
+  }
+}
diff --git a/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java b/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
new file mode 100644
index 000000000..fccc6ca0c
--- /dev/null
+++ b/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
@@ -0,0 +1,37 @@
+package co.elastic.otel.logging;
+
+import io.opentelemetry.javaagent.bootstrap.InternalLogger;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import javax.annotation.Nullable;
+
+/**
+ * Internal logger implementation that delegates to SLF4J
+ */
+public class Slf4jInternalLogger implements InternalLogger {
+  private final Logger logger;
+
+  private Slf4jInternalLogger(String name) {
+    this.logger = LoggerFactory.getLogger(name);
+  }
+
+  public static InternalLogger create(String name) {
+    return new Slf4jInternalLogger(name);
+  }
+
+  @Override
+  public boolean isLoggable(Level level) {
+    // TODO
+    return true;
+  }
+
+  @Override
+  public void log(Level level, String s, @Nullable Throwable throwable) {
+    // TODO
+  }
+
+  @Override
+  public String name() {
+    return logger.getName();
+  }
+}
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index dfb589ccc..6fc42d4fb 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -76,6 +76,9 @@ ant = "org.apache.ant:ant:1.10.15"
 # ASM is currently only used during compile-time, so it is okay to diverge from the version used in ByteBuddy
 asm = "org.ow2.asm:asm:9.8"
 
+slf4j-api = "org.slf4j:slf4j-api:2.0.17"
+log4j2 = "org.apache.logging.log4j:log4j-slf4j-impl:2.24.3"
+
 # Instrumented libraries
 openaiClient = "com.openai:openai-java:1.3.0"
 

From b3eff30ca55b52236c090352e5ce2ca0c0e8ffd3 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Wed, 16 Apr 2025 16:30:13 +0200
Subject: [PATCH 02/14] first working version

---
 ...tel.agent-packaging-conventions.gradle.kts |  3 ++
 buildscripts/allowed-licenses.json            |  2 +-
 custom/build.gradle.kts                       |  2 +-
 .../co/elastic/otel/logging/AgentLog.java     | 49 +++++++++++++++++++
 .../logging/ElasticLoggingCustomizer.java     | 16 +++++-
 .../otel/logging/Slf4jInternalLogger.java     | 35 +++++++++++--
 gradle/libs.versions.toml                     |  6 ++-
 7 files changed, 106 insertions(+), 7 deletions(-)
 create mode 100644 custom/src/main/java/co/elastic/otel/logging/AgentLog.java

diff --git a/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts b/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts
index 0c3c0ecf8..31725ce8d 100644
--- a/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts
+++ b/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts
@@ -182,6 +182,9 @@ tasks {
     dependsOn(isolateJavaagentLibs)
     configurations = listOf(bootstrapLibs, upstreamAgent)
 
+    // exclude slf4j-simple from the shadow jar as we use log4j2-slf4j instead
+    exclude("inst/io/opentelemetry/javaagent/slf4j/simple/**")
+
     from(isolateJavaagentLibs.get().outputs)
 
     archiveClassifier.set("")
diff --git a/buildscripts/allowed-licenses.json b/buildscripts/allowed-licenses.json
index 5646c0638..40847cc8f 100644
--- a/buildscripts/allowed-licenses.json
+++ b/buildscripts/allowed-licenses.json
@@ -11,7 +11,7 @@
     },
     {
       "moduleLicense": "MIT License",
-      "moduleVersion": "1.7.36",
+      "moduleVersion": ".*",
       "moduleName": "org.slf4j:slf4j-api"
     }
   ]
diff --git a/custom/build.gradle.kts b/custom/build.gradle.kts
index 950dd506b..afc157db3 100644
--- a/custom/build.gradle.kts
+++ b/custom/build.gradle.kts
@@ -22,7 +22,7 @@ dependencies {
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-tooling")
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-bootstrap")
   compileOnly(libs.slf4j.api)
-  implementation(libs.log4j2)
+  implementation(libs.bundles.log4j2)
   compileOnly(libs.bundles.semconv)
 
