Prototype Log4j2 Appender (#4375)

* WIP

* Use Log SDK snapshot

* Update to 1.9.0 release, refine

* Remove appenders from common log config

* Respond to PR feedback

* Update readme

* Switch to compileOnly log sdk dependency, use logger name as instrumentation name

* Switch to minimalist LogEvent mapping

* PR feedback

* PR feedback

instrumentation/log4j/log4j-2-common/testing/src/main/groovy/Log4j2Test.groovy

@@ -9,7 +9,7 @@ import io.opentelemetry.instrumentation.test.InstrumentationSpecification
 import org.apache.logging.log4j.LogManager
 
 abstract class Log4j2Test extends InstrumentationSpecification {
-  def cleanup() {
+  def setup() {
     ListAppender.get().clearEvents()
   }
 

instrumentation/log4j/log4j-2.13.2/library/README.md

@@ -1,15 +1,8 @@
 # Log4j 2 Integration
 
-This module integrates instrumentation with Log4j 2 by injecting the trace ID and span ID from a
+This module provides Log4j2 extensions related to OpenTelemetry.
-mounted span into
-Log4j's [context data](https://logging.apache.org/log4j/2.x/manual/thread-context.html).
 
-**Note**: Depending on your application, you may run into a [critical bug](https://issues.apache.org/jira/browse/LOG4J2-2838)
+To use it, add the module to your application's runtime classpath.
-with Log4j 2.13.2. If log messages show a `NullPointerException` when adding this instrumentation,
-please update to 2.13.3 or higher. The only change between 2.13.2 and 2.13.3 is the fix to this
-issue.
-
-To use it, just add the module to your application's runtime classpath.
 
 **Maven**
 
@@ -33,14 +26,26 @@ dependencies {
 }
 ```
 
-Log4j will automatically pick up our integration and will have these keys added to the context when
+## OpenTelemetry Context Data Provider
-a log statement is made when a span is active.
+
+`OpenTelemetryContextDataProvider` implements the Log4j2 `ContextDataProvider` SPI, and injects the
+trace ID and span ID from an active span into
+Log4j's [context data](https://logging.apache.org/log4j/2.x/manual/thread-context.html).
+
+**Note**: Depending on your application, you may run into
+a [critical bug](https://issues.apache.org/jira/browse/LOG4J2-2838)
+with Log4j 2.13.2. If log messages show a `NullPointerException` when adding this instrumentation,
+please update to 2.13.3 or higher. The only change between 2.13.2 and 2.13.3 is the fix to this
+issue.
+
+Log4j will automatically pick up the integration when you include this module. The following keys
+will be added to the context when a log statement is made when a span is active:
 
 - `trace_id`
 - `span_id`
 - `trace_flags`
 
-You can use these keys when defining an appender in your `log4j.xml` configuration, for example
+You can use these keys when defining an appender in your `log4j.xml` configuration, for example:
 
