三种简单的数据持久化技术

一、文件存储

保存文本到文件

private void save(String inputText) {
        FileOutputStream outputStream;
        BufferedWriter bufferedWriter = null;

        try {
            outputStream = openFileOutput("data",Context.MODE_PRIVATE);
            bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream));

            bufferedWriter.write(inputText);
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if (bufferedWriter != null){
                try {
                    bufferedWriter.flush();
                    bufferedWriter.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }

从文件中读取文本

private String load(){
        FileInputStream inputStream;
        BufferedReader bufferedReader = null;
        StringBuilder content = new StringBuilder();

        try {
            inputStream = openFileInput("data");
            bufferedReader = new BufferedReader(new InputStreamReader(inputStream));

            String line;
            while ((line = bufferedReader.readLine()) != null){
                content.append(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }finally {
            if (bufferedReader != null){
                try {
                    bufferedReader.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        return content.toString();
    }

二、SharedPreferences存储

SharedPreferences是使用键值对的方式来存储数据，支持不同数据类型存储

3种得到SharedPreferences对象的方法

• Context类中的getSharedPreferences()方法
• Activity类中的getSharedPreferences()方法
• PreFerenceManager类中的getDefaultSharedPreference()方法

保存数据到SharedPreferences

       SharedPreferences.Editor editor = getSharedPreferences("data",
                MODE_PRIVATE).edit();
        editor.putBoolean("love",true);
        editor.putFloat("money",1000000.0F);
        editor.putString("name","Mickey");
        editor.apply();

读取SharedPreferences中的数据

SharedPreferences sharedPreferences = getSharedPreferences("data",
                MODE_PRIVATE);
 //如果未找到项目的值，就会使用方法中传入的默认值来代替
 boolean love = sharedPreferences.getBoolean("love",true);
 float money = sharedPreferences.getFloat("money",0F);
 String name = sharedPreferences.getString("name","");

三、数据库存储

SQLite数据库存储

1、创建数据库

利用SQLiteOpenHelper帮助类创建数据库。
新建一个类去继承SQLiteOpenHelper抽象类，
重写onCreate()、onUpgrade()两个方法以实现创建和升级数据库的逻辑。
可以使用Android SDK 的ADB命令查看是否已在设备中成功创建了数据库

MyDataBasesHelper帮助类

package com.yxliu.demo.util;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
import android.widget.Toast;

public class MyDatabaseHelper extends SQLiteOpenHelper {

    private static final String CREATE_BOOK = "create table Book(" +
            "id integer primary key autoincrement," +
            "author text," +
            "price real," +
            "pages integer," +
            "name text)";
    private static final String CREATE_CATEGORY = "create table Category(" +
            "id integer primary key autoincrement," +
            "category_name text," +
            "category_code integer)";
    private Context mContext;

    public MyDatabaseHelper(Context context, String name, SQLiteDatabase.CursorFactory factory, int version) {
        super(context, name, factory, version);
        mContext = context;
    }


    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(CREATE_BOOK);
        db.execSQL(CREATE_CATEGORY);
        Toast.makeText(mContext,"Create succeeded",Toast.LENGTH_SHORT).show();
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("drop table if exists Book");
        db.execSQL("drop table if exists Category");
        onCreate(db);
    }
}

实例化dbHelper以创建BookStore.db

dbHelper = new MyDatabaseHelper(this, "BookStore.db", null, 4);
findViewById(R.id.btn_create_database).setOnClickListener(v -> dbHelper.getWritableDatabase());

2、升级数据库

数据库升级时如发现原来的表已存在就会直接报错，所以升级之前需要把原来的表删除，然后再创建

 @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        db.execSQL("drop table if exists Book");
        db.execSQL("drop table if exists Category");
        onCreate(db);
    }

3.向表中添加数据

 findViewById(R.id.btn_add_data).setOnClickListener(v -> {
            SQLiteDatabase db = dbHelper.getWritableDatabase();
            ContentValues values = new ContentValues();
//            第一条数据
            values.put("name", "First Love2");
            values.put("author", "屠格涅夫");
            values.put("pages", 600);
            values.put("price", 29.9);
            db.insert("Book", null, values);

            values.clear();
//            第二条数据
            values.put("name", "Sense and Sensibility");
            values.put("author", "Jane Austen");
            values.put("pages", 448);
            values.put("price", 52.10);
            db.insert("Book", null, values);

            Toast.makeText(this, "Add succeeded", Toast.LENGTH_SHORT).show();
        });

4、更新数据

 findViewById(R.id.btn_update_data).setOnClickListener(v -> {
            SQLiteDatabase db = dbHelper.getWritableDatabase();
            ContentValues values = new ContentValues();
            values.put("price", 19.99);
            db.update("Book", values, "name = ?", new String[]{"Sense and Sensibility"});

            Toast.makeText(SQLiteActivity.this,"update succeeded",Toast.LENGTH_SHORT).show();
        });

5、删除数据

 findViewById(R.id.btn_delete_data).setOnClickListener(v -> {
            SQLiteDatabase db = dbHelper.getWritableDatabase();
            db.delete("Book", "pages > ?", new String[]{"500"});

            Toast.makeText(SQLiteActivity.this,"delete succeeded",Toast.LENGTH_SHORT).show();
        });

6、查询数据

findViewById(R.id.btn_query_data).setOnClickListener(v -> {
            SQLiteDatabase db = dbHelper.getWritableDatabase();
            Cursor cursor = db.query("Book",null,null,null,null,null,null);
            if (cursor.moveToFirst()){
                do {
                    String name = cursor.getString(cursor.getColumnIndex("name"));
                    String author = cursor.getString(cursor.getColumnIndex("author"));
                    int pages = cursor.getInt(cursor.getColumnIndex("pages"));
                    double price = cursor.getDouble(cursor.getColumnIndex("price"));

