diff --git a/documentation/extensions/index.md b/documentation/extensions/index.md
index 91f291ca4..00a9680a7 100644
--- a/documentation/extensions/index.md
+++ b/documentation/extensions/index.md
@@ -122,6 +122,7 @@ Experimental Smack Extensions and currently supported XEPs of smack-experimental
| [Message Markup](messagemarkup.md) | [XEP-0394](https://xmpp.org/extensions/xep-0394.html) | 0.1.0 | Style message bodies while keeping body and markup information separated. |
| DNS Queries over XMPP (DoX) | [XEP-0418](https://xmpp.org/extensions/xep-0418.html) | 0.1.0 | Send DNS queries and responses over XMPP. |
| Message Fastening | [XEP-0422](https://xmpp.org/extensions/xep-0422.html) | 0.1.1 | Mark payloads on a message to be logistically fastened to a previous message. |
+| Fallback Indication | [XEP-0428](https://xmpp.org/extensions/xep-0428.html) | 0.1.0 | Declare body elements of a message as ignorable fallback for naive legacy clients. |
Unofficial XMPP Extensions
--------------------------
diff --git a/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationListener.java b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationListener.java
new file mode 100644
index 000000000..b7ef22525
--- /dev/null
+++ b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationListener.java
@@ -0,0 +1,33 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.jivesoftware.smackx.fallback_indication;
+
+import org.jivesoftware.smack.packet.Message;
+import org.jivesoftware.smackx.fallback_indication.element.FallbackIndicationElement;
+
+public interface FallbackIndicationListener {
+
+ /**
+ * Listener method that gets called when a {@link Message} containing a {@link FallbackIndicationElement} is received.
+ *
+ * @param message message
+ * @param indicator Fallback Indication
+ * @param fallbackBody body that is marked as fallback
+ */
+ void onFallbackIndicationReceived(Message message, FallbackIndicationElement indicator, String fallbackBody);
+
+}
diff --git a/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationManager.java b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationManager.java
new file mode 100644
index 000000000..da58e6ae9
--- /dev/null
+++ b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationManager.java
@@ -0,0 +1,174 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.jivesoftware.smackx.fallback_indication;
+
+import java.util.Map;
+import java.util.Set;
+import java.util.WeakHashMap;
+import java.util.concurrent.CopyOnWriteArraySet;
+
+import org.jivesoftware.smack.AsyncButOrdered;
+import org.jivesoftware.smack.ConnectionCreationListener;
+import org.jivesoftware.smack.Manager;
+import org.jivesoftware.smack.SmackException;
+import org.jivesoftware.smack.StanzaListener;
+import org.jivesoftware.smack.XMPPConnection;
+import org.jivesoftware.smack.XMPPConnectionRegistry;
+import org.jivesoftware.smack.XMPPException;
+import org.jivesoftware.smack.filter.AndFilter;
+import org.jivesoftware.smack.filter.StanzaExtensionFilter;
+import org.jivesoftware.smack.filter.StanzaFilter;
+import org.jivesoftware.smack.filter.StanzaTypeFilter;
+import org.jivesoftware.smack.packet.Message;
+import org.jivesoftware.smack.packet.MessageBuilder;
+import org.jivesoftware.smack.packet.Stanza;
+import org.jivesoftware.smackx.disco.ServiceDiscoveryManager;
+import org.jivesoftware.smackx.fallback_indication.element.FallbackIndicationElement;
+
+import org.jxmpp.jid.BareJid;
+import org.jxmpp.jid.EntityBareJid;
+
+/**
+ * Smacks API for XEP-0428: Fallback Indication.
+ * In some scenarios it might make sense to mark the body of a message as fallback for legacy clients.
+ * Examples are encryption mechanisms where the sender might include a hint for legacy clients stating that the
+ * body (eg. "This message is encrypted") should be ignored.
+ *
+ * @see XEP-0428: Fallback Indication
+ */
+public final class FallbackIndicationManager extends Manager {
+
+ private static final Map INSTANCES = new WeakHashMap<>();
+
+ static {
+ XMPPConnectionRegistry.addConnectionCreationListener(new ConnectionCreationListener() {
+ @Override
+ public void connectionCreated(XMPPConnection connection) {
+ getInstanceFor(connection);
+ }
+ });
+ }
+
+ private final Set listeners = new CopyOnWriteArraySet<>();
+ private final AsyncButOrdered asyncButOrdered = new AsyncButOrdered<>();
+ private final StanzaFilter fallbackIndicationElementFilter = new AndFilter(StanzaTypeFilter.MESSAGE,
+ new StanzaExtensionFilter(FallbackIndicationElement.ELEMENT, FallbackIndicationElement.NAMESPACE));
+
+ private final StanzaListener fallbackIndicationElementListener = new StanzaListener() {
+ @Override
+ public void processStanza(Stanza packet) {
+ Message message = (Message) packet;
+ FallbackIndicationElement indicator = FallbackIndicationElement.fromMessage(message);
+ String body = message.getBody();
+ asyncButOrdered.performAsyncButOrdered(message.getFrom().asBareJid(), () -> {
+ for (FallbackIndicationListener l : listeners) {
+ l.onFallbackIndicationReceived(message, indicator, body);
+ }
+ });
+ }
+ };
+
+ private FallbackIndicationManager(XMPPConnection connection) {
+ super(connection);
+ connection.addAsyncStanzaListener(fallbackIndicationElementListener, fallbackIndicationElementFilter);
+ ServiceDiscoveryManager.getInstanceFor(connection).addFeature(FallbackIndicationElement.NAMESPACE);
+ }
+
+ public static synchronized FallbackIndicationManager getInstanceFor(XMPPConnection connection) {
+ FallbackIndicationManager manager = INSTANCES.get(connection);
+ if (manager == null) {
+ manager = new FallbackIndicationManager(connection);
+ INSTANCES.put(connection, manager);
+ }
+ return manager;
+ }
+
+ /**
+ * Determine, whether or not a user supports Fallback Indications.
