diff --git a/README.md b/README.md
index 351fed1bf8..a9ec0423f8 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ Maven:
 ![Build Status](https://github.com/google/gson/actions/workflows/build.yml/badge.svg)
 
 ### Requirements
-#### Java version
+#### Minimum Java version
 - Gson 2.9.0 and newer: Java 7
 - Gson 2.8.9 and older: Java 6
 
diff --git a/gson/pom.xml b/gson/pom.xml
index 1b1eb0d117..c1be6a920b 100644
--- a/gson/pom.xml
+++ b/gson/pom.xml
@@ -35,33 +35,18 @@
         <executions>
           <execution>
             <id>default-compile</id>
-            <configuration>
-              <jdkToolchain>
-                <version>9</version>
-              </jdkToolchain>
-              <release>9</release>
-            </configuration>
-          </execution>
-          <execution>
-            <id>base-compile</id>
-            <goals>
-              <goal>compile</goal>
-            </goals>
             <configuration>
               <excludes>
+                <!-- module-info.java is compiled using ModiTect -->
                 <exclude>module-info.java</exclude>
               </excludes>
             </configuration>
           </execution>
         </executions>
-        <configuration>
-          <jdkToolchain>
-            <version>[1.5,9)</version>
-          </jdkToolchain>
-          <source>1.7</source>
-          <target>1.7</target>
-        </configuration>
       </plugin>
+      <!-- Note: Javadoc plugin has to be run in combination with >= `package` 
+        phase, e.g. `mvn package javadoc:javadoc`, otherwise it fails with
+        "Aggregator report contains named and unnamed modules" -->
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-surefire-plugin</artifactId>
@@ -82,11 +67,31 @@
         <artifactId>maven-javadoc-plugin</artifactId>
         <configuration>
           <excludePackageNames>com.google.gson.internal:com.google.gson.internal.bind</excludePackageNames>
-          <links>
-            <link>https://docs.oracle.com/javase/7/docs/api/</link>
-          </links>
         </configuration>
       </plugin>
+      <!-- Add module-info to JAR, see https://github.com/moditect/moditect#adding-module-descriptors-to-existing-jar-files -->
+      <!-- Uses ModiTect instead of separate maven-compiler-plugin executions 
+        for better Eclipse IDE support, see https://github.com/eclipse-m2e/m2e-core/issues/393 -->
+      <plugin>
+        <groupId>org.moditect</groupId>
+        <artifactId>moditect-maven-plugin</artifactId>
+        <version>1.0.0.RC2</version>
+        <executions>
+          <execution>
+            <id>add-module-info</id>
+            <phase>package</phase>
+            <goals>
+              <goal>add-module-info</goal>
+            </goals>
+            <configuration>
+              <jvmVersion>9</jvmVersion>
+              <module>
+                <moduleInfoFile>${project.build.sourceDirectory}/module-info.java</moduleInfoFile>
+              </module>
+            </configuration>
+          </execution>
+        </executions>
+      </plugin>
       <plugin>
         <groupId>biz.aQute.bnd</groupId>
         <artifactId>bnd-maven-plugin</artifactId>
@@ -104,6 +109,7 @@
         <artifactId>maven-jar-plugin</artifactId>
         <configuration>
           <archive>
+            <!-- Use existing manifest generated by BND plugin -->
             <manifestFile>${project.build.outputDirectory}/META-INF/MANIFEST.MF</manifestFile>
           </archive>
         </configuration>
diff --git a/gson/src/test/java/com/google/gson/functional/TreeTypeAdaptersTest.java b/gson/src/test/java/com/google/gson/functional/TreeTypeAdaptersTest.java
index dd412ea131..afcc4bccda 100644
--- a/gson/src/test/java/com/google/gson/functional/TreeTypeAdaptersTest.java
+++ b/gson/src/test/java/com/google/gson/functional/TreeTypeAdaptersTest.java
@@ -57,7 +57,7 @@ protected void setUp() {
         .registerTypeAdapter(Id.class, new IdTreeTypeAdapter())
         .create();
     course = new Course<HistoryCourse>(COURSE_ID, 4,
-        new Assignment<HistoryCourse>(null, null), createList(STUDENT1, STUDENT2));
+        new Assignment<HistoryCourse>(null, null), Arrays.asList(STUDENT1, STUDENT2));
   }
 
   public void testSerializeId() {
@@ -171,9 +171,4 @@ public Assignment(Id<Assignment<T>> id, T data) {
   private static class HistoryCourse {
     int numClasses;
   }
-
-  @SafeVarargs
-  private static <T> List<T> createList(T ...items) {
-    return Arrays.asList(items);
-  }
 }
diff --git a/pom.xml b/pom.xml
index 2c6dc9d617..0e888b64f2 100644
--- a/pom.xml
+++ b/pom.xml
@@ -28,7 +28,7 @@
 
