Android 连接Mysql数据库步骤（新手步骤）

1.新建项目project

2.运行，显示helloworld

3.复制mysql-connector-java-5.1.48.jar，到libs文件夹

4. Add as library

5. 搭建简单的xml显示框架，定义框架中的VIEW

public class MainActivity extends AppCompatActivity {
    private Button Btsearch;
    private TextView Texttime;
    private TextView Textdisplay;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Btsearch = findViewById(R.id.search);
        Texttime = findViewById(R.id.time);
        Textdisplay = findViewById(R.id.text_display);
        Btsearch.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                
            }
        });
    }
}

<TextView
    android:layout_width="match_parent"
    android:layout_height="50dp"
    android:text="数据库操作DEMO"
    android:textColor="@color/colorPrimaryDark"
    android:textSize="30dp"
    android:gravity="center"/>
<TextView
    android:id="@+id/time"
    android:layout_width="match_parent"
    android:layout_height="30dp"
    android:text="2020-05-21"
    android:textColor="@color/colorAccent"
    android:textSize="13dp"
    android:gravity="center"/>

<Button
    android:id="@+id/search"
    android:layout_width="120dp"
    android:layout_height="100dp"
    android:text="查询"

    />
<TextView
    android:id="@+id/text_display"
    android:layout_width="match_parent"
    android:layout_height="300dp"
    android:background="@color/colorPrimaryDark"
    android:text="Hello World!"
    android:textColor="#BEE8B6"
    android:textSize="20dp"
    android:layout_marginTop="20dp"
    android:paddingTop="20dp"
    />

6.定义Button的setOnClickListener 事件（Toast）

public void onClick(View v) {
    Toast toast = Toast.makeText(getApplicationContext(), "查询按钮被点击", Toast.LENGTH_SHORT);
    toast.show();
}

7.连接数据库

7.1寻找内存（需要try）ClassNotFoundException

Class.forName("com.mysql.jdbc.Driver");

7.2连接数据库（需要try）SQLException

java.sql.Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","****","123456");

7.3连接失败，异常Caused by: java.net.SocketException: Permission denied

这是一个经典错误， Socket不能对外连接，错误不会被报出，调试的时候，能看到Exception, 一般是抛出 java.net.socketexception permission denied这个异常。
只要你的程序版想联网，就会抛出这个异常，最终联网失败。
原因是： 需要访权问到网络必须要有权限，
在AndroidManifest.xml中，需要进行如下配置：

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

7.4连接失败，还是异常Caused by: android.os.NetworkOnMainThreadException

google不再允许网络请求（HTTP、Socket）等相关操作直接在Main Thread类中,所以和network有关的操作一般比较耗时,把这部分操作放到一个子线程里,然后用Handler消息机制与主线程通信.

7.5.给数据库操作加一个线程（连接数据库成功）

new Thread(new Runnable() {
    @Override
    public void run() {
        try {
            Class.forName("com.mysql.jdbc.Driver");
            System.out.println("ClassforName成功");
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
            System.out.println("ClassforName失败");
        }
        try {
            java.sql.Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
            cn.close();
            System.out.println("Connection连接数据库成功");
        } catch (SQLException e) {
            e.printStackTrace();
            System.out.println("Connection连接数据库失败");
        }
    }
}).start();

8.操作select数据库查找功能

8.1.建立一个数据操作对象

Statement st =cn.createStatement();

8.2.写查询SQL语句，执行查询SQL语句executeQuery()

ResultSet 是java中执行select后，返回的结果集类。
rs 就是结果集的变量。

这是利用来Java JDBC操作数据库源的写法。
st 实际是 Statement 对象
sql 是SQL语言，一百种用于操作数据库的语度言
st.executeQuery(sql);是执行这条知sql语言(例如: 从数据库中读取一些道信息)
ResultSet rs=st.executeQuery(sql);
ResultSet rs 存放的是从数据库中，返回来的数据结果。

String sql = "select B_Name from book";
ResultSet rs=st.executeQuery(sql);