 ```xml
 <?xml version="1.0" encoding="UTF-8"?>
@@ -58,3 +63,46 @@ You can use these keys when defining an appender in your `log4j.xml` configurati
   </Loggers>
 </Configuration>
 ```
+
+## OpenTelemetry Appender
+
+`OpenTelemetryAppender` is a Log4j2 [appender](https://logging.apache.org/log4j/2.x/manual/appenders.html) that can be used to forward log events to the [OpenTelemetry Log SDK](https://github.com/open-telemetry/opentelemetry-java/tree/main/sdk/logs).
+
+The following demonstrates how you might configure the appender in your `log4j.xml` configuration:
+
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<Configuration status="WARN" packages="io.opentelemetry.instrumentation.log4j.v2_13_2">
+  <Appenders>
+    <Console name="Console" target="SYSTEM_OUT">
+      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} traceId: %X{trace_id} spanId: %X{span_id} flags: %X{trace_flags} - %msg%n" />
+    </Console>
+    <OpenTelemetry name="OpenTelemetryAppender" />
+  </Appenders>
+  <Loggers>
+    <Root>
+      <AppenderRef ref="OpenTelemetryAppender" level="All" />
+      <AppenderRef ref="Console" level="All" />
+    </Root>
+  </Loggers>
+</Configuration>
+```
+
+Next, associate the `OpenTelemetryAppender` with a `SdkLogEmitterProvider` in your application:
+
+```
+SdkLogEmitterProvider logEmitterProvider =
+  SdkLogEmitterProvider.builder()
+    .setResource(Resource.create(...))
+    .addLogProcessor(...)
+    .build();
+OpenTelemetryLog4j.initialize(logEmitterProvider);
+```
+
+**Note:** In order to initialize the `OpenTelemetryAppender` your application must depend on the
+OpenTelemetry log sdk (`io.opentelemetry:opentelemetry-sdk-logs`).
+
+In this example Log4j2 logs will be sent to both the console appender and
+the `OpenTelemetryAppender`, which will drop the logs until `OpenTelemetryLog4j.initialize(..)` is
+called. Once initialized, logs will be emitted to a `LogEmitter` obtained from
+the `SdkLogEmitterProvider`.

instrumentation/log4j/log4j-2.13.2/library/build.gradle.kts

@@ -3,6 +3,8 @@ plugins {
 }
 
 dependencies {
+  compileOnly("io.opentelemetry:opentelemetry-sdk-logs")
+
   library("org.apache.logging.log4j:log4j-core:2.13.2")
 
   // Library instrumentation cannot be applied to 2.13.2 due to a bug in Log4J. The agent works
@@ -10,4 +12,7 @@ dependencies {
   testLibrary("org.apache.logging.log4j:log4j-core:2.13.3")
 
   testImplementation(project(":instrumentation:log4j:log4j-2-common:testing"))
+  testImplementation("io.opentelemetry:opentelemetry-sdk-logs")
+
+  testImplementation("org.mockito:mockito-core")
 }

instrumentation/log4j/log4j-2.13.2/library/src/main/java/io/opentelemetry/instrumentation/log4j/v2_13_2/LogEventMapper.java

@@ -0,0 +1,102 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.log4j.v2_13_2;
+
+import io.opentelemetry.api.common.AttributeKey;
+import io.opentelemetry.api.common.Attributes;
+import io.opentelemetry.api.common.AttributesBuilder;
+import io.opentelemetry.context.Context;
+import io.opentelemetry.sdk.logs.LogBuilder;
+import io.opentelemetry.sdk.logs.data.Severity;
+import java.util.concurrent.TimeUnit;
+import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.core.LogEvent;
+import org.apache.logging.log4j.core.time.Instant;
+import org.apache.logging.log4j.message.Message;
+
+final class LogEventMapper {
+
+  // Visible for testing
+  static final AttributeKey<String> ATTR_THROWABLE_MESSAGE =
+      AttributeKey.stringKey("throwable.message");
+
+  /**
+   * Map the {@link LogEvent} data model onto the {@link LogBuilder}. Unmapped fields include:
+   *
+   * <ul>
+   *   <li>Fully qualified class name - {@link LogEvent#getLoggerFqcn()}
+   *   <li>Thread name - {@link LogEvent#getThreadName()}
+   *   <li>Thread id - {@link LogEvent#getThreadId()}
+   *   <li>Thread priority - {@link LogEvent#getThreadPriority()}
