Room是什么

Room是Google提供的一个ORM库。（ORM就是object relational mapping,对象关系映射。将关系型数据库转化为对象来进行处理）

Room主要组件

Database:创建，管理数据库，创建Dao对象。
Entity:实体类和数据库中的表对应。
DAO:data access objects，数据访问对象，定义访问数据库的方法。

room_architecture.png

如何使用Room

1.引入相关依赖到项目中

dependencies {
    def room_version = "1.1.1"

    implementation "android.arch.persistence.room:runtime:$room_version"
    annotationProcessor "android.arch.persistence.room:compiler:$room_version" // use kapt for Kotlin

    // optional - RxJava support for Room
    implementation "android.arch.persistence.room:rxjava2:$room_version"

    // optional - Guava support for Room, including Optional and ListenableFuture
    implementation "android.arch.persistence.room:guava:$room_version"

    // Test helpers
    testImplementation "android.arch.persistence.room:testing:$room_version"
}

2.创建Entity

package com.architecture.demo.room;

import android.arch.persistence.room.ColumnInfo;
import android.arch.persistence.room.Entity;
import android.arch.persistence.room.ForeignKey;
import android.arch.persistence.room.Ignore;
import android.arch.persistence.room.Index;
import android.arch.persistence.room.PrimaryKey;
import android.text.TextUtils;

//指示数据表实体类
@Entity(tableName = "tb_student",//定义表名
        indices = @Index(value = {"class_id", "number"}),//定义索引
        foreignKeys = {@ForeignKey(entity = ClassEntity.class,
                parentColumns = "id",
                childColumns = "class_id")})//定义外键
public class StudentEntity {
    @PrimaryKey(autoGenerate = true) //定义主键
    public long id;
    @ColumnInfo(name = "name")//定义数据表中的字段名
    public String name;
    @ColumnInfo(name = "number")
    public int number;
    @ColumnInfo(name = "class_id")
    public String class_id;
    @Ignore//指示Room需要忽略的字段或方法
    public String ignoreText;

    public long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public int getNumber() {
        return number;
    }

    public void setNumber(int number) {
        this.number = number;
    }

    public String getClass_id() {
        return class_id;
    }

    public void setClass_id(String class_id) {
        this.class_id = class_id;
    }

    public String getIgnoreText() {
        return ignoreText;
    }

    public void setIgnoreText(String ignoreText) {
        this.ignoreText = ignoreText;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        StudentEntity that = (StudentEntity) o;
        return id == that.id &&
                number == that.number &&
                TextUtils.equals(name, that.name) &&
                TextUtils.equals(class_id, that.class_id) &&
                TextUtils.equals(ignoreText, that.ignoreText);
    }

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

3.创建Dao

package com.architecture.demo.room;

import android.arch.paging.DataSource;
import android.arch.persistence.room.Dao;
import android.arch.persistence.room.Delete;
import android.arch.persistence.room.Insert;
import android.arch.persistence.room.Query;
import android.arch.persistence.room.Update;

import java.util.List;

@Dao
public interface StudentDao {

    @Query("SELECT * FROM tb_student ORDER BY number ASC")
    List<StudentEntity> getAll();

    @Query("SELECT * FROM tb_student LIMIT :current,:count ")
    List<StudentEntity> getByLimit(int current, int count);

    @Query("SELECT COUNT(*) FROM tb_student")
    int getCount();

    @Query("SELECT Max(number) FROM tb_student")
    int getMaxNumber();

    @Insert
    void insert(StudentEntity... entities);

    @Delete
    void delete(StudentEntity entity);

    @Query("DELETE FROM tb_student")
    void deleteAll();

    @Update
    void update(StudentEntity entity);

    @Query("SELECT * FROM tb_student ORDER BY number DESC")
    DataSource.Factory<Integer, StudentEntity> loadStudent();
}