8.3.得到查询SQL结果

while (rs.next()){
    String mybook=rs.getString("B_Name");
    System.out.println(mybook);
}

8.4.建立handler （不能建立在Thread里面）

如果handler建立在线程里面，会报错
Can’t create handler inside thread that has not called Looper.prepare()

private Handler myhandler =new Handler(){
    public void handleMessage(Message msg){
        if(msg.what!=0){
            Textdisplay.setText(text);
        }
    }
};

8.5.通过handler返回结果显示在xml里

Integer i =0;
while (rs.next()){
    String mybook=rs.getString("B_Name");
    System.out.println(mybook);
    i++;
    if(i==1){
        text=mybook;
    }else {
        text=text+mybook+'\n';
    }
    myhandler.sendEmptyMessage(1);
}

9.操作insert数据库插入功能

9.1.Xml建立相应的editText

9.2.editText 定义控件，提取内容

id.addTextChangedListener(new TextWatcher() {
    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    @Override
    public void afterTextChanged(Editable s) {
	id=s.toString();
    }
});

9.3.插入按钮的监听事件

注意异常：
com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column ‘???’ in ‘field list’
可以用
System.out.print(sql);先将你的语句输出，这样就很容易发现有没有错误。

Btinsert.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Toast.makeText(getApplicationContext(),"插入按钮点击",Toast.LENGTH_SHORT).show();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try{
                    Class.forName("com.mysql.jdbc.Driver");
                    Connection cn =DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                    System.out.println("插入数据库Connection连接数据库成功");
                    Statement st = cn.createStatement();
                    String sql ="insert into book values("+id+",'"+name+"','小明','北京出版社','2020-05-21')";
                    System.out.println(sql);
                    st.execute(sql);
                    System.out.println("插入数据库成功");
                    text="插入"+name+"数据库成功";
                    myhandler.sendEmptyMessage(2);
                } catch (ClassNotFoundException | SQLException e) {
                    System.out.println("插入数据库失败");
                    e.printStackTrace();
                }
            }
        }).start();
    }
});

10.更新update数据库更新功能

10.1.Xml建立相应的editText,Button

10.2.editText 定义控件，提取内容

10.3.更新按钮的监听事件

Btupdate.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Toast.makeText(getApplicationContext(),"更新按钮点击",Toast.LENGTH_SHORT).show();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Class.forName("com.mysql.jdbc.Driver");
                    Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                    System.out.println("更新数据库Connection连接数据库成功");
                    Statement st = cn.createStatement();
                    String sql = "update book set B_Author='"+pname+"' where B_Name='"+name+"'";
                    System.out.println(sql);
                    st.execute(sql);
                    System.out.println("更新成功");
                    text="更新"+name+"的作者为"+pname+"成功";
                    myhandler.sendEmptyMessage(3);
                } catch (ClassNotFoundException | SQLException e) {
                    System.out.println("更新失败");
                    e.printStackTrace();
                }

            }
        }).start();
    }
});

11.删除delete数据库更新功能

11.1.Xml建立相应的Button

11.2.editText 定义控件，提取内容

11.3.更新按钮的监听事件

Btupdate.setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        Toast.makeText(getApplicationContext(),"更新按钮点击",Toast.LENGTH_SHORT).show();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Class.forName("com.mysql.jdbc.Driver");
                    Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                    System.out.println("更新数据库Connection连接数据库成功");
                    Statement st = cn.createStatement();
                    String sql = "delete from book where set B_Author='"+name+"'";
                    System.out.println(sql);
                    st.execute(sql);
                    System.out.println("删除成功");
                    text="删除书名为"+name+"成功";
                    myhandler.sendEmptyMessage(3);
                } catch (ClassNotFoundException | SQLException e) {
                    System.out.println("删除失败");
                    e.printStackTrace();
                }

            }
        }).start();
    }
});

12.安卓连接数据库增删查更操作代码(简易全功能版)

