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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。 org.eclipse.paho org.eclipse.paho.client.mqttv3 1.2.5 复制代码 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"; 复制代码然后创建 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); 复制代码说明 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。 org.bouncycastle bcpkix-jdk15on 1.70 复制代码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(); } } 复制代码参照如下设置 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); 复制代码 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); } } } 复制代码 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(); } } } 复制代码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:github.com/emqx/MQTT-C… 。 版权声明: 本文为 EMQ 原创,转载请注明出处。 原文链接:www.emqx.com/zh/blog/how… |
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