Encapsulate actual logging implementation better

conventions/src/main/kotlin/io.opentelemetry.instrumentation.javaagent-shadowing.gradle.kts

@@ -14,8 +14,6 @@ tasks.withType<ShadowJar>().configureEach {
 
   exclude("**/module-info.class")
 
-  // Prevents conflict with other SLF4J instances. Important for premain.
-  relocate("org.slf4j", "io.opentelemetry.javaagent.slf4j")
   // rewrite dependencies calling Logger.getLogger
   relocate("java.util.logging.Logger", "io.opentelemetry.javaagent.bootstrap.PatchLogger")
 

conventions/src/main/kotlin/io.opentelemetry.instrumentation.javaagent-testing.gradle.kts

@@ -92,12 +92,12 @@ class JavaagentTestArgumentsProvider(
     "-Dotel.metrics.exporter=otlp",
     // suppress repeated logging of "No metric data to export - skipping export."
     // since PeriodicMetricReader is configured with a short interval
-    "-Dio.opentelemetry.javaagent.slf4j.simpleLogger.log.io.opentelemetry.sdk.metrics.export.PeriodicMetricReader=INFO",
+    "-Dio.opentelemetry.javaagent.logging.simple.slf4j.simpleLogger.log.io.opentelemetry.sdk.metrics.export.PeriodicMetricReader=INFO",
     // suppress a couple of verbose ClassNotFoundException stack traces logged at debug level
-    "-Dio.opentelemetry.javaagent.slf4j.simpleLogger.log.io.grpc.internal.ServerImplBuilder=INFO",
-    "-Dio.opentelemetry.javaagent.slf4j.simpleLogger.log.io.grpc.internal.ManagedChannelImplBuilder=INFO",
-    "-Dio.opentelemetry.javaagent.slf4j.simpleLogger.log.io.perfmark.PerfMark=INFO",
-    "-Dio.opentelemetry.javaagent.slf4j.simpleLogger.log.io.grpc.Context=INFO"
+    "-Dio.opentelemetry.javaagent.logging.simple.slf4j.simpleLogger.log.io.grpc.internal.ServerImplBuilder=INFO",
+    "-Dio.opentelemetry.javaagent.logging.simple.slf4j.simpleLogger.log.io.grpc.internal.ManagedChannelImplBuilder=INFO",
+    "-Dio.opentelemetry.javaagent.logging.simple.slf4j.simpleLogger.log.io.perfmark.PerfMark=INFO",
+    "-Dio.opentelemetry.javaagent.logging.simple.slf4j.simpleLogger.log.io.grpc.Context=INFO"
   )
 }
 

examples/distro/gradle/shadow.gradle

@@ -1,6 +1,4 @@
 ext.relocatePackages = { shadowJar ->
-  // Prevents conflict with other SLF4J instances. Important for premain.
-  shadowJar.relocate 'org.slf4j', 'io.opentelemetry.javaagent.slf4j'
   // rewrite dependencies calling Logger.getLogger
   shadowJar.relocate 'java.util.logging.Logger', 'io.opentelemetry.javaagent.bootstrap.PatchLogger'
 

gradle-plugins/src/main/kotlin/io.opentelemetry.instrumentation.muzzle-check.gradle.kts

@@ -82,8 +82,6 @@ tasks.withType<ShadowJar>().configureEach {
 
   exclude("**/module-info.class")
 
-  // Prevents conflict with other SLF4J instances. Important for premain.
-  relocate("org.slf4j", "io.opentelemetry.javaagent.slf4j")
   // rewrite dependencies calling Logger.getLogger
   relocate("java.util.logging.Logger", "io.opentelemetry.javaagent.bootstrap.PatchLogger")
 

javaagent-bootstrap/build.gradle.kts

@@ -5,13 +5,9 @@ plugins {
 
 group = "io.opentelemetry.javaagent"
 
-val agentSlf4jVersion = "2.0.0"
-
 dependencies {
   implementation(project(":instrumentation-api"))
   implementation(project(":instrumentation-appender-api-internal"))
-  implementation("org.slf4j:slf4j-api:$agentSlf4jVersion")
-  implementation("org.slf4j:slf4j-simple:$agentSlf4jVersion")
 
   testImplementation(project(":testing-common"))
 }

javaagent-bootstrap/src/main/java/io/opentelemetry/javaagent/bootstrap/AgentClassLoader.java

@@ -33,9 +33,8 @@
  */
 public class AgentClassLoader extends URLClassLoader {
 
-  // NOTE it's important not to use slf4j in this class, because this class is used before slf4j is
-  // configured, and so using slf4j here would initialize slf4j-simple before we have a chance to
-  // configure the logging levels
+  // NOTE it's important not to use logging in this class, because this class is used before logging
+  // is initialized
 
   static {
     ClassLoader.registerAsParallelCapable();

javaagent-bootstrap/src/main/java/io/opentelemetry/javaagent/bootstrap/InternalLogger.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.javaagent.bootstrap;
+
+import java.util.concurrent.atomic.AtomicReference;
+import javax.annotation.Nullable;
+
+public abstract class InternalLogger {
+
+  private static final AtomicReference<Factory> loggerFactory =
+      new AtomicReference<>(NoopLoggerFactory.INSTANCE);
+
+  public static void initialize(Factory factory) {
+    if (!loggerFactory.compareAndSet(NoopLoggerFactory.INSTANCE, factory)) {
+      factory
+          .create(InternalLogger.class.getName())
+          .log(
+              Level.WARN,
+              "Developer error: logging system has already been initialized once",
+              null);
+    }
+  }
+
+  static InternalLogger getLogger(String name) {
+    return loggerFactory.get().create(name);
+  }
+
+  protected abstract boolean isLoggable(Level level);
+
+  protected abstract void log(Level level, String message, @Nullable Throwable error);
+
+  protected abstract String name();
+
+  public enum Level {
+    ERROR,
+    WARN,
+    INFO,
+    DEBUG,
+    TRACE
+  }
+
+  @FunctionalInterface
+  public interface Factory {
+
+    InternalLogger create(String name);
+  }
+}

javaagent-bootstrap/src/main/java/io/opentelemetry/javaagent/bootstrap/NoopLoggerFactory.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.javaagent.bootstrap;
+
+import javax.annotation.Nullable;
+
+final class NoopLoggerFactory implements InternalLogger.Factory {
+
+  static final InternalLogger.Factory INSTANCE = new NoopLoggerFactory();
+
+  private NoopLoggerFactory() {}
+
+  @Override
+  public InternalLogger create(String name) {
+    return new NoopLogger(name);
+  }
+
+  private static final class NoopLogger extends InternalLogger {
+
+    private final String name;
+
+    private NoopLogger(String name) {
+      this.name = name;
+    }
+
+    @Override
+    public boolean isLoggable(Level level) {
+      return false;
+    }
+
+    @Override
+    public void log(Level level, String message, @Nullable Throwable error) {}
+
+    @Override
+    protected String name() {
+      return name;
+    }
+  }
+}