Java - Jackson JSON Java Parser API

电玩女神 2022-11-15 03:59 120阅读 0赞

### 文章目录 ###

*  概述
 *  依赖
 *  Jackson JSON示例
 *   *  基础数据
     *  JSON转换为Java对象
     *  Jackson JSON –将JSON转换为Map

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70]

--------------------

# 概述 #

Jackson JSON Java Parser非常流行，并且也用于Spring框架。

Jackson JSON Parser API提供了将JSON转换为POJO对象的简便方法，并支持从JSON数据轻松转换为Map。

Jackson也支持泛型，并直接将它们从JSON转换为对象。

![在这里插入图片描述][20210331002434622.gif_pic_center]

--------------------

# 依赖 #

要在我们的项目中使用Jackson JSON Java API，我们可以将其添加到项目构建路径中

<dependency>
    	<groupId>com.fasterxml.jackson.core</groupId>
    	<artifactId>jackson-databind</artifactId>
    	<version>2.2.3</version>
    </dependency>

jackson-databind jar依赖于jackson-core和jackson-annotations库，因此，如果直接将它们添加到构建路径，请确保将所有三个添加在一起，否则会出现运行时错误。

![在这里插入图片描述][20210331002603225.gif_pic_center]

--------------------

# Jackson JSON示例 #

## 基础数据 ##

对于从JSON到POJO / Java对象转换的示例，我们将使用一个嵌套对象和数组的复杂示例。、

我们将在Java对象中使用数组，列表和Map进行转换。 json存储在文件employee.txt中，其结构如下

{ 
      "id": 123,
      "name": "Pankaj",
      "permanent": true,
      "address": { 
        "street": "Albany Dr",
        "city": "San Jose",
        "zipcode": 95129
      },
      "phoneNumbers": [
        123456,
        987654
      ],
      "role": "Manager",
      "cities": [
        "Los Angeles",
        "New York"
      ],
      "properties": { 
        "age": "29 years",
        "salary": "1000 USD"
      }
    }

有以下与json数据相对应的java类。

【Address 】

public class Address { 
    	
    	private String street;
    	private String city;
    	private int zipcode;
    	
    	public String getStreet() { 
    		return street;
    	}
    	public void setStreet(String street) { 
    		this.street = street;
    	}
    	public String getCity() { 
    		return city;
    	}
    	public void setCity(String city) { 
    		this.city = city;
    	}
    	public int getZipcode() { 
    		return zipcode;
    	}
    	public void setZipcode(int zipcode) { 
    		this.zipcode = zipcode;
    	}
    	
    	@Override
    	public String toString(){ 
    		return getStreet() + ", "+getCity()+", "+getZipcode();
    	}

【Employee 】

import java.util.Arrays;
    import java.util.List;
    import java.util.Map;
    
    public class Employee { 
    
    	private int id;
    	private String name;
    	private boolean permanent;
    	private Address address;
    	private long[] phoneNumbers;
    	private String role;
    	private List<String> cities;
    	private Map<String, String> properties;
    	
    	public int getId() { 
    		return id;
    	}
    	public void setId(int id) { 
    		this.id = id;
    	}
    	public String getName() { 
    		return name;
    	}
    	public void setName(String name) { 
    		this.name = name;
    	}
    	public boolean isPermanent() { 
    		return permanent;
    	}
    	public void setPermanent(boolean permanent) { 
    		this.permanent = permanent;
    	}
    	public Address getAddress() { 
    		return address;
    	}
    	public void setAddress(Address address) { 
    		this.address = address;
    	}
    	public long[] getPhoneNumbers() { 
    		return phoneNumbers;
    	}
    	public void setPhoneNumbers(long[] phoneNumbers) { 
    		this.phoneNumbers = phoneNumbers;
    	}
    	public String getRole() { 
    		return role;
    	}
    	public void setRole(String role) { 
    		this.role = role;
    	}
    	
    	
    	public List<String> getCities() { 
    		return cities;
    	}
    	public void setCities(List<String> cities) { 
    		this.cities = cities;
    	}
    	public Map<String, String> getProperties() { 
    		return properties;
    	}
    	public void setProperties(Map<String, String> properties) { 
    		this.properties = properties;
    	}
    