   implementation(libs.contribSpanStacktrace) {
diff --git a/custom/src/main/java/co/elastic/otel/logging/AgentLog.java b/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
new file mode 100644
index 000000000..7a5aa45c7
--- /dev/null
+++ b/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
@@ -0,0 +1,49 @@
+package co.elastic.otel.logging;
+
+import io.opentelemetry.javaagent.bootstrap.InternalLogger;
+import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.core.config.Configurator;
+
+public class AgentLog {
+
+  private AgentLog() {
+
+  }
+
+  /**
+   * Sets the agent log level at runtime
+   *
+   * @param level log level
+   */
+  public static void setLevel(InternalLogger.Level level) {
+    // Using log4j2 implementation allows to change the log level programmatically at runtime
+    // which is not directly possible through the slf4j API and simple implementation used in
+    // upstream distribution
+
+    Level rootLevel = toLog4jLevel(level);
+    Configurator.setRootLevel(rootLevel);
+
+    // when debugging we should avoid very chatty http client debug messages
+    if (rootLevel.intLevel() >= Level.DEBUG.intLevel()) {
+      Configurator.setLevel("okhttp3.internal.http2", Level.INFO);
+      Configurator.setLevel("okhttp3.internal.concurrent.TaskRunner", Level.INFO);
+    }
+  }
+
+  private static Level toLog4jLevel(InternalLogger.Level level) {
+    switch (level) {
+      case TRACE:
+        return Level.TRACE;
+      case DEBUG:
+        return Level.DEBUG;
+      case INFO:
+        return Level.INFO;
+      case WARN:
+        return Level.WARN;
+      case ERROR:
+        return Level.ERROR;
+      default:
+        throw new IllegalArgumentException("Unsupported level: " + level);
+    }
+  }
+}
diff --git a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
index 3b3195534..40ca5043e 100644
--- a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
+++ b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
@@ -4,6 +4,7 @@
 import io.opentelemetry.javaagent.bootstrap.InternalLogger;
 import io.opentelemetry.javaagent.tooling.LoggingCustomizer;
 import io.opentelemetry.javaagent.tooling.config.EarlyInitAgentConfig;
+import org.slf4j.LoggerFactory;
 
 @AutoService(LoggingCustomizer.class)
 public class ElasticLoggingCustomizer implements LoggingCustomizer {
@@ -15,8 +16,21 @@ public String name() {
   }
 
   @Override
-  public void init(EarlyInitAgentConfig earlyInitAgentConfig) {
+  public void init(EarlyInitAgentConfig earlyConfig) {
+
+    // trigger loading the slf4j provider from the agent CL, this should load log4j implementation
+    LoggerFactory.getILoggerFactory();
+
+    // make the agent internal logger delegate to slf4j, which will delegate to log4j
     InternalLogger.initialize(Slf4jInternalLogger::create);
+
+    // set debug logging when enabled through configuration to behave like the upstream distribution
+    if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
+      AgentLog.setLevel(InternalLogger.Level.DEBUG);
+    } else {
+      AgentLog.setLevel(InternalLogger.Level.INFO);
+    }
+
   }
 
   @Override
diff --git a/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java b/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
index fccc6ca0c..43ac07c40 100644
--- a/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
+++ b/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
@@ -21,13 +21,42 @@ public static InternalLogger create(String name) {
 
   @Override
   public boolean isLoggable(Level level) {
-    // TODO
-    return true;
+    switch (level) {
+      case TRACE:
+        return logger.isTraceEnabled();
+      case DEBUG:
+        return logger.isDebugEnabled();
+      case INFO:
+        return logger.isInfoEnabled();
+      case WARN:
+        return logger.isWarnEnabled();
+      case ERROR:
+        return logger.isErrorEnabled();
+      default:
+        throw new IllegalArgumentException("Unsupported level: " + level);
+    }
   }
 
   @Override
   public void log(Level level, String s, @Nullable Throwable throwable) {
-    // TODO
+    logger.atLevel(toSlf4jLevel(level)).setCause(throwable).log(s);
+  }
+
+  private static org.slf4j.event.Level toSlf4jLevel(Level level) {
+    switch (level) {
+      case TRACE:
+        return org.slf4j.event.Level.TRACE;
+      case DEBUG:
+        return org.slf4j.event.Level.DEBUG;
+      case INFO:
+        return org.slf4j.event.Level.INFO;
+      case WARN:
+        return org.slf4j.event.Level.WARN;
+      case ERROR:
+        return org.slf4j.event.Level.ERROR;
+      default:
+        throw new IllegalArgumentException("Unsupported level: " + level);
+    }
   }
 
   @Override
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index 6fc42d4fb..0f96c977a 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -4,6 +4,7 @@ jib = "3.4.5"
 spotless = "7.0.3"
 junit = "5.12.2"
 autoservice = "1.1.1"
+log4j2 = "2.24.3"
 
 # otel protocol (OTLP)
 opentelemetryProto = "1.3.2-alpha"
@@ -77,7 +78,8 @@ ant = "org.apache.ant:ant:1.10.15"
 asm = "org.ow2.asm:asm:9.8"
 
 slf4j-api = "org.slf4j:slf4j-api:2.0.17"
-log4j2 = "org.apache.logging.log4j:log4j-slf4j-impl:2.24.3"
+log4j2-slf4j = { group= "org.apache.logging.log4j", name="log4j-slf4j2-impl", version.ref="log4j2"}
+log4j2-core = { group= "org.apache.logging.log4j", name="log4j-core", version.ref="log4j2"}
 
 # Instrumented libraries
 openaiClient = "com.openai:openai-java:1.3.0"
@@ -85,6 +87,8 @@ openaiClient = "com.openai:openai-java:1.3.0"
 [bundles]
 
 semconv = ["opentelemetrySemconv", "opentelemetrySemconvIncubating"]
+log4j2 = ["log4j2-core", "log4j2-slf4j"]
+
 