12.1.MainActivity.java

package com.example.mysql_demo;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class MainActivity extends AppCompatActivity {
    private Button Btsearch,Btinsert,Btupdate,Btdelete;
    private EditText editid,editname,editpname;
    private TextView Texttime;
    private TextView Textdisplay;
    private String text,id,name,pname;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Btsearch = findViewById(R.id.search);
        Btinsert = findViewById(R.id.insert);
        Btupdate =findViewById(R.id.update);
        Btdelete = findViewById(R.id.delete);
        editid = findViewById(R.id.isbn);
        editname = findViewById(R.id.b_name);
        editpname = findViewById(R.id.p_name);
        Texttime = findViewById(R.id.time);
        Textdisplay = findViewById(R.id.text_display);
//textview滑动条
	Textdisplay.setMovementMethod(ScrollingMovementMethod.getInstance());
        //查询按钮点击事件
        Btsearch.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast toast = Toast.makeText(getApplicationContext(), "查询按钮被点击", Toast.LENGTH_SHORT);
                toast.show();
                text = "查询结果为：\n";
                //建一个子线程 数据库操作属于网络操作，不能用UI线程
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            //第一步  寻找这个驱动，JVM加入内存
                            Class.forName("com.mysql.jdbc.Driver");
                            System.out.println("ClassforName成功");
                            //第二步  通过connection数据库建立链接对象
                            java.sql.Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                            System.out.println("Connection连接数据库成功");
                            //第三步 通过cn链接对象 创建一个statement对象，操作对象
                            Statement st =cn.createStatement();
                            //第四步 写一个SQL语句
                            String sql = "select B_Name from book";
                            //第五步 执行SQL语句，返回结果集到rs对象
                            //statement类的executeQuery()方法来下达select指令以查询数据库
                            ResultSet rs=st.executeQuery(sql);
                            System.out.println(rs);
                            //第六步 返回结果集得到字符串，某一列
//                            Integer i =0;

                            while (rs.next()){
                                String mybook=rs.getString("B_Name");
                                System.out.println(mybook);
//                                i++;
//                                if(i==1){
//                                    text=mybook;
//                                }else {
                                    text=text+mybook+'\n';
//                                }
                                myhandler.sendEmptyMessage(1);
                            }
                          //class 异常
                        } catch (ClassNotFoundException e) {
                            e.printStackTrace();
                            System.out.println("ClassforName失败");
                            //Connection 异常
                        }  catch (SQLException e) {
                            e.printStackTrace();
                            System.out.println("Connection连接数据库失败");
                        }
                    }
                }).start();

            }
        });
        //插入按钮事件
        Btinsert.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(getApplicationContext(),"插入按钮点击",Toast.LENGTH_SHORT).show();
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try{
                            Class.forName("com.mysql.jdbc.Driver");
                            Connection cn =DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                            System.out.println("插入数据库Connection连接数据库成功");
                            Statement st = cn.createStatement();
                            String sql ="insert into book values("+id+",'"+name+"','小明','河南出版社','2020-05-21')";
                            System.out.println(sql);
                            st.execute(sql);
                            System.out.println("插入成功");
                            text="插入"+name+"数据库成功";
                            myhandler.sendEmptyMessage(2);
                        } catch (ClassNotFoundException | SQLException e) {
                            System.out.println("插入失败");
                            e.printStackTrace();
                        }
                    }
                }).start();
            }
        });
        //更新数据库
        Btupdate.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(getApplicationContext(),"更新按钮点击",Toast.LENGTH_SHORT).show();
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            Class.forName("com.mysql.jdbc.Driver");
                            Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                            System.out.println("更新数据库Connection连接数据库成功");
                            Statement st = cn.createStatement();
                            String sql = "update book set B_Author='"+pname+"' where B_Name='"+name+"'";
                            System.out.println(sql);
                            st.execute(sql);
                            System.out.println("更新成功");
                            text="更新"+name+"的作者为"+pname+"成功";
                            myhandler.sendEmptyMessage(3);
                        } catch (ClassNotFoundException | SQLException e) {
                            System.out.println("更新失败");
                            e.printStackTrace();
                        }

