Fixing log4j configuration initialization (#2343)

Author: eyalkoren

CHANGELOG.asciidoc

@@ -33,6 +33,7 @@ endif::[]
 [float]
 ===== Bug fixes
 * Fix module loading errors on J9 JVM - {pull}2341[#2341]
+* Fixing log4j configuration error - {pull}2343[#2343]
 
 [[release-notes-1.x]]
 === Java Agent version 1.x

apm-agent-core/src/main/java/co/elastic/apm/agent/logging/LoggingConfiguration.java

@@ -23,6 +23,7 @@
 import co.elastic.apm.agent.matcher.WildcardMatcher;
 import co.elastic.apm.agent.matcher.WildcardMatcherValueConverter;
 import org.apache.logging.log4j.Level;
+import org.apache.logging.log4j.core.config.ConfigurationFactory;
 import org.apache.logging.log4j.core.config.Configurator;
 import org.apache.logging.log4j.status.StatusLogger;
 import org.stagemonitor.configuration.ConfigurationOption;
@@ -300,16 +301,22 @@ public void assertValid(Boolean value) {
     public static void init(List<ConfigurationSource> sources, String ephemeralId) {
         // The initialization of log4j may produce errors if the traced application uses log4j settings (for
         // example - through file in the classpath or System properties) that configures specific properties for
-        // loading classes by name. Since we shade our usage of log4j, such non-shaded classes may not (and should not)
-        // be found on the classpath.
+        // loading custom classes by name, which may not be found within the agent ClassLoader's classpath.
         // All handled Exceptions should not prevent us from using log4j further, as the system falls back to a default
         // which we expect anyway. We take a calculated risk of ignoring such errors only through initialization time,
         // assuming that errors that will make the logging system non-usable won't be handled.
         String initialListenersLevel = System.setProperty(INITIAL_LISTENERS_LEVEL, "OFF");
         String initialStatusLoggerLevel = System.setProperty(INITIAL_STATUS_LOGGER_LEVEL, "OFF");
         String defaultListenerLevel = System.setProperty(DEFAULT_LISTENER_LEVEL, "OFF");
         try {
-            Configurator.initialize(new Log4j2ConfigurationFactory(sources, ephemeralId).getConfiguration());
+            // org.apache.logging.log4j.core.config.ConfigurationFactory is a singleton that allows overriding its instance
+            // through setConfigurationFactory. This API is not considered thread safe, but since we do it so early on when
+            // there is only the main thread it should be OK. Once we override the default factory instance, any logger
+            // created thereafter will be configured through our custom factory, regardless of its context.
+            // Initializing only per context (the caller class loader by default, can be changed to thread or other), for
+            // example through org.apache.logging.log4j.core.config.Configurator, means that loggers in non-initialized
+            // contexts will either get the app-configuration for log4j, if such exists, or none.
+            ConfigurationFactory.setConfigurationFactory(new Log4j2ConfigurationFactory(sources, ephemeralId));
         } catch (Throwable throwable) {
             System.err.println("[elastic-apm-agent] ERROR Failure during initialization of agent's log4j system: " + throwable.getMessage());
         } finally {

apm-agent-plugins/apm-log-shader-plugin/apm-log4j2-plugin/src/test/java/co/elastic/apm/agent/log4j2/LegacyLog4j2ShadingTest.java

@@ -19,12 +19,17 @@
 package co.elastic.apm.agent.log4j2;
 
 import co.elastic.apm.agent.TestClassWithDependencyRunner;
+import co.elastic.apm.agent.logging.LoggingConfiguration;
+import org.apache.logging.log4j.core.config.ConfigurationFactory;
 import org.junit.After;
+import org.junit.AfterClass;
 import org.junit.Before;
+import org.junit.BeforeClass;
 import org.junit.Ignore;
 import org.junit.Test;
 
 import java.io.IOException;
+import java.util.List;
 
 /**
  * This class only delegates tests to the current-version log4j2 tests through JUnit 4, so that it can be ran using
@@ -35,6 +40,16 @@
 @Ignore
 public class LegacyLog4j2ShadingTest extends Log4j2ShadingTest {
 
+    @BeforeClass
+    public static void resetConfigFactory() {
+        ConfigurationFactory.resetConfigurationFactory();
+    }
+
+    @AfterClass
+    public static void reInitLogging() {
+        LoggingConfiguration.init(List.of(), "");
+    }
+
     @Before
     @Override
     public void setup() throws Exception {

apm-agent-plugins/apm-log-shader-plugin/apm-log4j2-plugin/src/test/java/co/elastic/apm/agent/log4j2/Log4j2ShadingTest.java

@@ -20,19 +20,34 @@
 
 import co.elastic.apm.agent.log.shader.LogShadingInstrumentationTest;
 import co.elastic.apm.agent.log.shader.LoggerFacade;
+import co.elastic.apm.agent.logging.LoggingConfiguration;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 import org.apache.logging.log4j.Marker;
 import org.apache.logging.log4j.MarkerManager;
 import org.apache.logging.log4j.ThreadContext;
 import org.apache.logging.log4j.core.appender.RandomAccessFileAppender;
+import org.apache.logging.log4j.core.config.ConfigurationFactory;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.BeforeAll;
 
 import java.net.URI;
 import java.net.URISyntaxException;
+import java.util.List;
 import java.util.Objects;
 
 public class Log4j2ShadingTest extends LogShadingInstrumentationTest {
 
+    @BeforeAll
+    static void resetConfigFactory() {
+        ConfigurationFactory.resetConfigurationFactory();
+    }
+
+    @AfterAll
+    static void reInitLogging() {
+        LoggingConfiguration.init(List.of(), "");
+    }
+
     private static final Marker TEST_MARKER = MarkerManager.getMarker("TEST");
 
     @Override