+ *
+ * @param jid BareJid of the user.
+ * @return feature support
+ *
+ * @throws XMPPException.XMPPErrorException if a protocol level error happens
+ * @throws SmackException.NotConnectedException if the connection is not connected
+ * @throws InterruptedException if the thread is being interrupted
+ * @throws SmackException.NoResponseException if the server doesn't send a response in time
+ */
+ public boolean userSupportsFallbackIndications(EntityBareJid jid)
+ throws XMPPException.XMPPErrorException, SmackException.NotConnectedException, InterruptedException,
+ SmackException.NoResponseException {
+ return ServiceDiscoveryManager.getInstanceFor(connection())
+ .supportsFeature(jid, FallbackIndicationElement.NAMESPACE);
+ }
+
+ /**
+ * Determine, whether or not the server supports Fallback Indications.
+ *
+ * @return server side feature support
+ *
+ * @throws XMPPException.XMPPErrorException if a protocol level error happens
+ * @throws SmackException.NotConnectedException if the connection is not connected
+ * @throws InterruptedException if the thread is being interrupted
+ * @throws SmackException.NoResponseException if the server doesn't send a response in time
+ */
+ public boolean serverSupportsFallbackIndications()
+ throws XMPPException.XMPPErrorException, SmackException.NotConnectedException, InterruptedException,
+ SmackException.NoResponseException {
+ return ServiceDiscoveryManager.getInstanceFor(connection())
+ .serverSupportsFeature(FallbackIndicationElement.NAMESPACE);
+ }
+
+ /**
+ * Set the body of the message to the provided fallback message and add a {@link FallbackIndicationElement}.
+ *
+ * @param messageBuilder message builder
+ * @param fallbackMessageBody fallback message body
+ * @return builder with set body and added fallback element
+ */
+ public MessageBuilder addFallbackIndicationWithBody(MessageBuilder messageBuilder, String fallbackMessageBody) {
+ return addFallbackIndication(messageBuilder).setBody(fallbackMessageBody);
+ }
+
+ /**
+ * Add a {@link FallbackIndicationElement} to the provided message builder.
+ *
+ * @param messageBuilder message builder
+ * @return message builder with added fallback element
+ */
+ public MessageBuilder addFallbackIndication(MessageBuilder messageBuilder) {
+ return messageBuilder.addExtension(new FallbackIndicationElement());
+ }
+
+ /**
+ * Register a {@link FallbackIndicationListener} that gets notified whenever a message that contains a
+ * {@link FallbackIndicationElement} is received.
+ *
+ * @param listener listener to be registered.
+ */
+ public synchronized void addFallbackIndicationListener(FallbackIndicationListener listener) {
+ listeners.add(listener);
+ }
+
+ /**
+ * Unregister a {@link FallbackIndicationListener}.
+ *
+ * @param listener listener to be unregistered.
+ */
+ public synchronized void removeFallbackIndicationListener(FallbackIndicationListener listener) {
+ listeners.remove(listener);
+ }
+}
diff --git a/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/element/FallbackIndicationElement.java b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/element/FallbackIndicationElement.java
new file mode 100644
index 000000000..96b06fc99
--- /dev/null
+++ b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/element/FallbackIndicationElement.java
@@ -0,0 +1,53 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.jivesoftware.smackx.fallback_indication.element;
+
+import org.jivesoftware.smack.packet.ExtensionElement;
+import org.jivesoftware.smack.packet.Message;
+import org.jivesoftware.smack.packet.XmlEnvironment;
+import org.jivesoftware.smack.util.XmlStringBuilder;
+
+public class FallbackIndicationElement implements ExtensionElement {
+
+ public static final String NAMESPACE = "urn:xmpp:fallback:0";
+ public static final String ELEMENT = "fallback";
+
+ public static final FallbackIndicationElement INSTANCE = new FallbackIndicationElement();
+
+ @Override
+ public String getNamespace() {
+ return NAMESPACE;
+ }
+
+ @Override
+ public String getElementName() {
+ return ELEMENT;
+ }
+
+ @Override
+ public XmlStringBuilder toXML(XmlEnvironment xmlEnvironment) {
+ return new XmlStringBuilder(this).closeEmptyElement();
+ }
+
+ public static boolean hasFallbackIndication(Message message) {
+ return message.hasExtension(ELEMENT, NAMESPACE);
+ }
+
+ public static FallbackIndicationElement fromMessage(Message message) {
+ return message.getExtension(FallbackIndicationElement.class);
+ }
+}
diff --git a/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/element/package-info.java b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/element/package-info.java
new file mode 100644
index 000000000..192654509
--- /dev/null
+++ b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/element/package-info.java
@@ -0,0 +1,22 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.
