Consultar una API REST pública

por Rosa · Publicada abril 22, 2020 · Actualizado abril 23, 2020

A continuación, explicaré cómo consultar una API REST pública, ya sea desde el navegador como desde un programita en java.

La API que voy a utilizar es: https://jsonplaceholder.typicode.com, en concreto users.

Consultar desde el navegador

Para obtener la información que devuelve el servicio users, introduciremos en la barra de direcciones de un navegador: https://jsonplaceholder.typicode.com/users

El resultado es la salida de una lista de usuarios en formato json. Si quieres saber más de json, consulta su web oficial: https://www.json.org/json-es.html.

Salida de la consulta:

[
  {
    "id": 1,
    "name": "Leanne Graham",
    "username": "Bret",
    "email": "Sincere@april.biz",
    "address": {
      "street": "Kulas Light",
      "suite": "Apt. 556",
      "city": "Gwenborough",
      "zipcode": "92998-3874",
      "geo": {
        "lat": "-37.3159",
        "lng": "81.1496"
      }
    },
    "phone": "1-770-736-8031 x56442",
    "website": "hildegard.org",
    "company": {
      "name": "Romaguera-Crona",
      "catchPhrase": "Multi-layered client-server neural-net",
      "bs": "harness real-time e-markets"
    }
  },
  {
    "id": 2,
    "name": "Ervin Howell",
    "username": "Antonette",
    "email": "Shanna@melissa.tv",
    "address": {
      "street": "Victor Plains",
      "suite": "Suite 879",
      "city": "Wisokyburgh",
      "zipcode": "90566-7771",
      "geo": {
        "lat": "-43.9509",
        "lng": "-34.4618"
      }
    },
    "phone": "010-692-6593 x09125",
    "website": "anastasia.net",
    "company": {
      "name": "Deckow-Crist",
      "catchPhrase": "Proactive didactic contingency",
      "bs": "synergize scalable supply-chains"
    }
  },
…
…
…
]

Observa en la web principal de esta API https://jsonplaceholder.typicode.com/ los métodos disponibles (para nuestro caso sustituimos posts por users):

Routes

All HTTP methods are supported.

GET	/posts
GET	/posts/1
GET	/posts/1/comments
GET	/comments?postId=1
GET	/posts?userId=1
POST	/posts
PUT	/posts/1
PATCH	/posts/1
DELETE	/posts/1

Ejemplo:

Mostrar el usuario cuyo nombre sea Patricia Lebsack.

https://jsonplaceholder.typicode.com/users?name=Patricia Lebsack

Consultar desde programa java

Para obtener información más precisa y si los métodos disponibles no nos resultan útiles, crearemos un pequeño programita en java, donde accederos por código al servicio y parsearemos el resultado.

En este caso queremos obtener el usuario cuya ciudad sea: Aliyaview. Ten en cuenta que el resultado que devuelve está en formato json y no tiene un elemento root o padre y la complejidad a la hora de poder rescatar y tratar la información será algo mayor.

Pasos a realizar:

Crea un proyecto web dinámico en eclipse y configúralo con Maven.

A continuación incluyo las clases para la resolución:

PeticionRest.java

Clase java principal desde la que iniciaremos el programa.

package es.rosamarfil.cliente;

import java.util.ArrayList;
import java.util.List;

import javax.ws.rs.client.Client;
import javax.ws.rs.client.ClientBuilder;
import javax.ws.rs.client.WebTarget;
import javax.ws.rs.core.MediaType;

import org.json.JSONArray;
import org.json.JSONObject;

import es.rosamarfil.modelo.User;
import es.rosamarfil.modelo.User.Address_;
import es.rosamarfil.modelo.User.Company;
import es.rosamarfil.modelo.User.Geo;

public class PeticionREST {

	public static void main(String[] args) {

		String URL_API = "https://jsonplaceholder.typicode.com/users";

		// Cliente para la conexión
		Client client = ClientBuilder.newClient();
		// Definición de URL
		WebTarget target = client.target(URL_API);
		// Recogemos el resultado en una variable String
		String response = target.request(MediaType.APPLICATION_JSON).get(String.class);
		// Escribimos por consola el resultado de json
//		System.out.println(response);

		//Almacenamos la información del json en una lista java
		ArrayList<User> listUser = saveJsonToList(response);
		
		//Llamada a procedimiento que imprime el registro buscado
		showUserByCity("Aliyaview", listUser);
		
	}

	private static void showUserByCity(String string, ArrayList<User> listUser) {
		for (int i = 0; i < listUser.size(); i++) {
			if (listUser.get(i).getAddress().city.equalsIgnoreCase(string)){
				System.out.println("El usuario con ciudad " + string + " es: \n" + listUser.get(i));
			}
		}
		