    	@Override
    	public String toString(){ 
    		StringBuilder sb = new StringBuilder();
    		sb.append("***** Employee Details *****\n");
    		sb.append("ID="+getId()+"\n");
    		sb.append("Name="+getName()+"\n");
    		sb.append("Permanent="+isPermanent()+"\n");
    		sb.append("Role="+getRole()+"\n");
    		sb.append("Phone Numbers="+Arrays.toString(getPhoneNumbers())+"\n");
    		sb.append("Address="+getAddress()+"\n");
    		sb.append("Cities="+Arrays.toString(getCities().toArray())+"\n");
    		sb.append("Properties="+getProperties()+"\n");
    		sb.append("*****************************");
    		
    		return sb.toString();
    	}
    }

Employee是代表根json对象的Java bean。现在，让我们看看如何使用Jackson JSON解析器API将JSON转换为Java对象

## JSON转换为Java对象 ##

import java.io.File;
    import java.io.IOException;
    import java.io.StringWriter;
    import java.nio.file.Files;
    import java.nio.file.Paths;
    import java.util.ArrayList;
    import java.util.HashMap;
    import java.util.Iterator;
    import java.util.List;
    import java.util.Map;
    
    import com.fasterxml.jackson.core.type.TypeReference;
    import com.fasterxml.jackson.databind.JsonNode;
    import com.fasterxml.jackson.databind.ObjectMapper;
    import com.fasterxml.jackson.databind.SerializationFeature;
    import com.fasterxml.jackson.databind.node.ObjectNode;
    import com.journaldev.jackson.model.Address;
    import com.journaldev.jackson.model.Employee;
    
    
    public class JacksonObjectMapperExample { 
    
    	public static void main(String[] args) throws IOException { 
    		
    		//read json file data to String
    		byte[] jsonData = Files.readAllBytes(Paths.get("employee.txt"));
    		
    		//create ObjectMapper instance
    		ObjectMapper objectMapper = new ObjectMapper();
    		
    		//convert json string to object
    		Employee emp = objectMapper.readValue(jsonData, Employee.class);
    		
    		System.out.println("Employee Object\n"+emp);
    		
    		//convert Object to json string
    		Employee emp1 = createEmployee();
    		//configure Object mapper for pretty print
    		objectMapper.configure(SerializationFeature.INDENT_OUTPUT, true);
    		
    		//writing to console, can write to any output stream such as file
    		StringWriter stringEmp = new StringWriter();
    		objectMapper.writeValue(stringEmp, emp1);
    		System.out.println("Employee JSON is\n"+stringEmp);
    	}
    	
    	public static Employee createEmployee() { 
    
    		Employee emp = new Employee();
    		emp.setId(100);
    		emp.setName("David");
    		emp.setPermanent(false);
    		emp.setPhoneNumbers(new long[] {  123456, 987654 });
    		emp.setRole("Manager");
    
    		Address add = new Address();
    		add.setCity("Bangalore");
    		add.setStreet("BTM 1st Stage");
    		add.setZipcode(560100);
    		emp.setAddress(add);
    
    		List<String> cities = new ArrayList<String>();
    		cities.add("Los Angeles");
    		cities.add("New York");
    		emp.setCities(cities);
    
    		Map<String, String> props = new HashMap<String, String>();
    		props.put("salary", "1000 Rs");
    		props.put("age", "28 years");
    		emp.setProperties(props);
    
    		return emp;
    	}
    
    }

输出

Employee Object
    ***** Employee Details *****
    ID=123
    Name=Pankaj
    Permanent=true
    Role=Manager
    Phone Numbers=[123456, 987654]
    Address=Albany Dr, San Jose, 95129
    Cities=[Los Angeles, New York]
    Properties={ age=29 years, salary=1000 USD}
    *****************************
    Employee JSON is
    //printing same as above json file data

`com.fasterxml.jackson.databind.ObjectMapper`是Jackson API中最重要的类，它提供`readValue（）`和`writeValue（）`方法以将JSON转换为Java Object以及将Java Object转换为JSON。

ObjectMapper类可以重用，并且可以将其作为Singleton对象初始化一次。有很多重载版本的readValue（）和writeValue（）方法可用于字节数组，File，输入/输出流和Reader / Writer对象。

--------------------

## Jackson JSON –将JSON转换为Map ##

在data.txt文件中有一个如下所示的JSON对象：

{ 
      "name": "David",
      "role": "Manager",
      "city": "Los Angeles"
    }

2.2 Jackson JSON –读取特定的JSON密钥  
2.3 Jackson JSON –编辑JSON文档  
2.4 Jackson JSON流API示例

[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3lhbmdzaGFuZ3dlaQ_size_16_color_FFFFFF_t_70]: /images/20221022/594704a37a184335a62934e5c4723989.png
[20210331002434622.gif_pic_center]: https://img-blog.csdnimg.cn/20210331002434622.gif#pic_center
[20210331002603225.gif_pic_center]: https://img-blog.csdnimg.cn/20210331002603225.gif#pic_center