Android 应用数据存储简单来说有这么几种：
- 文件存储
- SharedPreference 存储
- SQLite 数据库存储
- 第三方的网上数据库存储
当需要本地存储大量数据的时候，文件存储频繁读取文件内容修改保存是很耗时，SharedPreference 无法支持大量数据。这时候本地原生的SQLite虽然符合，但是编程过程不太友好，所幸的是Google在其基础上的封装，就有了今天推荐的轻量级数据库——Room
导入依赖
implementation "android.arch.persistence.room:runtime:2.0.0"
annotationProcessor "android.arch.persistence.room:compiler:2.0.0"
创建数据库
在数据库中创建一张表，表中设计我们要操作的数据对象。在Room中对应的注解如下：
- @Database数据库：必须是扩展 RoomDatabase 的抽象类
- @Entity：表示数据库中的表
- @DAO：数据操作对象
创建一个学生数据库StudentDB, 这个数据库有一张学生表StudentEntity, StudentDao用于提供对学生表的各种增删查改。StudentDB代码如下：
@Database(entities = {StudentEntity.class}, version = 1 )
public abstract class StudentDB extends RoomDatabase {
public abstract StudentDao studentDao();
}
StudentEntity代码如下：
@Entity
public class StudentEntity {
@PrimaryKey
private long studentID;
private String name;
private int age;
public StudentEntity() {
}
@Ignore
public StudentEntity(long studentID, String name, int age) {
this.studentID = studentID;
this.name = name;
this.age = age;
}
public long getStudentID() {
return studentID;
}
public void setStudentID(long studentID) {
this.studentID = studentID;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "StudentEntity{" +
"studentID=" + studentID +
", name='" + name + '\'' +
", age=" + age +
'}';
}
}
@PrimaryKey是我们设置的主键，因为主键类型Long, INT，我们也可以写成@PrimaryKey(autoGenerate = true)，选择由数据库自动生成。StudentDao代码如下：
@Dao
public interface StudentDao {
@Insert(onConflict = OnConflictStrategy.REPLACE)
void insert(StudentEntity studentEntity);
@Insert(onConflict = OnConflictStrategy.REPLACE)
void insertList(List<StudentEntity> studentEntities);
@Query("delete from StudentEntity where studentID=:studentID")
void deleteStudent(long studentID);
@Query("select * from StudentEntity")
List<StudentEntity> getAll();
@Query("select * from StudentEntity where studentID=:studentID")
StudentEntity queryStudent(long studentID);
@Update
void updateStudent(StudentEntity studentEntity);
}
操作对象StudentDao 的增删查改（数据可以批量也可单独操作，如插入这边写了批量和单独的操作，其它也是类似，故不多写）
插入数据，只需要标记上Insert注解，onConflict = OnConflictStrategy.REPLACE表明插入一条数据如果主键已经存在，则可以直接替换旧的数据。
@Insert(onConflict = OnConflictStrategy.REPLACE)
void insert(StudentEntity studentEntity);
@Insert(onConflict = OnConflictStrategy.REPLACE)
void insertList(List<StudentEntity> studentEntities);
删除操作，可以执行我们写入的SQL语句
@Query("delete from StudentEntity where studentID=:studentID")
void deleteStudent(long studentID);
也可通过@Delete传入对象删除
@Delete
void deleteStudent(StudentEntity studentEntity);
查询，通过注解@Query 执行SQL语句
@Query("select * from StudentEntity")
List<StudentEntity> getAll();
@Query("select * from StudentEntity where studentID=:studentID")
StudentEntity queryStudent(long studentID);
更改，通过注解@Update传入对象更新数据
@Update
void updateStudent(StudentEntity studentEntity);
在主界面创建本地持久化的数据库。第一个传入的是上下文，第二个是我们注解了的@Database与扩展了RoomDatabase的类，第三个是创建的数据库文件的名称，是个字符串。
方法一：对数据增删查改需要在后台操作
StudentDB studentDB = Room.databaseBuilder(this, StudentDB.class, dataBaseName).build();
如果直接在UI线程操作，会报异常如下：
Cannot access database on the main thread since it may potentially lock the UI for a long period of time.
