[MQTT](https://www.emqx.com/zh/mqtt) 是一種基於釋出/訂閱模式的 **輕量級物聯網訊息傳輸協議** ，可在嚴重受限的硬體裝置和低頻寬、高延遲的網路上實現穩定傳輸。它憑藉簡單易實現、支援 QoS、報文小等特點，佔據了物聯網協議的半壁江山。

本文主要介紹如何在 Java 專案中使用 MQTT，實現客戶端與伺服器的連線、訂閱和收發訊息等功能。

## 引入客戶端庫

本文的開發環境為：

- 構建工具：[Maven](https://maven.apache.org/)
- IDE：[IntelliJ IDEA](https://www.jetbrains.com/idea/)
- Java 版本：JDK 1.8.0

本文將使用 [Eclipse Paho Java Client](https://github.com/eclipse/paho.mqtt.java) 作為客戶端，該客戶端是 Java 語言中使用最為廣泛的 MQTT 客戶端庫。

新增以下依賴到專案 pom.xml 檔案中。

```xml
<dependencies>
   <dependency>
       <groupid>org.eclipse.paho</groupid>
       <artifactid>org.eclipse.paho.client.mqttv3</artifactid>
       <version>1.2.5</version>
   </dependency>
</dependencies>
```

## 建立 MQTT 連線

### MQTT 伺服器

本文將使用 EMQX 提供的 [免費公共 MQTT 伺服器](https://www.emqx.com/zh/mqtt/public-mqtt5-broker)，該服務基於 EMQX 的 [MQTT 雲平臺](https://www.emqx.com/zh/cloud) 建立。伺服器接入資訊如下：

- Broker: **broker.emqx.io**（中國使用者可以使用 **broker-cn.emqx.io**）
- TCP Port: **1883**
- SSL/TLS Port: **8883**

### 普通 TCP 連線

設定 MQTT Broker 基本連線引數，使用者名稱、密碼為非必選引數。

```abnf
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";
```

然後建立 MQTT 客戶端並連線。

```reasonml
MqttClient client = new MqttClient(broker, clientid, new MemoryPersistence());
MqttConnectOptions options = new MqttConnectOptions();
options.setUserName(username);
options.setPassword(password.toCharArray());
client.connect(options);
```

說明

- MqttClient: 同步呼叫客戶端，使用阻塞方法通訊。
- MqttClientPersistence: 代表一個持久的資料儲存，用於在傳輸過程中儲存出站和入站的資訊，使其能夠傳遞到指定的 QoS。
- MqttConnectOptions: 連線選項，用於指定連線的引數，下面列舉一些常見的方法。
  - setUserName: 設定使用者名稱
  - setPassword: 設定密碼
  - setCleanSession: 設定是否清除會話
  - setKeepAliveInterval: 設定心跳間隔
  - setConnectionTimeout: 設定連線超時時間
  - setAutomaticReconnect: 設定是否自動重連

### TLS/SSL 連線

如果要使用自簽名證照進行 TLS/SSL 連線，需新增 [bcpkix-jdk15on](https://mvnrepository.com/artifact/org.bouncycastle/bcpkix-jdk15on) 到 pom.xml 檔案。

```xml

<dependency>
   <groupid>org.bouncycastle</groupid>
   <artifactid>bcpkix-jdk15on</artifactid>
   <version>1.70</version>
</dependency>
```

然後使用如下程式碼建立 `SSLUtils.java` 檔案。

```java
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();
  }
}
```

參照如下設定 `options`。

```reasonml
// 設定 SSL/TLS 連線地址
String broker = "ssl://broker.emqx.io:8883";
// 設定 socket factory
String caFilePath = "/cacert.pem";
String clientCrtFilePath = "/client.pem";
String clientKeyFilePath = "/client.key";
SSLSocketFactory socketFactory = getSocketFactory(caFilePath, clientCrtFilePath, clientKeyFilePath, "");
options.setSocketFactory(socketFactory);
```

## 釋出 MQTT 訊息

建立一個釋出客戶端類 `PublishSample`，該類將釋出一條 `Hello MQTT` 訊息至主題 `mqtt/test`。

```haxe
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());
           // 連線引數
           MqttConnectOptions options = new MqttConnectOptions();
           // 設定使用者名稱和密碼
           options.setUserName(username);
           options.setPassword(password.toCharArray());
           options.setConnectionTimeout(60);
      options.setKeepAliveInterval(60);
           // 連線
           client.connect(options);
           // 建立訊息並設定 QoS
           MqttMessage message = new MqttMessage(content.getBytes());
           message.setQos(qos);
           // 釋出訊息
           client.publish(topic, message);
           System.out.println("Message published");
           System.out.println("topic: " + topic);
           System.out.println("message content: " + content);
           // 關閉連線
           client.disconnect();
           // 關閉客戶端
           client.close();
      } catch (MqttException e) {
           throw new RuntimeException(e);
      }
  }
}
```

## 訂閱 MQTT 主題

建立一個訂閱客戶端類 `SubscribeSample`，該類將訂閱主題 `mqtt/test`。

```reasonml
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());
           // 連線引數
           MqttConnectOptions options = new MqttConnectOptions();
           options.setUserName(username);
           options.setPassword(password.toCharArray());
           options.setConnectionTimeout(60);
      options.setKeepAliveInterval(60);
           // 設定回撥
           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 說明：

- connectionLost(Throwable cause): 連線丟失時被呼叫
- messageArrived(String topic, MqttMessage message): 接收到訊息時被呼叫
- deliveryComplete(IMqttDeliveryToken token): 訊息傳送完成時被呼叫

## 測試

接下來執行 `SubscribeSample`，訂閱 `mqtt/test` 主題。 然後執行 `PublishSample`，釋出訊息到 `mqtt/test` 主題。 我們將會看到釋出端成功釋出訊息，同時訂閱端接收到訊息。

[![](http://img.blog.itpub.net/blog/2022/08/30/d8416cfd443bfb63.png?x-oss-process=style/bb)](https://www.emqx.com/zh/blog/how-to-use-mqtt-in-java)

至此，我們完成了在 Java 中使用 Paho Java Client 來作為 MQTT 客戶端連線到 [公共 MQTT 伺服器](https://www.emqx.com/zh/mqtt/public-mqtt5-broker)，並實現了測試客戶端與 MQTT 伺服器的連線、訊息釋出和訂閱。

完整程式碼請見：https://github.com/emqx/MQTT-Client-Examples/tree/master/mqtt-client-Java 。

>版權宣告： 本文為 EMQ 原創，轉載請註明出處。
>
>原文連結：[https://www.emqx.com/zh/blog/how-to-use-mqtt-in-java](https://www.emqx.com/zh/blog/how-to-use-mqtt-in-java)

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