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詳解Android中Room組件的使用

 更新時(shí)間：2023年08月11日 17:15:50   作者：故鄉(xiāng)的櫻花開(kāi)了  
Room 是在 SQLite 上提供了一個(gè)抽象層，以便在充分利用 SQLite 的強(qiáng)大功能的同時(shí)，能夠流暢地訪問(wèn)數(shù)據(jù)庫(kù)，這篇文章主要為大家介紹了Room組件的具體使用，需要的可以參考下

一.Android官方ORM數(shù)據(jù)庫(kù)Room

Android采用Sqlite作為數(shù)據(jù)庫(kù)存儲(chǔ)。但由于Sqlite代碼寫起來(lái)繁瑣且容易出錯(cuò)，因此Google推出了Room，其實(shí)Room就是在Sqlite上面再封裝了一層。下面是Room的架構(gòu)圖：

要想更好地理解上面的圖，我們先要理解幾個(gè)概念：Entity和Dao

Entity:實(shí)體，一個(gè)entity就對(duì)應(yīng)于數(shù)據(jù)庫(kù)中的一張表。Entity類是Sqlite中的表對(duì)java類的映射，例如有一個(gè)學(xué)生表，有id,name,age三個(gè)字段；那么對(duì)應(yīng)的就有一個(gè)學(xué)生類，有id,name,age三個(gè)成員變量和學(xué)生表中的字段進(jìn)行一一對(duì)應(yīng)。

Dao:即Data Access Object，數(shù)據(jù)訪問(wèn)對(duì)象，就是字面意思，可以通過(guò)他來(lái)訪問(wèn)數(shù)據(jù)庫(kù)中的數(shù)據(jù)。

那么所謂的ORM(Object Relational Mapping)，對(duì)象關(guān)系映射，就很好理解了。就是建立一個(gè)從數(shù)據(jù)庫(kù)表到j(luò)ava類的映射，表中的字段對(duì)應(yīng)類中的成員變量，表中的記錄對(duì)應(yīng)該類的一個(gè)實(shí)例。

二.Room數(shù)據(jù)庫(kù)的基本使用方法

1.在使用Room數(shù)據(jù)庫(kù)前，先要在app/build.gradle文件中導(dǎo)入以下的依賴：

implementation 'androidx.room:room-runtime:2.5.2'
annotationProcessor 'androidx.room:room-compiler:2.5.2'

2.創(chuàng)建一個(gè)關(guān)于學(xué)生的Entity，即創(chuàng)建一張學(xué)生表：

@Entity
public class Student {
    @PrimaryKey
    private Integer id;
    @ColumnInfo(name="name",typeAffinity = ColumnInfo.TEXT)
    private String name;
    @ColumnInfo(name="age",typeAffinity = ColumnInfo.INTEGER)
    private Integer age;
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public Student(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

@Entity注解用于將Student類和Room數(shù)據(jù)庫(kù)中的數(shù)據(jù)表對(duì)應(yīng)起來(lái)；@PrimaryKey注解即主鍵約束；@ColumnInfo注解可以設(shè)置該成員變量對(duì)應(yīng)的表中字段的名稱以及類型

需要注意的一點(diǎn)是get方法不可省略

3.針對(duì)上面的學(xué)生類Entity，我們需要定義一個(gè)Dao接口文件，以便對(duì)數(shù)據(jù)庫(kù)進(jìn)行訪問(wèn)，在接口的上方加上@Dao注解即可

@Entity
public class Student {
    @PrimaryKey
    private Integer id;
    @ColumnInfo(name="name",typeAffinity = ColumnInfo.TEXT)
    private String name;
    @ColumnInfo(name="age",typeAffinity = ColumnInfo.INTEGER)
    private Integer age;
    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
    public Student(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }
    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

4.定義好Entity和Dao后，接下來(lái)就是創(chuàng)建數(shù)據(jù)庫(kù)了，代碼如下：

@Database(entities = {Student.class},version = 1)
public abstract class MyDatabase extends RoomDatabase {
    private static final String DATABASE_NAME="my_db";
    private static MyDatabase myDatabase;
    public static synchronized MyDatabase getInstance(Context context){
        if(myDatabase==null){
            myDatabase= Room.databaseBuilder(context,MyDatabase.class,DATABASE_NAME).build();
        }
        return myDatabase;
    }
    @Override
    public void clearAllTables() {
    }
    @NonNull
    @Override
    protected InvalidationTracker createInvalidationTracker() {
        return null;
    }
    @NonNull
    @Override
    protected SupportSQLiteOpenHelper createOpenHelper(@NonNull DatabaseConfiguration databaseConfiguration) {
        return null;
    }
    public abstract StudentDao studentDao();
}

