LowDB 轻量级 JSON 本地数据库

作为轻量级的本地存储方式，对于构建不依赖服务器的小型项目，用LowDB存储和管理数据是十分理想的选择。在Nodejs, Electron and browser等一些小型项目中经常能看到LowDB的身影。

https://github.com/typicode/lowdb

npm install lowdb

或者：

yarn add lowdb

const low = require('lowdb')

const FileSync = require('lowdb/adapters/FileSync')// 有多种适配器可选择

const adapter = new FileSync('db.json')// 申明一个适配器

const db = low(adapter)

db.defaults({posts: [], user: {}, count: 0})

.write()

db.get('posts')

.push({id: 1, title: 'lowdb is awesome'})

.write()

db.set('user.name', 'typicode')

.write()

db.update('count', n =>n + 1)

.write()

运行程序会在项目中添加db.json文件，里面存储了添加的数据：

{

"posts": [

{

"id": 1,

"title": "lowdb is awesome"

}

],

"user": {

"name": "typicode"

},

"count": 1

}

lowdb是基于lodash构建的，所以可以使用任何lodash强大的函数，比如: _.get() 和 _.find()，并且可以串联地使用：

db.get('users')

.find({sex: 'male'})

.value()

函数 功能

low(adapter) 返回一个具有特定属性和功能的 lodash chain

db.[...].write() / .value() 写 / 读数据

db.getState() / .setState() 获取 / 设置数据库的状态

db._ 数据库lodash的实例，可以利用这个添加自己的函数或者第三方的mixins,比如lodash-id

db._.mixin({

second: function(array) {

return array[1]

}

})

db.get('posts')

.second()

.value()

针对lowdb自带的适配器：FileSync、FileAsync 和 LocalBrowser，有以下可选参数：

defaultValue: 文件不存在时的默认值；

serialize/deserialize: 写之前和读之后的 *** 作。

const adapter = new FilSync('db.json',{

serialize: (data) =>encrypt(JSON.stringify(data)),

deserialize: (data) =>JSON.parse(decrypt(data))

})

可以直接使用lodash的函数进行查询。需要注意的是有些 *** 作可能会导致原数据被修改，为了避免这种误 *** 作，需要使用 .cloneDeep()， *** 作都是惰性的，只有调用 .value()或 .write()后才会正式执行。

检查users是是否存在

db.has('users')

.value()

设置users

db.set('users', [])

.write()

排序、选择

db.get('users')

.filter({sex: 'male'})

.sortBy('age')

.take(5)

.value()

获取特定字段

db.get('users')

.map('name')

.value()

获取数量

db.get('users')

.size()

.value()

获取特定信息

db.get('users[0].name')

.value()

更新信息

db.get('users')

.find({name: 'Tom'})

.assign({name: 'Tim'})

.write()

删除信息

db.get('users')

.remove({name: 'Time'})

.write()

移除属性

db.unset('users.name)

.write()

深拷贝

db.get('users')

.cloneDeep()

.value()

可以使用 shortid 和 lodash-id 为数据库中的每一条记录创建唯一的id索引，然后通过id检索 *** 作记录：

const shortid = require('shortid')

const postId = db

.get('posts')

.push({ id: shortid.generate(), title: 'low!' })

.write()

.id

const post = db

.get('posts')

.find({ id: postId })

.value()

const lodashId = require('lodash-id')

const FileSync = require('lowdb/adapters/FileSync')

const adapter = new FileSync('db.json')

const db = low(adapter)

db._.mixin(lodashId)

// We need to set some default values, if the collection does not exist yet

// We also can store our collection

const collection = db

.defaults({ posts: [] })

.get('posts')

// Insert a new post...

const newPost = collection

.insert({ title: 'low!' })

.write()

// ...and retrieve it using its id

const post = collection

.getById(newPost.id)

.value()

low( ) 函数接受自定义的Adapter

class MyStorage {

constructor() {

// ...

}

read() {

// Should return data (object or array) or a Promise

}

write(data) {

// Should return nothing or a Promise

}

}

const adapter = new MyStorage(args)

const db = low(adapter)

==============================================

英文官网介绍，更加简洁

Lowdb 3 is a pure ESM package. If you're having trouble importing it in your project, please read this.

You can use TypeScript to type check your data.

You can also add lodash or other utility libraries to improve lowdb.

For CLI, server and browser usage, seeexamples/ directory.

Lowdb has two classes (for asynchronous and synchronous adapters).

Callsadapter.read() and sets db.data .

Note: JSONFile and JSONFileSync adapters will set db.data to null if file doesn't exist.

Callsadapter.write(db.data) .

Holds your db content. If you're using the adapters coming with lowdb, it can be any type supported byJSON.stringify .

For example:

Adapters for reading and writing JSON files.

In-memory adapters. Useful for speeding up unit tests.

