杰克逊–如何解析JSON

╰+哭是因爲堅強的太久メ 2023-02-15 05:06 29阅读 0赞

[Jackson][]提供了`writeValue()`和`readValue()`方法来将Java对象转换为JSON。

`mapper.writeValue` – Java对象转换为JSON

ObjectMapper mapper = new ObjectMapper();
    
    	// Java object to JSON file
    	mapper.writeValue(new File("c:\\test\\staff.json"), new Staff());
    
    	// Java object to JSON string, default compact-print
    	String jsonString = mapper.writeValueAsString(new Staff());
    	
    	// pretty-print
    	String jsonString2 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(new Staff());

`mapper.readValue` – JSON到Java对象

ObjectMapper mapper = new ObjectMapper();
    
    	//JSON file to Java object
    	Staff obj = mapper.readValue(new File("c:\\test\\staff.json"), Staff.class);
    
    	//JSON URL to Java object
    	Staff obj = mapper.readValue(new URL("http://some-domains/api/staff.json"), Staff.class);
    
    	//JSON string to Java Object
    	Staff obj = mapper.readValue("{'name' : 'mkyong'}", Staff.class);

*PS已在Jackson 2.9.8中测试*

## 1.下载杰克逊 ##

声明`jackson-databind` ，它将`jackson-annotations`和`jackson-core`

pom.xml

<dependency>
    		<groupId>com.fasterxml.jackson.core</groupId>
    		<artifactId>jackson-databind</artifactId>
    		<version>2.9.8</version>
    	</dependency>

Terminal

$ mvn dependency:tree
    
    \- com.fasterxml.jackson.core:jackson-databind:jar:2.9.8:compile
    [INFO]    +- com.fasterxml.jackson.core:jackson-annotations:jar:2.9.0:compile
    [INFO]    \- com.fasterxml.jackson.core:jackson-core:jar:2.9.8:compile

## 2. POJO ##

一个简单的Java对象POJO，供以后测试。

Staff.java

public class Staff {
    
        private String name;
        private int age;
        private String[] position;              //  Array
        private List<String> skills;            //  List
        private Map<String, BigDecimal> salary; //  Map
    
    	// getters , setters, some boring stuff
    }

## 3. Java对象到JSON ##

JacksonExample1.java

package com.mkyong;
    
    import com.fasterxml.jackson.databind.ObjectMapper;
    
    import java.io.File;
    import java.io.IOException;
    import java.math.BigDecimal;
    import java.util.Arrays;
    import java.util.HashMap;
    import java.util.Map;
    
    public class JacksonExample1 {
    
        public static void main(String[] args) {
    
            ObjectMapper mapper = new ObjectMapper();
    
            Staff staff = createStaff();
    
            try {
    
                // Java objects to JSON file
                mapper.writeValue(new File("c:\\test\\staff.json"), staff);
    
                // Java objects to JSON string - compact-print
                String jsonString = mapper.writeValueAsString(staff);
    
                System.out.println(jsonString);
    
                // Java objects to JSON string - pretty-print
                String jsonInString2 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff);
    
                System.out.println(jsonInString2);
    
            } catch (IOException e) {
                e.printStackTrace();
            }
    
        }
    
        private static Staff createStaff() {
    
            Staff staff = new Staff();
    
            staff.setName("mkyong");
            staff.setAge(38);
            staff.setPosition(new String[]{"Founder", "CTO", "Writer"});
            Map<String, BigDecimal> salary = new HashMap() {
        {
                put("2010", new BigDecimal(10000));
                put("2012", new BigDecimal(12000));
                put("2018", new BigDecimal(14000));
            }};
            staff.setSalary(salary);
            staff.setSkills(Arrays.asList("java", "python", "node", "kotlin"));
    
            return staff;
    
        }
    
    }

输出量

c:\\\\test\\\\staff.json

{"name":"mkyong","age":38,"position":["Founder","CTO","Writer"],"skills":["java","python","node","kotlin"],"salary":{"2018":14000,"2012":12000,"2010":10000}}

Terminal

{"name":"mkyong","age":38,"position":["Founder","CTO","Writer"],"skills":["java","python","node","kotlin"],"salary":{"2018":14000,"2012":12000,"2010":10000}}
    
    {
      "name" : "mkyong",
      "age" : 38,
      "position" : [ "Founder", "CTO", "Writer" ],
      "skills" : [ "java", "python", "node", "kotlin" ],
      "salary" : {
        "2018" : 14000,
        "2012" : 12000,
        "2010" : 10000
      }
    }

## 4. JSON到Java对象 ##

JacksonExample2.java

package com.mkyong;
    
    import com.fasterxml.jackson.databind.ObjectMapper;
    
    import java.io.File;
    import java.io.IOException;
    
    public class JacksonExample2 {
    
        public static void main(String[] args) {
    
            ObjectMapper mapper = new ObjectMapper();
    
            try {
    
                // JSON file to Java object
                Staff staff = mapper.readValue(new File("c:\\test\\staff.json"), Staff.class);
    
