MQTT 是一种基于发布/订阅模式的 轻量级物联网消息传输协议 ，可在严重受限的硬件设备和低带宽、高延迟的网络上实现稳定传输。它凭借简单易实现、支持 QoS、报文小等特点，占据了物联网协议的半壁江山。
本文主要介绍如何在 Java 项目中使用 MQTT，实现客户端与服务器的连接、订阅和收发消息等功能。
引入客户端库
本文的开发环境为：
- 构建工具：Maven
- IDE：IntelliJ IDEA
- Java 版本：JDK 1.8.0
本文将使用 Eclipse Paho Java Client 作为客户端，该客户端是 Java 语言中使用最为广泛的 MQTT 客户端库。
添加以下依赖到项目 pom.xml 文件中。
<dependencies>
<dependency>
<groupId>org.eclipse.paho</groupId>
<artifactId>org.eclipse.paho.client.mqttv3</artifactId>
<version>1.2.5</version>
</dependency>
</dependencies>创建 MQTT 连接
MQTT 服务器
本文将使用 EMQX 提供的 免费公共 MQTT 服务器，该服务基于 EMQX 的 MQTT 云平台 创建。服务器接入信息如下：
- Broker: broker.emqx.io（中国用户可以使用 broker-cn.emqx.io）
- TCP Port: 1883
- SSL/TLS Port: 8883
普通 TCP 连接
设置 MQTT Broker 基本连接参数，用户名、密码为非必选参数。
String broker = "tcp://broker.emqx.io:1883";
// TLS/SSL
// String broker = "ssl://broker.emqx.io:8883";
String username = "emqx";
String password = "public";
String clientid = "publish_client";然后创建 MQTT 客户端并连接。
MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
MqttConnectOptions options = new MqttConnectOptions();
options.setUserName(username);
options.setPassword(password.toCharArray());
client.connect(options);说明
- MqttClient: 同步调用客户端，使用阻塞方法通信。
- MqttClientPersistence: 代表一个持久的数据存储，用于在传输过程中存储出站和入站的信息，使其能够传递到指定的 QoS。
MqttConnectOptions: 连接选项，用于指定连接的参数，下面列举一些常见的方法。
- setUserName: 设置用户名
- setPassword: 设置密码
- setCleanSession: 设置是否清除会话
- setKeepAliveInterval: 设置心跳间隔
- setConnectionTimeout: 设置连接超时时间
- setAutomaticReconnect: 设置是否自动重连
TLS/SSL 连接
如果要使用自签名证书进行 TLS/SSL 连接，需添加 bcpkix-jdk15on 到 pom.xml 文件。
<!-- https://mvnrepository.com/artifact/org.bouncycastle/bcpkix-jdk15on -->
<dependency>
<groupId>org.bouncycastle</groupId>
<artifactId>bcpkix-jdk15on</artifactId>
<version>1.70</version>
</dependency>然后使用如下代码创建 SSLUtils.java 文件。
package io.emqx.mqtt;
import org.bouncycastle.jce.provider.BouncyCastleProvider;
import org.bouncycastle.openssl.PEMKeyPair;
import org.bouncycastle.openssl.PEMParser;
import org.bouncycastle.openssl.jcajce.JcaPEMKeyConverter;
import javax.net.ssl.KeyManagerFactory;
import javax.net.ssl.SSLContext;
import javax.net.ssl.SSLSocketFactory;
import javax.net.ssl.TrustManagerFactory;
import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FileReader;
import java.security.KeyPair;
import java.security.KeyStore;
import java.security.Security;
import java.security.cert.CertificateFactory;
import java.security.cert.X509Certificate;
public class SSLUtils {
public static SSLSocketFactory getSocketFactory(final String caCrtFile,
final String crtFile, final String keyFile, final String password)
throws Exception {
Security.addProvider(new BouncyCastleProvider());
// load CA certificate
X509Certificate caCert = null;
FileInputStream fis = new FileInputStream(caCrtFile);
BufferedInputStream bis = new BufferedInputStream(fis);
CertificateFactory cf = CertificateFactory.getInstance("X.509");
while (bis.available() > 0) {
caCert = (X509Certificate) cf.generateCertificate(bis);
}
// load client certificate
bis = new BufferedInputStream(new FileInputStream(crtFile));
X509Certificate cert = null;
while (bis.available() > 0) {
cert = (X509Certificate) cf.generateCertificate(bis);
}
// load client private key
PEMParser pemParser = new PEMParser(new FileReader(keyFile));
Object object = pemParser.readObject();
JcaPEMKeyConverter converter = new JcaPEMKeyConverter().setProvider("BC");
KeyPair key = converter.getKeyPair((PEMKeyPair) object);
pemParser.close();
// CA certificate is used to authenticate server
KeyStore caKs = KeyStore.getInstance(KeyStore.getDefaultType());
caKs.load(null, null);
caKs.setCertificateEntry("ca-certificate", caCert);
TrustManagerFactory tmf = TrustManagerFactory.getInstance("X509");
tmf.init(caKs);
// client key and certificates are sent to server so it can authenticate
KeyStore ks = KeyStore.getInstance(KeyStore.getDefaultType());
ks.load(null, null);
ks.setCertificateEntry("certificate", cert);
