1.Room框架

Room是Android Jetpack组件库中的一部分，它是一个SQLite数据库的抽象层，提供了更简单的API和更好的性能，适合于中大型应用程序。

2.Room的使用 

使用Room和之前使用SQLite搭建数据库的过程类似，但是更加简单了。

1）构建实体类

举个例子，我们要将书本信息存储到数据库中，那么可以先创建一个书本信息的新类，用于封装书本信息

在该实体类中，用注解表明实体：@Entity， 和对应表中的主键：@PrimaryKey

package com.example.study06.entity;


import androidx.room.Entity;
import androidx.room.PrimaryKey;

@Entity
public class BookInfo {
    @PrimaryKey(autoGenerate = true)
    private int id;

    private String name;
    private String isbn;
    private String publisher;

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getIsbn() {
        return isbn;
    }

    public String getPublisher() {
        return publisher;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setIsbn(String isbn) {
        this.isbn = isbn;
    }

    public void setPublisher(String publisher) {
        this.publisher = publisher;
    }

    @Override
    public String toString() {
        return "BookInfo{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", isbn='" + isbn + '\'' +
                ", publisher='" + publisher + '\'' +
                '}';
    }
}

2)创建Dao接口：data access operation

这个接口不需要实现具体的增删改查操作，只需要用注解标记好对应的操作。

 

package com.example.study06.Dao;


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

import com.example.study06.entity.BookInfo;

import java.util.List;

@Dao
public interface BookDao {
    @Insert
    void insert(BookInfo... book);

    @Delete
    void delete(BookInfo... book);

    @Query("delete from BookInfo")
    void deleteAll();

    @Update
    void update(BookInfo... book);

    @Query("select * from BookInfo")
    List<BookInfo> queryAll();

    @Query("select * from BookInfo where name=:name order by id desc limit 1")
    BookInfo queryByName(String name);

}

Room会自动生成一个实现该接口的类，这个类可以完成增删查改的操作：

3）创建数据库的抽象类

也是类似的，用注解写清楚该数据库涉及的实体类。

package com.example.study06.database;

import androidx.room.Database;
import androidx.room.RoomDatabase;

import com.example.study06.Dao.BookDao;
import com.example.study06.entity.BookInfo;

@Database(entities = {BookInfo.class}, version = 1, exportSchema = true)
public abstract class BookDataBase extends RoomDatabase {
    public abstract BookDao bookDao();
}

4）在Myapplication的OnCreate函数中创建数据库

这样数据库的实例就是全局变量

package com.example.study06;

import android.app.Application;

import androidx.room.Room;

import com.example.study06.database.BookDataBase;

import java.util.HashMap;

public class MyApplication extends Application {

    public static MyApplication myapp;
    public HashMap<String, String> infoMap = new HashMap<>();

    public BookDataBase bookDataBase;
    public static MyApplication getInstance(){
        return myapp;
    }

    @Override
    public void onCreate() {
        super.onCreate();
        myapp = this;
        bookDataBase = Room.databaseBuilder(this,BookDataBase.class,"book")
                .addMigrations()
                .allowMainThreadQueries()
                .build();
    }

    public BookDataBase getBookDataBase() {
        return bookDataBase;
    }
}

5）使用数据库

对BookInfo表进行增删查改

package com.example.study06;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

import com.example.study06.Dao.BookDao;
import com.example.study06.entity.BookInfo;

import java.util.List;

public class RoomWriteActivity extends AppCompatActivity implements View.OnClickListener {

    private EditText et_name;
    private EditText et_isbn;
    private EditText et_publisher;
    private BookDao bookDao;

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

        et_name = findViewById(R.id.et_name);
        et_isbn = findViewById(R.id.et_isbn);
        et_publisher = findViewById(R.id.et_publisher);

        findViewById(R.id.bt_save).setOnClickListener(this);
        findViewById(R.id.bt_delete).setOnClickListener(this);
        findViewById(R.id.bt_update).setOnClickListener(this);
        findViewById(R.id.bt_query).setOnClickListener(this);

        bookDao = MyApplication.getInstance().getBookDataBase().bookDao();

    }

    @Override
    public void onClick(View view) {
        String name = et_name.getText().toString();
        String isbn = et_isbn.getText().toString();
        String publisher = et_publisher.getText().toString();

        switch (view.getId()){
            case R.id.bt_save:
                BookInfo b1 = new BookInfo();
                b1.setIsbn(isbn);
                b1.setName(name);
                b1.setPublisher(publisher);
                bookDao.insert(b1);
                break;
            case R.id.bt_query:
                List<BookInfo> books = bookDao.queryAll();
                for(BookInfo b : books){
                    Log.d("test ", b.toString());
                }
                break;
            case R.id.bt_delete:

                BookInfo b2 = bookDao.queryByName(name);
                bookDao.delete(b2);
                break;
            case R.id.bt_update:
                BookInfo b3 = new BookInfo();
                BookInfo b4 = bookDao.queryByName(name);
                b3.setId(b4.getId());
                b3.setIsbn(isbn);
                b3.setName(name);
                b3.setPublisher(publisher);
                bookDao.update(b3);

                break;
        }
    }
}