Andriod --- JetPack ：Room 增删改查

Andriod — JetPack ：初识 JetPack

Andriod — JetPack ：LifeCycle 的诞生

Andriod — JetPack ：ViewModel 的诞生

Andriod — JetPack ：BaseObservable 与 ObservableField 双向绑定

Andriod — JetPack ：DataBinding + LiveData +ViewModel 简单实例

Andriod — JetPack ：Room 增删改查

Andriod — JetPack ：Room + ViewModel + LiveData 增删改查实例

Andriod — JetPack ：LiveData setValue 和 postValue 的区别

一、前言

Android 采用 SQlit 作为数据库存储，开源社区常用的 ORM (Object Relational Mapping) 库有 ORMLite、GreenDao 等。Room 和其他库一样，也是在 SQLite 上提供了一成封装。

这个 ORM 我们解释一下，因为我们用 Java 写的程序都是面向对象编程的（oop），而数据库是面向关系的，表与表之间的关系等等，那么我们怎么把这两者之间串联起来的就有了ORM，对象-关系-映射，这个东西。

而 Room 是由谷歌提出的，可以说是一统天下了，也是像 ORMLite、GreenDao的 ORM 库。

二、Romm 的应用

1. Room 的重要概念 Entity：实体类，对应的是数据库的一张表的结构，使用注解 @Entity 标记。 Dao：包含访问一系列访问数据库的方法，使用注解 @Dao 标记。 Database：数据库持有者，作为与应用持久化相关数据的低层连接的主要接入点。使用注解 @Database 标记，另外需要满足一下条件：定义的类必须是一个继承于 RoomDatabase 的抽象类，在注解中需要定义域数据库相关联的实体类列表。包好一个没有参数的抽象方法并且返回一个 Dao 对象。

2. 三者之间的关系

三、增删改查代码实例

build.gradle中添加 Room 依赖

// Room
implementation "androidx.room:room-runtime:2.2.5"
annotationProcessor "androidx.room:room-compiler:2.2.5"

activity_main.xml

item.xml

首先创建实体类Student.java

package com.example.room;

import androidx.room.ColumnInfo;
import androidx.room.Entity;
import androidx.room.Ignore;
import androidx.room.PrimaryKey;

@Entity(tableName = "student")
public class Student {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "id", typeAffinity = ColumnInfo.INTEGER)
    public int id;

    @ColumnInfo(name = "name", typeAffinity = ColumnInfo.TEXT)
    public String name;

    @ColumnInfo(name = "age", typeAffinity = ColumnInfo.INTEGER)
    public int age;

    public Student(int id, String name, int age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }
    @Ignore
    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }
    @Ignore
    public Student(int id) {
        this.id = id;
    }
}

StudentDao.java

package com.example.room;

import androidx.room.Dao;
import androidx.room.Delete;
import androidx.room.Insert;
import androidx.room.Query;
import androidx.room.Update;

import java.util.List;

@Dao
public interface StudentDao {
    @Insert
    void insertStudent(Student... students); // 可以传入多个 Student

    @Delete
    void deleteStudent(Student... students);

    @Update
    void updateStudent(Student... students);

    @Query("SELECT * FROM student")
    List getAllStudent();

    @Query("SELECT * FROM student WHERE id = :id")
    List getStudentById(Integer id);
}

MyDataBase.java

package com.example.room;

import android.content.Context;

import androidx.room.Database;
import androidx.room.Room;
import androidx.room.RoomDatabase;
// 一定是抽象类
@Database(entities = {Student.class}, version = 1, exportSchema = false)
public abstract class MyDataBase extends RoomDatabase {

    private static final String DATABASE_NAME = "my_db.db";
    private static MyDataBase mInstance;

    // 构建 database 单例
    public static synchronized MyDataBase getInstance(Context context) {
        if(mInstance == null) {
            mInstance = Room.databaseBuilder(context.getApplicationContext(), MyDataBase.class, DATABASE_NAME)
                    //.allowMainThreadQueries() // 允许在主线程操作数据库
                    .build();
        }
        return mInstance;
    }
    public abstract StudentDao getStudentDao();
}

