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org.eclipse.paho.client.mqttv3 (MQTT v3 Client API)
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<H2>
Package org.eclipse.paho.client.mqttv3
</H2>
Contains an API for connecting to a server which implements the MQTT V3
protocol.
<P>
<B>See:</B>
<BR>
<A HREF="#package_description"><B>Description</B></A>
<P>
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<TH ALIGN="left" COLSPAN=2><FONT SIZE="+2">
<B>Interface Summary</B></FONT></TH>
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<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttCallback.html" title="interface in org.eclipse.paho.client.mqttv3">MqttCallback</A></B></TD>
<TD>Asynchronous message listener. </TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClientPersistence.html" title="interface in org.eclipse.paho.client.mqttv3">MqttClientPersistence</A></B></TD>
<TD>Represents a persistent data store, used to store outbound and inbound messages while they
are in flight, enabling delivery to the QOS specified. </TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttDeliveryToken.html" title="interface in org.eclipse.paho.client.mqttv3">MqttDeliveryToken</A></B></TD>
<TD>Provides a mechanism for tracking the delivery of messages.
</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttPersistable.html" title="interface in org.eclipse.paho.client.mqttv3">MqttPersistable</A></B></TD>
<TD>Represents an object used to pass data to be persisted across the
<A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClientPersistence.html" title="interface in org.eclipse.paho.client.mqttv3"><CODE>MqttClientPersistence</CODE></A>
interface.
</TD>
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<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClient.html" title="class in org.eclipse.paho.client.mqttv3">MqttClient</A></B></TD>
<TD>Lightweight client for talking to a server via the MQTT version 3
protocol. </TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttConnectOptions.html" title="class in org.eclipse.paho.client.mqttv3">MqttConnectOptions</A></B></TD>
<TD>Stores options used when connecting to a server.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttDefaultFilePersistence.html" title="class in org.eclipse.paho.client.mqttv3">MqttDefaultFilePersistence</A></B></TD>
<TD>An implementation of the <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClientPersistence.html" title="interface in org.eclipse.paho.client.mqttv3"><CODE>MqttClientPersistence</CODE></A> interface that provides
file based persistence.
</TD>
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<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttMessage.html" title="class in org.eclipse.paho.client.mqttv3">MqttMessage</A></B></TD>
<TD>An MQTT message. </TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttTopic.html" title="class in org.eclipse.paho.client.mqttv3">MqttTopic</A></B></TD>
<TD>Represents a topic destination, used for publish/subscribe messaging.</TD>
</TR>
</TABLE>
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<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttException.html" title="class in org.eclipse.paho.client.mqttv3">MqttException</A></B></TD>
<TD>Thrown if an error occurs communicating with the server.</TD>
</TR>
<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttPersistenceException.html" title="class in org.eclipse.paho.client.mqttv3">MqttPersistenceException</A></B></TD>
<TD>This exception should be thrown by the implementor of the persistence
interface if there is a problem reading or writing persistent data.</TD>
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<TR BGCOLOR="white" CLASS="TableRowColor">
<TD WIDTH="15%"><B><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttSecurityException.html" title="class in org.eclipse.paho.client.mqttv3">MqttSecurityException</A></B></TD>
<TD>Thrown when a client is not authorized to perform an operation, or
if there is a problem with the security configuration.</TD>
</TR>
</TABLE>
<P>
<A NAME="package_description"><!-- --></A><H2>
Package org.eclipse.paho.client.mqttv3 Description
</H2>
<P>
Contains an API for connecting to a server which implements the MQTT V3
protocol.
<p>
The MQ Telemetry Transport (MQTT) is a lightweight publish/subscribe protocol
flowing over TCP/IP for remote sensors and control devices through low
bandwidth communications. MQTT is used by specialized applications on
small footprint devices that require a low bandwidth communication,
typically for remote data acquisition and process control.
</p>
<p>
A typical system might comprise several hundred client devices communicating
with a single server or "broker", where each client is identified
by a unique ID.
</p>
<p>The basic means of operating the client is as follows:</p>
<ol>
<li>Create an instance of <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClient.html" title="class in org.eclipse.paho.client.mqttv3"><CODE>MqttClient</CODE></A>, providing
the address of the server and a unique client identifier.</li>
<li><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClient.html#connect(org.eclipse.paho.client.mqttv3.MqttConnectOptions)"><CODE>Connect</CODE></A> to the server</li>
<li>Exchange messages with the server.
<ul>
<li>Send <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttMessage.html" title="class in org.eclipse.paho.client.mqttv3"><CODE>messages</CODE></A> to the server,
via <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttTopic.html" title="class in org.eclipse.paho.client.mqttv3"><CODE>topics</CODE></A>.</li>
<li>Receive messages from the server. This is done asynchronously
using
<A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClient.html#subscribe(java.lang.String, int)"><CODE>subscriptions</CODE></A>,
combined with a <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClient.html#setCallback(org.eclipse.paho.client.mqttv3.MqttCallback)"><CODE>callback</CODE></A>.