这里结合RxJava写出个方法一的简单调用。首先引入RxJava 2.0版本的依赖
implementation 'io.reactivex.rxjava2:rxjava:2.1.1'
implementation 'io.reactivex.rxjava2:rxandroid:2.1.1'
插入一条数据的代码
final StudentEntity studentEntity = new StudentEntity(1, "name", 18);
Observable.create(new ObservableOnSubscribe<StudentEntity>() {
@Override
public void subscribe(ObservableEmitter<StudentEntity> e) throws Exception {
studentDB.studentDao().insert(studentEntity);
//将插入成功的学生id数据传到主线程
e.onNext(studentDB.studentDao().queryStudent(studentEntity.getStudentID()));}})
.observeOn(AndroidSchedulers.mainThread())
.subscribeOn(Schedulers.io())
.subscribe(new Consumer<StudentEntity>() {
@Override
public void accept(StudentEntity studentEntity) throws Exception {
//显示插入成功的数据
tv_content.setText("插入数据："+studentEntity.toString());
}
});
方法二：通过设置allowMainThreadQueries()允许在主线程操作
StudentDB studentDB = Room.databaseBuilder(this, StudentDB.class, dataBaseName)
.allowMainThreadQueries()
.build();
可以直接调用增删查改
studentDB.studentDao().insert(StudentEntity);
studentDB.studentDao().deleteStudent(long studentID);
注意：由于数据库的操作是耗时的，画面在60fps则不会感觉到卡顿，假设大量数据的查询加绘制界面的时间超过16ms ，方法二会导致应用看起是卡顿，甚至ANR。对于是否允许UI线程运行数据库操作，取决小伙伴们自身开发的APP的需求
使用数据库
最后给出使用Room数据库MainActivity的代码
public class MainActivity extends AppCompatActivity implements View.OnClickListener {
private StudentDB studentDB;
private String dataBaseName = "StudentDB";
private TextView tv_content;
private Button btn_insert, btn_delete, btn_query, btn_update, btn_showAll;
private EditText edt_insert_num, edt_insert_name, edt_insert_age, edt_delete_num, edt_query_num,
edt_update_num, edt_update_name, edt_update_age;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initDataBase();
initView();
}
private void initDataBase() {
/**
* 后台操作
* */
//studentDB = Room.databaseBuilder(this, StudentDB.class, dataBaseName).build();
/**
* 主线程操作
* */
studentDB = Room.databaseBuilder(this, StudentDB.class, dataBaseName)
.allowMainThreadQueries()
.build();
}
private void initView() {
...
}
@Override
public void onClick(View v) {
...
switch (v.getId()) {
case R.id.btn_insert:
insertData();
break;
case R.id.btn_delete:
deleteData();
break;
case R.id.btn_query:
queryData();
break;
case R.id.btn_update:
updateData();
break;
case R.id.btn_showAll:
showAll();
break;
default:
}
}
private void showAll() {
// 在后台运行
/*Observable.create(new ObservableOnSubscribe<List<StudentEntity>>() {
@Override
public void subscribe(ObservableEmitter<List<StudentEntity>> emitter) throws Exception {
emitter.onNext(studentDB.studentDao().getAll());
}
})
.observeOn(AndroidSchedulers.mainThread())
.subscribeOn(Schedulers.io())
.subscribe(new Consumer<List<StudentEntity>>() {
@Override
public void accept(List<StudentEntity> data) throws Exception {
tv_content.setText(data.toString());
}
});
*/
// 运行到主线程：
tv_content.setText("展示所有数据："+ studentDB.studentDao().getAll().toString());
}
private void updateData() {
...
long studentID = Long.parseLong(edt_update_num.getText().toString());
String name = edt_update_name.getText().toString();
int age = Integer.parseInt(edt_update_age.getText().toString());
StudentEntity studentEntity = new StudentEntity(studentID, name, age);
tv_content.setText("更新数据："+studentEntity.toString());
studentDB.studentDao().updateStudent(studentEntity);
}
private void queryData() {
...
long studentID = Long.parseLong(edt_query_num.getText().toString());
StudentEntity studentEntity = studentDB.studentDao().queryStudent(studentID);
tv_content.setText("查询数据："+studentEntity.toString());
}
private void deleteData()
...
long studentID = Long.parseLong(edt_delete_num.getText().toString());
tv_content.setText("删除数据："+studentDB.studentDao().queryStudent(studentID));
studentDB.studentDao().deleteStudent(studentID);
}
private void insertData() {
...
long studentID = Long.parseLong(edt_insert_num.getText().toString());
String name = edt_insert_name.getText().toString();
int age = Integer.parseInt(edt_insert_age.getText().toString());
//数据库插入操作：当学号一样则替换
final StudentEntity studentEntity = new StudentEntity(studentID, name, age);
//final StudentEntity studentEntity = new StudentEntity(1, "name", 18);
/*Observable.create(new ObservableOnSubscribe<StudentEntity>() {
@Override
public void subscribe(ObservableEmitter<StudentEntity> e) throws Exception {
studentDB.studentDao().insert(studentEntity);
//studentDB.studentDao().queryStudent(studentEntity.getStudentID());
e.onNext(studentDB.studentDao().queryStudent(studentEntity.getStudentID()));
}})
.observeOn(AndroidSchedulers.mainThread())
.subscribeOn(Schedulers.io())
.subscribe(new Consumer<StudentEntity>() {
@Override
public void accept(StudentEntity studentEntity) throws Exception {
tv_content.setText("插入数据：" + studentEntity.toString());
}
});
*/
tv_content.setText("插入数据：" + studentEntity.toString());
studentDB.studentDao().insert(studentEntity);
}
}