                    }
                }).start();
            }
        });
        //删除数据库
        Btdelete.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(getApplicationContext(),"删除按钮点击",Toast.LENGTH_SHORT).show();
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        try {
                            Class.forName("com.mysql.jdbc.Driver");
                            Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                            System.out.println("删除数据库Connection连接数据库成功");
                            Statement st = cn.createStatement();
                            String sql = "delete from book where B_Name='"+name+"'";
                            System.out.println(sql);
                            st.execute(sql);
                            System.out.println("删除成功");
                            text="删除书名为"+name+"成功";
                            myhandler.sendEmptyMessage(4);
                        } catch (ClassNotFoundException | SQLException e) {
                            System.out.println("删除失败");
                            e.printStackTrace();
                        }

                    }
                }).start();
            }
        });
        //id文本框输入事件
        editid.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {

                Textdisplay.setText("输入结束后的内容为 [" + s.toString()+" ] 即将显示在屏幕上");
                id=s.toString();
            }
        });
        //图书文本框输入事件
        editname.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                Textdisplay.setText("输入结束后的内容为 [" + s.toString()+" ] 即将显示在屏幕上");
                name = s.toString();
            }
        });
        //作者文本框输入事件
        editpname.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {
                Textdisplay.setText("输入结束后的内容为 [" + s.toString()+" ] 即将显示在屏幕上");
                pname = s.toString();
            }
        });
    }


    private Handler myhandler =new Handler(){
        public void handleMessage(Message msg){
            if(msg.what==1){
                Textdisplay.setText(text);
            }if (msg.what == 2){
                Textdisplay.setText(text);
            }if (msg.what == 3){
                Textdisplay.setText(text);
            }if(msg.what == 4){
                Textdisplay.setText(text);
            }
        }
    };
}

12.2.activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    android:padding="20dp"
    tools:context=".MainActivity">

    <TextView
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:text="数据库操作DEMO"
        android:textColor="@color/colorPrimaryDark"
        android:textSize="30dp"
        android:gravity="center"/>
    <TextView
        android:id="@+id/time"
        android:layout_width="match_parent"
        android:layout_height="30dp"
        android:text="2020-05-21"
        android:textColor="@color/colorAccent"
        android:textSize="13dp"
        android:gravity="center"/>
    <EditText
        android:id="@+id/isbn"
        android:layout_width="200dp"
        android:layout_height="40dp"
        android:hint="ISBM"/>
    <EditText
        android:id="@+id/b_name"
        android:layout_width="200dp"
        android:layout_height="40dp"
        android:hint="图书名字"/>
    <EditText
        android:id="@+id/p_name"
        android:layout_width="200dp"
        android:layout_height="40dp"
        android:hint="作者名字"/>
    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:orientation="horizontal">
    <Button
        android:id="@+id/search"
        android:layout_width="80dp"
        android:layout_height="match_parent"
        android:text="查询"
        />
    <Button
        android:id="@+id/insert"
        android:layout_width="80dp"
        android:layout_height="match_parent"
        android:text="插入"
        />
    <Button
        android:id="@+id/update"
        android:layout_width="80dp"
        android:layout_height="match_parent"
        android:text="更新"
        />
    <Button
        android:id="@+id/delete"
        android:layout_width="80dp"
        android:layout_height="match_parent"
        android:text="删除"
        />
    </LinearLayout>
    <TextView
        android:id="@+id/text_display"
        android:layout_width="match_parent"
        android:layout_height="300dp"
        android:background="@color/colorPrimaryDark"
        android:text="Hello World!"
        android:textColor="#BEE8B6"
        android:textSize="20dp"
        android:layout_marginTop="20dp"
        android:paddingTop="20dp"
android:scrollbars="vertical"
        />

</LinearLayout>

12.3.AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.mysql_demo">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
    <uses-permission android:name="android.permission.INTERNET" />
</manifest>