 [plugins]
 

From c44ee17768bb79fed6bba1b0407043a436dd1fe4 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Wed, 16 Apr 2025 16:44:04 +0200
Subject: [PATCH 03/14] use log4j level directly

---
 .../co/elastic/otel/logging/AgentLog.java     | 24 +++----------------
 .../logging/ElasticLoggingCustomizer.java     |  5 ++--
 2 files changed, 6 insertions(+), 23 deletions(-)

diff --git a/custom/src/main/java/co/elastic/otel/logging/AgentLog.java b/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
index 7a5aa45c7..30a080edf 100644
--- a/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
+++ b/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
@@ -1,6 +1,5 @@
 package co.elastic.otel.logging;
 
-import io.opentelemetry.javaagent.bootstrap.InternalLogger;
 import org.apache.logging.log4j.Level;
 import org.apache.logging.log4j.core.config.Configurator;
 
@@ -15,35 +14,18 @@ private AgentLog() {
    *
    * @param level log level
    */
-  public static void setLevel(InternalLogger.Level level) {
+  public static void setLevel(Level level) {
     // Using log4j2 implementation allows to change the log level programmatically at runtime
     // which is not directly possible through the slf4j API and simple implementation used in
     // upstream distribution
 
-    Level rootLevel = toLog4jLevel(level);
-    Configurator.setRootLevel(rootLevel);
+    Configurator.setAllLevels("", level);
 
     // when debugging we should avoid very chatty http client debug messages
-    if (rootLevel.intLevel() >= Level.DEBUG.intLevel()) {
+    if (level.intLevel() >= Level.DEBUG.intLevel()) {
       Configurator.setLevel("okhttp3.internal.http2", Level.INFO);
       Configurator.setLevel("okhttp3.internal.concurrent.TaskRunner", Level.INFO);
     }
   }
 
-  private static Level toLog4jLevel(InternalLogger.Level level) {
-    switch (level) {
-      case TRACE:
-        return Level.TRACE;
-      case DEBUG:
-        return Level.DEBUG;
-      case INFO:
-        return Level.INFO;
-      case WARN:
-        return Level.WARN;
-      case ERROR:
-        return Level.ERROR;
-      default:
-        throw new IllegalArgumentException("Unsupported level: " + level);
-    }
-  }
 }
diff --git a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
index 40ca5043e..14715eb41 100644
--- a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
+++ b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
@@ -4,6 +4,7 @@
 import io.opentelemetry.javaagent.bootstrap.InternalLogger;
 import io.opentelemetry.javaagent.tooling.LoggingCustomizer;
 import io.opentelemetry.javaagent.tooling.config.EarlyInitAgentConfig;
+import org.apache.logging.log4j.Level;
 import org.slf4j.LoggerFactory;
 
 @AutoService(LoggingCustomizer.class)
@@ -26,9 +27,9 @@ public void init(EarlyInitAgentConfig earlyConfig) {
 
     // set debug logging when enabled through configuration to behave like the upstream distribution
     if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
-      AgentLog.setLevel(InternalLogger.Level.DEBUG);
+      AgentLog.setLevel(Level.DEBUG);
     } else {
-      AgentLog.setLevel(InternalLogger.Level.INFO);
+      AgentLog.setLevel(Level.INFO);
     }
 
   }

From 230cad1ef839ecd83d977d292a41d0a110830bb6 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 09:38:11 +0200
Subject: [PATCH 04/14] reformat

---
 .../co/elastic/otel/logging/AgentLog.java     | 40 ++++++++++++++++++-
 .../logging/ElasticLoggingCustomizer.java     | 27 ++++++++++---
 .../otel/logging/Slf4jInternalLogger.java     | 24 +++++++++--
 3 files changed, 80 insertions(+), 11 deletions(-)

diff --git a/custom/src/main/java/co/elastic/otel/logging/AgentLog.java b/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
index 30a080edf..5a9eae694 100644
--- a/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
+++ b/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
@@ -1,12 +1,49 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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
+ *
+ *   http://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 co.elastic.otel.logging;
 
 import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.core.appender.ConsoleAppender;
 import org.apache.logging.log4j.core.config.Configurator;
+import org.apache.logging.log4j.core.config.builder.api.ConfigurationBuilder;
+import org.apache.logging.log4j.core.config.builder.api.ConfigurationBuilderFactory;
+import org.apache.logging.log4j.core.config.builder.impl.BuiltConfiguration;
+import org.apache.logging.log4j.core.layout.PatternLayout;
 
 public class AgentLog {
 
-  private AgentLog() {
+  private static final String PATTERN = "%msg%n";
 
+  private AgentLog() {}
+
+  public static void init() {
+
+    ConfigurationBuilder<BuiltConfiguration> conf =
+        ConfigurationBuilderFactory.newConfigurationBuilder();
+    conf.add(
+        conf.newAppender("stdout", ConsoleAppender.PLUGIN_NAME)
+            .add(conf.newLayout(PatternLayout.class.getName()))
+            .addAttribute("pattern", PATTERN));
+
+    //    conf.add(conf.newRootLogger().add(conf.newAppenderRef("stdout")));
+
+    Configurator.initialize(conf.build());
   }
 
