Move ThreadContextMap implementations to log4j-core

We move `ThreadContextMap` alternative implementations to `log4j-core`.
These thread context maps are not useful to other `log4j-api`
implementations:

* `log4j-to-slf4j` delegates everything to SLF4J's `MDCAdapter`,
* `log4j-to-jul` uses the no-op context map.

Author: ppkarwasz

log4j-api-test/src/main/java/org/apache/logging/log4j/test/ThreadContextUtilityClass.java

@@ -112,8 +112,4 @@ public static void testPut() {
         ThreadContext.put("testKey", "testValue");
         assertEquals("testValue", ThreadContext.get("testKey"));
     }
-
-    public static void reset() {
-        ThreadContext.init();
-    }
 }

log4j-api-test/src/main/java/org/apache/logging/log4j/test/junit/InitializesThreadContext.java

@@ -0,0 +1,64 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF 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 org.apache.logging.log4j.test.spi;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+import java.time.Duration;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import org.apache.logging.log4j.spi.ThreadContextMap;
+
+/**
+ * Provides a set of utility methods to test implementations of {@link ThreadContextMap}.
+ */
+public abstract class AbstractThreadContextMapTest {
+
+    private static final String KEY = "key";
+
+    /**
+     * Implementations SHOULD not propagate the context to newly created threads by default.
+     *
+     * @param contextMap A {@link ThreadContextMap implementation}.
+     */
+    protected static void assertThreadLocalNotInheritable(final ThreadContextMap contextMap) {
+        contextMap.put(KEY, "threadLocalNotInheritableByDefault");
+        verifyThreadContextValueFromANewThread(contextMap, null);
+    }
+
+    /**
+     * Implementations MAY offer a configuration that propagates the context to newly created threads.
+     *
+     * @param contextMap A {@link ThreadContextMap implementation}.
+     */
+    protected static void assertThreadLocalInheritable(final ThreadContextMap contextMap) {
+        contextMap.put(KEY, "threadLocalInheritableIfConfigured");
+        verifyThreadContextValueFromANewThread(contextMap, "threadLocalInheritableIfConfigured");
+    }
+
+    private static void verifyThreadContextValueFromANewThread(
+            final ThreadContextMap contextMap, final String expected) {
+        final ExecutorService executorService = Executors.newSingleThreadExecutor();
+        try {
+            assertThat(executorService.submit(() -> contextMap.get(KEY)))
+                    .succeedsWithin(Duration.ofSeconds(1))
+                    .isEqualTo(expected);
+        } finally {
+            executorService.shutdown();
+        }
+    }
+}

log4j-api-test/src/main/java/org/apache/logging/log4j/test/junit/ThreadContextInitializer.java

@@ -23,21 +23,18 @@
 
 import org.apache.logging.log4j.spi.DefaultThreadContextMap;
 import org.apache.logging.log4j.test.ThreadContextUtilityClass;
-import org.apache.logging.log4j.test.junit.InitializesThreadContext;
 import org.apache.logging.log4j.test.junit.SetTestProperty;
 import org.apache.logging.log4j.test.junit.UsingThreadContextMap;
 import org.apache.logging.log4j.test.junit.UsingThreadContextStack;
 import org.junit.jupiter.api.AfterAll;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Tag;
 import org.junit.jupiter.api.Test;
-import org.junitpioneer.jupiter.SetSystemProperty;
 
 /**
  * Tests {@link ThreadContext}.
  */
 @SetTestProperty(key = DefaultThreadContextMap.INHERITABLE_MAP, value = "true")
-@InitializesThreadContext
 @UsingThreadContextMap
 @UsingThreadContextStack
 public class ThreadContextInheritanceTest {
@@ -63,31 +60,6 @@ public void testPush() {
         assertEquals(ThreadContext.pop(), "Hello", "Incorrect simple stack value");
     }
 
-    @Test
-    @SetSystemProperty(key = DefaultThreadContextMap.INHERITABLE_MAP, value = "true")
-    @InitializesThreadContext
-    public void testInheritanceSwitchedOn() throws Exception {
-        System.setProperty(DefaultThreadContextMap.INHERITABLE_MAP, "true");
-        try {
-            ThreadContext.clearMap();
-            ThreadContext.put("Greeting", "Hello");
-            StringBuilder sb = new StringBuilder();
-            TestThread thread = new TestThread(sb);
-            thread.start();
-            thread.join();
-            String str = sb.toString();
-            assertEquals("Hello", str, "Unexpected ThreadContext value. Expected Hello. Actual " + str);
-            sb = new StringBuilder();
-            thread = new TestThread(sb);
-            thread.start();
-            thread.join();
-            str = sb.toString();
-            assertEquals("Hello", str, "Unexpected ThreadContext value. Expected Hello. Actual " + str);
-        } finally {
-            System.clearProperty(DefaultThreadContextMap.INHERITABLE_MAP);
-        }
-    }
-
     @Test
     @Tag("performance")
     public void perfTest() {

log4j-api-test/src/main/java/org/apache/logging/log4j/test/spi/AbstractThreadContextMapTest.java

@@ -24,31 +24,17 @@
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Properties;
 import org.apache.logging.log4j.test.junit.UsingThreadContextMap;
+import org.apache.logging.log4j.test.spi.AbstractThreadContextMapTest;
+import org.apache.logging.log4j.util.PropertiesUtil;
 import org.junit.jupiter.api.Test;
 