   <properties>
     <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
-    <java.version>1.7</java.version>
+    <javaVersion>7</javaVersion>
   </properties>
 
   <scm>
@@ -68,23 +68,42 @@
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-compiler-plugin</artifactId>
           <version>3.8.1</version>
+          <configuration>
+            <release>${javaVersion}</release>
+            <jdkToolchain>
+              <version>[11,)</version>
+            </jdkToolchain>
+          </configuration>
         </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-javadoc-plugin</artifactId>
           <version>3.3.1</version>
+          <configuration>
+            <jdkToolchain>
+              <version>[11,)</version>
+            </jdkToolchain>
+            <!-- Exclude `missing` group because some tags have been omitted when they are redundant -->
+            <doclint>all,-missing</doclint>
+            <!-- Link against newer Java API Javadoc because most users likely 
+              use a newer Java version than the one used for building this project -->
+            <detectJavaApiLink>false</detectJavaApiLink>
+            <links>
+              <link>https://docs.oracle.com/en/java/javase/11/docs/api/</link>
+            </links>
+            <!-- Disable detection of offline links between Maven modules: 
+              (1) Only `gson` module is published, so for other modules Javadoc links don't 
+              matter much at the moment; (2) The derived URL for the modules is based on 
+              the project URL (= Gson GitHub repo) which is incorrect because it is not 
+              hosting the Javadoc (3) It might fail due to https://bugs.openjdk.java.net/browse/JDK-8212233 -->
+            <detectOfflineLinks>false</detectOfflineLinks>
+          </configuration>
         </plugin>
         <plugin>
           <groupId>org.apache.maven.plugins</groupId>
           <artifactId>maven-jar-plugin</artifactId>
           <version>3.2.0</version>
         </plugin>
-        <plugin>
-          <groupId>org.apache.felix</groupId>
-          <artifactId>maven-bundle-plugin</artifactId>
-          <version>5.1.3</version>
-          <inherited>true</inherited>
-        </plugin>
       </plugins>
     </pluginManagement>
     <plugins>
@@ -110,23 +129,4 @@
       </plugin>
     </plugins>
   </build>
-  <profiles>
-    <profile>
-      <id>doclint-java8-disable</id>
-      <activation>
-        <jdk>[1.8,)</jdk>
-      </activation>
-      <build>
-        <plugins>
-          <plugin>
-            <groupId>org.apache.maven.plugins</groupId>
-            <artifactId>maven-javadoc-plugin</artifactId>
-            <configuration>
-              <additionalparam>-Xdoclint:none</additionalparam>
-            </configuration>
-          </plugin>
-        </plugins>
-      </build>
-    </profile>
-  </profiles>
 </project>
diff --git a/proto/src/main/java/com/google/gson/protobuf/ProtoTypeAdapter.java b/proto/src/main/java/com/google/gson/protobuf/ProtoTypeAdapter.java
index d588d82404..742916b998 100644
--- a/proto/src/main/java/com/google/gson/protobuf/ProtoTypeAdapter.java
+++ b/proto/src/main/java/com/google/gson/protobuf/ProtoTypeAdapter.java
@@ -191,7 +191,7 @@ public static Builder newBuilder() {
   private static final com.google.protobuf.Descriptors.FieldDescriptor.Type ENUM_TYPE =
       com.google.protobuf.Descriptors.FieldDescriptor.Type.ENUM;
 
-  private static final ConcurrentMap<String, Map<Class<?>, Method>> mapOfMapOfMethods =
+  private static final ConcurrentMap<String, ConcurrentMap<Class<?>, Method>> mapOfMapOfMethods =
       new MapMaker().makeMap();
 
   private final EnumSerialization enumSerialization;
@@ -308,7 +308,7 @@ public Message deserialize(JsonElement json, Type typeOfT,
             }
           }
         }
-        return (Message) protoBuilder.build();
+        return protoBuilder.build();
       } catch (SecurityException e) {
         throw new JsonParseException(e);
       } catch (NoSuchMethodException e) {
@@ -396,10 +396,10 @@ private EnumValueDescriptor findValueByNameAndExtension(EnumDescriptor desc,
 
   private static Method getCachedMethod(Class<?> clazz, String methodName,
       Class<?>... methodParamTypes) throws NoSuchMethodException {
-    Map<Class<?>, Method> mapOfMethods = mapOfMapOfMethods.get(methodName);
+    ConcurrentMap<Class<?>, Method> mapOfMethods = mapOfMapOfMethods.get(methodName);
     if (mapOfMethods == null) {
       mapOfMethods = new MapMaker().makeMap();
-      Map<Class<?>, Method> previous =
+      ConcurrentMap<Class<?>, Method> previous =
           mapOfMapOfMethods.putIfAbsent(methodName, mapOfMethods);
       mapOfMethods = previous == null ? mapOfMethods : previous;
     }