   /**
@@ -27,5 +64,4 @@ public static void setLevel(Level level) {
       Configurator.setLevel("okhttp3.internal.concurrent.TaskRunner", Level.INFO);
     }
   }
-
 }
diff --git a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
index 14715eb41..28279d825 100644
--- a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
+++ b/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
@@ -1,3 +1,21 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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
+ *
+ *   http://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 co.elastic.otel.logging;
 
 import com.google.auto.service.AutoService;
@@ -22,21 +40,20 @@ public void init(EarlyInitAgentConfig earlyConfig) {
     // trigger loading the slf4j provider from the agent CL, this should load log4j implementation
     LoggerFactory.getILoggerFactory();
 
+    AgentLog.init();
+
     // make the agent internal logger delegate to slf4j, which will delegate to log4j
     InternalLogger.initialize(Slf4jInternalLogger::create);
 
+    AgentLog.setLevel(Level.INFO);
     // set debug logging when enabled through configuration to behave like the upstream distribution
     if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
       AgentLog.setLevel(Level.DEBUG);
-    } else {
-      AgentLog.setLevel(Level.INFO);
     }
-
   }
 
   @Override
-  public void onStartupSuccess() {
-  }
+  public void onStartupSuccess() {}
 
   @SuppressWarnings("CallToPrintStackTrace")
   @Override
diff --git a/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java b/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
index 43ac07c40..322e78507 100644
--- a/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
+++ b/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
@@ -1,13 +1,29 @@
+/*
+ * Licensed to Elasticsearch B.V. under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch B.V. licenses this file to you 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
+ *
+ *   http://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 co.elastic.otel.logging;
 
 import io.opentelemetry.javaagent.bootstrap.InternalLogger;
+import javax.annotation.Nullable;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
-import javax.annotation.Nullable;
 
-/**
- * Internal logger implementation that delegates to SLF4J
- */
+/** Internal logger implementation that delegates to SLF4J */
 public class Slf4jInternalLogger implements InternalLogger {
   private final Logger logger;
 

From 7e8163cd73a1bcd02fed1b03af02eb6048945a99 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 10:19:10 +0200
Subject: [PATCH 05/14] move to another module and cleanup

---
 .../co/elastic/otel/agent/ElasticAgent.java    |  1 +
 custom/build.gradle.kts                        |  3 +--
 internal-logging/build.gradle.kts              | 18 ++++++++++++++++++
 .../java/co/elastic/otel/logging/AgentLog.java | 11 +++++------
 .../otel/logging/ElasticLoggingCustomizer.java |  4 ++--
 .../otel/logging/Slf4jInternalLogger.java      |  0
 settings.gradle.kts                            |  1 +
 7 files changed, 28 insertions(+), 10 deletions(-)
 create mode 100644 internal-logging/build.gradle.kts
 rename {custom => internal-logging}/src/main/java/co/elastic/otel/logging/AgentLog.java (87%)
 rename {custom => internal-logging}/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java (100%)
 rename {custom => internal-logging}/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java (100%)

diff --git a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
index 5fa769114..7e6dad6a8 100644
--- a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
+++ b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
@@ -32,6 +32,7 @@ public class ElasticAgent {
    */
   public static void premain(String agentArgs, Instrumentation inst) {
 
+    // must match value returned by ElasticLoggingCustomizer#getName
     System.setProperty("otel.javaagent.logging", "elastic");
 
     OpenTelemetryAgent.premain(agentArgs, inst);
diff --git a/custom/build.gradle.kts b/custom/build.gradle.kts
index afc157db3..fbde0d5ca 100644
--- a/custom/build.gradle.kts
+++ b/custom/build.gradle.kts
@@ -11,6 +11,7 @@ dependencies {
   implementation(project(":inferred-spans"))
   implementation(project(":universal-profiling-integration"))
   implementation(project(":resources"))
+  implementation(project(":internal-logging"))
   instrumentations.forEach {
     implementation(project(it))
   }
@@ -21,8 +22,6 @@ dependencies {
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-extension-api")
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-tooling")
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-bootstrap")
-  compileOnly(libs.slf4j.api)
-  implementation(libs.bundles.log4j2)
   compileOnly(libs.bundles.semconv)
 
