diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index c82e759c2..a38b26f4c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -14,6 +14,7 @@ jobs:
       matrix:
         java:
           - 11
+          - 15
 
     steps:
       - name: Checkout
diff --git a/build.gradle b/build.gradle
index 47e5e334e..de0d5323f 100644
--- a/build.gradle
+++ b/build.gradle
@@ -152,6 +152,7 @@ allprojects {
 		commonsIoVersion = '2.6'
 		bouncyCastleVersion = '1.68'
 		guavaVersion = '30.1-jre'
+		mockitoVersion = '3.7.7'
 
 		if (project.hasProperty("useSonatype")) {
 			useSonatype = project.getProperty("useSonatype").toBoolean()
@@ -255,7 +256,7 @@ allprojects {
 	}
 
 	jacoco {
-		toolVersion = "0.8.3"
+		toolVersion = "0.8.6"
 	}
 
 	jacocoTestReport {
@@ -273,7 +274,10 @@ allprojects {
 			// since the one paramater addStringOption doesn't seem to
 			// work, we extra add '-quiet', which is added anyway by
 			// gradle.
-			options.addStringOption('Xdoclint:all', '-quiet')
+			// TODO: This enables all doclint check but
+			// 'missing'. Re-enable 'missing' once every public API in
+			// Smack has a javadoc comment.
+			options.addStringOption('Xdoclint:accessibility,html,reference,syntax', '-quiet')
 
 			// Treat warnings as errors.
 			// See also https://bugs.openjdk.java.net/browse/JDK-8200363
@@ -285,12 +289,16 @@ allprojects {
 		tasks.withType(Javadoc) {
 			options.addStringOption('-release', javaMajor)
 
-			// Fix for javadoc search. If not set, the search result would direct to
-			// javadoc/undefined/org/jivesoftware/smack/altconnections/HttpLookupMethod.html
-			// instead of
-			// javadoc/org/jivesoftware/smack/altconnections/HttpLookupMethod.html
-			// https://stackoverflow.com/a/53732633/194894
-			options.addBooleanOption("-no-module-directories", true)
+			// The -no-modules-directories javadoc option was removed in Java 13
+			// See https://bugs.openjdk.java.net/browse/JDK-8215582
+			if (JavaVersion.current() < JavaVersion.VERSION_13) {
+				// Fix for javadoc search. If not set, the search result would direct to
+				// javadoc/undefined/org/jivesoftware/smack/altconnections/HttpLookupMethod.html
+				// instead of
+				// javadoc/org/jivesoftware/smack/altconnections/HttpLookupMethod.html
+				// https://stackoverflow.com/a/53732633/194894
+				options.addBooleanOption("-no-module-directories", true)
+			}
 		}
 		tasks.withType(JavaCompile) {
 			options.compilerArgs.addAll([
@@ -312,10 +320,10 @@ tasks.withType(Javadoc) {
 		// The smack-extensions subproject uses mockito in its fest
 		// fixtures, and we want to have mockito also available in
 		// test, so we use API here.
-		testFixturesApi "org.mockito:mockito-core:3.3.3"
+		testFixturesApi "org.mockito:mockito-core:${mockitoVersion}"
 		
 		// To mock final classes
-		testImplementation 'org.mockito:mockito-inline:3.3.3'
+		testImplementation "org.mockito:mockito-inline:${mockitoVersion}"
 		testImplementation 'com.jamesmurty.utils:java-xmlbuilder:1.2'
 
 		errorprone 'com.google.errorprone:error_prone_core:2.5.1'
diff --git a/smack-android-extensions/src/main/java/org/jivesoftware/smackx/ping/android/ServerPingWithAlarmManager.java b/smack-android-extensions/src/main/java/org/jivesoftware/smackx/ping/android/ServerPingWithAlarmManager.java
index 1d7e6b826..a81f51ade 100644
--- a/smack-android-extensions/src/main/java/org/jivesoftware/smackx/ping/android/ServerPingWithAlarmManager.java
+++ b/smack-android-extensions/src/main/java/org/jivesoftware/smackx/ping/android/ServerPingWithAlarmManager.java
@@ -79,6 +79,12 @@ public final class ServerPingWithAlarmManager extends Manager {
         });
     }
 
+    /**
+     * Get the instance of this manager for the given connection.
+     *
+     * @param connection the connection.
+     * @return the instance of this manager for the given connection.
+     */
     public static synchronized ServerPingWithAlarmManager getInstanceFor(XMPPConnection connection) {
         ServerPingWithAlarmManager serverPingWithAlarmManager = INSTANCES.get(connection);
         if (serverPingWithAlarmManager == null) {
@@ -104,6 +110,11 @@ public final class ServerPingWithAlarmManager extends Manager {
         mEnabled = enabled;
     }
 