	}

	private static ArrayList<User> saveJsonToList(String response) {

		ArrayList<User> listUser = new ArrayList<User>();
		
		User user;
		Address_ address = null;
		Geo geo = null;
		Company company = null;
		
		JSONObject jsonUser;

		JSONArray jsonarray = new JSONArray(response);

		for (int i = 0; i < jsonarray.length(); i++) {

			jsonUser = jsonarray.getJSONObject(i);

			user = new User();
			user.setId(jsonUser.getInt(User.ID));
			user.setName(jsonUser.getString(User.NAME));
			user.setUsername(jsonUser.getString(User.USERNAME));
			user.setEmail(jsonUser.getString(User.EMAIL));
			user.setPhone(jsonUser.getString(User.PHONE));
			user.setWebsite(jsonUser.getString(User.WEBSITE));

			if (jsonUser.get(User.ADDRESS) != null) {
				JSONObject jsonAddress = jsonUser.getJSONObject(User.ADDRESS);
				address = new User().new Address_();
				address.setStreet(jsonAddress.getString(User.STREET));
				address.setSuite(jsonAddress.getString(User.SUITE));
				address.setCity(jsonAddress.getString(User.CITY));
				address.setZipcode(jsonAddress.getString(User.ZIPCODE));

				if (jsonAddress.get(User.GEO) != null) {
					JSONObject jsonGeo = jsonAddress.getJSONObject(User.GEO);

					geo = new User().new Geo();
					geo.setLat(jsonGeo.getString(User.LAT));
					geo.setLng(jsonGeo.getString(User.LNG));
					address.setGeo(geo);
				}
			}

			if (jsonUser.get(User.COMPANY) != null) {
				JSONObject jsonCompany = jsonUser.getJSONObject(User.COMPANY);
				company = new User().new Company();
				company.setName(jsonCompany.getString(User.C_NAME));
				company.setCatchPhrase(jsonCompany.getString(User.CATCHPHRASE));
				company.setBs(jsonCompany.getString(User.BS));

			}

			user.setAddress(address);
			user.setCompany(company);

			listUser.add(user);
		}

//		imprimirLista(listUser);
		return listUser;
	}

	private static void imprimirLista(List<User> listUser) {
		for (int i = 0; i < listUser.size(); i++) {
			System.out.println(listUser.get(i));
		}

	}
}

User.java

Clase que representa a cada usuario. Este tipo de clases suelen llamarse Bean.

package es.rosamarfil.modelo;

import java.io.Serializable;

public class User implements Serializable {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	
	// User
	public static String ID = "id";
	public static String NAME = "name";
	public static String USERNAME = "username";
	public static String EMAIL = "email";
	public static String ADDRESS = "address";
	public static String PHONE = "phone";
	public static String WEBSITE = "website";
	public static String COMPANY = "company";

	// Address
	public static String STREET = "street";
	public static String SUITE = "suite";
	public static String CITY = "city";
	public static String ZIPCODE = "zipcode";
	public static String GEO = "geo";

	// Geo
	public static String LAT = "lat";
	public static String LNG = "lng";

	// Company
	public static String C_NAME = "name";
	public static String CATCHPHRASE = "catchPhrase";
	public static String BS = "bs";

	public int id;

	public String name;

	public String username;

	// @JsonProperty(value = "email")
	public String email;

	public Address_ address;

	public String phone;

	public String website;

	public Company company;

	public User(Address_ address, Company company) {
		super();
		this.address = address;
		this.company = company;
	}

	public User() {
	}

	public void setId(int i) {
		this.id = i;
	}

	public int getId() {
		return id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public String getUsername() {
		return username;
	}

	public void setUsername(String username) {
		this.username = username;
	}

	public String getEmail() {
		return email;
	}

	public void setEmail(String email) {
		this.email = email;
	}

	public Address_ getAddress() {
		return address;
	}

	public void setAddress(Address_ address) {
		this.address = address;
	}

	public String getPhone() {
		return phone;
	}

	public void setPhone(String phone) {
		this.phone = phone;
	}

	public String getWebsite() {
		return website;
	}

	public void setWebsite(String website) {
		this.website = website;
	}

	public Company getCompany() {
		return company;
	}

	public void setCompany(Company company) {
		this.company = company;
	}


	public class Address_ {

		public String street;

		public String suite;

		public String city;

		public String zipcode;

		public Geo geo;