   implementation(libs.contribSpanStacktrace) {
diff --git a/internal-logging/build.gradle.kts b/internal-logging/build.gradle.kts
new file mode 100644
index 000000000..b9ac365f1
--- /dev/null
+++ b/internal-logging/build.gradle.kts
@@ -0,0 +1,18 @@
+plugins {
+  id("elastic-otel.library-packaging-conventions")
+}
+
+dependencies {
+
+  compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-tooling")
+  compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-bootstrap")
+  compileOnly(libs.slf4j.api)
+  implementation(libs.bundles.log4j2)
+
+  // needs to be added in order to allow access to AgentListener interface
+  // this is currently required because autoconfigure is currently not exposed to the extension API.
+  compileOnly("io.opentelemetry:opentelemetry-sdk-extension-autoconfigure")
+}
+
+tasks {
+}
diff --git a/custom/src/main/java/co/elastic/otel/logging/AgentLog.java b/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
similarity index 87%
rename from custom/src/main/java/co/elastic/otel/logging/AgentLog.java
rename to internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
index 5a9eae694..9394ead31 100644
--- a/custom/src/main/java/co/elastic/otel/logging/AgentLog.java
+++ b/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
@@ -19,12 +19,10 @@
 package co.elastic.otel.logging;
 
 import org.apache.logging.log4j.Level;
-import org.apache.logging.log4j.core.appender.ConsoleAppender;
 import org.apache.logging.log4j.core.config.Configurator;
 import org.apache.logging.log4j.core.config.builder.api.ConfigurationBuilder;
 import org.apache.logging.log4j.core.config.builder.api.ConfigurationBuilderFactory;
 import org.apache.logging.log4j.core.config.builder.impl.BuiltConfiguration;
-import org.apache.logging.log4j.core.layout.PatternLayout;
 
 public class AgentLog {
 
@@ -36,10 +34,11 @@ public static void init() {
 
     ConfigurationBuilder<BuiltConfiguration> conf =
         ConfigurationBuilderFactory.newConfigurationBuilder();
-    conf.add(
-        conf.newAppender("stdout", ConsoleAppender.PLUGIN_NAME)
-            .add(conf.newLayout(PatternLayout.class.getName()))
-            .addAttribute("pattern", PATTERN));
+
+    //    conf.add(
+    //        conf.newAppender("stdout", ConsoleAppender.PLUGIN_NAME)
+    //            .add(conf.newLayout(PatternLayout.class.getName()))
+    //            .addAttribute("pattern", PATTERN));
 
     //    conf.add(conf.newRootLogger().add(conf.newAppenderRef("stdout")));
 
diff --git a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java b/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
similarity index 100%
rename from custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
rename to internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
index 28279d825..e6cbd9186 100644
--- a/custom/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
+++ b/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
@@ -40,11 +40,11 @@ public void init(EarlyInitAgentConfig earlyConfig) {
     // trigger loading the slf4j provider from the agent CL, this should load log4j implementation
     LoggerFactory.getILoggerFactory();
 
-    AgentLog.init();
-
     // make the agent internal logger delegate to slf4j, which will delegate to log4j
     InternalLogger.initialize(Slf4jInternalLogger::create);
 
+    AgentLog.init();
+
     AgentLog.setLevel(Level.INFO);
     // set debug logging when enabled through configuration to behave like the upstream distribution
     if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
diff --git a/custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java b/internal-logging/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
similarity index 100%
rename from custom/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
rename to internal-logging/src/main/java/co/elastic/otel/logging/Slf4jInternalLogger.java
diff --git a/settings.gradle.kts b/settings.gradle.kts
index 05c3ef69b..68c1a8577 100644
--- a/settings.gradle.kts
+++ b/settings.gradle.kts
@@ -19,6 +19,7 @@ include("custom")
 include("instrumentation")
 include("instrumentation:openai-client-instrumentation:instrumentation-1.1")
 include("inferred-spans")
+include("internal-logging")
 include("resources")
 include("runtime-attach")
 include("smoke-tests")

From 6c7fcf2da8a6e25a7a5c828c5254ad73151fe6e5 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 11:10:11 +0200
Subject: [PATCH 06/14] relocate internal logging impl.

---
 ...tel.agent-packaging-conventions.gradle.kts |  2 +-
 custom/build.gradle.kts                       |  2 +-
 internal-logging/build.gradle.kts             | 19 +++++++++++++++++++
 3 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts b/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts
index 31725ce8d..704bc1b30 100644
--- a/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts
+++ b/buildSrc/src/main/kotlin/elastic-otel.agent-packaging-conventions.gradle.kts
@@ -182,7 +182,7 @@ tasks {
     dependsOn(isolateJavaagentLibs)
     configurations = listOf(bootstrapLibs, upstreamAgent)
 
-    // exclude slf4j-simple from the shadow jar as we use log4j2-slf4j instead
+    // exclude slf4j-simple from the shadow jar as we use log4j2-slf4j with internal-logging instead
     exclude("inst/io/opentelemetry/javaagent/slf4j/simple/**")
 