@Database注解用于告訴系統(tǒng)這是Room數(shù)據(jù)庫(kù)對(duì)象，entities屬性用于指定該數(shù)據(jù)庫(kù)有哪些表，version用于指定數(shù)據(jù)庫(kù)的版本號(hào)數(shù)據(jù)庫(kù)類需要繼承RoomDatabase類，并結(jié)合單例模式完成創(chuàng)建。

到這里，數(shù)據(jù)庫(kù)和表就創(chuàng)建完成了，接下來(lái)就看看如何對(duì)數(shù)據(jù)庫(kù)進(jìn)行增刪改查了。

5.結(jié)合ViewModel和LiveData，對(duì)數(shù)據(jù)庫(kù)進(jìn)行增刪改查，并且數(shù)據(jù)庫(kù)表的記錄發(fā)生變化時(shí)，頁(yè)面可以及時(shí)收到通知，并更新頁(yè)面。

LiveData通常和ViewModel一起使用，ViewModel用于存儲(chǔ)頁(yè)面的數(shù)據(jù)，因此我們可以把數(shù)據(jù)庫(kù)的實(shí)例化放到ViewModel中，但數(shù)據(jù)庫(kù)的實(shí)例化需要用到Context對(duì)象，因此我們不宜直接用ViewModel,而應(yīng)該用其子類AndroidViewModel。

public class StudentViewModel extends AndroidViewModel {
    private MyDatabase myDatabase;
    private LiveData<List<Student>> liveDataStudents;
    public StudentViewModel(@NonNull Application application) {
        super(application);
        myDatabase=MyDatabase.getInstance(application);
        liveDataStudents=myDatabase.studentDao().getAllStudents();
    }
    public LiveData<List<Student>> getLiveDataStudents(){
        return liveDataStudents;
    }
    public void insertStudent(Student student){
        myDatabase.studentDao().insertStudent(student);
    }
    public void deleteStudent(Student student){
        myDatabase.studentDao().deleteStudent(student);
    }
    public void updateStudent(Student student){
        myDatabase.studentDao().updataStudent(student);
    }
    public Student selectStudentById(Integer id){
        return myDatabase.studentDao().selectStudentById(id);
    }
}

6.在Activity中實(shí)例化StudentViewModel，并進(jìn)行增刪改查操作，并監(jiān)聽(tīng)LiveData的變化。

public class MainActivity extends AppCompatActivity implements View.OnClickListener {
    private Button btn_insert,btn_delete,btn_update,btn_select;
    private TextView tv_display;
    private StudentViewModel studentViewModel;
    private ExecutorService executor = Executors.newSingleThreadExecutor();
    private Student student;
    @SuppressLint("MissingInflatedId")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv_display=findViewById(R.id.tv_display);
        btn_delete=findViewById(R.id.btn_delete);
        btn_insert=findViewById(R.id.btn_insert);
        btn_update=findViewById(R.id.btn_update);
        btn_select=findViewById(R.id.btn_select);
        btn_select.setOnClickListener(this);
        btn_insert.setOnClickListener(this);
        btn_delete.setOnClickListener(this);
        btn_update.setOnClickListener(this);
        studentViewModel=new ViewModelProvider(this,new MyViewModelFactory(getApplication())).get(StudentViewModel.class);
        studentViewModel.getLiveDataStudents().observe(this, new Observer<List<Student>>() {
            @Override
            public void onChanged(List<Student> students) {
                tv_display.setText(students+"");
            }
        });
    }
    @Override
    public void onClick(View view) {
        switch(view.getId()){
            case R.id.btn_delete:
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        studentViewModel.deleteStudent(new Student(1,"jack",20));
                    }
                });
                break;
            case R.id.btn_update:
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        studentViewModel.updateStudent(new Student(1,"zhangsan",32));
                    }
                });
                break;
            case R.id.btn_insert:
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        studentViewModel.insertStudent(new Student(1,"lisi",22));
                    }
                });
                break;
            case R.id.btn_select:
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        student = studentViewModel.selectStudentById(1);
                        Log.i("test",student.toString());
                    }
                });
                break;
        }
    }
}
public class MyViewModelFactory implements ViewModelProvider.Factory {
    private Application application;
    public MyViewModelFactory(Application application){
        this.application=application;
    }
    @NonNull
    @Override
    public <T extends ViewModel> T create(@NonNull Class<T> modelClass) {
        return (T)new StudentViewModel(application);
    }
}

運(yùn)行應(yīng)用程序，對(duì)數(shù)據(jù)庫(kù)進(jìn)行增刪改操作時(shí)，onChanged方法就會(huì)回調(diào)，然后在這個(gè)方法中對(duì)頁(yè)面進(jìn)行更新即可。

以上就是詳解Android中Room組件的使用的詳細(xì)內(nèi)容，更多關(guān)于Android Room組件的資料請(qǐng)關(guān)注腳本之家其它相關(guān)文章！

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