 /**
  * Tests the {@code DefaultThreadContextMap} class.
  */
 @UsingThreadContextMap
-public class DefaultThreadContextMapTest {
-
-    @Test
-    public void testEqualsVsSameKind() {
-        final DefaultThreadContextMap map1 = createMap();
-        final DefaultThreadContextMap map2 = createMap();
-        assertEquals(map1, map1);
-        assertEquals(map2, map2);
-        assertEquals(map1, map2);
-        assertEquals(map2, map1);
-    }
-
-    @Test
-    public void testHashCodeVsSameKind() {
-        final DefaultThreadContextMap map1 = createMap();
-        final DefaultThreadContextMap map2 = createMap();
-        assertEquals(map1.hashCode(), map2.hashCode());
-    }
+class DefaultThreadContextMapTest extends AbstractThreadContextMapTest {
 
     @Test
     public void testDoesNothingIfConstructedWithUseMapIsFalse() {
@@ -214,4 +200,17 @@ public void testToStringShowsMapContext() {
         map.put("key2", "value2");
         assertEquals("{key2=value2}", map.toString());
     }
+
+    @Test
+    void threadLocalNotInheritableByDefault() {
+        assertThreadLocalNotInheritable(new DefaultThreadContextMap());
+    }
+
+    @Test
+    void threadLocalInheritableIfConfigured() {
+        final Properties props = new Properties();
+        props.setProperty("log4j2.isThreadContextMapInheritable", "true");
+        final PropertiesUtil util = new PropertiesUtil(props);
+        assertThreadLocalInheritable(new DefaultThreadContextMap(true, util));
+    }
 }

log4j-api-test/src/test/java/org/apache/logging/log4j/ThreadContextInheritanceTest.java

@@ -46,7 +46,9 @@
     <bnd-module-name>org.apache.logging.log4j</bnd-module-name>
     <bnd-extra-package-options>
       <!-- Not exported by most OSGi system bundles, hence we use the system classloader to load `sun.reflect.Reflection` -->
-      !sun.reflect
+      !sun.reflect,
+      <!-- Annotations only -->
+      org.jspecify.*;resolution:=optional
     </bnd-extra-package-options>
     <bnd-extra-module-options>
       <!-- Used in StringBuilders through reflection -->
@@ -57,6 +59,13 @@
 
   </properties>
   <dependencies>
+
+    <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <scope>provided</scope>
+    </dependency>
+
     <dependency>
       <groupId>org.osgi</groupId>
       <artifactId>org.osgi.core</artifactId>

log4j-api-test/src/test/java/org/apache/logging/log4j/spi/DefaultThreadContextMapTest.java

@@ -23,15 +23,12 @@
 import java.util.Iterator;
 import java.util.List;
 import java.util.Map;
-import org.apache.logging.log4j.internal.map.StringArrayThreadContextMap;
 import org.apache.logging.log4j.message.ParameterizedMessage;
-import org.apache.logging.log4j.spi.CleanableThreadContextMap;
 import org.apache.logging.log4j.spi.DefaultThreadContextMap;
 import org.apache.logging.log4j.spi.DefaultThreadContextStack;
 import org.apache.logging.log4j.spi.MutableThreadContextStack;
 import org.apache.logging.log4j.spi.ReadOnlyThreadContextMap;
 import org.apache.logging.log4j.spi.ThreadContextMap;
-import org.apache.logging.log4j.spi.ThreadContextMap2;
 import org.apache.logging.log4j.spi.ThreadContextMapFactory;
 import org.apache.logging.log4j.spi.ThreadContextStack;
 import org.apache.logging.log4j.util.PropertiesUtil;
@@ -268,21 +265,11 @@ public static void putIfNull(final String key, final String value) {
      *
      * <p>If the current thread does not have a context map it is
      * created as a side effect.</p>
-     * @param m The map.
+     * @param map The map.
      * @since 2.7
      */
-    public static void putAll(final Map<String, String> m) {
-        if (contextMap instanceof ThreadContextMap2) {
-            ((ThreadContextMap2) contextMap).putAll(m);
-        } else if (contextMap instanceof DefaultThreadContextMap) {
-            ((DefaultThreadContextMap) contextMap).putAll(m);
-        } else if (contextMap instanceof StringArrayThreadContextMap) {
-            ((StringArrayThreadContextMap) contextMap).putAll(m);
-        } else {
-            for (final Map.Entry<String, String> entry : m.entrySet()) {
-                contextMap.put(entry.getKey(), entry.getValue());
-            }
-        }
+    public static void putAll(final Map<String, String> map) {
+        contextMap.putAll(map);
     }
 
     /**
@@ -316,17 +303,7 @@ public static void remove(final String key) {
      * @since 2.8
      */
     public static void removeAll(final Iterable<String> keys) {
-        if (contextMap instanceof CleanableThreadContextMap) {
-            ((CleanableThreadContextMap) contextMap).removeAll(keys);
-        } else if (contextMap instanceof DefaultThreadContextMap) {
-            ((DefaultThreadContextMap) contextMap).removeAll(keys);
-        } else if (contextMap instanceof StringArrayThreadContextMap) {
-            ((StringArrayThreadContextMap) contextMap).removeAll(keys);
-        } else {
-            for (final String key : keys) {
-                contextMap.remove(key);
-            }
-        }
+        contextMap.removeAll(keys);
     }
 
     /**