【JSON】Jackson初学，及常用的例子

现在很多公司的项目都基于SOA架构，系统间的调用有许多方式，其中一种常见的是用HTTP协议、以JSON格式返回结果。

这使得JSON的使用更加普遍。而市面上处理JSON的框架五花八门，常见的有JSONObject、GSON、Jackson等。

现在我们来学习Jackson，及记录常用的一些例子

版本说明

<dependencies>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-core</artifactId>
        <version>2.1.4</version>
    </dependency>
    <dependency>
        <groupId>com.fasterxml.jackson.core</groupId>
        <artifactId>jackson-databind</artifactId>
        <version>2.1.4</version>
    </dependency>
</dependencies>

最简单的例子

将Bean对象转换为JSON，在还原回来。（为了得到指定格式的日期格式，我指定了日期格式）

package com.nicchagil.demo.No001最简单的例子;

import java.io.IOException;
import java.sql.Timestamp;
import java.text.SimpleDateFormat;
import java.util.Calendar;

import org.junit.Test;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Call {

    @Test
    public void writeJson() throws JsonProcessingException {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:SS"));
        
        String json = objectMapper.writeValueAsString(new User(999, "Nick Huang", new Timestamp(Calendar.getInstance().getTimeInMillis())));
        System.out.println(json);
    }
    
    @Test
    public void readJson() throws IOException {
        String json = "{"id":999,"name":"Nick Huang","birthday":"2015-10-21 15:45:673"}";
        
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:SS"));
        
        User user = objectMapper.readValue(json, User.class);
        System.out.println(user);
    }

}
View Code

还需要一个Bean类型

package com.nicchagil.demo.No001最简单的例子;

import java.sql.Timestamp;

public class User {

    private Integer id;
    private String name;
    private Timestamp birthday;

    public Integer getId() {
        return id;
    }

    public void setId(Integer id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Timestamp getBirthday() {
        return birthday;
    }

    public void setBirthday(Timestamp birthday) {
        this.birthday = birthday;
    }

    public User(Integer id, String name, Timestamp birthday) {
        super();
        this.id = id;
        this.name = name;
        this.birthday = birthday;
    }

    public User() {
        super();
    }

    @Override
    public String toString() {
        StringBuilder builder = new StringBuilder();
        builder.append("User [id=").append(id).append(", name=").append(name)
                .append(", birthday=").append(birthday).append("]");
        return builder.toString();
    }
    
}
View Code

 日志如下

{"id":999,"name":"Nick Huang","birthday":"2015-10-22 17:34:318"}
User [id=999, name=Nick Huang, birthday=2015-10-21 15:45:00.673]
View Code

有泛型、集合的情况，如何指定转换的结构

package com.nicchagil.demo.No002含泛型和集合;

import java.io.IOException;
import java.sql.Timestamp;
import java.util.Calendar;
import java.util.HashMap;
import java.util.Map;

import org.junit.Test;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Call {

    @Test
    public void writeJson() throws JsonProcessingException {
        ObjectMapper objectMapper = new ObjectMapper();
        
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("Nick", new User(999, "Nick Huang", new Timestamp(Calendar.getInstance().getTimeInMillis())));
        map.put("Robbin", new User(998, "Robbin", new Timestamp(Calendar.getInstance().getTimeInMillis())));
        
        String json = objectMapper.writeValueAsString(map);
        System.out.println(json);
    }
    
    @Test
    public void readJson() throws IOException {
        String json = "{"Robbin":{"id":998,"name":"Robbin","birthday":1445415635096},"Nick":{"id":999,"name":"Nick Huang","birthday":1445415635096}}";
        
        ObjectMapper objectMapper = new ObjectMapper();
        
        Map<String, User> map = objectMapper.readValue(json, new TypeReference<Map<String, User>>() {});
        System.out.println(map);
    }

