场景

以前工作中，习惯于将对象转换为 json。并一直感觉这种转换非常的方便。

因为借助 fastjson, gjson, jackson 等工具就是一个方法就搞定。

以前写一些数据的抓取，我也会写一大堆代码，去构建这个对象。

后来知道有 mongo 这种文档型数据库，觉得也挺方便。

但是 xml 有 xmlPath, 我为什么一直都没有想到 json 有 json path 呢？？

问题

比如 google 返回的 json 如下

[[["Hello, Li Yinhe. ","你好啊，李银河。",null,null,3],["Beautiful stories are always too far away. ","美丽的故事总是太遥远。",null,null,3],["Grass along the Qingqing River, seeing the Vega.","青青河边草，遥看织女星。",null,null,3],[null,null,null,"Nǐ hǎo a, lǐ yínhé. Měilì de gùshì zǒng shì tài yáoyuǎn. Qīngqīng hé biān cǎo, yáo kàn zhīnǚxīng."]],null,"zh-CN",null,null,[["你好啊，李银河。",null,[["Hello, Li Yinhe.",0,true,false]],[[0,8]],"你好啊，李银河。",0,0],["美丽的故事总是太遥远。",null,[["Beautiful stories are always too far away.",0,true,false],["Beautiful story always too far away.",0,true,false]],[[0,11]],"美丽的故事总是太遥远。",0,0],["青青河边草，遥看织女星。",null,[["Grass along the Qingqing River, seeing the Vega.",0,true,false],["青青河边草, very thick Vega.",0,true,false]],[[0,12]],"青青河边草，遥看织女星。",0,0]],1,null,[["zh-CN"],null,[1],["zh-CN"]]]

我构建了半天都感觉这个对象不一般，而且第一个数组是会动态变化的。

我关心的字段信息，只是翻译后的字段信息。

然后就去查了一下，发现了 json path。

核心方法介绍

JsonPath表达式始终引用JSON结构，其方式与XPath表达式与XML文档结合使用的方式相同。

JsonPath中的“根成员对象”始终称为 $，无论它是对象还是数组。

访问

可以使用点访问符号

$.store.book[0].title

也可以使用中括号访问

$['store']['book'][0]['title']

操作

Operator	Description
`$`	The root element to query. This starts all path expressions.
`@`	The current node being processed by a filter predicate.
`*`	Wildcard. Available anywhere a name or numeric are required.
`..`	Deep scan. Available anywhere a name is required.
`.<name>`	Dot-notated child
`['<name>' (, '<name>')]`	Bracket-notated child or children
`[<number> (, <number>)]`	Array index or indexes
`[start:end]`	Array slice operator
`[?(<expression>)]`	Filter expression. Expression must evaluate to a boolean value.

函数

可以在路径的尾端调用函数 - 函数的输入是路径表达式的输出。

功能输出由功能本身决定。

Function	Description	Output
min()	Provides the min value of an array of numbers	Double
max()	Provides the max value of an array of numbers	Double
avg()	Provides the average value of an array of numbers	Double
stddev()	Provides the standard deviation value of an array of numbers	Double
length()	Provides the length of an array	Integer

过滤运算符

过滤器是用于过滤数组的逻辑表达式。

典型的过滤器是 [?(@.age > 18)] ，其中@表示当前正在处理的项目。

可以使用逻辑运算符 && 和 || 创建更复杂的过滤器。

字符串文字必须用单引号或双引号括起来 ([?(@.color == 'blue')] or [?(@.color == "blue")])。

Operator	Description
==	left is equal to right (note that 1 is not equal to ‘1’)
!=	left is not equal to right
<	left is less than right
<=	left is less or equal to right
>	left is greater than right
>=	left is greater than or equal to right
=~	left matches regular expression [?(@.name =~ /foo.*?/i)]
in	left exists in right [?(@.size in [‘S’, ‘M’])]
nin	left does not exists in right
subsetof	left is a subset of right [?(@.sizes subsetof [‘S’, ‘M’, ‘L’])]
anyof	left has an intersection with right [?(@.sizes anyof [‘M’, ‘L’])]
noneof	left has no intersection with right [?(@.sizes noneof [‘M’, ‘L’])]
size	size of left (array or string) should match right
empty	left (array or string) should be empty

使用 JsonPath

maven 引入

<dependency>
    <groupId>com.jayway.jsonpath</groupId>
    <artifactId>json-path</artifactId>
    <version>2.4.0</version>
</dependency>

入门案例

{"store":{"book":[{"category":"reference","author":"Nigel Rees","title":"Sayings of the Century","price":8.95},{"category":"fiction","author":"Evelyn Waugh","title":"Sword of Honour","price":12.99},{"category":"fiction","author":"Herman Melville","title":"Moby Dick","isbn":"0-553-21311-3","price":8.99},{"category":"fiction","author":"J. R. R. Tolkien","title":"The Lord of the Rings","isbn":"0-395-19395-8","price":22.99}],"bicycle":{"color":"red","price":19.95}},"expensive":10}

解析

JsonPath (click link to try)	Result
$.store.book[*].author	The authors of all books
$..author	All authors
$.store.*	All things, both books and bicycles
$.store..price	The price of everything
$..book[2]	The third book
$..book[-2]	The second to last book
$..book[0,1]	The first two books
$..book[:2]	All books from index 0 (inclusive) until index 2 (exclusive)
$..book[1:2]	All books from index 1 (inclusive) until index 2 (exclusive)
$..book[-2:]	Last two books
$..book[2:]	Book number two from tail
$..book[?(@.isbn)]	All books with an ISBN number
$.store.book[?(@.price < 10)]	All books in store cheaper than 10
$..book[?(@.price <= $[‘expensive’])]	All books in store that are not “expensive”
$..book[?(@.author =~ /.*REES/i)]	All books matching regex (ignore case)
$..*	Give me every thing
$..book.length()	The number of books

读取文档

使用JsonPath最简单最直接的方法是通过静态读取API。

List<String> authors = JsonPath.read(json, "$.store.book[*].author");

个人实战

获取所有中文的翻译

很简单的一句话即可：

List<String> stringList = JsonPath.read(json, "$.[0][:-1][0]");

信息如下：

Hello, Li Yinhe. 
Beautiful stories are always too far away. 
Grass along the Qingqing River, seeing the Vega.

性能优化

如果我们需要提取 json 中的多个数值，可以首先初始化一下 context，然后再做处理。

final String json = "{}";

// 避免多次解析
final ReadContext jsonContext = JsonPath.parse(json);

// 根据路径获取指定的值
Object value = jsonContext.read(jsonPath);

jsonpath 报错的问题

发现 jsonPath 在处理值的时候，如果值不存在，会直接报错。

怎么解决呢？

解决方案

Configuration configuration = Configuration.builder().options(Option.DEFAULT_PATH_LEAF_TO_NULL).build();
val value = JsonPath.parse(document, configuration).read("$.root.family.hasChild"));