Jackson中json格式的字符串与对象的互相转换方式

2022-11-13 11:11:12 对象字符串转换

json格式的字符串与对象的互相转换

Jackson 简介

Jackson是一个简单基于Java应用库，Jackson可以轻松的将Java对象转换成json字符串和xml文档，同样也可以将json、xml转换成Java对象。Jackson所依赖的jar包较少，简单易用并且性能也要相对高些，并且Jackson社区相对比较活跃，更新速度也比较快。

jackson特点

容易使用 - jackson api提供了一个高层次外观，以简化常用的用例。
无需创建映射 - API提供了默认的映射大部分对象序列化。
性能高 - 快速，低内存占用，适合大型对象图表或系统。
干净的JSON - jackson创建一个干净和紧凑的JSON结果，这是让人很容易阅读。
不依赖 - 库不需要任何其他的库，除了jdk。
开源代码 - jackson是开源的，可以免费使用。

json格式的字符串与对象的转换

使用Jackson必须要在项目导入对应的jar包。

Book.java文件

package cn.hestyle.bean;
public class Book {
	private String id;
	private String name;
	private double price;
	private int pnum;
	private String cateGory;
	private String description;
	
	public Book() {
		super();
		// TODO Auto-generated constructor stub
	}
	public Book(String id, String name, double price, int pnum, String category, String description) {
		super();
		this.id = id;
		this.name = name;
		this.price = price;
		this.pnum = pnum;
		this.category = category;
		this.description = description;
	}
	public String getId() {
		return id;
	}
	public void setId(String id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public double getPrice() {
		return price;
	}
	public void setPrice(double price) {
		this.price = price;
	}
	public int getPnum() {
		return pnum;
	}
	public void setPnum(int pnum) {
		this.pnum = pnum;
	}
	public String getCategory() {
		return category;
	}
	public void setCategory(String category) {
		this.category = category;
	}
	public String getDescription() {
		return description;
	}
	public void setDescription(String description) {
		this.description = description;
	}
	@Override
	public String toString() {
		return "Book [id=" + id + ", name=" + name + ", price=" + price + ", pnum=" + pnum + ", category=" + category
				+ ", description=" + description + "]";
	}
}

package cn.hestyle.utils;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import cn.hestyle.bean.Book;
public class JacksonDemo01 {
	public static void main(String[] args) throws JsonGenerationException, JsonMappingException, IOException {
		ObjectMapper objectMapper = new ObjectMapper();
		//将对象转换为json格式的字符串
		Book book = new Book("1001", "Java入门到入土", 99.9, 100, "编程开发", "你懂得！");
		String bookJsonStr = objectMapper.writeValueAsString(book);
		System.out.println(bookJsonStr);
		//将集合、数组对象转换为json格式的字符串
		List<Book> bookList = new ArrayList<Book>();
		bookList.add(new Book("1001", "Java入门到入土", 99.9, 100, "编程开发", "你懂得！"));
		bookList.add(new Book("1002", "Python入门到入土", 89.9, 100, "编程开发", "你懂得！"));
		bookList.add(new Book("1003", "c++入门到入土", 89.9, 200, "编程开发", "你懂得！"));
		String bookListJsonStr = objectMapper.writeValueAsString(bookList);
		System.out.println(bookListJsonStr);
		
		//将json格式的字符串转化为对象
		//字符串中含有双引号，需要使用反斜杠转义
		String bookString = "{\"id\":\"1001\",\"name\":\"Java入门到入土\",\"price\":99.9,\"pnum\":100}";
		//Book必须有无参的构造方法
		Book book2 = objectMapper.readValue(bookString, Book.class);
		System.out.println(book2);
	}
}

控制台输出：

Jackson进行json转对象,对象转json总结

在前后端分离的项目中，后端经常涉及到json与对象互转的场景。阿里巴巴的Fastjson是好用，但是，因为技术领导的原因（可能因为fastjson1的各种问题很多吧），不让用，所以就需要选择其他技术栈。当前比较常用的是SpringBoot自带的Jackson或者谷歌的Gson。

下面，做一下使用Jackson的总结。

JavaBean准备

1.Company，公司

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Company {
    private String companyName;
    private List<WEBSite> webSites;
}

2.WebSite，站点

@Data
@NoArgsConstructor
@AllArgsConstructor
public class WebSite {
    private String webSiteName;
    private List<User> users;
}

3.User，用户

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    private String userId;
    private String username;
    private String passWord;
}

json字符串和对象互转

对象转json字符串

不管你是什么结构的对象，想转json，直接objectMapper.writeValueAsString()搞定。

    
    @Test
    public void testObject2Json2() {
        User user2 = new User("1","上海辟谣专属队","职业辟谣，不信谣，不传谣，呵呵");
        List<User> users = new ArrayList<>();
        users.add(user2);
        WebSite webSite = new WebSite();
        webSite.setWebSiteName("xxxxxx.com");
        webSite.setUsers(users);
        List<WebSite> webSites = new ArrayList<>();
        webSites.add(webSite);
        Company company = new Company();
        company.setCompanyName("yyyyyy");
        company.setWebSites(webSites);
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            String jsonString = objectMapper.writeValueAsString(company);
            System.out.println(jsonString);
        } catch (JsonProcessingException e) {
            log.error("error: ", e);
        }
    }

