Java - Jackson JSON Java Parser API-CFANZ编程社区

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

Java - Jackson JSON Java Parser API_json转换

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