359 lines
7.3 KiB
Markdown
359 lines
7.3 KiB
Markdown
# Room数据库
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## 目录
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- [Room架构](#room架构)
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- [Entity定义](#entity定义)
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- [DAO接口](#dao接口)
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- [Database配置](#database配置)
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- [Room迁移](#room迁移)
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- [Room与LiveData](#room与livedata)
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- [Room最佳实践](#room最佳实践)
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- [面试常见问题](#面试常见问题)
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---
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## Room架构
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### Room 组件
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```
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┌─────────────┐
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│ Entity │ ←─── 数据模型
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└──────┬──────┘
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│
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┌──────▼──────┐
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│ DAO │ ←─── 数据访问接口
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└──────┬──────┘
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│
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┌──────▼──────┐
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│ Database │ ←─── 数据库配置
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└─────────────┘
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```
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### Room 优势
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1. **编译时检查**：SQL 语句编译时检查
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2. **类型安全**：类型安全的数据访问
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3. **支持 LiveData**：自动更新 UI
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4. **支持 RxJava**：响应式编程
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5. **简化代码**：减少样板代码
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---
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## Entity定义
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### Entity 注解
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```java
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// Entity：数据模型
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@Entity(tableName = "user")
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public class User {
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@PrimaryKey(autoGenerate = true)
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private int id;
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@ColumnInfo(name = "user_name")
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private String name;
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private String email;
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private int age;
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// Getters and Setters
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public int getId() { return id; }
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public void setId(int id) { this.id = id; }
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public String getName() { return name; }
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public void setName(String name) { this.name = name; }
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public String getEmail() { return email; }
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public void setEmail(String email) { this.email = email; }
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public int getAge() { return age; }
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public void setAge(int age) { this.age = age; }
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}
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```
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### Entity 注解选项
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```java
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@Entity(
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tableName = "user",
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indices = {@Index("name"), @Index(value = {"name", "age"})},
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foreignKeys = {
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@ForeignKey(
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entity = Department.class,
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parentColumns = "id",
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childColumns = "department_id"
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)
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}
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)
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public class User {
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@PrimaryKey
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private int id;
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@ColumnInfo(name = "department_id")
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private int departmentId;
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}
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```
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---
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## DAO接口
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### DAO 定义
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```java
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// DAO：数据访问对象
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@Dao
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public interface UserDao {
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// 查询
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@Query("SELECT * FROM user")
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List<User> getAllUsers();
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@Query("SELECT * FROM user WHERE id = :id")
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User getUserById(int id);
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@Query("SELECT * FROM user WHERE age > :age")
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List<User> getUsersByAge(int age);
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// 插入
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@Insert
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void insertUser(User user);
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@Insert
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void insertUsers(User... users);
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// 更新
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@Update
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void updateUser(User user);
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// 删除
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@Delete
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void deleteUser(User user);
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@Query("DELETE FROM user WHERE id = :id")
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void deleteUserById(int id);
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}
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```
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### 复杂查询
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```java
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@Dao
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public interface UserDao {
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// 多表查询
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@Query("SELECT * FROM user u INNER JOIN department d ON u.department_id = d.id")
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List<UserWithDepartment> getUsersWithDepartment();
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// 参数查询
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@Query("SELECT * FROM user WHERE name LIKE :name AND age > :age")
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List<User> searchUsers(String name, int age);
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// 返回 LiveData
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@Query("SELECT * FROM user")
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LiveData<List<User>> getAllUsersLive();
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// 返回 Flowable（RxJava）
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@Query("SELECT * FROM user")
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Flowable<List<User>> getAllUsersFlowable();
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}
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```
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---
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## Database配置
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### Database 定义
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```java
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// Database：数据库配置
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@Database(
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entities = {User.class, Department.class},
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version = 1,
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exportSchema = false
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)
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public abstract class AppDatabase extends RoomDatabase {
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public abstract UserDao userDao();
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public abstract DepartmentDao departmentDao();
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private static AppDatabase instance;