+   *   <li>Thread priority - {@link LogEvent#getThreadPriority()}
+   *   <li>Thrown details (stack trace, class name) - {@link LogEvent#getThrown()}
+   *   <li>Marker - {@link LogEvent#getMarker()}
+   *   <li>Nested diagnostic context - {@link LogEvent#getContextStack()}
+   *   <li>Mapped diagnostic context - {@link LogEvent#getContextData()}
+   * </ul>
+   */
+  static void mapLogEvent(LogBuilder builder, LogEvent logEvent) {
+    // TODO: map the LogEvent more completely when semantic conventions allow it
+    AttributesBuilder attributes = Attributes.builder();
+
+    // message
+    Message message = logEvent.getMessage();
+    if (message != null) {
+      builder.setBody(message.getFormattedMessage());
+    }
+
+    // time
+    Instant instant = logEvent.getInstant();
+    if (instant != null) {
+      builder.setEpoch(
+          TimeUnit.MILLISECONDS.toNanos(instant.getEpochMillisecond())
+              + instant.getNanoOfMillisecond(),
+          TimeUnit.NANOSECONDS);
+    }
+
+    // level
+    Level level = logEvent.getLevel();
+    if (level != null) {
+      builder.setSeverity(levelToSeverity(level));
+      builder.setSeverityText(logEvent.getLevel().name());
+    }
+
+    // throwable
+    Throwable throwable = logEvent.getThrown();
+    if (throwable != null) {
+      attributes.put(ATTR_THROWABLE_MESSAGE, throwable.getMessage());
+    }
+
+    // span context
+    builder.setContext(Context.current());
+
+    builder.setAttributes(attributes.build());
+  }
+
+  private static Severity levelToSeverity(Level level) {
+    switch (level.getStandardLevel()) {
+      case ALL:
+        return Severity.TRACE;
+      case TRACE:
+        return Severity.TRACE2;
+      case DEBUG:
+        return Severity.DEBUG;
+      case INFO:
+        return Severity.INFO;
+      case WARN:
+        return Severity.WARN;
+      case ERROR:
+        return Severity.ERROR;
+      case FATAL:
+        return Severity.FATAL;
+      case OFF:
+        return Severity.UNDEFINED_SEVERITY_NUMBER;
+    }
+    return Severity.UNDEFINED_SEVERITY_NUMBER;
+  }
+
+  private LogEventMapper() {}
+}

instrumentation/log4j/log4j-2.13.2/library/src/main/java/io/opentelemetry/instrumentation/log4j/v2_13_2/OpenTelemetryAppender.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.log4j.v2_13_2;
+
+import io.opentelemetry.sdk.logs.LogBuilder;
+import io.opentelemetry.sdk.logs.SdkLogEmitterProvider;
+import java.io.Serializable;
+import java.util.concurrent.atomic.AtomicReference;
+import org.apache.logging.log4j.core.Appender;
+import org.apache.logging.log4j.core.Core;
+import org.apache.logging.log4j.core.Filter;
+import org.apache.logging.log4j.core.Layout;
+import org.apache.logging.log4j.core.LogEvent;
+import org.apache.logging.log4j.core.appender.AbstractAppender;
+import org.apache.logging.log4j.core.config.Property;
+import org.apache.logging.log4j.core.config.plugins.Plugin;
+import org.apache.logging.log4j.core.config.plugins.PluginBuilderFactory;
+
+@Plugin(
+    name = OpenTelemetryAppender.PLUGIN_NAME,
+    category = Core.CATEGORY_NAME,
+    elementType = Appender.ELEMENT_TYPE)
+public class OpenTelemetryAppender extends AbstractAppender {
+
+  static final String PLUGIN_NAME = "OpenTelemetry";
+
+  @PluginBuilderFactory
+  public static <B extends Builder<B>> B builder() {
+    return new Builder<B>().asBuilder();
+  }
+
+  static class Builder<B extends Builder<B>> extends AbstractAppender.Builder<B>
+      implements org.apache.logging.log4j.core.util.Builder<OpenTelemetryAppender> {
+
+    @Override
+    public OpenTelemetryAppender build() {
+      OpenTelemetryAppender appender =
+          new OpenTelemetryAppender(
+              getName(), getLayout(), getFilter(), isIgnoreExceptions(), getPropertyArray());
