Java中的JSON数据绑定框架Jackson使用介绍

时间 2019-11-16

标签 java json 数据绑定框架 jackson 使用介绍栏目 Java 繁體版

原文原文链接

Github 系列文章地址java

Jackson

jackson-databind文档python

Jackson能够轻松的将Java对象转换成json对象和xml文档，一样也能够将json、xml转换成Java对象。在项目中若是要引入Jackson，能够直接利用Maven或者Gradle引入：git

<properties>
  ...
  <!-- Use the latest version whenever possible. -->
  <jackson.version>2.7.0</jackson.version>
  ...
</properties>

<dependencies>
  ...
  <dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>${jackson.version}</version>
  </dependency>
  ...
</dependencies>

注意，databind项目已经自动依赖了jackson-core与jackson-annotation，不须要额外重复引入。github

Convert Java to JSON

首先声明有一个简单的POJO:json

// Note: can use getters/setters as well; here we just use public fields directly:
public class MyValue {
  public String name;
  public int age;
  // NOTE: if using getters/setters, can keep fields `protected` or `private`
}

而后建立一个ObjectMapper实例用于进行转化：api

ObjectMapper mapper = new ObjectMapper(); // create once, reuse

MyValue value = mapper.readValue(new File("data.json"), MyValue.class);
// or:
value = mapper.readValue(new URL("http://some.com/api/entry.json"), MyValue.class);
// or:
value = mapper.readValue("{\"name\":\"Bob\", \"age\":13}", MyValue.class);

咱们能够参考一个实例，将某个Staff的信息转化为JSON而后写入到文件中，首先来定义实体类：app

package com.mkyong.json;

import java.math.BigDecimal;
import java.util.List;

public class Staff {

    private String name;
    private int age;
    private String position;
    private BigDecimal salary;
    private List<String> skills;

    //getters and setters

而后具体的将Java实体类转化为JSON的语句为：less

package com.mkyong.json;

import java.io.File;
import java.io.IOException;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Jackson2Example {

    public static void main(String[] args) {
        Jackson2Example obj = new Jackson2Example();
        obj.run();
    }

    private void run() {
        ObjectMapper mapper = new ObjectMapper();

        Staff staff = createDummyObject();

        try {
            // Convert object to JSON string and save into a file directly
            mapper.writeValue(new File("D:\\staff.json"), staff);

            // Convert object to JSON string
            String jsonInString = mapper.writeValueAsString(staff);
            System.out.println(jsonInString);

            // Convert object to JSON string and pretty print
            jsonInString = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff);
            System.out.println(jsonInString);

        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private Staff createDummyObject() {

        Staff staff = new Staff();

        staff.setName("mkyong");
        staff.setAge(33);
        staff.setPosition("Developer");
        staff.setSalary(new BigDecimal("7500"));

        List<String> skills = new ArrayList<>();
        skills.add("java");
        skills.add("python");

        staff.setSkills(skills);

        return staff;

    }

}

最终的输出为：函数

//new json file is created in D:\\staff.json"

{"name":"mkyong","age":33,"position":"Developer","salary":7500,"skills":["java","python"]}

{
  "name" : "mkyong",
  "age" : 33,
  "position" : "Developer",
  "salary" : 7500,
  "skills" : [ "java", "python" ]
}

Properties:属性处理

Rename:属性重命名

public class Name {
  @JsonProperty("firstName")
  public String _first_name;
}

在将Name实体类转化为JSON的时候，就会变成：this

{ "firstName" : "Bob" }

Ignore:属性忽略

public class Value {
  public int value;
  @JsonIgnore public int internalValue;
}

最终生成的JSON是以下格式：

{ "value" : 42 }

也能够在类的头部统一声明:

@JsonIgnoreProperties({ "extra", "uselessValue" })
public class Value {
  public int value;
}

那么以下的JSON字符串也是能够被转化为该实体类的:

{ "value" : 42, "extra" : "fluffy", "uselessValue" : -13 }

对于意外地未知属性，也能够统一忽略：

@JsonIgnoreProperties(ignoreUnknown=true)
public class PojoWithAny {
  public int value;
}

@JsonView:动态控制展现的成员变量

首先定义一个简单的View控制类：

package com.mkyong.json;

public class Views {

    public static class Normal{};
    
    public static class Manager extends Normal{};

}

在下面的代码实现中，若是是选择了Normal View，那么salary属性将会被隐藏，而在Manager View状态下，任何属性都会被展现。

package com.mkyong.json;

import java.math.BigDecimal;
import java.util.List;
import com.fasterxml.jackson.annotation.JsonView;

public class Staff {

    @JsonView(Views.Normal.class)
    private String name;

    @JsonView(Views.Normal.class)
    private int age;

    @JsonView(Views.Normal.class)
    private String position;

    @JsonView(Views.Manager.class)
    private BigDecimal salary;

    @JsonView(Views.Normal.class)
    private List<String> skills;

在进行Object转化为JSON的过程当中，进行视图控制：

package com.mkyong.json;

import java.io.IOException;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Jackson2Example {

    public static void main(String[] args) {
        Jackson2Example obj = new Jackson2Example();
        obj.run();
    }

    private void run() {
        ObjectMapper mapper = new ObjectMapper();