                    Log.d("SQLiteActivity---","Book name is " + name);
                    Log.d("SQLiteActivity---","Book author is " + author);
                    Log.d("SQLiteActivity---","Book pages is " + pages);
                    Log.d("SQLiteActivity---","Book price is " + price);
                }while (cursor.moveToNext());
            }
            cursor.close();

            Toast.makeText(SQLiteActivity.this,"Query succeeded",Toast.LENGTH_SHORT).show();
        });

7、使用SQL操作数据库

• 添加数据

db.execSQL("insert into Book (name, author, pages,price) values (?, ?, ?, ?)", new String[]{"Sense and Sensibility", "Jane Austen", "448", "52.10"});

• 更新数据

 db.execSQL("update Book set price = ? where name = ?",new String[]{"19.99","Sense and Sensibility"});

• 删除数据

db.execSQL("delete from Book where pages > ?",new String[]{"550"});

• 查询数据

db.rawQuery("select * from Book",null);

---

LitePal数据库存储

1、集成LitePal

在app/build.gradle添加依赖

implementation 'org.litepal.android:core:2.0.0'

2、初始化LitePal

• 在AndroidManifest注册LitePalApplication

android:name="org.litepal.LitePalApplication"

• 如果已有Application,则已存在的Application继承LitePalApplication，或者在已存在Application的onCreate()方法中调用LitePal.initialize(context)方法

 private static Context mContext;

    @Override
    public void onCreate() {
        super.onCreate();

        mContext = getApplicationContext();
        LitePal.initialize(mContext);
    }

3、配置映射模型

• 创建javaBean并继承LitePalSupport类(LitePal 2.0版本)

package com.yxliu.demo.model;

import org.litepal.crud.LitePalSupport;
import java.io.Serializable;

public class Book extends LitePalSupport implements Serializable {
    private Integer id;
    private String name;
    private String author;
    private Double price;
    private Integer pages;
    private String press;

    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 String getAuthor() {
        return author;
    }

    public void setAuthor(String author) {
        this.author = author;
    }

    public Double getPrice() {
        return price;
    }

    public void setPrice(Double price) {
        this.price = price;
    }

    public Integer getPages() {
        return pages;
    }

    public void setPages(Integer pages) {
        this.pages = pages;
    }

    public String getPress() {
        return press;
    }

    public void setPress(String press) {
        this.press = press;
    }
}

• 在assets文件夹中添加LitePal.xml

<?xml version="1.0" encoding="utf-8"?>
<litepal>
    <dbname value="BookStore" />
    <version value="1" />
    <list>
        <mapping class="com.yxliu.demo.model.Book"/>
    </list>
</litepal>

• 调用LitePal.getDatabase()启动数据库

findViewById(R.id.btn_pal_create_database).setOnClickListener(v -> LitePal.getDatabase());

4、LitePal的更新数据库

修改表：只需要修改对应的javaBean
新增表：新增对应的javaBean然后在LitePal.xml的<list>的标签下配置mapping
最后修改版本号大于之前版本号即可

<?xml version="1.0" encoding="utf-8"?>
<litepal>
    <dbname value="BookStore" />
    <version value="2" />
    <list>
        <mapping class="com.yxliu.demo.model.Book"/>
        <mapping class="com.yxliu.demo.model.Category"/>
    </list>
</litepal>

5、LitePal的CRUD

• 新增

 findViewById(R.id.btn_pal_add_data).setOnClickListener(v -> {
            Book book = new Book();
            book.setName("Sense and Sensibility");
            book.setAuthor("Jane Austen");
            book.setPages(448);
            book.setPrice(52.99);
            book.setPress("Unknow");
            book.save();

            Book book2 = new Book();
            book2.setName("First Love");
            book2.setAuthor("屠格涅夫");
            book2.setPages(448);
            book2.setPrice(52.99);
            book2.setPress("Unknow");
            book2.save();
        });

• 修改

findViewById(R.id.btn_pal_update_data).setOnClickListener(v -> {
           Book book = new Book();
           book.setPress("Anchor");
           book.setPrice(9.99);
           book.updateAll("name = ? and author = ?", "Sense and Sensibility", "Jane Austen");
       });

• 删除

findViewById(R.id.btn_pal_delete_data).setOnClickListener(v -> LitePal.deleteAll(Book.class, "price > ?", "10"));

• 查询

findViewById(R.id.btn_pal_query_data).setOnClickListener(v -> {
//            其他查询语句
//            List<Book> bookList = LitePal.select("name","author").find(Book.class);
//            List<Book> bookList = LitePal.where("pages > ?","400").find(Book.class);
//            List<Book> bookList = LitePal.order("price desc").find(Book.class);
//            List<Book> bookList = LitePal.limit(3).find(Book.class);
//            offset(1)查询结果的偏移量为一
//            List<Book> bookList = LitePal.limit(3).offset(1).find(Book.class);
            List<Book> bookList = LitePal.select("name","author","pages")
                    .where("pages > ?","400")
                    .order("pages")
                    .limit(3)
                    .offset(1)
                    .find(Book.class);

//            List<Book> bookList = LitePal.findAll(Book.class);
            for (Book book : bookList) {
                Log.d("LitePalActivity---", "Book name is " + book.getName());
                Log.d("LitePalActivity---", "Book author is " + book.getAuthor());
                Log.d("LitePalActivity---", "Book pages is " + book.getPages());
                Log.d("LitePalActivity---", "Book press is " + book.getPress());
                Log.d("LitePalActivity---", "Book price is " + book.getPrice());
            }
        });

持续更新中...