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public static synchronized AppDatabase getInstance(Context context) {
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if (instance == null) {
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instance = Room.databaseBuilder(
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context.getApplicationContext(),
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AppDatabase.class,
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"app_database"
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).build();
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}
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return instance;
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}
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}
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```
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### Database 配置选项
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```java
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Room.databaseBuilder(context, AppDatabase.class, "database_name")
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.allowMainThreadQueries() // 允许主线程查询（不推荐）
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.fallbackToDestructiveMigration() // 破坏性迁移
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.addMigrations(MIGRATION_1_2, MIGRATION_2_3) // 迁移
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.build();
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```
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---
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## Room迁移
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### 数据库迁移
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```java
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// 版本1 → 版本2
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static final Migration MIGRATION_1_2 = new Migration(1, 2) {
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@Override
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public void migrate(SupportSQLiteDatabase database) {
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database.execSQL("ALTER TABLE user ADD COLUMN phone TEXT");
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}
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};
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// 版本2 → 版本3
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static final Migration MIGRATION_2_3 = new Migration(2, 3) {
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@Override
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public void migrate(SupportSQLiteDatabase database) {
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database.execSQL("CREATE TABLE address (" +
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"id INTEGER PRIMARY KEY AUTOINCREMENT," +
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"user_id INTEGER," +
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"address TEXT" +
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")");
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}
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};
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// 配置迁移
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Room.databaseBuilder(context, AppDatabase.class, "database_name")
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.addMigrations(MIGRATION_1_2, MIGRATION_2_3)
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.build();
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```
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---
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## Room与LiveData
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### LiveData 集成
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```java
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// DAO 返回 LiveData
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@Dao
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public interface UserDao {
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@Query("SELECT * FROM user")
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LiveData<List<User>> getAllUsers();
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@Query("SELECT * FROM user WHERE id = :id")
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LiveData<User> getUserById(int id);
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}
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// 观察数据变化
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userDao.getAllUsers().observe(this, users -> {
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adapter.updateUsers(users);
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});
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```
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### RxJava 集成
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```java
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// 添加依赖
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dependencies {
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implementation 'androidx.room:room-rxjava2:2.3.0'
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}
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// DAO 返回 Flowable
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@Dao
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public interface UserDao {
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@Query("SELECT * FROM user")
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Flowable<List<User>> getAllUsers();
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}
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// 使用
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userDao.getAllUsers()
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.subscribeOn(Schedulers.io())
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.observeOn(AndroidSchedulers.mainThread())
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.subscribe(users -> {
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adapter.updateUsers(users);
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});
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```
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---
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## Room最佳实践
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### 1. 使用单例模式
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```java
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// 数据库使用单例
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public static synchronized AppDatabase getInstance(Context context) {
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if (instance == null) {
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instance = Room.databaseBuilder(...).build();
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}
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return instance;
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}
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```
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### 2. 在后台线程操作
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```java
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// ✅ 正确：在后台线程操作
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new Thread(() -> {
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userDao.insertUser(user);
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}).start();
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// ❌ 错误：在主线程操作（除非使用 allowMainThreadQueries）
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userDao.insertUser(user);
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```
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### 3. 使用事务
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```java
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@Dao
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public interface UserDao {
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@Transaction
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@Query("SELECT * FROM user")
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List<User> getAllUsers();
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}
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```
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---
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## 面试常见问题
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### Q1: Room 的优势？
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**答案：**
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1. 编译时检查 SQL
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2. 类型安全
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3. 支持 LiveData
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4. 支持 RxJava
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5. 简化代码
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### Q2: Room 的组件？
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**答案：**
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1. **Entity**：数据模型
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2. **DAO**：数据访问接口
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3. **Database**：数据库配置
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### Q3: Room 迁移？
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**答案：**
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- 使用 Migration 类处理数据库升级
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- 定义迁移规则
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- 在 Database 配置中添加迁移
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### Q4: Room 和 SQLite 的区别？
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**答案：**
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- **Room**：SQLite 的封装，提供类型安全、编译时检查
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- **SQLite**：底层数据库，需要手动编写 SQL
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---
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*最后更新：2024年*
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