+      OpenTelemetryLog4j.registerInstance(appender);
+      return appender;
+    }
+  }
+
+  private final AtomicReference<SdkLogEmitterProvider> sdkLogEmitterProviderRef =
+      new AtomicReference<>();
+
+  private OpenTelemetryAppender(
+      String name,
+      Layout<? extends Serializable> layout,
+      Filter filter,
+      boolean ignoreExceptions,
+      Property[] properties) {
+    super(name, filter, layout, ignoreExceptions, properties);
+  }
+
+  @Override
+  public void append(LogEvent event) {
+    SdkLogEmitterProvider logEmitterProvider = sdkLogEmitterProviderRef.get();
+    if (logEmitterProvider == null) {
+      // appender hasn't been initialized
+      return;
+    }
+    LogBuilder builder =
+        logEmitterProvider.logEmitterBuilder(event.getLoggerName()).build().logBuilder();
+    LogEventMapper.mapLogEvent(builder, event);
+    builder.emit();
+  }
+
+  void initialize(SdkLogEmitterProvider sdkLogEmitterProvider) {
+    if (!sdkLogEmitterProviderRef.compareAndSet(null, sdkLogEmitterProvider)) {
+      throw new IllegalStateException("OpenTelemetryAppender has already been initialized.");
+    }
+  }
+}

instrumentation/log4j/log4j-2.13.2/library/src/main/java/io/opentelemetry/instrumentation/log4j/v2_13_2/OpenTelemetryLog4j.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.log4j.v2_13_2;
+
+import io.opentelemetry.sdk.logs.SdkLogEmitterProvider;
+import java.util.ArrayList;
+import java.util.List;
+import javax.annotation.Nullable;
+import javax.annotation.concurrent.GuardedBy;
+
+public final class OpenTelemetryLog4j {
+
+  private static final Object lock = new Object();
+
+  @GuardedBy("lock")
+  private static SdkLogEmitterProvider sdkLogEmitterProvider;
+
+  @GuardedBy("lock")
+  @Nullable
+  private static Throwable initializeCaller;
+
+  @GuardedBy("lock")
+  private static final List<OpenTelemetryAppender> APPENDERS = new ArrayList<>();
+
+  public static void initialize(SdkLogEmitterProvider sdkLogEmitterProvider) {
+    List<OpenTelemetryAppender> instances;
+    synchronized (lock) {
+      if (OpenTelemetryLog4j.sdkLogEmitterProvider != null) {
+        throw new IllegalStateException(
+            "OpenTelemetryLog4j.initialize has already been called. OpenTelemetryLog4j.initialize "
+                + "must be called only once. Previous invocation set to cause of this exception.",
+            initializeCaller);
+      }
+      OpenTelemetryLog4j.sdkLogEmitterProvider = sdkLogEmitterProvider;
+      instances = new ArrayList<>(APPENDERS);
+      initializeCaller = new Throwable();
+    }
+    for (OpenTelemetryAppender instance : instances) {
+      instance.initialize(sdkLogEmitterProvider);
+    }
+  }
+
+  static void registerInstance(OpenTelemetryAppender appender) {
+    synchronized (lock) {
+      if (sdkLogEmitterProvider != null) {
+        appender.initialize(sdkLogEmitterProvider);
+      }
+      APPENDERS.add(appender);
+    }
+  }
+
+  // Visible for testing
+  static void resetForTest() {
+    synchronized (lock) {
+      sdkLogEmitterProvider = null;
+      APPENDERS.clear();
+    }
+  }
+
+  private OpenTelemetryLog4j() {}
+}

instrumentation/log4j/log4j-2.13.2/library/src/test/java/io/opentelemetry/instrumentation/log4j/v2_13_2/OpenTelemetryAppenderConfigTest.java

@@ -0,0 +1,121 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.log4j.v2_13_2;
+
+import static io.opentelemetry.instrumentation.log4j.v2_13_2.LogEventMapper.ATTR_THROWABLE_MESSAGE;
+import static org.assertj.core.api.AssertionsForInterfaceTypes.assertThat;
+
+import io.opentelemetry.api.common.Attributes;
+import io.opentelemetry.api.trace.Span;
+import io.opentelemetry.api.trace.SpanContext;
+import io.opentelemetry.context.Scope;
+import io.opentelemetry.sdk.common.InstrumentationLibraryInfo;
+import io.opentelemetry.sdk.logs.SdkLogEmitterProvider;
