Java中的JSON數(shù)據(jù)綁定框架Jackson使用介紹

mengera88 發(fā)布于2019-08-14 14:06 / 1318人閱讀

摘要：系列文章地址文檔可以輕松的將對象轉(zhuǎn)換成對象和文檔，同樣也可以將轉(zhuǎn)換成對象。在項目中如果要引入，可以直接利用或者引入注意，項目已經(jīng)自動依賴了與，不需要額外重復(fù)引入。

Github 系列文章地址

Jackson

jackson-databind文檔

Jackson可以輕松的將Java對象轉(zhuǎn)換成json對象和xml文檔，同樣也可以將json、xml轉(zhuǎn)換成Java對象。在項目中如果要引入Jackson，可以直接利用Maven或者Gradle引入：


  ...
  
  2.7.0
  ...



  ...
  
    com.fasterxml.jackson.core
    jackson-databind
    ${jackson.version}
  
  ...

注意，databind項目已經(jīng)自動依賴了jackson-core與jackson-annotation，不需要額外重復(fù)引入。

Convert Java to JSON

首先聲明有一個簡單的POJO:

// 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`
}

然后創(chuàng)建一個ObjectMapper實例用于進(jìn)行轉(zhuǎn)化：

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);

我們可以參考一個實例，將某個Staff的信息轉(zhuǎn)化為JSON然后寫入到文件中，首先來定義實體類：


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 skills;

    //getters and setters

然后具體的將Java實體類轉(zhuǎn)化為JSON的語句為：

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

        staff.setSkills(skills);

        return staff;

    }

}

最終的輸出為：


//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:屬性處理 Rename:屬性重命名

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

在將Name實體類轉(zhuǎn)化為JSON的時候，就會變成：

{ "firstName" : "Bob" }

Ignore:屬性忽略

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

最終生成的JSON是如下格式：

{ "value" : 42 }

也可以在類的頭部統(tǒng)一聲明:

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

那么如下的JSON字符串也是可以被轉(zhuǎn)化為該實體類的:

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

對于意外地未知屬性，也可以統(tǒng)一忽略：

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

@JsonView:動態(tài)控制展示的成員變量

首先定義一個簡單的View控制類：

package com.mkyong.json;

public class Views {

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

}

在下面的代碼實現(xiàn)中，如果是選擇了Normal View，那么salary屬性將會被隱藏，而在Manager View狀態(tài)下，任何屬性都會被展示。

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 skills;

在進(jìn)行Object轉(zhuǎn)化為JSON的過程中，進(jìn)行視圖控制：

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

        staff.setSkills(skills);

        return staff;

    }

}

最終輸出的結(jié)果為:

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

將JSON轉(zhuǎn)化為Java的實體類同樣需要用到ObjectMapper對象：


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

而如果我們要將JSON轉(zhuǎn)化為Java中的List或者M(jìn)ap的話，可以采用如下方式：

//將某個JSON轉(zhuǎn)化為List

String json = "[{"name":"mkyong"}, {"name":"laplap"}]";
List list = mapper.readValue(json, new TypeReference>(){});
//將某個JSON轉(zhuǎn)化為Map

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

還是來看一個實例，：


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();
        }
    }

}

最終的輸出為：

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數(shù)據(jù)類型，類似于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:自定義構(gòu)造器

默認(rèn)情況下，Jackson使用默認(rèn)的構(gòu)造器創(chuàng)建新的對象，不過你也可以使用@JsonCreator與@JsonProperty注解來自定義對象創(chuàng)建函數(shù)與值的綁定。

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 delegate) {
      _x = (Integer) delegate.get("x");
      _y = (Integer) delegate.get("y");
   }
}

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