                // JSON string to Java object
                String jsonInString = "{\"name\":\"mkyong\",\"age\":37,\"skills\":[\"java\",\"python\"]}";
                Staff staff2 = mapper.readValue(jsonInString, Staff.class);
    
                // compact print
                System.out.println(staff2);
    
                // pretty print
                String prettyStaff1 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff2);
    
                System.out.println(prettyStaff1);
    
    
            } catch (IOException e) {
                e.printStackTrace();
            }
    
        }
    
    }

输出量

Staff{name='mkyong', age=37, position=null, skills=[java, python], salary=null}
    
    {
      "name" : "mkyong",
      "age" : 37,
      "position" : null,
      "skills" : [ "java", "python" ],
      "salary" : null
    }

## 5. @JsonProperty – JSON字段命名 ##

5.1默认

public class Staff {
    
        private String name;
    	private int age;

输出量

{"name":"abc", "age":38}

5.2使用`@JsonProperty`更改属性名称

public class Staff {
    
        @JsonProperty("custom_name")
        private String name;
    	private int age;

输出量

{"custom_name":"abc", "age":38}

## 6. @JsonInclude –忽略空字段 ##

默认情况下，Jackson将包含`null`字段。

{
      "name" : "mkyong",
      "age" : 38,
      "position" : null,
      "skills" : null,
      "salary" : null
    }

6.1 `@JsonInclude`在课程级别。

Staff.java

import com.fasterxml.jackson.annotation.JsonInclude;
    
    											//	ignore null fields , class level
    @JsonInclude(JsonInclude.Include.NON_NULL) 	//  ignore all null fields
    public class Staff {
    
        private String name;
        private int age;
        private String[] position;              
        private List<String> skills;           
        private Map<String, BigDecimal> salary; 
    	//...

6.2 `@JsonInclude`在字段级别。

Staff.java

import com.fasterxml.jackson.annotation.JsonInclude;
    
    public class Staff {
    
        private String name;
        private int age;
        
    	@JsonInclude(JsonInclude.Include.NON_NULL) //ignore null field on this property only
        private String[] position;              
        
        private List<String> skills;           
    	
        private Map<String, BigDecimal> salary;

输出量

{
      "name" : "mkyong",
      "age" : 38,
      "skill" : null,
      "salary" : null
    }

6.3全球。

ObjectMapper mapper = new ObjectMapper();
    
    	// ignore all null fields globally
    	mapper.setSerializationInclusion(JsonInclude.Include.NON_NULL);

> **注意**  
> 有关[如何使用Jackson忽略空字段的][Jackson 1]更多示例

## 7. @JsonView ##

7.1 `@JsonView`用于限制不同用户的字段显示。 例如：

CompanyViews.java

package com.mkyong;
    
    public class CompanyViews {
    
        public static class Normal{};
    
        public static class Manager extends Normal{};
        
    }

普通视图仅显示姓名和年龄，经理视图可以显示全部。

Staff.java

import com.fasterxml.jackson.annotation.JsonView;
    
    import java.math.BigDecimal;
    import java.util.Arrays;
    import java.util.List;
    import java.util.Map;
    
    public class Staff {
    
        @JsonView(CompanyViews.Normal.class)
        private String name;
    
        @JsonView(CompanyViews.Normal.class)
        private int age;
    
        @JsonView(CompanyViews.Manager.class)
        private String[] position;
    
        @JsonView(CompanyViews.Manager.class)
        private List<String> skills;
    
        @JsonView(CompanyViews.Manager.class)
        private Map<String, BigDecimal> salary;

JacksonExample.java

package com.mkyong;
    
    import com.fasterxml.jackson.databind.ObjectMapper;
    import com.fasterxml.jackson.databind.SerializationFeature;
    
    import java.io.IOException;
    import java.math.BigDecimal;
    import java.util.Arrays;
    import java.util.HashMap;
    import java.util.Map;
    
    public class JacksonExample {
    
        public static void main(String[] args) {
    
            ObjectMapper mapper = new ObjectMapper();
    
            Staff staff = createStaff();
    
            try {
    
                // to enable pretty print
                mapper.enable(SerializationFeature.INDENT_OUTPUT);
    
                // normal
                String normalView = mapper
    				.writerWithView(CompanyViews.Normal.class)
    				.writeValueAsString(staff);
    
                System.out.format("Normal views\n%s\n", normalView);
    
                // manager
                String managerView = mapper
    				.writerWithView(CompanyViews.Manager.class)
    				.writeValueAsString(staff);
    
                System.out.format("Manager views\n%s\n", managerView);
    
