https://spring.io/guides/gs/consuming-rest/ https://github.com/spring-guides/gs-consuming-rest

This guide walks you through the process of creating an application that consumes a RESTful web service.

What You Will Build

You will build an application that uses Spring’s RestTemplate to retrieve a random Spring Boot quotation at https://quoters.apps.pcfone.io/api/random.

What You Need

  • About 15 minutes
  • A favorite text editor or IDE
  • JDK 1.8 or later
  • Maven 3.8 +
  • You can also import the code straight into your IDE:

To start from scratch, move on to Starting with Spring Initializr.
To skip the basics, do the following:

When you finish, you can check your results against the code in gs-consuming-rest/complete.

Starting with Spring Initializr

If you use Maven, visit the Spring Initializr to generate a new project with the required dependency (Spring Web).

The following listing shows the pom.xml file created when you choose Maven:

  1. <?xml version="1.0" encoding="UTF-8"?>
  2. <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  3.     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  4.     <modelVersion>4.0.0</modelVersion>
  5.     <parent>
  6.         <groupId>org.springframework.boot</groupId>
  7.         <artifactId>spring-boot-starter-parent</artifactId>
  8.         <version>2.4.3</version>
  9.         <relativePath/> <!-- lookup parent from repository -->
  10.     </parent>
  11.     <groupId>com.example</groupId>
  12.     <artifactId>consuming-rest</artifactId>
  13.     <version>0.0.1-SNAPSHOT</version>
  14.     <name>consuming-rest</name>
  15.     <description>Demo project for Spring Boot</description>
  17.     <properties>
  18.         <java.version>1.8</java.version>
  19.     </properties>
  21.     <dependencies>
  22.         <dependency>
  23.             <groupId>org.springframework.boot</groupId>
  24.             <artifactId>spring-boot-starter-web</artifactId>
  25.         </dependency>
  27.         <dependency>
  28.             <groupId>org.springframework.boot</groupId>
  29.             <artifactId>spring-boot-starter-test</artifactId>
  30.             <scope>test</scope>
  31.         </dependency>
  32.     </dependencies>
  34.     <build>
  35.         <plugins>
  36.             <plugin>
  37.                 <groupId>org.springframework.boot</groupId>
  38.                 <artifactId>spring-boot-maven-plugin</artifactId>
  39.             </plugin>
  40.         </plugins>
  41.     </build>
  43. </project>

These build files can be this simple because spring-boot-starter-web includes everything you need to build a web application, including the Jackson classes you need to work with JSON.

Manual Initialization (optional)

If you want to initialize the project manually rather than use the links shown earlier, follow the steps given below:

  1. Navigate to https://start.spring.io. This service pulls in all the dependencies you need for an application and does most of the setup for you.
  2. Choose Maven and the language you want to use. This guide assumes that you chose Java.
  3. Click Dependencies and select Spring Web.
  4. Click Generate.
  5. Download the resulting ZIP file, which is an archive of a web application that is configured with your choices.’

If your IDE has the Spring Initializr integration, you can complete this process from your IDE.

Fetching a REST Resource

With project setup complete, you can create a simple application that consumes a RESTful service.

A RESTful service has been stood up at https://quoters.apps.pcfone.io/api/random. It randomly fetches quotations about Spring Boot and returns them as JSON documents.

If you request that URL through a web browser or curl, you receive a JSON document that looks something like this:

{
   type: "success",
   value: {
      id: 10,
      quote: "Really loving Spring Boot, makes stand alone Spring apps easy."
   }
}

That is easy enough but not terribly useful when fetched through a browser or through curl.

A more useful way to consume a REST web service is programmatically. To help you with that task, Spring provides a convenient template class called RestTemplate. RestTemplate makes interacting with most RESTful services a one-line incantation. And it can even bind that data to custom domain types.