+import io.opentelemetry.sdk.logs.data.LogData;
+import io.opentelemetry.sdk.logs.data.Severity;
+import io.opentelemetry.sdk.logs.export.InMemoryLogExporter;
+import io.opentelemetry.sdk.logs.export.SimpleLogProcessor;
+import io.opentelemetry.sdk.resources.Resource;
+import io.opentelemetry.sdk.trace.SdkTracerProvider;
+import java.time.Instant;
+import java.util.List;
+import java.util.concurrent.TimeUnit;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.apache.logging.log4j.ThreadContext;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class OpenTelemetryAppenderConfigTest {
+
+  private static final Logger logger = LogManager.getLogger("TestLogger");
+
+  private static InMemoryLogExporter logExporter;
+  private static Resource resource;
+  private static InstrumentationLibraryInfo instrumentationLibraryInfo;
+
+  @BeforeAll
+  static void setupAll() {
+    logExporter = InMemoryLogExporter.create();
+    resource = Resource.getDefault();
+    instrumentationLibraryInfo = InstrumentationLibraryInfo.create("TestLogger", null);
+
+    SdkLogEmitterProvider logEmitterProvider =
+        SdkLogEmitterProvider.builder()
+            .setResource(resource)
+            .addLogProcessor(SimpleLogProcessor.create(logExporter))
+            .build();
+    OpenTelemetryLog4j.initialize(logEmitterProvider);
+  }
+
+  @AfterAll
+  static void afterAll() {
+    OpenTelemetryLog4j.resetForTest();
+  }
+
+  @BeforeEach
+  void setup() {
+    logExporter.reset();
+    ThreadContext.clearAll();
+  }
+
+  @Test
+  void logNoSpan() {
+    logger.info("log message 1");
+
+    List<LogData> logDataList = logExporter.getFinishedLogItems();
+    assertThat(logDataList).hasSize(1);
+    LogData logData = logDataList.get(0);
+    assertThat(logData.getResource()).isEqualTo(resource);
+    assertThat(logData.getInstrumentationLibraryInfo()).isEqualTo(instrumentationLibraryInfo);
+    assertThat(logData.getBody().asString()).isEqualTo("log message 1");
+    assertThat(logData.getAttributes()).isEqualTo(Attributes.empty());
+  }
+
+  @Test
+  void logWithSpan() {
+    Span span1 = runWithSpan("span1", () -> logger.info("log message 1"));
+
+    logger.info("log message 2");
+
+    Span span2 = runWithSpan("span2", () -> logger.info("log message 3"));
+
+    List<LogData> logDataList = logExporter.getFinishedLogItems();
+    assertThat(logDataList).hasSize(3);
+    assertThat(logDataList.get(0).getSpanContext()).isEqualTo(span1.getSpanContext());
+    assertThat(logDataList.get(1).getSpanContext()).isEqualTo(SpanContext.getInvalid());
+    assertThat(logDataList.get(2).getSpanContext()).isEqualTo(span2.getSpanContext());
+  }
+
+  private static Span runWithSpan(String spanName, Runnable runnable) {
+    Span span = SdkTracerProvider.builder().build().get("tracer").spanBuilder(spanName).startSpan();
+    try (Scope unused = span.makeCurrent()) {
+      runnable.run();
+    } finally {
+      span.end();
+    }
+    return span;
+  }
+
+  @Test
+  void logWithExtras() {
+    logger.info("log message 1", new IllegalStateException("Error!"));
+
+    List<LogData> logDataList = logExporter.getFinishedLogItems();
+    assertThat(logDataList).hasSize(1);
+    LogData logData = logDataList.get(0);
+    assertThat(logData.getResource()).isEqualTo(resource);
+    assertThat(logData.getInstrumentationLibraryInfo()).isEqualTo(instrumentationLibraryInfo);
+    assertThat(logData.getBody().asString()).isEqualTo("log message 1");
+    assertThat(logData.getEpochNanos())
+        .isGreaterThan(TimeUnit.MILLISECONDS.toNanos(Instant.now().toEpochMilli() - 1000));
+    assertThat(logData.getSeverity()).isEqualTo(Severity.INFO);
+    assertThat(logData.getSeverityText()).isEqualTo("INFO");
+    assertThat(logData.getAttributes()).isEqualTo(Attributes.of(ATTR_THROWABLE_MESSAGE, "Error!"));
+  }
+}