13 代码修改（精简）

13.1.修改OnClickListener为接口实现，重写OnClick

public class MainActivity extends AppCompatActivity implements View.OnClickListener

protected void onCreate(Bundle savedInstanceState) {
//第一种myclick实现 用 implements OnclickListener
//所有用this代替
Btsearch.setOnClickListener(this);
Btinsert.setOnClickListener(this);
Btupdate.setOnClickListener(this);
Btdelete.setOnClickListener(this);
//设置Tag标签为1，标识查询
//1--查询 2--插入 3--更新 4--删除
Btsearch.setTag(1);
Btinsert.setTag(2);
Btupdate.setTag(3);
Btdelete.setTag(4);
}



@Override
public void onClick(View v) {
    //由于多个按钮作用不太一样，所以先识别Tag标签来区分每个按钮,由于Tag为object对象，所以需要强制装换Integer
    //1--查询 2--插入 3--更新 4--删除
    Integer tagid = (Integer) v.getTag();
    if(tagid==1){
        Toast toast = Toast.makeText(getApplicationContext(), "查询按钮被点击", Toast.LENGTH_SHORT);
        toast.show();
        text = "查询结果为：\n";
        //建一个子线程 数据库操作属于网络操作，不能用UI线程
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    //第一步  寻找这个驱动，JVM加入内存
                    Class.forName("com.mysql.jdbc.Driver");
                    System.out.println("ClassforName成功");
                    //第二步  通过connection数据库建立链接对象
                    java.sql.Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                    System.out.println("Connection连接数据库成功");
                    //第三步 通过cn链接对象 创建一个statement对象，操作对象
                    Statement st =cn.createStatement();
                    //第四步 写一个SQL语句
                    String sql = "select B_Name from book";
                    //第五步 执行SQL语句，返回结果集到rs对象
                    //statement类的executeQuery()方法来下达select指令以查询数据库
                    ResultSet rs=st.executeQuery(sql);
                    System.out.println(rs);
                    //第六步 返回结果集得到字符串，某一列
                    while (rs.next()){
                        String mybook=rs.getString("B_Name");
                        System.out.println(mybook);
                        text=text+mybook+'\n';
                        myhandler.sendEmptyMessage(1);
                    }
                    //class 异常
                } catch (ClassNotFoundException e) {
                    e.printStackTrace();
                    System.out.println("ClassforName失败");
                    //Connection 异常
                }  catch (SQLException e) {
                    e.printStackTrace();
                    System.out.println("Connection连接数据库失败");
                }
            }
        }).start();

    }if(tagid==2){
        Toast.makeText(getApplicationContext(),"插入按钮点击",Toast.LENGTH_SHORT).show();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try{
                    Class.forName("com.mysql.jdbc.Driver");
                    Connection cn =DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                    System.out.println("插入数据库Connection连接数据库成功");
                    Statement st = cn.createStatement();
                    String sql ="insert into book values("+id+",'"+name+"','小明','河南出版社','2020-05-21')";
                    System.out.println(sql);
                    st.execute(sql);
                    System.out.println("插入成功");
                    text="插入"+name+"数据库成功";
                    myhandler.sendEmptyMessage(2);
                } catch (ClassNotFoundException | SQLException e) {
                    System.out.println("插入失败");
                    e.printStackTrace();
                }
            }
        }).start();
    }
    if(tagid==3){
        Toast.makeText(getApplicationContext(),"更新按钮点击",Toast.LENGTH_SHORT).show();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Class.forName("com.mysql.jdbc.Driver");
                    Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                    System.out.println("更新数据库Connection连接数据库成功");
                    Statement st = cn.createStatement();
                    String sql = "update book set B_Author='"+pname+"' where B_Name='"+name+"'";
                    System.out.println(sql);
                    st.execute(sql);
                    System.out.println("更新成功");
                    text="更新"+name+"的作者为"+pname+"成功";
                    myhandler.sendEmptyMessage(3);
                } catch (ClassNotFoundException | SQLException e) {
                    System.out.println("更新失败");
                    e.printStackTrace();
                }

