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

What you’ll build

You’ll build an application that uses Spring’s RestTemplate to retrieve a random Spring Boot quotation at http://gturnquist-quoters.cfapps.io/api/random.

What you’ll need

How to complete this guide

Like most Spring Getting Started guides, you can start from scratch and complete each step, or you can bypass basic setup steps that are already familiar to you. Either way, you end up with working code.

To start from scratch, move on to Build with Gradle.

To skip the basics, do the following:

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

Build with Gradle

Build with Gradle

First you set up a basic build script. You can use any build system you like when building apps with Spring, but the code you need to work with Gradle and Maven is included here. If you’re not familiar with either, refer to Building Java Projects with Gradle or Building Java Projects with Maven.

Create the directory structure

In a project directory of your choosing, create the following subdirectory structure; for example, with mkdir -p src/main/java/hello on *nix systems:

└── src
    └── main
        └── java
            └── hello

Create a Gradle build file

Below is the initial Gradle build file.

build.gradle

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath("org.springframework.boot:spring-boot-gradle-plugin:1.5.2.RELEASE")
    }
}

apply plugin: 'java'
apply plugin: 'eclipse'
apply plugin: 'idea'
apply plugin: 'org.springframework.boot'

jar {
    baseName = 'gs-consuming-rest'
    version =  '0.1.0'
}

repositories {
    mavenCentral()
}

sourceCompatibility = 1.8
targetCompatibility = 1.8

dependencies {
    compile("org.springframework.boot:spring-boot-starter")
    compile("org.springframework:spring-web")
    compile("com.fasterxml.jackson.core:jackson-databind")
    testCompile("junit:junit")
}

The Spring Boot gradle plugin provides many convenient features:

  • It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.

  • It searches for the public static void main() method to flag as a runnable class.

  • It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

Build with Maven

Build with Maven

First you set up a basic build script. You can use any build system you like when building apps with Spring, but the code you need to work with Maven is included here. If you’re not familiar with Maven, refer to Building Java Projects with Maven.

Create the directory structure

In a project directory of your choosing, create the following subdirectory structure; for example, with mkdir -p src/main/java/hello on *nix systems:

└── src
    └── main
        └── java
            └── hello

pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<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>org.springframework</groupId>
    <artifactId>gs-consuming-rest</artifactId>
    <version>0.1.0</version>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.2.RELEASE</version>
    </parent>

    <properties>
        <java.version>1.8</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
        </dependency>
    </dependencies>


    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

The Spring Boot Maven plugin provides many convenient features:

  • It collects all the jars on the classpath and builds a single, runnable "über-jar", which makes it more convenient to execute and transport your service.

  • It searches for the public static void main() method to flag as a runnable class.

  • It provides a built-in dependency resolver that sets the version number to match Spring Boot dependencies. You can override any version you wish, but it will default to Boot’s chosen set of versions.

Build with your IDE

Build with your IDE

Fetch 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 http://gturnquist-quoters.cfapps.io/api/random. It randomly fetches quotes about Spring Boot and returns them as a JSON document.

If you request that URL through your web browser or curl, you’ll 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."
   }
}

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, create a domain class to contain the data that you need.

src/main/java/hello/Quote.java

package hello;

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 +
                '}';
    }
}

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

In order for you to directly bind your data to your custom types, you need to specify the variable name exact same as the key in the JSON Document returned from the API. In case your variable name and key in JSON doc are not matching, you need to use @JsonProperty annotation to specify the exact key of JSON document.

An additional class is needed to embed the inner quotation itself.

src/main/java/hello/Value.java

package hello;

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 simply maps onto other data fields.

Make the application executable

Although it is possible to package this service as a traditional WAR file for deployment to an external application server, the simpler approach demonstrated below creates a standalone application. You package everything in a single, executable JAR file, driven by a good old Java main() method. Along the way, you use Spring’s support for embedding the Tomcat servlet container as the HTTP runtime, instead of deploying to an external instance.

Now you can write the Application class that uses RestTemplate to fetch the data from our Spring Boot quotation service.

src/main/java/hello/Application.java

public class Application {

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

    public static void main(String args[]) {
        RestTemplate restTemplate = new RestTemplate();
        Quote quote = restTemplate.getForObject("http://gturnquist-quoters.cfapps.io/api/random", Quote.class);
        log.info(quote.toString());
    }

}

Because the Jackson JSON processing library is in the classpath, RestTemplate will use it (via a message converter) to convert the incoming JSON data into a Quote object. From there, the contents of the Quote object will be logged to the console.

Here you’ve only used RestTemplate to make an HTTP GET request. But RestTemplate also supports other HTTP verbs such as POST, PUT, and DELETE.

Managing the Application Lifecycle with Spring Boot

So far we haven’t used Spring Boot in our application, but there are some advantages in doing so, and it isn’t hard to do. One of the advantages is that we might want to let Spring Boot manage the message converters in the RestTemplate, so that customizations are easy to add declaratively. To do that we use @SpringBootApplication on the main class and convert the main method to start it up, like in any Spring Boot application. Finally we move the RestTemplate to a CommandLineRunner callback so it is executed by Spring Boot on startup:

src/main/java/hello/Application.java

package hello;

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 Application {

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

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

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

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

The RestTemplateBuilder is injected by Spring, and if you use it to create a RestTemplate then you will benefit from all the autoconfiguration that happens in Spring Boot with message converters and request factories. We also extract the RestTemplate into a @Bean to make it easier to test (it can be mocked more easily that way).

Build an executable JAR

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

If you are using Gradle, you can run the application using ./gradlew bootRun. Or you can build the JAR file using ./gradlew build. Then you can run the JAR file:

java -jar build/libs/gs-consuming-rest-0.1.0.jar

If you are using Maven, you can run the application using ./mvnw spring-boot:run. Or you can build the JAR file with ./mvnw clean package. Then you can run the JAR file:

java -jar target/gs-consuming-rest-0.1.0.jar
The procedure above will create a runnable JAR. You can also opt to build a classic WAR file instead.

You should see output like the following, with a random quote:

2015-09-23 14:22:26.415  INFO 23613 --- [main] hello.Application  : Quote{type='success', value=Value{id=12, [email protected] with @springframework is pure productivity! Who said in #java one has to write double the code than in other langs? #newFavLib'}}
If you see the error Could not extract response: no suitable HttpMessageConverter found for response type [class hello.Quote] it’s possible 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 standard system properties http.proxyHost and http.proxyPort to values appropriate for your environment.

Summary

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

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