instrumentation/log4j/log4j-2.13.2/library/src/test/java/io/opentelemetry/instrumentation/log4j/v2_13_2/OpenTelemetryAppenderTest.java

@@ -0,0 +1,61 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.log4j.v2_13_2;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+import io.opentelemetry.sdk.logs.SdkLogEmitterProvider;
+import io.opentelemetry.sdk.logs.export.InMemoryLogExporter;
+import io.opentelemetry.sdk.logs.export.SimpleLogProcessor;
+import org.apache.logging.log4j.ThreadContext;
+import org.apache.logging.log4j.core.LogEvent;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+class OpenTelemetryAppenderTest {
+
+  private OpenTelemetryAppender appender;
+  private InMemoryLogExporter logExporter;
+  private SdkLogEmitterProvider logEmitterProvider;
+  private LogEvent logEvent;
+
+  @AfterAll
+  static void afterAll() {
+    OpenTelemetryLog4j.resetForTest();
+  }
+
+  @BeforeEach
+  void setup() {
+    appender = OpenTelemetryAppender.builder().setName("my-otel-appender").build();
+
+    logEvent = mock(LogEvent.class);
+    when(logEvent.getContextStack()).thenReturn(mock(ThreadContext.ContextStack.class));
+
+    logExporter = InMemoryLogExporter.create();
+    logEmitterProvider =
+        SdkLogEmitterProvider.builder()
+            .addLogProcessor(SimpleLogProcessor.create(logExporter))
+            .build();
+  }
+
+  @Test
+  void append_Uninitialized() {
+    appender.append(logEvent);
+
+    assertThat(logExporter.getFinishedLogItems()).isEmpty();
+  }
+
+  @Test
+  void append_Initialized() {
+    appender.initialize(logEmitterProvider);
+    appender.append(logEvent);
+
+    assertThat(logExporter.getFinishedLogItems()).hasSize(1);
+  }
+}

instrumentation/log4j/log4j-2.13.2/library/src/test/java/io/opentelemetry/instrumentation/log4j/v2_13_2/OpenTelemetryLog4jTest.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.instrumentation.log4j.v2_13_2;
+
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+
+import io.opentelemetry.sdk.logs.SdkLogEmitterProvider;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.Test;
+
+class OpenTelemetryLog4jTest {
+
+  @AfterAll
+  static void afterAll() {
+    OpenTelemetryLog4j.resetForTest();
+  }
+
+  @Test
+  void registerAndInitialize() {
+    OpenTelemetryAppender appender1 = mock(OpenTelemetryAppender.class);
+    OpenTelemetryAppender appender2 = mock(OpenTelemetryAppender.class);
+    OpenTelemetryLog4j.registerInstance(appender1);
+    OpenTelemetryLog4j.registerInstance(appender2);
+
+    OpenTelemetryLog4j.initialize(SdkLogEmitterProvider.builder().build());
+
+    verify(appender1).initialize(any());
+    verify(appender2).initialize(any());
+
+    assertThatCode(() -> OpenTelemetryLog4j.initialize(SdkLogEmitterProvider.builder().build()))
+        .isInstanceOf(IllegalStateException.class)
+        .hasMessageContaining("OpenTelemetryLog4j.initialize has already been called.");
+  }
+}

instrumentation/log4j/log4j-2.13.2/library/src/test/resources/log4j2-test.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<Configuration status="WARN" packages="com.example.appender,io.opentelemetry.instrumentation.log4j.v2_13_2">
+  <Appenders>
+    <Console name="Console" target="SYSTEM_OUT">
+      <PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} traceId: %X{trace_id} spanId: %X{span_id} flags: %X{trace_flags} - %msg%n" />
+    </Console>
+    <ListAppender name="ListAppender" />
+    <OpenTelemetry name="OpenTelemetryAppender" />
+  </Appenders>
+  <Loggers>
+    <Logger name="TestLogger" level="All">
+      <AppenderRef ref="OpenTelemetryAppender" level="All" />
+      <AppenderRef ref="ListAppender" level="All" />
+      <AppenderRef ref="Console" level="All" />
+    </Logger>
+    <Root>
+      <AppenderRef ref="Console" level="All" />
+    </Root>
+  </Loggers>
+</Configuration>