4创建Database

package com.architecture.demo.room;


import android.arch.persistence.db.SupportSQLiteDatabase;
import android.arch.persistence.room.Database;
import android.arch.persistence.room.Room;
import android.arch.persistence.room.RoomDatabase;
import android.content.Context;
import android.support.annotation.NonNull;
import android.util.Log;

import com.architecture.demo.util.CONSTANT;

@Database(entities = {ClassEntity.class, StudentEntity.class}, version = 1)
public abstract class DemoRoomDatabase extends RoomDatabase {
    private static DemoRoomDatabase instance = null;

    public static DemoRoomDatabase getInstance(Context context) {
        if (instance == null) {
            instance = Room.databaseBuilder(context, DemoRoomDatabase.class, "room_database").addCallback(new RoomDatabase.Callback() {
                @Override
                public void onCreate(@NonNull SupportSQLiteDatabase db) {
                    super.onCreate(db);
                    Log.i(CONSTANT.TAG_ROOM, "onCreate:" + db.getPath());
                }

                @Override
                public void onOpen(@NonNull SupportSQLiteDatabase db) {
                    super.onOpen(db);
                    Log.i(CONSTANT.TAG_ROOM, "onOpen:" + db.getPath());
                }
            }).allowMainThreadQueries().build();
        }
        return instance;
    }

    public abstract StudentDao studentDao();
}

5.使用Room向插入数据，查找数据

package com.architecture.demo;

import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.v4.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;

import com.architecture.demo.paging.PagingFragment;
import com.architecture.demo.room.ClassDao;
import com.architecture.demo.room.ClassEntity;
import com.architecture.demo.room.DemoRoomDatabase;
import com.architecture.demo.room.StudentDao;
import com.architecture.demo.room.StudentEntity;

public class RoomAndPagingActivity extends AppCompatActivity implements View.OnClickListener {
    DemoRoomDatabase mRoomDatabase;
    private Button btn1;
    private Button btn2;
    private Button btn3;
    private Button btn4;
    public static int NUMBER;
    PagingFragment mPagingFragment;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_room);
        btn1 = findViewById(R.id.text1);
        btn2 = findViewById(R.id.text2);
        btn3 = findViewById(R.id.text3);
        btn4 = findViewById(R.id.text4);
        btn1.setOnClickListener(this);
        btn2.setOnClickListener(this);
        btn3.setOnClickListener(this);
        btn4.setOnClickListener(this);
        mPagingFragment = PagingFragment.getInstance();
        FragmentTransaction fragmentTransaction = getSupportFragmentManager().beginTransaction();
        fragmentTransaction.add(R.id.content_data, mPagingFragment);
        fragmentTransaction.commit();
        mRoomDatabase = DemoRoomDatabase.getInstance(getApplicationContext());
        NUMBER = mRoomDatabase.studentDao().getMaxNumber();
    }

    @Override
    public void onClick(View v) {
        if (v == btn1) {
            ClassDao classDao = mRoomDatabase.classDao();
            ClassEntity classEntity = new ClassEntity();
            classEntity.setName("三年二班");
            classDao.insert(classEntity);
        } else if (v == btn2) {
            add10();
        } else if (v == btn3) {
            StudentDao studentDao = mRoomDatabase.studentDao();
            StudentEntity student = getStudentEntity();
            studentDao.insert(student);
        } else if (v == btn4) {
            StudentDao studentDao = mRoomDatabase.studentDao();
            studentDao.deleteAll();
            NUMBER = 0;
        }
        if (mPagingFragment != null) {
            mPagingFragment.refreshData();
        }
    }

    private void add10() {
        for (int i = 0; i < 10; i++) {
            StudentDao studentDao = mRoomDatabase.studentDao();
            StudentEntity student = getStudentEntity();
            studentDao.insert(student);
        }
    }

    @NonNull
    private StudentEntity getStudentEntity() {
        NUMBER++;
        StudentEntity student = new StudentEntity();
        student.setName("小闹" + NUMBER);
        student.setNumber(NUMBER);
        student.setClass_id("1");
        return student;
    }
}

通过以上步骤既可完成Room的简单使用。
Demo地址：
https://github.com/CongYiMing/ArchitectureDemo