</li>
</ul>
</li>
<li><A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttClient.html#disconnect()"><CODE>Disconnect</CODE></A> from the server.</li>
</ol>
</p>
<h3>Examples</h3>
<h4>Publish a message, at most once</h4>
<p>In the following code snippet, a simple message is published to the server,
using defaults wherever possible.</p>
<code><pre> MqttClient client = new MqttClient("tcp://localhost:1883", "SampleClient");
client.connect();
MqttMessage message = new MqttMessage("Hello world");
message.setQos(0);
client.getTopic("foo/bar").publish(message);
client.disconnect();</pre></code>
<p>This is the fastest way of publishing messages,
but is also the least reliable. There is no assurance that the message will actually be
delivered, and no exception will be thrown if the server refuses the message, or if the
network connection fails.</p>
<h4>Publish a message, once and only once (single threaded)</h4>
<p>In the following code snippet, a message is published to the server
at QoS 2, which is the most reliable delivery mechanism.
In order to achieve this, the message needs to be persisted by the client.
A default implementation of persistence is provided by <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttDefaultFilePersistence.html" title="class in org.eclipse.paho.client.mqttv3"><CODE>MqttDefaultFilePersistence</CODE></A>.
This will be used by the client unless an alternative is supplied.</p>
<p>Publishing reliably is more complex, and most of the code is used to handle
error conditions. The key thing to notice is the two-stage delivery: the first
stage gives the message to the client. After this, the client is responsible for
delivering the message to the server. The second stage is about determining
success or failure.</p>
<code><pre>
MqttClient client = null;
try {
// Create a client to communicate with a broker at the specified address
client = new MqttClient("tcp://localhost:1883", "SampleClient");
// Connect to the broker
client.connect();
} catch (MqttException ex) {
System.err.println("Could not connect");
}
if ((client != null) && client.isConnected()) {
MqttTopic topic = client.getTopic("foo/bar");
MqttDeliveryToken token = null;
// Create message and set quality of service to deliver the message once
MqttMessage message = new MqttMessage("Hello world");
message.setQos(2);
try {
// Give the message to the client for publishing. For QoS 2,
// this will involve multiple network calls, which will happen
// asynchronously after this method has returned.
token = topic.publish(message);
}
catch (MqttException ex) {
// Client has not accepted the message due to a failure
// Depending on the exception's reason code, we could always retry
System.err.println("Failed to send message");
}
if (token != null) {
boolean keepTrying = true;
do {
try {
// Wait for the message delivery to complete
token.waitForCompletion();
System.out.println("Message delivery complete");
}
catch (MqttException deliveryException) {
int reasonCode = deliveryException.getReasonCode();
// TODO: Retry the message, or decide to stop trying
System.err.println("Message delivery failed");
if (client.isConnected() == false) {
try {
// Re-connect to the server
client.connect();
}
catch (MqttException connectException) {
// Can't reconnect, so give up. If and when the
// client does reconnect, the message delivery
// will automatically continue
keepTrying = false;
}
}
}
} while (!token.isComplete() && keepTrying);
}
}</pre></code>
<h4>Publish a message, once and only once (multi-threaded)</h4>
<p>In the following code snippet, a message is published to the server
at QoS 2, which is the most reliable delivery mechanism.
The application uses a <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttCallback.html" title="interface in org.eclipse.paho.client.mqttv3"><CODE>callback</CODE></A> to asynchronously be notified
when delivery of a message has been <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttCallback.html#deliveryComplete(org.eclipse.paho.client.mqttv3.MqttDeliveryToken)"><CODE>completed</CODE></A>.</p>
<p>
A default implementation of persistence is provided by <A HREF="../../../../../org/eclipse/paho/client/mqttv3/MqttDefaultFilePersistence.html" title="class in org.eclipse.paho.client.mqttv3"><CODE>MqttDefaultFilePersistence</CODE></A>.
This will be used by the client unless an alternative is supplied.</p>
<code><pre>
public class SampleClient implements MqttCallback {
public void run() {
MqttClient client = null;
try {
// Create a client to communicate with a broker at the specified address
client = new MqttClient("tcp://localhost:1883", "SampleClient");
// Connect to the broker
client.connect();
// Setup a callback
client.setCallback(this);
} catch (MqttException ex) {
System.err.println("Could not connect");
}
if ((client != null) && client.isConnected()) {
MqttTopic topic = client.getTopic("foo/bar");
// Create message and set quality of service to deliver the message once
MqttMessage message = new MqttMessage("Hello world");
message.setQos(2);
try {
// Give the message to the client for publishing. For QoS 2, this
// will involve multiple network calls, which will happen
// asynchronously after this method has returned.
topic.publish(message);
} catch (MqttException ex) {
// Client has not accepted the message due to a failure
// Depending on the exception's reason code, we could always retry
System.err.println("Failed to send message");
}
}
}
public void connectionLost(Throwable cause) {
// TODO: Implement reconnection logic
System.err.println("Connection lost");
}
public void deliveryComplete(MqttDeliveryToken token) {
System.out.println("Delivery complete");
}
public void messageArrived(MqttTopic topic, MqttMessage message) throws Exception {
}
}</pre></code>
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