     from(isolateJavaagentLibs.get().outputs)
diff --git a/custom/build.gradle.kts b/custom/build.gradle.kts
index fbde0d5ca..6f9f6ac45 100644
--- a/custom/build.gradle.kts
+++ b/custom/build.gradle.kts
@@ -11,7 +11,7 @@ dependencies {
   implementation(project(":inferred-spans"))
   implementation(project(":universal-profiling-integration"))
   implementation(project(":resources"))
-  implementation(project(":internal-logging"))
+  implementation(project(":internal-logging", configuration = "shadow"))
   instrumentations.forEach {
     implementation(project(it))
   }
diff --git a/internal-logging/build.gradle.kts b/internal-logging/build.gradle.kts
index b9ac365f1..3de61c03f 100644
--- a/internal-logging/build.gradle.kts
+++ b/internal-logging/build.gradle.kts
@@ -1,5 +1,9 @@
+import com.github.jengelman.gradle.plugins.shadow.tasks.ShadowJar
+import com.github.jengelman.gradle.plugins.shadow.transformers.Log4j2PluginsCacheFileTransformer
+
 plugins {
   id("elastic-otel.library-packaging-conventions")
+  id("com.gradleup.shadow")
 }
 
 dependencies {
@@ -15,4 +19,19 @@ dependencies {
 }
 
 tasks {
+  val shadowJar by existing(ShadowJar::class) {
+    // required for META-INF/services files relocation
+    mergeServiceFiles()
+
+    transform(Log4j2PluginsCacheFileTransformer::class.java)
+
+    // relocate slf4j and log4j for internal logging to prevent any conflict
+    relocate("org.slf4j", "co.elastic.otel.logging.slf4j")
+    relocate("org.apache.logging.log4j", "co.elastic.otel.logging.log4j")
+    relocate("org.apache.logging.slf4j", "co.elastic.otel.logging.log4j.slf4j")
+  }
+
+  assemble {
+    dependsOn(shadowJar)
+  }
 }

From a01182c39ca6794d3138eb0d472f74a0ad579d79 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 11:10:24 +0200
Subject: [PATCH 07/14] provide elastic config option for log level

---
 .../otel/logging/ElasticLoggingCustomizer.java | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java b/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
index e6cbd9186..0ab6f4f58 100644
--- a/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
+++ b/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
@@ -45,11 +45,21 @@ public void init(EarlyInitAgentConfig earlyConfig) {
 
     AgentLog.init();
 
-    AgentLog.setLevel(Level.INFO);
-    // set debug logging when enabled through configuration to behave like the upstream distribution
-    if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
-      AgentLog.setLevel(Level.DEBUG);
+    String levelConfig = earlyConfig.getString("elastic.otel.javaagent.log.level");
+    if (levelConfig == null) {
+      // explicit level not set
+      AgentLog.setLevel(Level.INFO);
+      // set debug logging when enabled through configuration to behave like the upstream distribution
+      if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
+        AgentLog.setLevel(Level.DEBUG);
+      }
+    } else {
+      // explicit level provided
+      Level level = Level.getLevel(levelConfig);
+      AgentLog.setLevel(level == null ? Level.INFO : level);
     }
+
+
   }
 
   @Override

From 4cf00481beab51c6bee9989c90fe6b26dec333e7 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 11:14:57 +0200
Subject: [PATCH 08/14] simplify custom log level usage

---
 .../logging/ElasticLoggingCustomizer.java     | 23 ++++++++-----------
 1 file changed, 10 insertions(+), 13 deletions(-)

diff --git a/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java b/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
index 0ab6f4f58..93ae61e15 100644
--- a/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
+++ b/internal-logging/src/main/java/co/elastic/otel/logging/ElasticLoggingCustomizer.java
@@ -45,21 +45,18 @@ public void init(EarlyInitAgentConfig earlyConfig) {
 
     AgentLog.init();
 
-    String levelConfig = earlyConfig.getString("elastic.otel.javaagent.log.level");
-    if (levelConfig == null) {
-      // explicit level not set
-      AgentLog.setLevel(Level.INFO);
-      // set debug logging when enabled through configuration to behave like the upstream distribution
-      if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
-        AgentLog.setLevel(Level.DEBUG);
-      }
+    Level level = null;
+    if (earlyConfig.getBoolean("otel.javaagent.debug", false)) {
+      // set debug logging when enabled through configuration to behave like the upstream
+      // distribution
+      level = Level.DEBUG;
     } else {
-      // explicit level provided
-      Level level = Level.getLevel(levelConfig);
-      AgentLog.setLevel(level == null ? Level.INFO : level);
+      String levelConfig = earlyConfig.getString("elastic.otel.javaagent.log.level");
+      if (levelConfig != null) {
+        level = Level.getLevel(levelConfig);
+      }
     }
-
-
+    AgentLog.setLevel(level != null ? level : Level.INFO);
   }
 
   @Override

From a2e07dc4ef1b115f8c2150da93f3f27f4615b7e4 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 11:54:05 +0200
Subject: [PATCH 09/14] set custom log format

---
 .../main/java/co/elastic/otel/logging/AgentLog.java | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java b/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
index 9394ead31..bb8a8ff02 100644
--- a/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
+++ b/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
@@ -26,7 +26,7 @@
 
 public class AgentLog {
 
-  private static final String PATTERN = "%msg%n";
+  private static final String PATTERN = "%d{DEFAULT} [%t] %-5level %logger{36} - %msg{nolookups}%n";
 
   private AgentLog() {}
 
@@ -35,14 +35,13 @@ public static void init() {
     ConfigurationBuilder<BuiltConfiguration> conf =
         ConfigurationBuilderFactory.newConfigurationBuilder();
 