StudentRecycleViewAdapter.java

package com.example.room;

import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;

import androidx.annotation.NonNull;
import androidx.recyclerview.widget.RecyclerView;

import java.util.List;

public class StudentRecycleViewAdapter extends RecyclerView.Adapter {

    List students;

    public void setStudents(List students) {
        this.students = students;
    }

    public StudentRecycleViewAdapter(List students) {
        this.students = students;
    }

    @NonNull
    @Override
    public MyViewHolder onCreateViewHolder(@NonNull ViewGroup parent, int viewType) {
        View root = LayoutInflater.from(parent.getContext()).inflate(R.layout.item,parent, false);
        return new MyViewHolder(root);
    }

    @Override
    public void onBindViewHolder(@NonNull MyViewHolder holder, int position) {
        Student student = students.get(position);

        holder.tvId.setText(String.valueOf(student.id));
        holder.name.setText(student.name);
        holder.age.setText(String.valueOf(student.age));
    }

    @Override
    public int getItemCount() {
        return students == null ? 0 :students.size();
    }

    public class MyViewHolder extends RecyclerView.ViewHolder {
        private TextView tvId, name, age;

        public MyViewHolder(View view) {
            super(view);
            tvId = view.findViewById(R.id.tvId);
            name = view.findViewById(R.id.name);
            age = view.findViewById(R.id.age);
        }
    }
}

MainActivity.java

package com.example.room;

import androidx.appcompat.app.AppCompatActivity;
import androidx.recyclerview.widget.LinearLayoutManager;
import androidx.recyclerview.widget.RecyclerView;

import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {

    private StudentRecycleViewAdapter adapter;
    private StudentDao studentDao;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        List students = new ArrayList();
        RecyclerView recyclerView = findViewById(R.id.rv_01);

        adapter = new StudentRecycleViewAdapter(students);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setAdapter(adapter);

        MyDataBase myDataBase = MyDataBase.getInstance(this);
        studentDao = myDataBase.getStudentDao();

    }
    // Room 不允许在主线程操作数据库，如果非在主线程操作数据库，请修改 MyDataBase 文件


    // 增加
    public void mInsert(View view) {
        Student s1 = new Student("Jack", 20);
        Student s2 = new Student("Rose", 22);
        new  InsertStudentTask(studentDao).execute(s1,s2);
    }

    // 删除
    public void mDelete(View view) {
        Student s1 = new Student(1);
        new DeleteStudentTask(studentDao).execute(s1);
    }

    // 修改
    public void mUpdate(View view) {
        Student s1 = new Student(3, "Jason", 21);
        new UpdateStudentTask(studentDao).execute(s1);
    }

    // 查询
    public void mQuery(View view) {
        new GetAllStudentTask(studentDao).execute();
    }


    // 异步线程添加学生
    class InsertStudentTask extends AsyncTask {
        private StudentDao studentDao;

        public InsertStudentTask(StudentDao studentDao) {
            this.studentDao = studentDao;
        }

        @Override
        protected Void doInBackground(Student... students) {
            studentDao.insertStudent(students);
            return null;
        }
    }

    // 异步修改学生
    class UpdateStudentTask extends AsyncTask {
        private StudentDao studentDao;

        public UpdateStudentTask(StudentDao studentDao) {
            this.studentDao = studentDao;
        }

        @Override
        protected Void doInBackground(Student... students) {
            studentDao.updateStudent(students);
            return null;
        }
    }

    // 异步删除学生
    class DeleteStudentTask extends AsyncTask {
        private StudentDao studentDao;

        public DeleteStudentTask(StudentDao studentDao) {
            this.studentDao = studentDao;
        }

        @Override
        protected Void doInBackground(Student... students) {
            studentDao.deleteStudent(students);
            return null;
        }
    }

    // 异步获取学生信息
    class GetAllStudentTask extends AsyncTask {
        private StudentDao studentDao;

        public GetAllStudentTask(StudentDao studentDao) {
            this.studentDao = studentDao;
        }

        @Override
        protected Void doInBackground(Student... students) {
            List list = studentDao.getAllStudent();
            adapter.setStudents(list);
            return null;
        }

        // 更新主线程
        @Override
        protected void onPostExecute(Void aVoid) {
            super.onPostExecute(aVoid);
            adapter.notifyDataSetChanged();
        }
    }
}

效果图如下：

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