ks.setKeyEntry("private-key", key.getPrivate(), password.toCharArray(),
new java.security.cert.Certificate[]{cert});
KeyManagerFactory kmf = KeyManagerFactory.getInstance(KeyManagerFactory
.getDefaultAlgorithm());
kmf.init(ks, password.toCharArray());
// finally, create SSL socket factory
SSLContext context = SSLContext.getInstance("TLSv1.2");
context.init(kmf.getKeyManagers(), tmf.getTrustManagers(), null);
return context.getSocketFactory();
}
}参照如下设置 options。
// 设置 SSL/TLS 连接地址
String broker = "ssl://broker.emqx.io:8883";
// 设置 socket factory
String caFilePath = "/cacert.pem";
String clientCrtFilePath = "/client.pem";
String clientKeyFilePath = "/client.key";
SSLSocketFactory socketFactory = getSocketFactory(caFilePath, clientCrtFilePath, clientKeyFilePath, "");
options.setSocketFactory(socketFactory);发布 MQTT 消息
创建一个发布客户端类 PublishSample，该类将发布一条 Hello MQTT 消息至主题 mqtt/test。
package io.emqx.mqtt;
import org.eclipse.paho.client.mqttv3.MqttClient;
import org.eclipse.paho.client.mqttv3.MqttConnectOptions;
import org.eclipse.paho.client.mqttv3.MqttException;
import org.eclipse.paho.client.mqttv3.MqttMessage;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
public class PublishSample {
public static void main(String[] args) {
String broker = "tcp://broker.emqx.io:1883";
String topic = "mqtt/test";
String username = "emqx";
String password = "public";
String clientid = "publish_client";
String content = "Hello MQTT";
int qos = 0;
try {
MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
// 连接参数
MqttConnectOptions options = new MqttConnectOptions();
// 设置用户名和密码
options.setUserName(username);
options.setPassword(password.toCharArray());
options.setConnectionTimeout(60);
options.setKeepAliveInterval(60);
// 连接
client.connect(options);
// 创建消息并设置 QoS
MqttMessage message = new MqttMessage(content.getBytes());
message.setQos(qos);
// 发布消息
client.publish(topic, message);
System.out.println("Message published");
System.out.println("topic: " + topic);
System.out.println("message content: " + content);
// 关闭连接
client.disconnect();
// 关闭客户端
client.close();
} catch (MqttException e) {
throw new RuntimeException(e);
}
}
}订阅 MQTT 主题
创建一个订阅客户端类 SubscribeSample，该类将订阅主题 mqtt/test。
package io.emqx.mqtt;
import org.eclipse.paho.client.mqttv3.*;
import org.eclipse.paho.client.mqttv3.persist.MemoryPersistence;
public class SubscribeSample {
public static void main(String[] args) {
String broker = "tcp://broker.emqx.io:1883";
String topic = "mqtt/test";
String username = "emqx";
String password = "public";
String clientid = "subscribe_client";
int qos = 0;
try {
MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
// 连接参数
MqttConnectOptions options = new MqttConnectOptions();
options.setUserName(username);
options.setPassword(password.toCharArray());
options.setConnectionTimeout(60);
options.setKeepAliveInterval(60);
// 设置回调
client.setCallback(new MqttCallback() {
public void connectionLost(Throwable cause) {
System.out.println("connectionLost: " + cause.getMessage());
}
public void messageArrived(String topic, MqttMessage message) {
System.out.println("topic: " + topic);
System.out.println("Qos: " + message.getQos());
System.out.println("message content: " + new String(message.getPayload()));
}
public void deliveryComplete(IMqttDeliveryToken token) {
System.out.println("deliveryComplete---------" + token.isComplete());
}
});
client.connect(options);
client.subscribe(topic, qos);
} catch (Exception e) {
e.printStackTrace();
}
}
}MqttCallback 说明：
- connectionLost(Throwable cause): 连接丢失时被调用
- messageArrived(String topic, MqttMessage message): 接收到消息时被调用
- deliveryComplete(IMqttDeliveryToken token): 消息发送完成时被调用
测试
接下来运行 SubscribeSample，订阅 mqtt/test 主题。 然后运行 PublishSample，发布消息到 mqtt/test 主题。 我们将会看到发布端成功发布消息，同时订阅端接收到消息。
至此，我们完成了在 Java 中使用 Paho Java Client 来作为 MQTT 客户端连接到 公共 MQTT 服务器，并实现了测试客户端与 MQTT 服务器的连接、消息发布和订阅。
完整代码请见：https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-Java。
版权声明： 本文为 EMQ 原创，转载请注明出处。
