Android--Jetpack--数据库Room详解一

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一，定义

Room也是一个ORM框架，它在SQLite上提供了一个抽象层，屏蔽了部分底层的细节，使用对象对数据库进行操作，进行CRUD就像对象调用方法一样的简单。

二，角色介绍

谷歌官方给出了一张图片，可以更加直观的了解Room的组成部分：

?从上图可以看出，Room主要由三部分组成：

1，Room Database 数据库：底层连接的主要接入点，创建数据库就靠它了

2，Data Access Objects DAO：在DAO中会有一系列对数据库进行CRUD的方法声明

3，Entity 实体类：是对象与数据表的对应表现，设计实体类，并最后转化为对应的数据表

可以这么理解，我们通过实体类的中Getter和Setter方法对数据进行变更操作，然后Room Database使用DAO中的方法，对表中的数据进行对应的操作。从而屏蔽了繁琐的数据库原生操作。当然这其中使用了注解进行功能的标注。

三，基本使用

1，在app的build.gradle里面添加依赖：

def room_version = "2.2.6"
implementation "androidx.room:room-runtime:$room_version"
annotationProcessor "androidx.room:room-compiler:$room_version" // use kapt for Kotlin
// optional - RxJava support for Room
implementation "androidx.room:room-rxjava2:$room_version"
// optional - Guava support for Room, including Optional and ListenableFuture
implementation "androidx.room:room-guava:$room_version"
// Test helpers
testImplementation "androidx.room:room-testing:$room_version"

2，创建表Entity：

@Entity
public class YuanZhen {

    @PrimaryKey(autoGenerate = true)
    private int id;

    @ColumnInfo(name ="name")
    private String name;

    @ColumnInfo(name ="age")
    private int age;

    @ColumnInfo(name ="address")
    private String address;

    @Ignore
    private String sex;

    public YuanZhen(String name, int age, String address) {
        this.name = name;
        this.age = age;
        this.address = address;
    }

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

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

    public void setAge(int age) {
        this.age = age;
    }

    public void setAddress(String address) {
        this.address = address;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String getAddress() {
        return address;
    }

    @Override
    public String toString() {
        return "YuanZhen{" +
            "id=" + id +
            ", name='" + name + '\'' +
            ", age=" + age +
            ", address='" + address + '\'' +
            ", sex='" + sex + '\'' +
            '}';
    }
}

上面其中几个注解的含义：

①，@Entity?表示数据库中的表

②，@PrimaryKey?表示主键，autoGenerate?表示自增

③，@ColumnInfo?表示字段，name?表示字段名称

④，@Ignore 表示一个属性不加入生成表的字段，只是临时使用

3，创建Dao:

@Dao
public interface YuanZhenDao {
    
    @Insert
    void insert(YuanZhen... yuanzhens);
    
    @Delete
    void delete(YuanZhen yuanZhen);
    
    @Update
    void update(YuanZhen yuanZhen);
    
    @Query("select * from YuanZhen")
    List<YuanZhen> getAll();
}

这里面包含了增上改查。

其中几个注解的含义：

①，@Dao?表示访问?DB?的方法，需要声明为接口或抽象类，编译阶段将生成 _Impl 实现类，此处则将生成 YuanZhenDao_Impl.java 文件

②,@Insert、@Delete、@Update 、?@Query?分别表示数据库的增删改查方法

4，创建Room Database 数据库：

@Database(entities = {YuanZhen.class},version = 1,exportSchema = false)
public abstract class MyDatabase extends RoomDatabase {

    public abstract YuanZhenDao yuanZhenDao();
    
}

其中几个注解的含义：

①，@Database：表示数据库的定义

②，entities： 表示数据库中包含的表

③，version：表示数据库版本号

5，使用：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {
                MyDatabase myDatabase = Room.databaseBuilder(getApplicationContext(),MyDatabase.class,"YuanZhenDb").build();
                YuanZhenDao yuanZhenDao = myDatabase.yuanZhenDao();
                yuanZhenDao.insert(new YuanZhen("yz",20,"淄博市"));
                yuanZhenDao.insert(new YuanZhen("yz1",22,"淄博市"));