		public String getStreet() {
			return street;
		}

		public void setStreet(String street) {
			this.street = street;
		}

		public String getSuite() {
			return suite;
		}

		public void setSuite(String suite) {
			this.suite = suite;
		}

		public String getCity() {
			return city;
		}

		public void setCity(String city) {
			this.city = city;
		}

		public String getZipcode() {
			return zipcode;
		}

		public void setZipcode(String zipcode) {
			this.zipcode = zipcode;
		}

		public Geo getGeo() {
			return geo;
		}

		public void setGeo(Geo geo) {
			this.geo = geo;
		}

		@Override
		public String toString() {
			return "Address_ [getStreet()=" + getStreet() + ", getSuite()=" + getSuite() + ", getCity()=" + getCity()
					+ ", getZipcode()=" + getZipcode() + ", getGeo()=" + getGeo() + "]";
		}

	}
	
	public class Company {

		public String name;
		public String catchPhrase;

		public String bs;

		public String getName() {
			return name;
		}

		public void setName(String name) {
			this.name = name;
		}

		public String getCatchPhrase() {
			return catchPhrase;
		}

		public void setCatchPhrase(String catchPhrase) {
			this.catchPhrase = catchPhrase;
		}

		public String getBs() {
			return bs;
		}

		public void setBs(String bs) {
			this.bs = bs;
		}

		@Override
		public String toString() {
			return "Company [getName()=" + getName() + ", getCatchPhrase()=" + getCatchPhrase() + ", getBs()=" + getBs()
					+ "]";
		}
	}

	public class Geo {

		public String lat;

		public String lng;

		public String getLat() {
			return lat;
		}

		public void setLat(String string) {
			this.lat = string;
		}

		public String getLng() {
			return lng;
		}

		public void setLng(String lng) {
			this.lng = lng;
		}

	}

	@Override
	public String toString() {
		return "User [id=" + getId() + ", name=" + getName()
				+ ", username=" + getUsername() + ", email=" + getEmail() + ", \naddress=" + getAddress()
				+ ", phone=" + getPhone() + ", website=" + getWebsite() + ", \ncompany=" + getCompany()
				+ "]";
	}

}

pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>ClienteRest</groupId>
	<artifactId>ClienteRest</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>war</packaging>
	<build>
		<sourceDirectory>src</sourceDirectory>
		<plugins>
			<plugin>
				<artifactId>maven-compiler-plugin</artifactId>
				<version>3.8.0</version>
				<configuration>
					<source>1.8</source>
					<target>1.8</target>
				</configuration>
			</plugin>
			<plugin>
				<artifactId>maven-war-plugin</artifactId>
				<version>3.2.1</version>
				<configuration>
					<warSourceDirectory>WebContent</warSourceDirectory>
				</configuration>
			</plugin>
		</plugins>
	</build>
	<dependencies>

		<!-- https://mvnrepository.com/artifact/javax.ws.rs/javax.ws.rs-api -->
		<dependency>
			<groupId>javax.ws.rs</groupId>
			<artifactId>javax.ws.rs-api</artifactId>
			<version>2.1.1</version>
		</dependency>

		<!-- https://mvnrepository.com/artifact/org.glassfish.jersey.core/jersey-client -->
		<dependency>
			<groupId>org.glassfish.jersey.core</groupId>
			<artifactId>jersey-client</artifactId>
			<version>2.12</version>
		</dependency>

		<!-- https://mvnrepository.com/artifact/org.json/json -->
		<dependency>
			<groupId>org.json</groupId>
			<artifactId>json</artifactId>
			<version>20180813</version>
		</dependency>

		<!-- https://mvnrepository.com/artifact/org.scalastuff/json-parser -->
		<dependency>
			<groupId>org.scalastuff</groupId>
			<artifactId>json-parser_2.11</artifactId>
			<version>2.0.2</version>
		</dependency>


        </dependencies>
</project>

Resultado:

El usuario con ciudad Aliyaview es: 
User [id=8, name=Nicholas Runolfsdottir V, username=Maxime_Nienow, email=Sherwood@rosamond.me, 
address=Address_ [getStreet()=Ellsworth Summit, getSuite()=Suite 729, getCity()=Aliyaview, getZipcode()=45169, getGeo()=es.rosamarfil.modelo.User$Geo@671a5887], phone=586.493.6943 x140, website=jacynthe.com, 
company=Company [getName()=Abernathy Group, getCatchPhrase()=Implemented secondary concept, getBs()=e-enable extensible e-tailers]]

Evidentemente se puede formatear la salida para que quede más ordenado, se puede dividir la clase User.java en varias clases, etc. ya al gusto de cada uno. Pero a groso modo, ya has aprendido a recoger la respuesta de un WebService Rest y almacenarla en una lista Java.