            }
        }).start();
    }
    if(tagid==4){
        Toast.makeText(getApplicationContext(),"删除按钮点击",Toast.LENGTH_SHORT).show();
        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Class.forName("com.mysql.jdbc.Driver");
                    Connection cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
                    System.out.println("删除数据库Connection连接数据库成功");
                    Statement st = cn.createStatement();
                    String sql = "delete from book where B_Name='"+name+"'";
                    System.out.println(sql);
                    st.execute(sql);
                    System.out.println("删除成功");
                    text="删除书名为"+name+"成功";
                    myhandler.sendEmptyMessage(4);
                } catch (ClassNotFoundException | SQLException e) {
                    System.out.println("删除失败");
                    e.printStackTrace();
                }

            }
        }).start();
    }

}

13.2.修改后的代码（Onclick接口实现，afterTextchange接口实现，连接数据库方法实现，增删更查更新数据库方法实现）

package com.example.mysql_demo;

import androidx.appcompat.app.AppCompatActivity;

import android.nfc.Tag;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.text.Editable;
import android.text.TextWatcher;
import android.text.method.ScrollingMovementMethod;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class MainActivity extends AppCompatActivity implements View.OnClickListener,TextWatcher{
    private Button Btsearch,Btinsert,Btupdate,Btdelete;
    private EditText editid,editname,editpname;
    private TextView Texttime;
    private TextView Textdisplay;
    private String text,id,name,pname;
    java.sql.Connection cn= null;
    Statement st= null;
    ResultSet rs= null;



    @Override
     public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }

    @Override
    public void afterTextChanged(Editable s) {
        Textdisplay.setText("输入结束后的内容为 [" + s.toString()+" ] 即将显示在屏幕上");
        //依靠getEditableText()判断是哪个文本框
        if(s==editid.getEditableText()){
            id=s.toString();
        }
        if(s==editpname.getEditableText()){
            pname=s.toString();
        }
        if(s==editname.getEditableText()){
            name=s.toString();
        }
    }

    public void MysqlConnect(){

        try {
            //第一步  寻找这个驱动，JVM加入内存
            Class.forName("com.mysql.jdbc.Driver");
            System.out.println("ClassforName成功");
            //第二步  通过connection数据库建立链接对象
            cn = DriverManager.getConnection("jdbc:mysql://192.168.1.4/testdb","***","123456");
            System.out.println("Connection连接数据库成功");
            //第三步 通过cn链接对象 创建一个statement对象，操作对象
            st =cn.createStatement();
            //class 异常
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
            System.out.println("ClassforName失败");
            text="ClassforName失败";
//            myhandler.sendEmptyMessage(1);
            //Connection 异常
        }  catch (SQLException e) {
            e.printStackTrace();
            System.out.println("Connection连接数据库失败");
            text="Connection连接数据库失败";
//            myhandler.sendEmptyMessage(1);
        }

    }
    public void Mysqlupdate(String sql){
        if (sql.indexOf("select")==0){
            try {
                MysqlConnect();
                System.out.println("执行第111111111111111111111111111111111111111步");
                rs=st.executeQuery(sql);
                //class 异常
            } catch (SQLException e) {
                e.printStackTrace();
                    System.out.println("查询数据库失败");
//            text="Connection连接数据库失败";
//            myhandler.sendEmptyMessage(1);
            }
        }else{
            try {
                MysqlConnect();
                System.out.println("执行第222222222222222222222222222222222222222222步");
                st.execute(sql);
                //class 异常
            } catch (SQLException e) {
                e.printStackTrace();
                if(sql.indexOf("insert")==0){
                    System.out.println("插入数据库失败");
                }
                if(sql.indexOf("update")==0){
                    System.out.println("更新数据库失败");
                }
                if(sql.indexOf("delete")==0){
                    System.out.println("删除数据库失败");
                }
//            text="Connection连接数据库失败";
//            myhandler.sendEmptyMessage(1);
            }
        }
    }