+    /**
+     * Check if this manager is enabled.
+     *
+     * @return <code>true</code> if this manager is enabled, <code>false</code> otherwise.
+     */
     public boolean isEnabled() {
         return mEnabled;
     }
diff --git a/smack-android/src/main/java/org/jivesoftware/smack/android/AndroidSmackInitializer.java b/smack-android/src/main/java/org/jivesoftware/smack/android/AndroidSmackInitializer.java
index 81a228ce0..a34fb1776 100644
--- a/smack-android/src/main/java/org/jivesoftware/smack/android/AndroidSmackInitializer.java
+++ b/smack-android/src/main/java/org/jivesoftware/smack/android/AndroidSmackInitializer.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2014-2018 Florian Schmaus
+ * Copyright © 2014-2021 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,8 +39,8 @@ public class AndroidSmackInitializer implements SmackInitializer {
     @Override
     public List<Exception> initialize() {
         SmackConfiguration.setDefaultHostnameVerifier(new StrictHostnameVerifier());
-        Base64.setEncoder(AndroidBase64Encoder.getInstance());
-        Base64UrlSafeEncoder.setEncoder(AndroidBase64UrlSafeEncoder.getInstance());
+        Base64.setEncoder(AndroidBase64Encoder.INSTANCE);
+        Base64UrlSafeEncoder.setEncoder(AndroidBase64UrlSafeEncoder.INSTANCE);
         return null;
     }
 
diff --git a/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64Encoder.java b/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64Encoder.java
index 8e4d47d6c..6ae2cfb6b 100644
--- a/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64Encoder.java
+++ b/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64Encoder.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2014-2019 Florian Schmaus
+ * Copyright © 2014-2021 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,7 +24,10 @@ import android.util.Base64;
  */
 public final class AndroidBase64Encoder implements org.jivesoftware.smack.util.stringencoder.Base64.Encoder {
 
-    private static AndroidBase64Encoder instance = new AndroidBase64Encoder();
+    /**
+     * An instance of this encoder.
+     */
+    public static AndroidBase64Encoder INSTANCE = new AndroidBase64Encoder();
 
     private static final int BASE64_ENCODER_FLAGS = Base64.NO_WRAP;
 
@@ -32,10 +35,6 @@ public final class AndroidBase64Encoder implements org.jivesoftware.smack.util.s
         // Use getInstance()
     }
 
-    public static AndroidBase64Encoder getInstance() {
-        return instance;
-    }
-
     @Override
     public byte[] decode(String string) {
         return Base64.decode(string, Base64.DEFAULT);
diff --git a/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64UrlSafeEncoder.java b/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64UrlSafeEncoder.java
index b83d3014d..70c144e66 100644
--- a/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64UrlSafeEncoder.java
+++ b/smack-android/src/main/java/org/jivesoftware/smack/util/stringencoder/android/AndroidBase64UrlSafeEncoder.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2014-2019 Florian Schmaus
+ * Copyright © 2014-2021 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -22,9 +22,16 @@ import org.jivesoftware.smack.util.stringencoder.StringEncoder;
 
 import android.util.Base64;
 
+/**
+ * An URL-safe Base64 encoder.
+ * @author Florian Schmaus
+ */
 public final class AndroidBase64UrlSafeEncoder implements StringEncoder<String> {
 
-    private static AndroidBase64UrlSafeEncoder instance = new AndroidBase64UrlSafeEncoder();
+    /**
+     * An instance of this encoder.
+     */
+    public static AndroidBase64UrlSafeEncoder INSTANCE = new AndroidBase64UrlSafeEncoder();
 
     private static final int BASE64_ENCODER_FLAGS = Base64.URL_SAFE | Base64.NO_WRAP;
 
@@ -32,10 +39,6 @@ public final class AndroidBase64UrlSafeEncoder implements StringEncoder<String>
         // Use getInstance()
     }
 