}
View Code

日志如下

{"Robbin":{"id":998,"name":"Robbin","birthday":1445506610097},"Nick":{"id":999,"name":"Nick Huang","birthday":1445506610097}}
{Robbin=User [id=998, name=Robbin, birthday=2015-10-21 16:20:35.096], Nick=User [id=999, name=Nick Huang, birthday=2015-10-21 16:20:35.096]}
View Code

更复杂的结构呢

比如Map里面的不同key值对应的类型不同（我这个只是土方法，有好方法的童靴提示下哦）(>_<)

package com.nicchagil.demo.No003复杂结构;

import java.io.IOException;
import java.sql.Timestamp;
import java.util.Calendar;
import java.util.HashMap;
import java.util.Map;

import org.junit.Test;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.nicchagil.demo.No001最简单的例子.User;

public class Call {

    @Test
    public void writeJson() throws JsonProcessingException {
        ObjectMapper objectMapper = new ObjectMapper();
        
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("code", "success");
        map.put("data", new User(999, "Nick Huang", new Timestamp(Calendar.getInstance().getTimeInMillis())));
        
        String json = objectMapper.writeValueAsString(map);
        System.out.println(json);
    }
    
    @Test
    public void readJson() throws IOException {
        String json = "{"data":{"id":999,"name":"Nick Huang","birthday":1445502596155},"code":"success"}";
        
        ObjectMapper objectMapper = new ObjectMapper();
        
        Map<String, Object> map = objectMapper.readValue(json, Map.class);
        Object data = map.get("data");
        User user = objectMapper.convertValue(data, User.class);
        map.put("data", user);
        
        System.out.println(map);
        
        /* 打印出类型 */
        System.out.println("code's type : " + map.get("code").getClass().toString());
        System.out.println("data's type : " + map.get("data").getClass().toString());
    }

}
View Code

日志如下

{"data":{"id":999,"name":"Nick Huang","birthday":1445506936594},"code":"success"}
{data=User [id=999, name=Nick Huang, birthday=2015-10-22 16:29:56.155], code=success}
code's type : class java.lang.String
data's type : class com.nicchagil.demo.No001最简单的例子.User
View Code

简单的工具类

import java.io.IOException;
import java.sql.Timestamp;
import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;

import org.junit.Test;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.core.type.TypeReference;
import com.fasterxml.jackson.databind.ObjectMapper;

public class JSONUtil {
    
    /* 公共的ObjectMapper */
    private static ObjectMapper commonObjectMapper = new ObjectMapper();
    
    static {
        commonObjectMapper.setDateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
    }
    
    /**
     * 转JSON
     */
    public static String toJSON(Object obj) {
        String json = null;
        try {
            json = commonObjectMapper.writeValueAsString(obj);
        } catch (JsonProcessingException e) {
            throw new RuntimeException("Bean->JSON转换失败");
        }
        return json;
    }
    
    /**
     * 转Bean
     */
    public static <T> T toBean(String json, TypeReference<T> valueTypeRef) {
        T t = null;
        try {
            t = commonObjectMapper.readValue(json, valueTypeRef);
        } catch (IOException e) {
            throw new RuntimeException("JSON->Bean转换失败");
        }
        return t;
    }
    
    @Test
    public void toJSONTest() {
        Map<String, Object> map = new HashMap<String, Object>();
        map.put("id", "123");
        map.put("name", "Nick Huang");
        map.put("createTime", new Date());
        map.put("updateTime", new Timestamp(new Date().getTime()));
        map.put("sendTime", Calendar.getInstance());
        
        System.out.println(JSONUtil.toJSON(map));
    }
    
    @Test
    public void toBeanTest() {
        System.out.println(JSONUtil.toBean("{"createTime":"2017-07-26 10:09:58","name":"Nick Huang","updateTime":"2017-07-26 10:09:58","id":"123","sendTime":"2017-07-26 10:09:58"}",  
                new TypeReference<Map<String, Object>>() {}));
    }

}
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原文地址：https://www.cnblogs.com/nick-huang/p/4901738.html