diff --git a/.circleci/config.yml b/.circleci/config.yml
index 40ffd98e..d979b645 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -44,12 +44,10 @@ jobs:
     docker:
       - image: cimg/openjdk:17.0
     <<: *default_steps
-
-## Fails with "Source option 7 is no longer supported. Use 8 or later."
-#  openjdk21:
-#    docker:
-#      - image: cimg/openjdk:21.0
-#    <<: *default_steps
+  openjdk21:
+    docker:
+      - image: cimg/openjdk:21.0
+    <<: *default_steps
 
   windows-openjdk12:
     executor: win/default
@@ -79,7 +77,7 @@ workflows:
       - openjdk11
       - openjdk13
       - openjdk17
-#      - openjdk21
+      - openjdk21
       - windows-openjdk12
       - openjdk8-jnr-exclude
       - openjdk8-jnr-latest
diff --git a/pom.xml b/pom.xml
index 1183d265..b20937f1 100644
--- a/pom.xml
+++ b/pom.xml
@@ -14,7 +14,8 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.scm.id>github</project.scm.id>
-        <java.version>1.6</java.version>
+        <java.version>1.7</java.version>
+        <jdk.version>1.7</jdk.version>
         <!-- 3.1.0 is last version to support Java 7 -->
         <maven-wrapper-plugin.version>3.1.0</maven-wrapper-plugin.version>
         <!-- 2.22.2 is last version to support Java 7 -->
@@ -168,6 +169,10 @@
             <activation>
                 <jdk>[16.0,)</jdk>
             </activation>
+            <properties>
+                <!-- Fix for JDK 21 which fails with "Source option 7 is no longer supported. Use 8 or later. -->
+                <jdk.version>1.8</jdk.version>
+            </properties>
             <build>
                 <plugins>
                     <plugin>
@@ -256,7 +261,7 @@
                         <configuration>
                             <rules>
                                 <requireJavaVersion>
-                                    <version>[1.7.0-0,1.9.0-0),[9.0,18.0-0)</version>
+                                    <version>[1.7.0-0,1.9.0-0),[9.0,22.0-0)</version>
                                 </requireJavaVersion>
                             </rules>
                         </configuration>
@@ -266,7 +271,7 @@
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-source-plugin</artifactId>
-                <version>2.4</version>
+                <version>3.3.0</version>
                 <executions>
                     <execution>
                         <id>attach-sources</id>
@@ -281,8 +286,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.8.1</version>
                 <configuration>
-                    <source>1.7</source>
-                    <target>1.7</target>
+                    <source>${jdk.version}</source>
+                    <target>${jdk.version}</target>
                 </configuration>
             </plugin>
             <plugin>
@@ -290,7 +295,7 @@
                 <artifactId>maven-javadoc-plugin</artifactId>
                 <version>3.1.0</version>
                 <configuration>
-                    <source>1.7</source>
+                    <source>${jdk.version}</source>
                 </configuration>
                 <executions>
                     <execution>