Jetpack学习-5-Room

Room是什么

持久性库在SQLite基础上提供抽象层，充分利用SQLite强大功能，更强的数据库访问机制。
Room是基于SQLite封装的一个框架。

简单使用

引入room，模块build.gradle增加配置

dependencies {
   ...
    implementation 'androidx.room:room-runtime:2.2.5'
    annotationProcessor 'androidx.room:room-compiler:2.2.5'
}

Entity

// @Entity 标识这个类用于建表，表名(默认不写，则用类名小写作为表名)
@Entity(tableName = "student")
public class Student {
    // 主键，是否自增长
    @PrimaryKey(autoGenerate = true)
    public int id;
    // 表中字段，表中字段名
    @ColumnInfo(name = "name")
    public String name;
    // 表中字段，默认使用下面字段名age
    @ColumnInfo
    public int age;

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

Dao

@Dao
public interface StudentDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    void insert(Student... students);

    @Delete
    void delete(Student student);

    @Update
    void update(Student student);

    @Query("SELECT * FROM student")
    List<Student> getAll();

    @Query("SELECT * FROM student WHERE name = :name")
    List<Student> findByName(String name);

    @Query("SELECT * FROM student WHERE id in (:ids)")
    List<Student> findByIds(int[] ids);
}

DataBase

@Database(entities = {Student.class},version = 1)
public abstract class DataBase extends RoomDatabase {
    public abstract StudentDao studentDao();
}

使用

public class RoomActivity extends AppCompatActivity {
    private static final String TAG = "Room-";
    private StudentDao dao;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_room);
        DataBase dataBase = Room.databaseBuilder(getApplicationContext(), DataBase.class,
                "room").allowMainThreadQueries().build();
        dao = dataBase.studentDao();
        String[] permissions = {Manifest.permission.WRITE_EXTERNAL_STORAGE};
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            requestPermissions(permissions, 1);
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
        if (requestCode == 1) {
            Log.d(TAG, "onRequestPermissionsResult: " + requestCode);
            for (int i = 0; i < grantResults.length; i++) {
                if (grantResults[i] != PackageManager.PERMISSION_GRANTED) {
                    Log.d(TAG, "onRequestPermissionsResult-需要开启存储权限");
                }
            }
        }
        super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    }

    /**
     * 在Activity中所有对数据库的操作(增、删、改、查)都不可以在主线程中进行,除非在数据库的Builder上调用了allowMainThreadQueries(),
     * 或者所有的操作都在子线程中完成,否则程序会崩溃报以下错误
     * Cannot access database on the main thread since it may potentially lock the UI for a long period of time.
     *
     * @param view
     */
    public void queryAll(View view) {
        List<Student> students = dao.getAll();
        Log.d(TAG, "queryAll: " + students);
    }

    public void insert(View view) {
        Student student = new Student();
        student.age = 18;
        student.name = "小明";
        dao.insert(student);
        Log.d(TAG, "insert:" + student.toString());
    }

    /**
     * 删除必须指定id，否则删除不成功(id不存在也是删除不成功的)
     *
     * @param view
     */
    public void delete(View view) {
        Student student = new Student();
        student.age = 18;
        student.name = "小明";
        student.id = 1;
        dao.delete(student);
        Log.d(TAG, "insert:" + student.toString());
    }

    /**
     * 更新必须指定id，否则不成功(id不存在也是更新不成功的)
     *
     * @param view
     */
    public void update(View view) {
        Student student = new Student();
        student.age = 18;
        student.name = "小明-1";
        student.id = 1;
        dao.update(student);
        Log.d(TAG, "insert:" + student.toString());
    }

    public void queryByName(View view) {
        Log.d(TAG, "queryByName: " + dao.findByName("小明"));
    }

    public void queryByIds(View view) {
        Log.d(TAG, "queryByName: " + dao.findByIds(new int[]{1, 2, 3}));
    }
}

当然这是基础用法，有关数据库升级、多表关联、查询部分字段等随后写。

数据库信息导出

允许表信息导出json文件。应该在版本控制中保存该文件，代表数据库历史，允许创建就版本数据库用于测试。配置如下build.gradle

android{
    defaultConfig {
        ···
        javaCompileOptions {
            annotationProcessorOptions {
                arguments = ["room.schemaLocation":
                                     "$projectDir/schemas".toString()]
            }
        }
    }
    // 用于测试
    sourceSets {
        androidTest.assets.srcDirs += files("$projectDir/schemas".toString())
    }
}

schemas路径：项目名/app/schemas

image.png

{
  "formatVersion": 1,
  "database": {
    "version": 1,
    "identityHash": "acca4b709e6c8b9b88d8328be36b9032",
    "entities": [
      {
        "tableName": "student",
        "createSql": "CREATE TABLE IF NOT EXISTS `${TABLE_NAME}` (`id` INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, `name` TEXT, `age` INTEGER NOT NULL)",
        "fields": [
          {
            "fieldPath": "id",
            "columnName": "id",
            "affinity": "INTEGER",
            "notNull": true
          },
          {
            "fieldPath": "name",
            "columnName": "name",
            "affinity": "TEXT",
            "notNull": false
          },
          {
            "fieldPath": "age",
            "columnName": "age",
            "affinity": "INTEGER",
            "notNull": true
          }
        ],
        "primaryKey": {
          "columnNames": [
            "id"
          ],
          "autoGenerate": true
        },
        "indices": [],
        "foreignKeys": []
      }
    ],
    "views": [],
    "setupQueries": [
      "CREATE TABLE IF NOT EXISTS room_master_table (id INTEGER PRIMARY KEY,identity_hash TEXT)",
      "INSERT OR REPLACE INTO room_master_table (id,identity_hash) VALUES(42, 'acca4b709e6c8b9b88d8328be36b9032')"
    ]
  }
}

原理

大量用了注解，Room是根据注解处理器辅助生成类文件。