jackson序列化和反序列化Json

jackson包提供了java对象与json相互转换的API。

jackson转换机制

Jackson要求java对象是一个POJO对象，即它是一个普通JavaBean对象。此外，如果字段是用private修饰的，则必须有getXXX()方法，否则字段用public修饰。

json常见格式如下

{
    "key1" : value,
    "key2" : [...],
    "key3" : {...}
}

jackson把JavaBean对象的每个字段映射为json的键，json键值由JavaBean的getXXX()方法确定。

json键值从形式上看，可以分为基本类型（字符串、数值）、数组、字典。当JavaBean的字段声明为基本类型时对应json的基本类型，当JavaBean声明为数组或链表时对应json的数组类型，当JavaBean声明为字典或对象时对应json的字典类型。

序列化

定义一个符合JavaBean规则的类

package com.weixia.Json;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

public class Bean {
	private String name;
	
	private int[] stature;
	
	private Friend friend;
	
	private ArrayList<String> song;
	
	private Map<String,Integer> score = new HashMap<String,Integer>();
	
	public Bean(String name) {
		this.name = name;
	}
	
	public String getName() {
		return this.name;
	}
	
	public void setStature(int[] stature) {
		this.stature = stature;
	}
	
	public int[] getStature() {
		return this.stature;
	}
	
	public void setSong(ArrayList<String> song) {
		this.song = song;
	}
	
	public ArrayList<String> getSong() {
		return this.song;
	}
	
	public void setFriend(Friend friend) {
		this.friend = friend;
	}
	
	public Friend getFriend() {
		return this.friend;
	}
	
	public void addScore(String subject,Integer score) {
		this.score.put(subject, score);
	}
	
	public Map getScore() {
		return this.score;
	}
}


class Friend {
	public String name;
	public int age;
}

将Bean对象序列化为json如下

package com.weixia.Json;

import java.util.ArrayList;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.annotation.JsonInclude.Include;

public class App 
{
    public static void main( String[] args ) throws Exception
    {
        Bean bean = new Bean("Album");
    	ObjectMapper mapper = new ObjectMapper();
    	
    	bean.setStature(new int[] {88,60,89});
    	
    	ArrayList<String> song = new ArrayList<String>();
    	song.add("奇异恩典");
    	song.add("东京的都");
    	bean.setSong(song);
    	
    	Friend friend = new Friend();
    	friend.name = "小明";
    	friend.age = 24;
    	bean.setFriend(friend);
    	
    	bean.addScore("Math", 100);
    	bean.addScore("PE", 88);
    	
        mapper.configure(SerializationFeature.INDENT_OUTPUT, true);	//格式化输出
        mapper.configure(SerializationFeature.ORDER_MAP_ENTRIES_BY_KEYS, true);	//键按自然顺序输出
        mapper.setSerializationInclusion(Include.NON_EMPTY);	//忽略POJO中属性为空的字段
        mapper.writeValue(System.out, bean);
    }
}

反序列化

json文件如下：

{
    "name": "weixia",
    "age": 24,
    "stature":[89,66,89],
    "friend":{
        "name":"zhiye",
        "age":24
    },
    "test":""
}

定义一个符合JaveBean规则的类

package com.weixia.Json;

public class People {
	private String name;
	private int age;
	private Friend friend;
	private int[] stature;
	public String getName() {
		return this.name;
	}
	
	public int getAge() {
		return this.age;
	}
	
	public Friend getFriend() {
		return this.friend;
	}
	
	public int[] getStature() {
		return this.stature;
	}
}


class Friend {
	public String name;
	public int age;
}

将json反序列化为java对象

package com.weixia.Json;

import java.io.File;

import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;

public class Json2Bean {

	public static void main(String[] args) throws Exception {
		ObjectMapper mapper = new ObjectMapper();
		mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); //忽略未知的属性
		People people = mapper.readValue(new File("F:\test.json"), People.class);
		
		System.out.println(people.getName());
		System.out.println(people.getAge());
		System.out.println(people.getFriend());
		int[] stature = people.getStature();
		for (int num : stature) {
			System.out.println(num);
		}
	}

}