Jackson中json格式的字符串與對象的互相轉(zhuǎn)換方式

更新時間：2022年07月05日 14:21:24 作者：hestyle

這篇文章主要介紹了Jackson中json格式的字符串與對象的互相轉(zhuǎn)換方式，具有很好的參考價值，希望對大家有所幫助。如有錯誤或未考慮完全的地方，望不吝賜教

json格式的字符串與對象的互相轉(zhuǎn)換

Jackson 簡介

Jackson是一個簡單基于Java應(yīng)用庫，Jackson可以輕松的將Java對象轉(zhuǎn)換成json字符串和xml文檔，同樣也可以將json、xml轉(zhuǎn)換成Java對象。Jackson所依賴的jar包較少，簡單易用并且性能也要相對高些，并且Jackson社區(qū)相對比較活躍，更新速度也比較快。

jackson特點

容易使用 - jackson API提供了一個高層次外觀，以簡化常用的用例。
無需創(chuàng)建映射 - API提供了默認(rèn)的映射大部分對象序列化。
性能高 - 快速，低內(nèi)存占用，適合大型對象圖表或系統(tǒng)。
干凈的JSON - jackson創(chuàng)建一個干凈和緊湊的JSON結(jié)果，這是讓人很容易閱讀。
不依賴 - 庫不需要任何其他的庫，除了JDK。
開源代碼 - jackson是開源的，可以免費使用。

json格式的字符串與對象的轉(zhuǎn)換

使用Jackson必須要在項目導(dǎo)入對應(yīng)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();
		//將對象轉(zhuǎn)換為json格式的字符串
		Book book = new Book("1001", "Java入門到入土", 99.9, 100, "編程開發(fā)", "你懂得！");
		String bookJsonStr = objectMapper.writeValueAsString(book);
		System.out.println(bookJsonStr);
		//將集合、數(shù)組對象轉(zhuǎn)換為json格式的字符串
		List<Book> bookList = new ArrayList<Book>();
		bookList.add(new Book("1001", "Java入門到入土", 99.9, 100, "編程開發(fā)", "你懂得！"));
		bookList.add(new Book("1002", "Python入門到入土", 89.9, 100, "編程開發(fā)", "你懂得！"));
		bookList.add(new Book("1003", "C++入門到入土", 89.9, 200, "編程開發(fā)", "你懂得！"));
		String bookListJsonStr = objectMapper.writeValueAsString(bookList);
		System.out.println(bookListJsonStr);
		
		//將json格式的字符串轉(zhuǎn)化為對象
		//字符串中含有雙引號，需要使用反斜杠轉(zhuǎn)義
		String bookString = "{\"id\":\"1001\",\"name\":\"Java入門到入土\",\"price\":99.9,\"pnum\":100}";
		//Book必須有無參的構(gòu)造方法
		Book book2 = objectMapper.readValue(bookString, Book.class);
		System.out.println(book2);
	}
}

控制臺輸出：

Jackson進(jìn)行json轉(zhuǎn)對象,對象轉(zhuǎn)json總結(jié)

在前后端分離的項目中，后端經(jīng)常涉及到j(luò)son與對象互轉(zhuǎn)的場景。阿里巴巴的Fastjson是好用，但是，因為技術(shù)領(lǐng)導(dǎo)的原因（可能因為fastjson1的各種問題很多吧），不讓用，所以就需要選擇其他技術(shù)棧。當(dāng)前比較常用的是SpringBoot自帶的Jackson或者谷歌的Gson。

下面，做一下使用Jackson的總結(jié)。

JavaBean準(zhǔn)備

1.Company，公司

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

2.WebSite，站點

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

3.User，用戶

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

json字符串和對象互轉(zhuǎn)

對象轉(zhuǎn)json字符串

不管你是什么結(jié)構(gòu)的對象，想轉(zhuǎn)json，直接objectMapper.writeValueAsString()搞定。

    /**
     * 序列化：對象轉(zhuǎn)json字符串，包含多個數(shù)組
     *
     */
    @Test
    public void testObject2Json2() {
        User user2 = new User("1","上海辟謠專屬隊","職業(yè)辟謠，不信謠，不傳謠，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);
        }
    }

