46. Room数据库的使用

引用：

implementation "androidx.room:room-runtime:2.0.0-beta01"
annotationProcessor "androidx.room:room-compiler:2.0.0-beta01"

一.定义表(@Entity)

定义一张PersonBean表

1.每张表必须有至少一个字段使用@PrimaryKey标识（autoGenerate=true 自增长）

2.表中每一列使用@ColumnInfo标识，并且推荐显示使用name参数定义好表的列名

3.如果你的列名使用private修复，那么必须提供set get方法

4.@Ignore，表示一个属性不加入生成表的字段，只是临时使用

@Entity
public class PersonBean {

    @PrimaryKey(autoGenerate = true)
    private int id;
    @ColumnInfo(name = "version")
    private String version;
    @ColumnInfo(name = "name")
    private String name;
    @ColumnInfo(name = "myadress")
    private String address;

    public int getId() {
        return id;
    }

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

    public String getVersion() {
        return version;
    }

    public void setVersion(String version) {
        this.version = version;
    }

    public String getName() {
        return name;
    }

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

    public String getAddress() {
        return address;
    }

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

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

二.定义数据库访问对象（@Dao）

全部通过注解定义，Query方法需要指定sql语句

@Dao
public interface PersonInfoDao {

    @Insert
    void insert(PersonBean personBean);

    @Delete
    void delete(PersonBean personBean);

    @Update
    void update(PersonBean personBean);

    @Query(value = "select * from PersonBean where name like :name")
    PersonBean findByName(String name);

    @Query(value = "select * from PersonBean")
    List<PersonBean> findAll();
}

三.定义数据库

定义一个抽象类继承RoomDatabase，提供dao获取接口

1.entities指定包含的数据表

2.version指定表的版本，和版本升级有关

@Database(entities = {PersonBean.class, version = 1, exportSchema = false)
public abstract class PersonInfoDb extends RoomDatabase {
    public abstract PersonInfoDao getPersonDao();
}

四.Build一下后通过apt生成辅助类

五.使用

PersonInfoDb personInfoDb = Room.databaseBuilder(getApplicationContext(), PersonInfoDb.class, "renzhenming").build();
            PersonInfoDao personDao = personInfoDb.getPersonDao();
            for (int i = 0; i < 100; i++) {
                PersonBean p = new PersonBean();
                p.setAddress("上海陆家嘴" + i + "号");
                p.setName("张三" + i);
                p.setVersion("" + i);

                personDao.insert(p);
            }
            List<PersonBean> p = personDao.findAll();
            System.out.println("renzhenming: " + p);        

六.定义一个表作为另一个表的外键

指定teacher中teacher_id为PersonBean id的外键，onDelete = CASCADE,onUpdate = CASCADE标识当PersonBean删除后者更新后，Teacher中对应id的数据也同步进行删除和更新

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

Entity(foreignKeys = @ForeignKey(entity = PersonBean.class,parentColumns = "id",
        childColumns = "teacher_id",onDelete = CASCADE,onUpdate = CASCADE))
public class Teacher {
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "teacher_id")
}

七.嵌套类

Teacher中嵌套了Father，通过prefix = "father"指定Father表中列的前缀，防止重复

@Entity
public class Teacher {
    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "teacher_id")
    private int id;
    @ColumnInfo(name = "teacher_name")
    private String name;
    @ColumnInfo(name = "teacher_age")
    private int age;
    @Embedded(prefix = "father")
    private Father father;
}

@Entity
public class Father {

    @PrimaryKey(autoGenerate = true)
    @ColumnInfo(name = "id")
    private int id;
    @ColumnInfo(name = "name")
    private String name;
    @ColumnInfo(name = "age")
    private int age;
}

八.数据库升级addMigrations(Migration)

MyDatabase instance = Room.databaseBuilder(context.getApplicationContext(), MyDatabase.class, "renzm_upgrade")
     //可以强制在主线程运行数据库操作
     .allowMainThreadQueries()
     //强制升级
     //.fallbackToDestructiveMigration()
     .addMigrations(MIGRATION_1_2)
     .build();

    //进行数据库升级 version 1 to version 2
static final Migration MIGRATION_1_2 = new Migration(1, 2) {
     @Override
     public void migrate(@NonNull SupportSQLiteDatabase database) {
         //在这里用sql脚本完成数据变化
         database.execSQL("alter table myteacher add column flag integer not null default 1");
     }
};