Make InternalLogger an interface

instrumentation/internal/internal-slf4j/javaagent/src/main/java/io/opentelemetry/javaagent/instrumentation/internal/slf4j/Slf4jApplicationLoggerBridge.java

@@ -27,7 +27,7 @@ public InternalLogger create(String name) {
     return new Slf4jApplicationLogger(name);
   }
 
-  static final class Slf4jApplicationLogger extends InternalLogger {
+  static final class Slf4jApplicationLogger implements InternalLogger {
 
     private final Logger slf4jLogger;
 

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

@@ -5,36 +5,25 @@
 
 package io.opentelemetry.javaagent.bootstrap;
 
-import java.util.concurrent.atomic.AtomicReference;
 import javax.annotation.Nullable;
 
-public abstract class InternalLogger {
+public interface 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 void initialize(Factory factory) {
+    InternalLoggerFactoryHolder.initialize(factory);
   }
 
   static InternalLogger getLogger(String name) {
-    return loggerFactory.get().create(name);
+    return InternalLoggerFactoryHolder.get().create(name);
   }
 
-  public abstract boolean isLoggable(Level level);
+  boolean isLoggable(Level level);
 
-  public abstract void log(Level level, String message, @Nullable Throwable error);
+  void log(Level level, String message, @Nullable Throwable error);
 
-  public abstract String name();
+  String name();
 
-  public enum Level {
+  enum Level {
     ERROR,
     WARN,
     INFO,
@@ -43,7 +32,7 @@ public enum Level {
   }
 
   @FunctionalInterface
-  public interface Factory {
+  interface Factory {
 
     InternalLogger create(String name);
   }

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

@@ -0,0 +1,31 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.javaagent.bootstrap;
+
+import java.util.concurrent.atomic.AtomicReference;
+
+final class InternalLoggerFactoryHolder {
+
+  private static final AtomicReference<InternalLogger.Factory> loggerFactory =
+      new AtomicReference<>(NoopLoggerFactory.INSTANCE);
+
+  static void initialize(InternalLogger.Factory factory) {
+    if (!loggerFactory.compareAndSet(NoopLoggerFactory.INSTANCE, factory)) {
+      factory
+          .create(InternalLogger.class.getName())
+          .log(
+              InternalLogger.Level.WARN,
+              "Developer error: logging system has already been initialized once",
+              null);
+    }
+  }
+
+  static InternalLogger.Factory get() {
+    return loggerFactory.get();
+  }
+
+  private InternalLoggerFactoryHolder() {}
+}

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

@@ -18,7 +18,7 @@ public InternalLogger create(String name) {
     return new NoopLogger(name);
   }
 
-  private static final class NoopLogger extends InternalLogger {
+  private static final class NoopLogger implements InternalLogger {
 
     private final String name;
 

javaagent-internal-logging-application/src/main/java/io/opentelemetry/javaagent/logging/application/ApplicationLogger.java

@@ -8,7 +8,7 @@
 import io.opentelemetry.javaagent.bootstrap.InternalLogger;
 import javax.annotation.Nullable;
 
-final class ApplicationLogger extends InternalLogger {
+final class ApplicationLogger implements InternalLogger {
 
   private final InMemoryLogStore inMemoryLogStore;
   private final String name;

javaagent-internal-logging-application/src/test/java/io/opentelemetry/javaagent/logging/application/ConcurrentIntegrationTest.java

@@ -61,7 +61,7 @@ void shouldLogEverything() throws InterruptedException {
         .hasSize(count.get());
   }
 
-  private static final class TestLogger extends InternalLogger {
+  private static final class TestLogger implements InternalLogger {
 
     private final String name;
     private final List<Integer> logMessages;

javaagent-internal-logging-simple/src/main/java/io/opentelemetry/javaagent/logging/simple/Slf4jSimpleLogger.java

@@ -10,7 +10,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
-final class Slf4jSimpleLogger extends InternalLogger {
+final class Slf4jSimpleLogger implements InternalLogger {
 
   static Slf4jSimpleLogger create(String name) {
     return new Slf4jSimpleLogger(name);