4,JsonPath进阶实战--字符串转POJO

1,json数据和POJO的关系

目前微服务非常火,前后端分离成为主流趋势,接口返回的数据主要以json数据为主。后端研发需要将POJO转为json字符串。微服务之间的调用或调用第三方接口返回的json数据需要转为POJO对象进行逻辑处理。

目前使用的比较多的json和POJO互转的工具包有：
FastJson （每年都要爆出多次漏洞）
FastJson2
Gson
Jackson （Spring内置）
Hutool
json-lib
JsonPath (本次主角)

2,JsonPath返回结果

在java中使用JsonPath时,知道返回的结果类型是很重要的。JsonPath将自动尝试将结果强制转换为调用方所期望的类型。

//Will throw an java.lang.ClassCastException    
List list = JsonPath.parse(json).read("$.store.book[0].author")

//Works fine
String author = JsonPath.parse(json).read("$.store.book[0].author")

计算路径时需要知道路径是明确的还是不明确的,明确的返回单个值,不明确的返回一个List.

不明确的路径有如下几种情况:

• .. - 深度扫描操作
• ?() - 表达式
• [, (, )] - 多个数组索引

3,JsonPath提供的MappingProvider和JsonProvider

默认情况下，一个简单的对象映射器由MappingProvider SPI提供。JsonPath提供了默认的实现： JsonSmartJsonProvider 和 JsonSmartMappingProvider。但是不能将json字符串转为POJO对象。

JsonPath提供的MappingProvider如下：

• JsonSmartMappingProvider(默认)
• GsonMappingProvider
• JacksonMappingProvider
• JakartaMappingProvider
• JsonOrgMappingProvider
• TapestryMappingProvider

JsonPath提供的JsonProvider如下：

• JsonSmartJsonProvider (默认)
• GsonJsonProvider
• JacksonJsonNodeJsonProvider
• JacksonJsonProvider
• JakartaJsonProvider
• JsonOrgJsonProvider
• TapestryJsonProvider

4,JsonPath配置，支持json转POJO

在初始化应用程序时才能更改所配置默认值。强烈反对在运行时进行更改，尤其是在多线程应用程序中。

Configuration.setDefaults(new Configuration.Defaults() {

    private final JsonProvider jsonProvider = new JacksonJsonProvider();
    private final MappingProvider mappingProvider = new JacksonMappingProvider();
      
    @Override
    public JsonProvider jsonProvider() {
        return jsonProvider;
    }

    @Override
    public MappingProvider mappingProvider() {
        return mappingProvider;
    }
    
    @Override
    public Set options() {
        return EnumSet.noneOf(Option.class);
    }
});

注意：
JacksonJsonProvider 要求 com.fasterxml.jackson.core:jackson-databind:2.4.5
GsonJsonProvider 要求 com.google.code.gson:gson:2.3.1

5,实战 准备数据

{
    "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
}

准备POJO对象

@Data
public static class Book {

	
	private String category;
	
	private String author;
	
	private String title;
	
	private Double price;

	
	private String isbn;
}

@Data
public static class Bicycle {
	
	private String color;
	
	private Double price;
}

@Data
public static class Store {
	
	private List book;
	
	private Bicycle bicycle;
}

@Data
public static  class Demo {
	
	private Store store;

	
	private Integer expensive;

}

maven 坐标

  
    org.projectlombok  
    lombok  
    1.18.16  


  
   cn.hutool  
   hutool-all  
   5.3.9  



    com.jayway.jsonpath
    json-path
    2.7.0


  
  
   com.fasterxml.jackson.core  
   jackson-databind  
   2.13.1  

5.1,json转Book

代码：

@Test
public void json2Pojo(){
	Configuration.setDefaults(new Configuration.Defaults() {
		@Override
		public MappingProvider mappingProvider() {
			// 需要引入 jackson-databind jar
			return new JacksonMappingProvider();
		}

		@Override
		public JsonProvider jsonProvider() {
			return new JacksonJsonProvider();
		}

		@Override
		public Set options() {
			return EnumSet.noneOf(Option.class);
		}
	});
	Book book = JsonPath.parse(jsonData).read("$.store.book[0]", Book.class);
	Console.log(JSONUtil.toJsonStr(book));
}

输出：

{"author":"Nigel Rees","title":"Sayings of the Century","price":8.95,"category":"reference"}

5.2,json转List

代码：

@Test
public void json2List(){
	Configuration.setDefaults(new Configuration.Defaults() {
		@Override
		public MappingProvider mappingProvider() {
			// 需要引入 jackson-databind jar
			return new JacksonMappingProvider();
		}

		@Override
		public JsonProvider jsonProvider() {
			return new JacksonJsonProvider();
		}

		@Override
		public Set options() {
			return EnumSet.noneOf(Option.class);
		}
	});
	TypeRef> typeRef = new TypeRef>(){};
	List books = JsonPath.parse(jsonData).read("$..book[*]", typeRef);
	Console.log(JSONUtil.toJsonStr(books));
}

输出：

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

5.3,json转复杂POJO

代码：

@Test
public void json2ComplexPojo(){
	Configuration.setDefaults(new Configuration.Defaults() {
		@Override
		public MappingProvider mappingProvider() {
			// 需要引入 jackson-databind jar
			return new JacksonMappingProvider();
		}

		@Override
		public JsonProvider jsonProvider() {
			return new JacksonJsonProvider();
		}

		@Override
		public Set options() {
			return EnumSet.noneOf(Option.class);
		}
	});
	TypeRef typeRef = new TypeRef(){};
	// $ 代表整个json字符串
	Demo demo = JsonPath.parse(jsonData).read("$", typeRef);
	Console.log(JSONUtil.toJsonStr(demo));
}

输出：

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

6,总结

• json转嵌套多或复杂的POJO，使用TypeRef。
• JsonPath可以结合断言实现对json数据过滤。
• JsonPath返回的List数据是按json字符串出现的顺序返回，即有序的。