    @Override
    public void onClick(View v) {
        //由于多个按钮作用不太一样，所以先识别Tag标签来区分每个按钮,由于Tag为object对象，所以需要强制装换Integer
        //1--查询 2--插入 3--更新 4--删除
        Integer tagid = (Integer) v.getTag();
        if(tagid==1){
            Toast toast = Toast.makeText(getApplicationContext(), "查询按钮被点击", Toast.LENGTH_SHORT);
            toast.show();
            text = "查询结果为：\n";
            //建一个子线程 数据库操作属于网络操作，不能用UI线程
            new Thread(new Runnable() {
                @Override
                public void run() {
                    String sql = "select B_Name from book";
                    Mysqlupdate(sql);
                    try {
                        //第六步 返回结果集得到字符串，某一列
                        while (rs.next()) {
                            String mybook = rs.getString("B_Name");
                            System.out.println(mybook);
                            text = text + mybook + '\n';
                            myhandler.sendEmptyMessage(1);
                        }
                    } catch (SQLException e) {
                        e.printStackTrace();
                    }
                }
            }).start();
        }
        if(tagid==2){
            Toast.makeText(getApplicationContext(),"插入按钮点击",Toast.LENGTH_SHORT).show();
            new Thread(new Runnable() {
                @Override
                public void run() {
                        String sql ="insert into book values("+id+",'"+name+"','小明','河南出版社','2020-05-21')";
                        Mysqlupdate(sql);
                        text="插入"+name+"数据库成功";
                        myhandler.sendEmptyMessage(2);
                }
            }).start();
        }
        if(tagid==3){
            Toast.makeText(getApplicationContext(),"更新按钮点击",Toast.LENGTH_SHORT).show();
            new Thread(new Runnable() {
                @Override
                public void run() {
                        String sql = "update book set B_Author='"+pname+"' where B_Name='"+name+"'";
                        Mysqlupdate(sql);
                        text="更新"+name+"的作者为"+pname+"成功";
                        myhandler.sendEmptyMessage(3);
                }
            }).start();
        }
        if(tagid==4){
            Toast.makeText(getApplicationContext(),"删除按钮点击",Toast.LENGTH_SHORT).show();
            new Thread(new Runnable() {
                @Override
                public void run() {
                        String sql = "delete from book where B_Name='"+name+"'";
                        Mysqlupdate(sql);
                        text="删除书名为"+name+"成功";
                        myhandler.sendEmptyMessage(4);
                }
            }).start();
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Btsearch = findViewById(R.id.search);
        Btinsert = findViewById(R.id.insert);
        Btupdate =findViewById(R.id.update);
        Btdelete = findViewById(R.id.delete);
        editid = findViewById(R.id.isbn);
        editname = findViewById(R.id.b_name);
        editpname = findViewById(R.id.p_name);
        Texttime = findViewById(R.id.time);
        Textdisplay = findViewById(R.id.text_display);
        //textview滑动条
        Textdisplay.setMovementMethod(ScrollingMovementMethod.getInstance());
        //第一种myclick实现 用 implements OnclickListener
        //所有用this代替
        Btsearch.setOnClickListener(this);
        Btinsert.setOnClickListener(this);
        Btupdate.setOnClickListener(this);
        Btdelete.setOnClickListener(this);
        //设置Tag标签
        //1--查询 2--插入 3--更新 4--删除
        Btsearch.setTag(1);
        Btinsert.setTag(2);
        Btupdate.setTag(3);
        Btdelete.setTag(4);
        //所有文本 输入框用接口implements TextWatcher实现
        editid.addTextChangedListener(this);
        editpname.addTextChangedListener(this);
        editname.addTextChangedListener(this);


    }

    private Handler myhandler =new Handler(){
        public void handleMessage(Message msg){
            if(msg.what!=0){
                Textdisplay.setText(text);
            }
        }
    };

}