輸出：

{"companyName":"yyyyyy","webSites":[{"webSiteName":"xxxxxx.com","users":[{"userId":"1","username":"上海辟謠專屬隊","password":"職業(yè)辟謠，不信謠，不傳謠，呵呵"}]}]}

json字符串轉(zhuǎn)對象

注意：這種情形是json對象字符串，不能是json數(shù)組字符串，后面會說到。

    @Test
    public void testJson2Object() {
        String json = "{\"companyName\":\"yyyyyy\",\"webSites\":[" +
                "{\"webSiteName\":\"xxxxxx.com\",\"users\":[{\"userId\":\"1\",\"username\":\"上海辟謠專屬隊\",\"password\":\"職業(yè)辟謠，不信謠，不傳謠，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);
        }
    }

輸出：

上海辟謠專屬隊

json數(shù)組字符串和數(shù)組（集合）對象互轉(zhuǎn)

有個大坑

數(shù)組對象轉(zhuǎn)json數(shù)組字符串

    /**
     * 序列化：數(shù)組對象轉(zhuǎn)json數(shù)組類型的字符串
     *
     */
    @Test
    public void testObjectArray2JsonArrayString() {
        User user1 = new User("1","上海帶刀滬衛(wèi)","帶刀大佬");
        User user2 = new User("1","上海辟謠專屬隊","職業(yè)辟謠，不信謠，不傳謠，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);
        }
    }

輸出：

[{"userId":"1","username":"上海帶刀滬衛(wèi)","password":"帶刀大佬"},{"userId":"1","username":"上海辟謠專屬隊","password":"職業(yè)辟謠，不信謠，不傳謠，呵呵"}]

json數(shù)組字符串轉(zhuǎn)數(shù)組對象

先演示一下錯誤的寫法，先演示一下錯誤的寫法，先演示一下錯誤的寫法，重要的事情說三遍！

    /**
     * 反序列化：數(shù)組類型的json字符串轉(zhuǎn)對象數(shù)組
     *
     */
    @Test
    public void testJsonArrayString2ObjectArray() {
        String json = "[" +
                "{\"userId\":\"1\",\"username\":\"上海帶刀滬衛(wèi)\",\"password\":\"帶刀大佬\(zhòng)"}" +
                ",{\"userId\":\"1\",\"username\":\"上海辟謠專屬隊\",\"password\":\"職業(yè)辟謠，不信謠，不傳謠，呵呵\"}" +
                "]";
        ObjectMapper objectMapper = new ObjectMapper();
        try {
			//錯誤寫法
            List<User> list = objectMapper.readValue(json, List.class);
            list.forEach(user -> {
                System.out.println(user.getUsername());
            });
        } catch (JsonProcessingException e) {
            log.error("error: ", e);
        }
    }

輸出異常：

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

因為實際轉(zhuǎn)成了map類型，所以使用List方式取值是錯的！

正確做法：

    /**
     * 反序列化：數(shù)組類型的json字符串轉(zhuǎn)對象數(shù)組
     *
     */
    @Test
    public void testJsonArrayString2ObjectArray() {
        String json = "[" +
                "{\"userId\":\"1\",\"username\":\"上海帶刀滬衛(wèi)\",\"password\":\"帶刀大佬\(zhòng)"}" +
                ",{\"userId\":\"1\",\"username\":\"上海辟謠專屬隊\",\"password\":\"職業(yè)辟謠，不信謠，不傳謠，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);
        }
    }

最終輸出：

上海帶刀滬衛(wèi)

上海辟謠專屬隊

同時，對多層嵌套的數(shù)組也適用：

    @Test
    public void testJsonArrayString2ObjectArray2() {
        String json = "[" +
                    "{" +
                    "\"companyName\":\"yyyyyy\",\"webSites\":[" +
                            "{\"webSiteName\":\"xxxxxx.com\",\"users\":[" +
                                "{\"userId\":\"1\",\"username\":\"上海辟謠專屬隊\",\"password\":\"職業(yè)辟謠，不信謠，不傳謠，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);
        }
    }

以上為個人經(jīng)驗，希望能給大家一個參考，也希望大家多多支持腳本之家。

您可能感興趣的文章: