文章目录
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。
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。
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