输出：

{"companyName":"yyyyyy","webSites":[{"webSiteName":"xxxxxx.com","users":[{"userId":"1","username":"上海辟谣专属队","password":"职业辟谣，不信谣，不传谣，呵呵"}]}]}

json字符串转对象

注意：这种情形是json对象字符串，不能是json数组字符串，后面会说到。

    @Test
    public void testJson2Object() {
        String json = "{\"companyName\":\"yyyyyy\",\"webSites\":[" +
                "{\"webSiteName\":\"xxxxxx.com\",\"users\":[{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}]}" +
                "]}";
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            Company company = objectMapper.readValue(json, Company.class);
            System.out.println(company.getWebSites().get(0).getUsers().get(0).getUsername());
        } catch (JsonProcessingException e) {
            log.error("error: ", e);
        }
    }

输出：

上海辟谣专属队

json数组字符串和数组（集合）对象互转

有个大坑

数组对象转json数组字符串


    
    @Test
    public void testObjectArray2JsonArrayString() {
        User user1 = new User("1","上海带刀沪卫","带刀大佬");
        User user2 = new User("1","上海辟谣专属队","职业辟谣，不信谣，不传谣，呵呵");
        List<User> users = new ArrayList<>();
        users.add(user1);
        users.add(user2);
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            String json = objectMapper.writeValueAsString(users);
            System.out.println(json);
        } catch (JsonProcessingException e) {
            log.error("error: ", e);
        }
    }

输出：

[{"userId":"1","username":"上海带刀沪卫","password":"带刀大佬"},{"userId":"1","username":"上海辟谣专属队","password":"职业辟谣，不信谣，不传谣，呵呵"}]

json数组字符串转数组对象

先演示一下错误的写法，先演示一下错误的写法，先演示一下错误的写法，重要的事情说三遍！

    
    @Test
    public void testJsonArrayString2ObjectArray() {
        String json = "[" +
                "{\"userId\":\"1\",\"username\":\"上海带刀沪卫\",\"password\":\"带刀大佬\"}" +
                ",{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}" +
                "]";
        ObjectMapper objectMapper = new ObjectMapper();
        try {
			//错误写法
            List<User> list = objectMapper.readValue(json, List.class);
            list.forEach(user -> {
                System.out.println(user.getUsername());
            });
        } catch (JsonProcessingException e) {
            log.error("error: ", e);
        }
    }

输出异常：

java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to com.xywei.springboot.springtestdemo.entity.User

   at java.util.ArrayList.forEach(ArrayList.java:1259)
   at com.xywei.springboot.springtestdemo.junit.TestUseJackson.testJsonArrayString2ObjectArray(TestUseJackson.java:160)
   at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
   at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
   at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
   at java.lang.reflect.Method.invoke(Method.java:498)
   at org.junit.platfORM.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688)
   at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
   at

#省略大段……
Process finished with exit code -1

因为实际转成了map类型，所以使用List方式取值是错的！

正确做法：

    
    @Test
    public void testJsonArrayString2ObjectArray() {
        String json = "[" +
                "{\"userId\":\"1\",\"username\":\"上海带刀沪卫\",\"password\":\"带刀大佬\"}" +
                ",{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}" +
                "]";
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            TypeReference<List<User>> typeReference = new TypeReference<List<User>>() {
            };
            List<User> list = objectMapper.readValue(json, typeReference);
            list.forEach(user -> {
                System.out.println(user.getUsername());
            });
        } catch (JsonProcessingException e) {
            log.error("error: ", e);
        }
    }

最终输出：

上海带刀沪卫

上海辟谣专属队

同时，对多层嵌套的数组也适用：

    @Test
    public void testJsonArrayString2ObjectArray2() {
        String json = "[" +
                    "{" +
                    "\"companyName\":\"yyyyyy\",\"webSites\":[" +
                            "{\"webSiteName\":\"xxxxxx.com\",\"users\":[" +
                                "{\"userId\":\"1\",\"username\":\"上海辟谣专属队\",\"password\":\"职业辟谣，不信谣，不传谣，呵呵\"}" +
                                "]" +
                            "}" +
                        "]" +
                    "}" +
                "]";
        ObjectMapper objectMapper = new ObjectMapper();
        try {
            TypeReference<List<Company>> typeReference = new TypeReference<List<Company>>() {
            };
            List<Company> list = objectMapper.readValue(json, typeReference);
            list.forEach(company -> {
                System.out.println(company.getWebSites().get(0).getUsers().get(0).getUsername());
            });
        } catch (JsonProcessingException e) {
            log.error("error: ", e);
        }
    }

以上为个人经验，希望能给大家一个参考，也希望大家多多支持。

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