# Overview

Smack is a library for client-to-server XMPP connections to perform real-time communications and data exchange. This includes, but is not limited to, instant messaging and group chat. More generically speaking, it allows you to easily exchange data in various ways: For example fire-and-forget, publish-subscribe, between human and non-human endpoints. The use cases include M2M, IoT, and many more.

Smack is a pure Java library, open-source and highly modular. It runs on Android and Java SE. The API strives to be easy to use but yet powerful.

## Key Advantages

Smack is extremely simple to use, yet provides a powerful API. Sending a text message to a user can be accomplished in only a few lines of code.

    {@code
    AbstractXMPPConnection connection = new XMPPTCPConnection("mtucker", "password", "jabber.org");
    connection.connect().login();

    Message message = connection.getStanzaFactory()
       .buildMessageStanza()
       .to("jsmith@igniterealtime.org")
       .setBody("Howdy! How are you?")
       .build();

    connection.sendStanza(message);

    connection.disconnect();
    }

Smack doesn't force you to code at the protcol level of XMPP. The library provides intelligent higher level constructs, often called {@link org.jivesoftware.smack.Manager}, which let you program more efficiently. Other examples of those constructs are the Chat and Roster classes.

Smack comes with APIs for easy machine-to-machine communication. You can set any number of properties on each message, including properties that are Java objects.

## License

Smack is open-source and most parts are under the Apache License 2.0, which means you can incorporate Smack into your commercial or non-commercial applications. Some parts of Smack may be under a different open-source license. Please refer to the individual subprojects for their license statement.

Note that the Apache License 2.0 requires that the contents of a NOICE text file are shown "…within a display generated by the Derivative Works, if and wherever such third-party notices normally appear.". Smack comes with such a NOTICE file. The content of Smack's NOTICE file can conveniently be retrieved using {@link org.jivesoftware.smack.Smack#getNoticeStream}.

## About XMPP

XMPP (eXtensible Messaging and Presence Protocol) is an open protocol standardized by the Internet Engineering Task Force (IETF) and supported and extended by the XMPP Standards Foundation (XSF).

## Smack Modules

Smack is meant to be easily embedded into any existing Java application. The library ships as several modules to provide more flexibility over which features applications require.

- smack-core -- provides core XMPP functionality. All XMPP features that are part of the XMPP RFCs are included.
- smack-im -- provides functionality defined in RFC 6121 (XMPP-IM), like the Roster.
- {@link org.jivesoftware.smack.tcp smack-tcp} -- support for XMPP over TCP. Includes XMPPTCPConnection class, which you usually want to use
- smack-extensions -- support for many of the extensions (XEPs) defined by the XMPP Standards Foundation, including multi-user chat, file transfer, user search, etc. The extensions are documented in the [extensions manual](extensions/index.md).
- smack-experimental -- support for experimental extensions (XEPs) defined by the XMPP Standards Foundation. The API and functionality of those extensions should be considered as unstable.
- smack-legacy -- support for legacy extensions (XEPs) defined by the XMPP Standards Foundation.
- smack-bosh -- support for BOSH (XEP-0124). This code should be considered as beta.
- smack-resolver-minidns -- support for resolving DNS SRV records with the help of MiniDNS. Ideal for platforms that do not support the javax.naming API. Also supports [DNSSEC](dnssec.md) TODO: Fix link.
- smack-resolver-dnsjava -- support for resolving DNS SRV records with the help of dnsjava.
- smack-resolver-javax -- support for resolving DNS SRV records with the javax namespace API.
- smack-debug -- an enhanced GUI debugger for protocol traffic. It will automatically be used when found in the classpath and when [debugging](debugging.md) is enabled.

## Main API Entry Points

- {@link org.jivesoftware.smack.XMPPConnection}
- {@link org.jivesoftware.smack.tcp.XMPPTCPConnection}
- {@link org.jivesoftware.smack.roster.Roster}

## Smack Extensions

Since the X in XMPP stands for eXstensible, Smack comes with many built-in extensions for XMPP. Click

> {@link org.jivesoftware.smackx}

for an overview of all supporteted XMPP extensions of Smack.

Some selected extensions are

- {@link org.jivesoftware.smackx.muc.MultiUserChat Multi-User Chat (XEP-0045)}
- {@link org.jivesoftware.smackx.carbons.CarbonManager Message Carbons (XEP-0289)}
- {@link org.jivesoftware.smackx.omemo OMEMO (XEP-0384)}

## Configuration

Smack has an initialization process that involves 2 phases.

- Initializing system properties - Initializing all the system properties accessible through the class **SmackConfiguration**. These properties are retrieved by the _getXXX_ methods on that class.
- Initializing startup classes - Initializing any classes meant to be active at startup by instantiating the class, and then calling the _initialize_ method on that class if it extends **SmackInitializer**. If it does not extend this interface, then initialization will have to take place in a static block of code which is automatically executed when the class is loaded.

Initialization is accomplished via a configuration file. By default, Smack will load the one embedded in the Smack jar at _org.jivesoftware.smack/smack- config.xml_. This particular configuration contains a list of initializer classes to load. All manager type classes that need to be initialized are contained in this list of initializers.