-    public static AndroidBase64UrlSafeEncoder getInstance() {
-        return instance;
-    }
-
     @Override
     public String encode(String string) {
         return Base64.encodeToString(string.getBytes(StandardCharsets.UTF_8), BASE64_ENCODER_FLAGS);
diff --git a/smack-android/src/main/java/org/jivesoftware/smackx/debugger/android/AndroidDebugger.java b/smack-android/src/main/java/org/jivesoftware/smackx/debugger/android/AndroidDebugger.java
index c1a355def..068dab202 100644
--- a/smack-android/src/main/java/org/jivesoftware/smackx/debugger/android/AndroidDebugger.java
+++ b/smack-android/src/main/java/org/jivesoftware/smackx/debugger/android/AndroidDebugger.java
@@ -1,6 +1,6 @@
 /**
  *
- * Copyright © 2014-2017 Florian Schmaus
+ * Copyright © 2014-2021 Florian Schmaus
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,6 +34,11 @@ import android.util.Log;
  */
 public class AndroidDebugger extends AbstractDebugger {
 
+    /**
+     * Constructs a new Smack debugger for Android.
+     *
+     * @param connection the connection to debug.
+     */
     public AndroidDebugger(XMPPConnection connection) {
         super(connection);
     }
diff --git a/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/Item.java b/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/Item.java
index 0cb4db2e3..571bb10d6 100644
--- a/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/Item.java
+++ b/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/Item.java
@@ -27,7 +27,9 @@ import org.jivesoftware.smackx.pubsub.provider.ItemProvider;
  * PubSub node.  An <code>Item</code> has several properties that are dependent
  * on the configuration of the node to which it has been or will be published.
  *
- * <h3>An Item received from a node (via {@link LeafNode#getItems()} or {@link LeafNode#addItemEventListener(org.jivesoftware.smackx.pubsub.listener.ItemEventListener)}</h3>
+ * <p>
+ * <b>An Item received from a node (via {@link LeafNode#getItems()} or {@link LeafNode#addItemEventListener(org.jivesoftware.smackx.pubsub.listener.ItemEventListener)}</b>
+ * </p>
  * <ul>
  * <li>Will always have an id (either user or server generated) unless node configuration has both
  * {@link ConfigureForm#isPersistItems()} and {@link ConfigureForm#isDeliverPayloads()}set to false.
@@ -35,7 +37,9 @@ import org.jivesoftware.smackx.pubsub.provider.ItemProvider;
  * to true, otherwise it will be null.
  * </ul>
  *
- * <h3>An Item created to send to a node (via {@link LeafNode#publish()} or {@link LeafNode#publish()}</h3>
+ * <p>
+ * <b>An Item created to send to a node (via {@link LeafNode#publish()} or {@link LeafNode#publish()}</b>
+ * </p>
  * <ul>
  * <li>The id is optional, since the server will generate one if necessary, but should be used if it is
  * meaningful in the context of the node.  This value must be unique within the node that it is sent to, since
diff --git a/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/PayloadItem.java b/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/PayloadItem.java
index 6cb4210d8..94462de7c 100644
--- a/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/PayloadItem.java
+++ b/smack-extensions/src/main/java/org/jivesoftware/smackx/pubsub/PayloadItem.java
@@ -28,7 +28,9 @@ import org.jivesoftware.smackx.pubsub.provider.ItemProvider;
  * pubsub node.  An <code>Item</code> has several properties that are dependent
  * on the configuration of the node to which it has been or will be published.
  *
- * <h3>An Item received from a node (via {@link LeafNode#getItems()} or {@link LeafNode#addItemEventListener(org.jivesoftware.smackx.pubsub.listener.ItemEventListener)}</h3>
+ * <p>
+ * <b>An Item received from a node (via {@link LeafNode#getItems()} or {@link LeafNode#addItemEventListener(org.jivesoftware.smackx.pubsub.listener.ItemEventListener)}</b>
+ * </p>
  * <ul>
  * <li>Will always have an id (either user or server generated) unless node configuration has both
  * {@link ConfigureForm#isPersistItems()} and {@link ConfigureForm#isDeliverPayloads()}set to false.</li>
@@ -36,7 +38,9 @@ import org.jivesoftware.smackx.pubsub.provider.ItemProvider;
  * to true, otherwise it will be null.</li>
  * </ul>
  *
- * <h3>An Item created to send to a node (via {@link LeafNode#publish()}</h3>
+ * <p>
+ * <b>An Item created to send to a node (via {@link LeafNode#publish()}</b>
+ * </p>
  * <ul>
  * <li>The id is optional, since the server will generate one if necessary, but should be used if it is
  * meaningful in the context of the node.  This value must be unique within the node that it is sent to, since
@@ -45,6 +49,7 @@ import org.jivesoftware.smackx.pubsub.provider.ItemProvider;
  * to true.</li>
  * </ul>
  *
+ *
  * <p>To customise the payload object being returned from the {@link #getPayload()} method, you can
  * add a custom parser as explained in {@link ItemProvider}.</p>
  *