-    //    conf.add(
-    //        conf.newAppender("stdout", ConsoleAppender.PLUGIN_NAME)
-    //            .add(conf.newLayout(PatternLayout.class.getName()))
-    //            .addAttribute("pattern", PATTERN));
+    conf.add(
+        conf.newAppender("stdout", "Console")
+            .add(conf.newLayout("PatternLayout").addAttribute("pattern", PATTERN)));
 
-    //    conf.add(conf.newRootLogger().add(conf.newAppenderRef("stdout")));
+    conf.add(conf.newRootLogger().add(conf.newAppenderRef("stdout")));
 
-    Configurator.initialize(conf.build());
+    Configurator.initialize(conf.build(false));
   }
 
   /**

From 9c01a438517bc188013e3f6770ab2ab14d266445 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 14:16:31 +0200
Subject: [PATCH 10/14] cleanup

---
 custom/build.gradle.kts                                   | 1 -
 .../src/main/java/co/elastic/otel/logging/AgentLog.java   | 8 ++++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/custom/build.gradle.kts b/custom/build.gradle.kts
index 34f0835aa..d4f59d0e0 100644
--- a/custom/build.gradle.kts
+++ b/custom/build.gradle.kts
@@ -21,7 +21,6 @@ dependencies {
   compileOnly("io.opentelemetry:opentelemetry-sdk-extension-autoconfigure-spi")
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-extension-api")
   compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-tooling")
-  compileOnly("io.opentelemetry.javaagent:opentelemetry-javaagent-bootstrap")
   compileOnly(libs.bundles.semconv)
 
   implementation(libs.contribSpanStacktrace) {
diff --git a/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java b/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
index bb8a8ff02..3db95705f 100644
--- a/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
+++ b/internal-logging/src/main/java/co/elastic/otel/logging/AgentLog.java
@@ -28,6 +28,9 @@ public class AgentLog {
 
   private static final String PATTERN = "%d{DEFAULT} [%t] %-5level %logger{36} - %msg{nolookups}%n";
 
+  // logger is an empty string
+  private static final String ROOT_LOGGER_NAME = "";
+
   private AgentLog() {}
 
   public static void init() {
@@ -52,11 +55,12 @@ public static void init() {
   public static void setLevel(Level level) {
     // Using log4j2 implementation allows to change the log level programmatically at runtime
     // which is not directly possible through the slf4j API and simple implementation used in
-    // upstream distribution
+    // upstream distribution.
 
-    Configurator.setAllLevels("", level);
+    Configurator.setAllLevels(ROOT_LOGGER_NAME, level);
 
     // when debugging we should avoid very chatty http client debug messages
+    // this behavior is replicated from the upstream distribution.
     if (level.intLevel() >= Level.DEBUG.intLevel()) {
       Configurator.setLevel("okhttp3.internal.http2", Level.INFO);
       Configurator.setLevel("okhttp3.internal.concurrent.TaskRunner", Level.INFO);

From 32cf14cc0470ce45587e38b70bb39a7afaf57285 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 15:18:17 +0200
Subject: [PATCH 11/14] attempt to preserve upstream log options

---
 .../co/elastic/otel/agent/ElasticAgent.java   | 22 +++++++++++++++----
 internal-logging/build.gradle.kts             |  3 ---
 2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
index 7e6dad6a8..aee24202f 100644
--- a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
+++ b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
@@ -20,10 +20,14 @@
 
 import io.opentelemetry.javaagent.OpenTelemetryAgent;
 import java.lang.instrument.Instrumentation;
+import java.util.Locale;
 
 /** Elastic agent entry point, delegates to OpenTelemetry agent */
 public class ElasticAgent {
 
+  private static final String OTEL_JAVAAGENT_LOGGING = "otel.javaagent.logging";
+  private static final String OTEL_JAVAAGENT_LOGGING_ENV = "OTEL_JAVAAGENT_LOGGING";
+
   /**
    * Entry point for -javaagent JVM argument attach
    *
@@ -31,10 +35,7 @@ public class ElasticAgent {
    * @param inst instrumentation
    */
   public static void premain(String agentArgs, Instrumentation inst) {
-
-    // must match value returned by ElasticLoggingCustomizer#getName
-    System.setProperty("otel.javaagent.logging", "elastic");
-
+    initLogging();
     OpenTelemetryAgent.premain(agentArgs, inst);
   }
 
@@ -45,6 +46,7 @@ public static void premain(String agentArgs, Instrumentation inst) {
    * @param inst instrumentation
    */
   public static void agentmain(String agentArgs, Instrumentation inst) {
+    initLogging();
     OpenTelemetryAgent.agentmain(agentArgs, inst);
   }
 
