Jackson

Jackson可以轻松的将Java对象转换成json对象和xml文档，同样也可以将json、xml转换成Java对象。在项目中如果要引入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项目已经自动依赖了jackson-core与jackson-annotation，不需要额外重复引入。

Convert Java to JSON

首先声明有一个简单的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`

}

然后创建一个ObjectMapper实例用于进行转化：

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);

我们可以参考一个实例，将某个Staff的信息转化为JSON然后写入到文件中，首先来定义实体类：

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

然后具体的将Java实体类转化为JSON的语句为：

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;

}

}

最终的输出为：

//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：

{ "firstName" : "Bob" }

Ignore:属性忽略

public class Value {

public int value;

@JsonIgnore public int internalValue;

}

最终生成的JSON是如下格式：

{ "value" : 42 }

也可以在类的头部统一声明:

@JsonIgnoreProperties({ "extra", "uselessValue" })

public class Value {

public int value;

}

那么如下的JSON字符串也是可以被转化为该实体类的:

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

对于意外地未知属性，也可以统一忽略：

@JsonIgnoreProperties(ignoreUnknown=true)

public class PojoWithAny {

public int value;

}

@JsonView:动态控制展示的成员变量

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，如果是选择了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;

在进行Object转化为JSON的过程中，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 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的实体类同样需要用到ObjectMapper对象：

mapper.writeValue(new File("result.json"), myResultObject);

// or:

byte[] jsonBytes = mapper.writeValueAsBytes(myResultObject);

// or:

String jsonString = mapper.writeValueAsString(myResultObject);

而如果我们要将JSON转化为Java中的List或者Map的话，可以采用如下方式：

//将某个JSON转化为List

String json = "[{\"name\":\"mkyong\"}, {\"name\":\"laplap\"}]";

List list = mapper.readValue(json, new TypeReference>(){});

//将某个JSON转化为Map

String json = "{\"name\":\"mkyong\", \"age\":33}";

Map map = mapper.readValue(json, new TypeReference>(){});

还是来看一个实例，：

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，类似于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

默认情况下，Jackson使用默认的构造器创建新的对象，不过你也可以使用@JsonCreator与@JsonProperty注解来自定义对象创建函数与值的绑定。

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 delegate) {

_x = (Integer) delegate.get("x");

_y = (Integer) delegate.get("y");

}

}