» Assume we have simple REST API available at the endpoint https://dummy.restapiexample.com/api/v1/employee/1. We will use ObjectMapper and StdDeserializer to extract the JSON response.

Note: You can use the internal JSON Viewer tool here to perform simple manipulations on the JSON file.

» There are various methods and libraries for performing HTTP requests in Java here, including Apache HttpClient, Java 11's HttpClient, Spring's RestTemplate, and OkHttp.

[pom.xml]

<dependency>
  <groupId>org.apache.httpcomponents</groupId>
  <artifactId>httpclient</artifactId>
  <version>4.5.13</version>
</dependency>
<dependency>
  <groupId>com.fasterxml.jackson.core</groupId>
  <artifactId>jackson-databind</artifactId>
  <version>2.13.0</version>
</dependency>

[App.java]

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpGet request = new HttpGet("https://dummy.restapiexample.com/api/v1/employee/1");
HttpResponse response = httpClient.execute(request);
HttpEntity entity = response.getEntity();
String result = EntityUtils.toString(entity);

// TODO

» Install the dependencies needed to use ObjectMapper.

[pom.xml]

<dependency> 
    <groupId>com.fasterxml.jackson.core</groupId> 
    <artifactId>jackson-databind</artifactId> 
    <version>2.13.3</version> 
</dependency>

[App.java]

...
ObjectMapper mapper = new ObjectMapper();
SimpleModule module = new SimpleModule();
module.addDeserializer(Map.class, new EmployeeDeserializer());
mapper.registerModule(module);

Map<String, Object> employeeData = mapper.readValue(result, Map.class);

// Print the extracted information
System.out.println("Employee Name: " + employeeData.get("employee_name"));
System.out.println("Employee Salary: " + employeeData.get("employee_salary"));
System.out.println("Employee Age: " + employeeData.get("employee_age"));

» Instead of using SimpleModule, avoid it and directly deserialize the response into a custom class that matches the structure of the JSON data. Below is the revised approach:

[Employee.java]

class Employee {
    @JsonProperty("employee_name")
    private String employeeName;

    @JsonProperty("employee_salary")
    private int employeeSalary;

    @JsonProperty("employee_age")
    private int employeeAge;

    // Getters and Setters
    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public int getEmployeeSalary() {
        return employeeSalary;
    }

    public void setEmployeeSalary(int employeeSalary) {
        this.employeeSalary = employeeSalary;
    }

    public int getEmployeeAge() {
        return employeeAge;
    }

    public void setEmployeeAge(int employeeAge) {
        this.employeeAge = employeeAge;
    }
}

[ApiResponse.java]

class ApiResponse {
    private String status;
    private Employee data;
    private String message;

    // Getters and Setters
    public String getStatus() {
        return status;
    }

    public void setStatus(String status) {
        this.status = status;
    }

    public Employee getData() {
        return data;
    }

    public void setData(Employee data) {
        this.data = data;
    }

    public String getMessage() {
        return message;
    }

    public void setMessage(String message) {
        this.message = message;
    }
}

[App.java]

ObjectMapper mapper = new ObjectMapper();
ApiResponse apiResponse = mapper.readValue(result, ApiResponse.class);

Employee employee = apiResponse.getData();

System.out.println("Employee Name: " + employee.getEmployeeName());
System.out.println("Employee Salary: " + employee.getEmployeeSalary());
System.out.println("Employee Age: " + employee.getEmployeeAge());