1添加依赖

在项目中找到下图文件

打开文件

如下

plugins {
    alias(libs.plugins.android.application)
}

android {
    namespace = "com.example.mqtt_04"
    compileSdk = 34

    defaultConfig {
        applicationId = "com.example.mqtt_04"
        minSdk = 27
        targetSdk = 34
        versionCode = 1
        versionName = "1.0"

        testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
    }

    buildTypes {
        release {
            isMinifyEnabled = false
            proguardFiles(getDefaultProguardFile("proguard-android-optimize.txt"), "proguard-rules.pro")
        }
    }
    compileOptions {
        sourceCompatibility = JavaVersion.VERSION_1_8
        targetCompatibility = JavaVersion.VERSION_1_8
    }
}

dependencies {

    implementation(libs.appcompat)
    implementation(libs.material)
    implementation(libs.activity)
    implementation(libs.constraintlayout)
    testImplementation(libs.junit)
    androidTestImplementation(libs.ext.junit)
    androidTestImplementation(libs.espresso.core)
}

在dependencies节点中添加如下语句

implementation("org.eclipse.paho","org.eclipse.paho.client.mqttv3","1.2.5")

此文件中上方会显示是否同步

点击同步

依赖会自动下载

2添加网路权限

找到如下文件

在文件中在

<application

节点的上方添加权限选中的是要添加的权限

 <uses-permission android:name="android.permission.INTERNET" />
    <uses-permission android:name="android.permission.WAKE_LOCK" />
    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

权限添加完成

3创建MQTT类

在类中添加 连接 订阅  发布 和 关闭 方法

package com.example.mqtt_04;

import android.os.Message;

import org.eclipse.paho.client.mqttv3.IMqttDeliveryToken;
import org.eclipse.paho.client.mqttv3.MqttCallback;
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;

import java.nio.charset.StandardCharsets;

public class MQTT_xie_yi {
    private static MqttClient client;
    private static MqttConnectOptions options;
    //建立连接
    public static void Mqtt_init() throws MqttException {
        //创建客户端对象
        client=new MqttClient("tcp://(写入自己的IP):(写入IP的端口号)","app_MQTT_04",new MemoryPersistence());
        //创建连接
        options=new MqttConnectOptions();
        options.setCleanSession(true);
        //可以设置如用户名  密码 等等

        //将配置给客户端
        client.connect(options);
    }
    //订阅
    public static String  SubClient() throws MqttException {
        final String[] a1 = {""};
        client.setCallback(new MqttCallback() {
            @Override
            public void connectionLost(Throwable cause) {
                //连接断开是
            }

            @Override
            public void messageArrived(String topic, MqttMessage message) throws Exception {
                //接收到信息
                a1[0] =new String(message.getPayload());
            }

            @Override
            public void deliveryComplete(IMqttDeliveryToken token) {
                //接收信息完成后
            }
        });
        //开始订阅
        client.subscribe("wd");
        //返回数据
        return a1[0];
    }
    //发布
    public static void PubClient(String a2) throws MqttException {
        //创建消息对象
        MqttMessage mqttMessage=new MqttMessage(("信息为:"+a2).getBytes(StandardCharsets.UTF_8));
        //设置消息等级
        mqttMessage.setQos(0);
        //发布消息
        client.publish("wd",mqttMessage);
    }
    //关闭客户端//会将此线程关闭在没有重建一条线程去运行会退出页面
    public static void guan_bi() throws MqttException {
        //断开连接
        client.disconnect();
        //关闭客户端
        client.close();
    }
}

主函数中调用即可

要将IP改为自己的IP