Synchronous adapter forwindow.localStorage .

Adapters for reading and writing text. Useful for creating custom adapters.

If you've published an adapter for lowdb, feel free to create a PR to add it here.

You may want to create an adapter to writedb.data to YAML, XML, encrypt data, a remote storage, ...

An adapter is a simple class that just needs to expose two methods:

For example, let's say you have some async storage and want to create an adapter for it:

Seesrc/adapters/ for more examples.

To create an adapter for another format than JSON, you can useTextFile or TextFileSync .

For example:

Lowdb doesn't support Node's cluster module.

If you have large JavaScript objects ( ~10-100MB ) you may hit some performance issues. This is because whenever you call db.write , the whole db.data is serialized using JSON.stringify and written to storage.

Depending on your use case, this can be fine or not. It can be mitigated by doing batch operations and callingdb.write only when you need it.

If you plan to scale, it's highly recommended to use databases like PostgreSQL or MongoDB instead.

直接读写文件，再把读出来的文件内容格式化成json，再用JDBC、Mybatis或者其他框架将json数据存入数据库。

假设实体类是这样的：

public class ElectSet {

public String xueqi

public String xuenian

public String startTime

public String endTime

public int menshu

public String isReadDB

//{"xueqi":,"xuenian":,"startTime":,"endTime":,"renshu":,"isReadDB":}

public String getXueqi() {

return xueqi

}

public void setXueqi(String xueqi) {

this.xueqi = xueqi

}

public String getXuenian() {

return xuenian

}

public void setXuenian(String xuenian) {

this.xuenian = xuenian

}

public String getStartTime() {

return startTime

}

public void setStartTime(String startTime) {

this.startTime = startTime

}

public String getEndTime() {

return endTime

}

public void setEndTime(String endTime) {

this.endTime = endTime

}

public int getMenshu() {

return menshu

}

public void setMenshu(int menshu) {

this.menshu = menshu

}

public String getIsReadDB() {

return isReadDB

}

public void setIsReadDB(String isReadDB) {

this.isReadDB = isReadDB

}

}

有一个json格式的文件，存的信息如下：

Sets.json:

{"xuenian":"2007-2008","xueqi":"1","startTime":"2009-07-19 08:30","endTime":"2009-07-22 18:00","menshu":"10","isReadDB":"Y"}

具体 *** 作：

/*

* 取出文件内容，填充对象

*/

public ElectSet findElectSet(String path){

ElectSet electset=new ElectSet()

String sets=ReadFile(path)//获得json文件的内容

JSONObject jo=JSONObject.fromObject(sets)//格式化成json对象

//System.out.println("------------" jo)

//String name = jo.getString("xuenian")

//System.out.println(name)

electset.setXueqi(jo.getString("xueqi"))

electset.setXuenian(jo.getString("xuenian"))

electset.setStartTime(jo.getString("startTime"))

electset.setEndTime(jo.getString("endTime"))

electset.setMenshu(jo.getInt("menshu"))

electset.setIsReadDB(jo.getString("isReadDB"))

return electset

}

//设置属性，并保存

public boolean setElect(String path,String sets){

try {

writeFile(path,sets)

return true

} catch (IOException e) {

// TODO Auto-generated catch block

e.printStackTrace()

return false

}

}

//读文件，返回字符串

public String ReadFile(String path){

File file = new File(path)

BufferedReader reader = null

String laststr = ""

try {

//System.out.println("以行为单位读取文件内容，一次读一整行：")

reader = new BufferedReader(new FileReader(file))

String tempString = null

int line = 1

//一次读入一行，直到读入null为文件结束

while ((tempString = reader.readLine()) != null) {

//显示行号

System.out.println("line " line ": " tempString)

laststr = laststr tempString

line

}

reader.close()

} catch (IOException e) {

e.printStackTrace()

} finally {

if (reader != null) {

try {

reader.close()

} catch (IOException e1) {

}

}

}

return laststr

}

将获取到的字符串，入库即可。

json数据仅仅只能用于展示display，如果用于条件查询，数据更新其效率是很低的，而且难于优化，不要尝试在json字段上进行查询优化。

虽然mysql5.7支持了json类型，但mysql作为关系型数据库，对标准化的column-per-value支持更好，包括数据类型限制、长度限制，唯一索引限制，查询索引优化，外键关联，关联查询支持，运算支持等，这些都是json中key无法达到的。

将常用的查询字段从json数据中剥离出来形成单独的字段，虽然可以改善查询问题，但你最好有先见之明，如果后期进行剥离就会涉及代码修改和数据迁移，遇到多版本的话，还可能出现数据冗余的问题，处理不好还会出现数据不一致问题，并不仅仅这么简单，一定慎用。

存储json的text类型性能并不乐观。

大JSON的解析性能同样不乐观，而且对于中文数据，纯JSON太占空间了。

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