@@ -57,5 +59,17 @@ public static void main(String[] args) {
     OpenTelemetryAgent.main(args);
   }
 
+  private static void initLogging() {
+
+    // do not override explicitly provided configuration
+    if (System.getProperty(OTEL_JAVAAGENT_LOGGING) != null
+        || System.getenv(OTEL_JAVAAGENT_LOGGING_ENV) != null) {
+      return;
+    }
+
+    // must match value returned by ElasticLoggingCustomizer#getName
+    System.setProperty(OTEL_JAVAAGENT_LOGGING, "elastic");
+  }
+
   private ElasticAgent() {}
 }
diff --git a/internal-logging/build.gradle.kts b/internal-logging/build.gradle.kts
index 3de61c03f..a88867c41 100644
--- a/internal-logging/build.gradle.kts
+++ b/internal-logging/build.gradle.kts
@@ -13,9 +13,6 @@ dependencies {
   compileOnly(libs.slf4j.api)
   implementation(libs.bundles.log4j2)
 
-  // needs to be added in order to allow access to AgentListener interface
-  // this is currently required because autoconfigure is currently not exposed to the extension API.
-  compileOnly("io.opentelemetry:opentelemetry-sdk-extension-autoconfigure")
 }
 
 tasks {

From b866dd1e99c3907d7c9f494063e90e1e7712a95b Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Thu, 17 Apr 2025 15:19:06 +0200
Subject: [PATCH 12/14] cleanup

---
 .../src/main/java/co/elastic/otel/agent/ElasticAgent.java        | 1 -
 1 file changed, 1 deletion(-)

diff --git a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
index aee24202f..7d50c5673 100644
--- a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
+++ b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
@@ -20,7 +20,6 @@
 
 import io.opentelemetry.javaagent.OpenTelemetryAgent;
 import java.lang.instrument.Instrumentation;
-import java.util.Locale;
 
 /** Elastic agent entry point, delegates to OpenTelemetry agent */
 public class ElasticAgent {

From d6d14e402b2ed62f42be4878a6ada9b17f01f577 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Tue, 22 Apr 2025 15:03:38 +0200
Subject: [PATCH 13/14] disable system properties and env variables for log4j

---
 internal-logging/build.gradle.kts | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/internal-logging/build.gradle.kts b/internal-logging/build.gradle.kts
index a88867c41..969717157 100644
--- a/internal-logging/build.gradle.kts
+++ b/internal-logging/build.gradle.kts
@@ -20,6 +20,10 @@ tasks {
     // required for META-INF/services files relocation
     mergeServiceFiles()
 
+    // Excluding property source SPI prevents log4j system properties and env variables that might
+    // be set at the application level to change the behavior of this internal log4j instance.
+    exclude("**/*.log4j.util.PropertySource")
+
     transform(Log4j2PluginsCacheFileTransformer::class.java)
 
     // relocate slf4j and log4j for internal logging to prevent any conflict

From 0b05c841bb2eb1710310bf23545af3431b491644 Mon Sep 17 00:00:00 2001
From: Sylvain Juge <763082+SylvainJuge@users.noreply.github.com>
Date: Tue, 22 Apr 2025 15:20:31 +0200
Subject: [PATCH 14/14] prevent explicit 'simple' config issue

---
 .../java/co/elastic/otel/agent/ElasticAgent.java     | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
index 7d50c5673..793e19adc 100644
--- a/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
+++ b/agent/entrypoint/src/main/java/co/elastic/otel/agent/ElasticAgent.java
@@ -26,6 +26,7 @@ public class ElasticAgent {
 
   private static final String OTEL_JAVAAGENT_LOGGING = "otel.javaagent.logging";
   private static final String OTEL_JAVAAGENT_LOGGING_ENV = "OTEL_JAVAAGENT_LOGGING";
+  private static final String OTEL_JAVAAGENT_LOGGING_DEFAULT = "simple";
 
   /**
    * Entry point for -javaagent JVM argument attach
@@ -60,9 +61,10 @@ public static void main(String[] args) {
 
   private static void initLogging() {
 
-    // do not override explicitly provided configuration
-    if (System.getProperty(OTEL_JAVAAGENT_LOGGING) != null
-        || System.getenv(OTEL_JAVAAGENT_LOGGING_ENV) != null) {
+    // Do not override explicitly provided configuration unless it's using the default as the
+    // 'simple' provider is not included in this distribution and that triggers an SLF4j error.
+    if (isLoggingNotDefault(System.getProperty(OTEL_JAVAAGENT_LOGGING))
+        || isLoggingNotDefault(System.getenv(OTEL_JAVAAGENT_LOGGING_ENV))) {
       return;
     }
 
@@ -70,5 +72,9 @@ private static void initLogging() {
     System.setProperty(OTEL_JAVAAGENT_LOGGING, "elastic");
   }
 
+  private static boolean isLoggingNotDefault(String value) {
+    return value != null && !OTEL_JAVAAGENT_LOGGING_DEFAULT.equals(value);
+  }
+
   private ElasticAgent() {}
 }