            } catch (IOException e) {
                e.printStackTrace();
            }
    
        }
    
        private static Staff createStaff() {
    
            Staff staff = new Staff();
    
            staff.setName("mkyong");
            staff.setAge(38);
            staff.setPosition(new String[]{"Founder", "CTO", "Writer"});
            Map<String, BigDecimal> salary = new HashMap() {
        {
                put("2010", new BigDecimal(10000));
                put("2012", new BigDecimal(12000));
                put("2018", new BigDecimal(14000));
            }};
            staff.setSalary(salary);
            staff.setSkills(Arrays.asList("java", "python", "node", "kotlin"));
    
            return staff;
    
        }
    
    }

输出量

Normal views
    {
      "name" : "mkyong",
      "age" : 38
    }
    
    Manager views
    {
      "name" : "mkyong",
      "age" : 38,
      "position" : [ "Founder", "CTO", "Writer" ],
      "skills" : [ "java", "python", "node", "kotlin" ],
      "salary" : {
        "2018" : 14000,
        "2012" : 12000,
        "2010" : 10000
      }
    }

> **注意**  
> 阅读此[Jackson @JsonView示例][Jackson _JsonView]

## 8. @JsonIgnore和@JsonIgnoreProperties ##

默认情况下，Jackson包含所有字段，甚至包括`static`或`transient`字段。

8.1 `@JsonIgnore`忽略字段级别的字段。

import com.fasterxml.jackson.annotation.JsonIgnore;
    
    public class Staff {
    
        private String name;
        private int age;
        private String[] position;
    	
        @JsonIgnore
        private List<String> skills;
    	
        @JsonIgnore
        private Map<String, BigDecimal> salary;

输出量

{
      "name" : "mkyong",
      "age" : 38,
      "position" : [ "Founder", "CTO", "Writer" ]
    }

8.2 `@JsonIgnoreProperties`忽略类级别的字段。

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
    
    @JsonIgnoreProperties({"salary", "position"})
    public class Staff {
    
        private String name;
        private int age;
        private String[] position;
        private List<String> skills;
        private Map<String, BigDecimal> salary;

输出量

{
      "name" : "mkyong",
      "age" : 38,
      "skills" : [ "java", "python", "node", "kotlin" ]
    }

## 9.常见问题 ##

9.1 [将JSON数组字符串转换为List][JSON_List]

String json = "[{\"name\":\"mkyong\", \"age\":38}, {\"name\":\"laplap\", \"age\":5}]";
    
    	List<Staff> list = Arrays.asList(mapper.readValue(json, Staff[].class));
    	
    	// or like this:
    	// List<Staff> list = mapper.readValue(json, new TypeReference<List<Staff>>(){});

9.2 [将JSON字符串转换为Map][JSON_Map]

String json = "{\"name\":\"mkyong\", \"age\":\"33\"}";
                
    	Map<String, String> map = mapper.readValue(json, Map.class);
    	
    	// or like this:
    	//Map<String, String> map = mapper.readValue(json, new TypeReference<Map<String, String>>(){});
    
    	map.forEach((k, v) -> System.out.format("[key]:%s \t[value]:%s\n", k, v));

输出量

[key]:name 	[value]:mkyong
    [key]:age 	[value]:33

9.3如果某些复杂的JSON结构无法轻松映射到Java类怎么办？  
**答：**尝试使用[Jackson TreeModel][]将JSON数据转换为`JsonNode` ，以便我们可以轻松地添加，更新或删除JSON节点。

## 参考文献 ##

*  [杰克逊数据绑定官方网站][Jackson]
 *  [杰克逊2 –将Java对象与JSON相互转换][2 _Java_JSON]
 *  [Gson –如何解析JSON][Gson _JSON]

标签： [杰克逊][Link 1] [java][] [json][]

> 翻译自: [https://mkyong.com/java/jackson-how-to-parse-json/][https_mkyong.com_java_jackson-how-to-parse-json]

[Jackson]: https://github.com/FasterXML/jackson-databind/
[Jackson 1]: https://www.mkyong.com/java/jackson-how-to-ignore-null-fields/
[Jackson _JsonView]: https://www.mkyong.com/java/jackson-jsonview-examples/
[JSON_List]: https://www.mkyong.com/java/jackson-convert-json-array-string-to-list/
[JSON_Map]: https://www.mkyong.com/java/how-to-convert-java-map-to-from-json-jackson/
[Jackson TreeModel]: https://www.mkyong.com/java/jackson-tree-model-example/
[2 _Java_JSON]: https://www.mkyong.com/java/jackson-2-convert-java-object-to-from-json/
[Gson _JSON]: https://www.mkyong.com/java/how-to-parse-json-with-gson/
[Link 1]: https://mkyong.com/tag/jackson/
[java]: https://mkyong.com/tag/java/
[json]: https://mkyong.com/tag/json/
[https_mkyong.com_java_jackson-how-to-parse-json]: https://mkyong.com/java/jackson-how-to-parse-json/