First, you need to create a domain class to contain the data that you need. The following listing shows the Quote class, which you can use as your domain class:
src/main/java/com/example/consumingrest/Quote.java

package com.example.consumingrest;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public class Quote {

  private String type;
  private Value value;

  public Quote() {
  }

  public String getType() {
    return type;
  }

  public void setType(String type) {
    this.type = type;
  }

  public Value getValue() {
    return value;
  }

  public void setValue(Value value) {
    this.value = value;
  }

  @Override
  public String toString() {
    return "Quote{" +
        "type='" + type + '\'' +
        ", value=" + value +
        '}';
  }
}

This simple Java class has a handful of properties and matching getter methods. It is annotated with @JsonIgnoreProperties from the Jackson JSON processing library to indicate that any properties not bound in this type should be ignored.

To directly bind your data to your custom types, you need to specify the variable name to be exactly the same as the key in the JSON document returned from the API. In case your variable name and key in JSON doc do not match, you can use @JsonProperty annotation to specify the exact key of the JSON document. (This example matches each variable name to a JSON key, so you do not need that annotation here.)

You also need an additional class, to embed the inner quotation itself. The Value class fills that need and is shown in the following listing (at src/main/java/com/example/consumingrest/Value.java):

package com.example.consumingrest;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown = true)
public class Value {

  private Long id;
  private String quote;

  public Value() {
  }

  public Long getId() {
    return this.id;
  }

  public String getQuote() {
    return this.quote;
  }

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

  public void setQuote(String quote) {
    this.quote = quote;
  }

  @Override
  public String toString() {
    return "Value{" +
        "id=" + id +
        ", quote='" + quote + '\'' +
        '}';
  }
}

This uses the same annotations but maps onto other data fields.

Finishing the Application

The Initalizr creates a class with a main() method. The following listing shows the class the Initializr creates (at src/main/java/com/example/consumingrest/ConsumingRestApplication.java):

package com.example.consumingrest;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class ConsumingRestApplication {

    public static void main(String[] args) {
        SpringApplication.run(ConsumingRestApplication.class, args);
    }

}

Now you need to add a few other things to the ConsumingRestApplication class to get it to show quotations from our RESTful source. You need to add:

  • A logger, to send output to the log (the console, in this example).
  • A RestTemplate, which uses the Jackson JSON processing library to process the incoming data.
  • A CommandLineRunner that runs the RestTemplate (and, consequently, fetches our quotation) on startup.

The following listing shows the finished ConsumingRestApplication class (at src/main/java/com/example/consumingrest/ConsumingRestApplication.java):

package com.example.consumingrest;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;

@SpringBootApplication
public class ConsumingRestApplication {

    private static final Logger log = LoggerFactory.getLogger(ConsumingRestApplication.class);

    public static void main(String[] args) {
        SpringApplication.run(ConsumingRestApplication.class, args);
    }

    @Bean
    public RestTemplate restTemplate(RestTemplateBuilder builder) {
        return builder.build();
    }

    @Bean
    public CommandLineRunner run(RestTemplate restTemplate) throws Exception {
        return args -> {
            Quote quote = restTemplate.getForObject(
                    "https://quoters.apps.pcfone.io/api/random", Quote.class);
            log.info(quote.toString());
        };
    }
}

Running the Application

You can run the application from the command line with Maven. You can also build a single executable JAR file that contains all the necessary dependencies, classes, and resources and run that. Building an executable jar makes it easy to ship, version, and deploy the service as an application throughout the development lifecycle, across different environments, and so forth.

To use Maven, you can run the application by using ./mvnw spring-boot:run. Alternatively, you can build the JAR file with ./mvnw clean package and then run the JAR file, as follows:

$ java -jar target/gs-consuming-rest-0.1.0.jar

The steps described here create a runnable JAR. You can also build a classic WAR file.

You should see output similar to the following but with a random quotation:

2019-08-22 14:06:46.506  INFO 42940 --- [           main] c.e.c.ConsumingRestApplication           : Quote{type='success', value=Value{id=1, quote='Working with Spring Boot is like pair-programming with the Spring developers.'}}

If you see an error that reads, Could not extract response: no suitable HttpMessageConverter found for response type [class com.example.consumingrest.Quote], it is possible that you are in an environment that cannot connect to the backend service (which sends JSON if you can reach it). Maybe you are behind a corporate proxy. Try setting the http.proxyHost and http.proxyPort system properties to values appropriate for your environment.

Summary

Congratulations! You have just developed a simple REST client by using Spring Boot.