JsonPath：针对json的强大的规则解析与参数查找工具

项目特点

GitHub项目地址：https://github.com/json-path/JsonPath
主要功能：

1. 将Json字符串转为Java Map对象（这个不算什么，FastJson之类的工具都可以）
2. 通过强大的规则表达式定位字段，返回字段值或值集合（很厉害）

支持的规则表达式以及示例（选自项目readme）：

JsonPath (点击测试)	结果
$.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
$.store.book[?(@.price < 10)]	All books in store cheaper than 10
..book[?(@.price<=$..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

实战测试

就用项目readme文档提供的例子进行测试：

json格式字符串，点击查看

{
    "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
}

POM中引入依赖：

<dependency>
    <groupId>com.jayway.jsonpath</groupId>
    <artifactId>json-path</artifactId>
    <version>2.7.0</version>
</dependency>

写一个单元测试类：

@SpringBootTest
@RunWith(MockitoJUnitRunner.class)
public class JsonPathTest {
    private final String jsonStr = "...";
    private final Object context = Configuration.defaultConfiguration().jsonProvider().parse(jsonStr);

    @Test
    public void test(){}
}

其中：

1. 第二个注解可以做到启动SpringBoot测试类而无需启动SpringApplication，并且也可以做到组件注入等功能。
2. jsonStr用来指代之前所述的json字符串，这里简略写。
3. context实际上是由json字符串通过默认配置生成的一个Map<String, Object>。

先看看context的类型：

System.out.println("context的类型：" + context.getClass().getSimpleName() + "\n");
------------------------------------------------------------------
output: context的类型：LinkedHashMap

用法1：通过key定位一个value：

jsonpath = "$.store.book[1].price";
result = JsonPath.read(testMap, jsonpath);
System.out.println("contents on " + jsonpath + ": " + result + "\n");
------------------------------------------------------------------
output: contents on $.store.book[1].price: 12.99

用法2：通过key定位一组value：

jsonpath = "$.store.book[*].price";
result = JsonPath.read(context, jsonpath);
System.out.println("contents on " + jsonpath + ": " + result + "\n");
------------------------------------------------------------------
output: contents on $.store.book[*].price: [8.95,12.99,8.99,22.99]

这里如果去查看result的type，会发现它是JSONArray类型，是List<Object>接口的实现，当List用就可以了。

用法3：向下递归查找：

jsonpath = "$..price";
result = JsonPath.read(context, jsonpath);
System.out.println("contents on " + jsonpath + ": " + result);
------------------------------------------------------------------
output: contents on $..price: [8.95,12.99,8.99,22.99,19.95]

在测试的时候想起一个问题，设想这样的场景，如果我们已经有了这样一个Map，只需要JsonPath提供的规则表达式解析与查找能力，那么想要使用JsonPath是不是需要Map->JsonString->context->规则解析->查找结果？答案是不需要的。
我们已经知道这个context本就是一个Map类型，那么是不是可以省去中间转为json string的步骤呢？我们测试看看：

//1. 先将json解析为一个Map
Map<String, Object> testMap = (Map) JSONObject.parseObject(JSON_STRING);
jsonpath = "$..price";
//2. 再将Map传入read方法，看是否能查找到结果：
result = JsonPath.read(testMap, jsonpath);
System.out.println("contents on " + jsonpath + ": " + result);
------------------------------------------------------------------
output: contents on $..price: [8.95,12.99,8.99,22.99,19.95]

可以看出，JsonPath的搜索规则解析能力是支持对普通Map进行查找的。