                List<YuanZhen> all = yuanZhenDao.getAll();
                System.out.println("输出："+all.toString());
            }
        }).start();
    }
}

输出结果：

I/System.out: 输出：[YuanZhen{id=1, name='yz', age=20, address='淄博市', sex='null'}, YuanZhen{id=2, name='yz1', age=22, address='淄博市', sex='null'}, YuanZhen{id=3, name='yz', age=20, address='淄博市', sex='null'}

这就是基本使用。

四，条件查询

如果我们想查询某一条数据的话，需要在dao里面增加条件查询：

@Dao
public interface YuanZhenDao {

    @Insert
    void insert(YuanZhen... yuanzhens);

    @Delete
    void delete(YuanZhen yuanZhen);

    @Update
    void update(YuanZhen yuanZhen);

    @Query("select * from YuanZhen")
    List<YuanZhen> getAll();

    @Query("select * from YuanZhen where name like :name")
    YuanZhen getByName(String name);

    @Query("select * from YuanZhen where age in(:ages)")
    List<YuanZhen> getByAges(int[] ages);
}

使用：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {
                MyDatabase myDatabase = Room.databaseBuilder(getApplicationContext(),MyDatabase.class,"YuanZhenDb").build();
                YuanZhenDao yuanZhenDao = myDatabase.yuanZhenDao();
                yuanZhenDao.insert(new YuanZhen("yz",20,"淄博市"));
                yuanZhenDao.insert(new YuanZhen("yz1",22,"淄博市"));

            

                YuanZhen yz = yuanZhenDao.getByName("yz");
                System.out.println("输出ByName："+yz.toString());
                int[] ages ={20,22};
                List<YuanZhen> byAges = yuanZhenDao.getByAges(ages);
                System.out.println("输出ByAges："+byAges.toString());

            }
        }).start();
    }
}

输出：

I/System.out: 输出ByName：YuanZhen{id=1, name='yz', age=20, address='淄博市', sex='null'}
I/System.out: 输出ByAges：[YuanZhen{id=1, name='yz', age=20, address='淄博市', sex='null'}, YuanZhen{id=2, name='yz1', age=22, address='淄博市', sex='null'}

如果只想查姓名和地址，那么可以新建一个类：

public class YuanZhenNew {

    @ColumnInfo(name = "name")
    public String name;

    @ColumnInfo(name = "address")
    public String address;

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

    public void setAddress(String address) {
        this.address = address;
    }

    public String getName() {
        return name;
    }

    public String getAddress() {
        return address;
    }

    public YuanZhenNew(String name, String address) {
        this.name = name;
        this.address = address;
    }

    @Override
    public String toString() {
        return "YuanZhenNew{" +
                "name='" + name + '\'' +
                ", address='" + address + '\'' +
                '}';
    }
}

在dao中查询：

@Dao
public interface YuanZhenDao {

    @Insert
    void insert(YuanZhen... yuanzhens);

    @Delete
    void delete(YuanZhen yuanZhen);

    @Update
    void update(YuanZhen yuanZhen);

    @Query("select * from YuanZhen")
    List<YuanZhen> getAll();

    @Query("select * from YuanZhen where name like :name")
    YuanZhen getByName(String name);

    @Query("select * from YuanZhen where age in(:ages)")
    List<YuanZhen> getByAges(int[] ages);

    @Query("select name,address from YuanZhen ")
    public List<YuanZhenNew> getNew();
}

使用：

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        new Thread(new Runnable() {
            @Override
            public void run() {
                MyDatabase myDatabase = Room.databaseBuilder(getApplicationContext(),MyDatabase.class,"YuanZhenDb").build();
                YuanZhenDao yuanZhenDao = myDatabase.yuanZhenDao();
                yuanZhenDao.insert(new YuanZhen("yz",20,"淄博市"));
                yuanZhenDao.insert(new YuanZhen("yz1",22,"淄博市"));

                
                List<YuanZhenNew> aNew = yuanZhenDao.getNew();
                System.out.println("输出aNew："+aNew.toString());

            }
        }).start();
    }
}

输出：

I/System.out: 输出aNew：[YuanZhenNew{name='yz', address='淄博市'}, YuanZhenNew{name='yz1', address='淄博市'}]

更多高级用法下章继续讲解