        Staff staff = createDummyObject();

        try {

            // Salary will be hidden
            System.out.println("Normal View");
            String normalView = mapper.writerWithView(Views.Normal.class).writeValueAsString(staff);
            System.out.println(normalView);

            String jsonInString = "{\"name\":\"mkyong\",\"age\":33,\"position\":\"Developer\",\"salary\":7500,\"skills\":[\"java\",\"python\"]}";
            Staff normalStaff = mapper.readerWithView(Views.Normal.class).forType(Staff.class).readValue(jsonInString);
            System.out.println(normalStaff);

            // Display everything
            System.out.println("\nManager View");
            String managerView = mapper.writerWithView(Views.Manager.class).writeValueAsString(staff);
            System.out.println(managerView);

            Staff managerStaff = mapper.readerWithView(Views.Manager.class).forType(Staff.class).readValue(jsonInString);
            System.out.println(managerStaff);

        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private Staff createDummyObject() {

        Staff staff = new Staff();

        staff.setName("mkyong");
        staff.setAge(33);
        staff.setPosition("Developer");
        staff.setSalary(new BigDecimal("7500"));

        List<String> skills = new ArrayList<>();
        skills.add("java");
        skills.add("python");

        staff.setSkills(skills);

        return staff;

    }

}

最终输出的结果为:

Normal View
{"name":"mkyong","age":33,"position":"Developer","skills":["java","python"]}
Staff [name=mkyong, age=33, position=Developer, salary=null, skills=[java, python]]

Manager View
{"name":"mkyong","age":33,"position":"Developer","salary":7500,"skills":["java","python"]}
Staff [name=mkyong, age=33, position=Developer, salary=7500, skills=[java, python]]

Convert JSON to Java

将JSON转化为Java的实体类一样须要用到ObjectMapper对象：

mapper.writeValue(new File("result.json"), myResultObject);
// or:
byte[] jsonBytes = mapper.writeValueAsBytes(myResultObject);
// or:
String jsonString = mapper.writeValueAsString(myResultObject);

而若是咱们要将JSON转化为Java中的List或者Map的话，能够采用以下方式：

//将某个JSON转化为List

String json = "[{\"name\":\"mkyong\"}, {\"name\":\"laplap\"}]";
List<Staff> list = mapper.readValue(json, new TypeReference<List<Staff>>(){});
//将某个JSON转化为Map

String json = "{\"name\":\"mkyong\", \"age\":33}";
Map<String, Object> map = mapper.readValue(json, new TypeReference<Map<String,Object>>(){});

仍是来看一个实例，：

package com.mkyong.json;

import java.io.File;
import java.io.IOException;
import java.math.BigDecimal;
import java.util.ArrayList;
import java.util.List;

import com.fasterxml.jackson.core.JsonGenerationException;
import com.fasterxml.jackson.databind.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Jackson2Example {

    public static void main(String[] args) {
        Jackson2Example obj = new Jackson2Example();
        obj.run();
    }

    private void run() {
        ObjectMapper mapper = new ObjectMapper();

        try {

            // Convert JSON string from file to Object
            Staff staff = mapper.readValue(new File("D:\\staff.json"), Staff.class);
            System.out.println(staff);

            // Convert JSON string to Object
            String jsonInString = "{\"name\":\"mkyong\",\"salary\":7500,\"skills\":[\"java\",\"python\"]}";
            Staff staff1 = mapper.readValue(jsonInString, Staff.class);
            System.out.println(staff1);

            //Pretty print
            String prettyStaff1 = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(staff1);
            System.out.println(prettyStaff1);
            
        } catch (JsonGenerationException e) {
            e.printStackTrace();
        } catch (JsonMappingException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

}

最终的输出为：

Staff [name=mkyong, age=33, position=Developer, salary=7500, skills=[java, python]]

Staff [name=mkyong, age=0, position=null, salary=7500, skills=[java, python]]

{
  "name" : "mkyong",
  "age" : 0,
  "position" : null,
  "salary" : 7500,
  "skills" : [ "java", "python" ]
}

Tree Model:抽象的JSON数据类型，相似于FastJSON中的JSONObject

// can be read as generic JsonNode, if it can be Object or Array; or,
// if known to be Object, as ObjectNode, if array, ArrayNode etc:
ObjectNode root = mapper.readTree("stuff.json");
String name = root.get("name").asText();
int age = root.get("age").asInt();

// can modify as well: this adds child Object as property 'other', set property 'type'
root.with("other").put("type", "student");
String json = mapper.writeValueAsString(root);

// with above, we end up with something like as 'json' String:
// {
//   "name" : "Bob", "age" : 13,
//   "other" : {
//      "type" : "student"
//   }
// }

Constructor:自定义构造器

默认状况下，Jackson使用默认的构造器建立新的对象，不过你也能够使用@JsonCreator与@JsonProperty注解来自定义对象建立函数与值的绑定。

public class CtorPOJO {
   private final int _x, _y;

   @JsonCreator
   public CtorPOJO(@JsonProperty("x") int x, @JsonProperty("y") int y) {
      _x = x;
      _y = y;
   }
}

public class DelegatingPOJO {
   private final int _x, _y;

   @JsonCreator
   public DelegatingPOJO(Map<String,Object> delegate) {
      _x = (Integer) delegate.get("x");
      _y = (Integer) delegate.get("y");
   }
}