+ */
+
+/**
+ * Smack's API for XEP-0428: Fallback Indication.
+ * Extension Elements.
+ */
+package org.jivesoftware.smackx.fallback_indication.element;
diff --git a/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/package-info.java b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/package-info.java
new file mode 100644
index 000000000..11dfe6e80
--- /dev/null
+++ b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/package-info.java
@@ -0,0 +1,21 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.
+ */
+
+/**
+ * Smack's API for XEP-0428: Fallback Indication.
+ */
+package org.jivesoftware.smackx.fallback_indication;
diff --git a/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/provider/FallbackIndicationElementProvider.java b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/provider/FallbackIndicationElementProvider.java
new file mode 100644
index 000000000..66bc9fb9e
--- /dev/null
+++ b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/provider/FallbackIndicationElementProvider.java
@@ -0,0 +1,35 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.jivesoftware.smackx.fallback_indication.provider;
+
+import java.io.IOException;
+
+import org.jivesoftware.smack.packet.XmlEnvironment;
+import org.jivesoftware.smack.parsing.SmackParsingException;
+import org.jivesoftware.smack.provider.ExtensionElementProvider;
+import org.jivesoftware.smack.xml.XmlPullParser;
+import org.jivesoftware.smack.xml.XmlPullParserException;
+import org.jivesoftware.smackx.fallback_indication.element.FallbackIndicationElement;
+
+public class FallbackIndicationElementProvider extends ExtensionElementProvider {
+
+ @Override
+ public FallbackIndicationElement parse(XmlPullParser parser, int initialDepth, XmlEnvironment xmlEnvironment)
+ throws XmlPullParserException, IOException, SmackParsingException {
+ return FallbackIndicationElement.INSTANCE;
+ }
+}
diff --git a/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/provider/package-info.java b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/provider/package-info.java
new file mode 100644
index 000000000..ea9827989
--- /dev/null
+++ b/smack-experimental/src/main/java/org/jivesoftware/smackx/fallback_indication/provider/package-info.java
@@ -0,0 +1,22 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.
+ */
+
+/**
+ * Smack's API for XEP-0428: Fallback Indication.
+ * Element Providers.
+ */
+package org.jivesoftware.smackx.fallback_indication.provider;
diff --git a/smack-experimental/src/main/resources/org.jivesoftware.smack.experimental/experimental.providers b/smack-experimental/src/main/resources/org.jivesoftware.smack.experimental/experimental.providers
index d594578a4..d97f52ce5 100644
--- a/smack-experimental/src/main/resources/org.jivesoftware.smack.experimental/experimental.providers
+++ b/smack-experimental/src/main/resources/org.jivesoftware.smack.experimental/experimental.providers
@@ -299,6 +299,13 @@
org.jivesoftware.smackx.message_fastening.provider.FasteningElementProvider
+
+
+ fallback
+ urn:xmpp:fallback:0
+ org.jivesoftware.smackx.fallback_indication.provider.FallbackIndicationElementProvider
+
+
query
diff --git a/smack-experimental/src/test/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationTest.java b/smack-experimental/src/test/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationTest.java
new file mode 100644
index 000000000..342e783b5
--- /dev/null
+++ b/smack-experimental/src/test/java/org/jivesoftware/smackx/fallback_indication/FallbackIndicationTest.java
@@ -0,0 +1,42 @@
+/**
+ *
+ * Copyright 2020 Paul Schaub
+ *
+ * Licensed 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.jivesoftware.smackx.fallback_indication;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+import org.jivesoftware.smack.packet.Message;
+import org.jivesoftware.smack.packet.MessageBuilder;
+
+import org.jivesoftware.smackx.fallback_indication.element.FallbackIndicationElement;
+
+import org.junit.jupiter.api.Test;
+
+public class FallbackIndicationTest {
+
+ @Test
+ public void testFallbackIndicationElementFromMessageTest() {
+ Message messageWithoutFallback = MessageBuilder.buildMessage()
+ .build();
+ assertNull(FallbackIndicationElement.fromMessage(messageWithoutFallback));
+
+ Message messageWithFallback = MessageBuilder.buildMessage()
+ .addExtension(new FallbackIndicationElement())
+ .build();
+ assertNotNull(FallbackIndicationElement.fromMessage(messageWithFallback));
+ }
+}