This guide walks you through creating asynchronous queries to GitHub. The focus is on the asynchronous part, a feature often used when scaling services.

What You Will build

You will build a lookup service that queries GitHub user information and retrieves data through GitHub’s API. One approach to scaling services is to run expensive jobs in the background and wait for the results by using Java’s CompletableFuture interface. Java’s CompletableFuture is an evolution from the regular Future. It makes it easy to pipeline multiple asynchronous operations and merge them into a single asynchronous computation.

What You 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 Starting with Spring Initializr.

To skip the basics, do the following:

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

Starting with Spring Initializr

For all Spring applications, you should start with the Spring Initializr. The Initializr offers a fast way to pull in all the dependencies you need for an application and does a lot of the set up for you. This example needs only the Spring Web dependency. The following image shows the Initializr set up for this sample project:

initializr
The preceding image shows the Initializr with Maven chosen as the build tool. You can also use Gradle. It also shows values of com.example and async-method as the Group and Artifact, respectively. You will use those values throughout the rest of this sample.

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

<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.1.8.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.example</groupId>
	<artifactId>async-method</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>async-method</name>
	<description>Demo project for Spring Boot</description>

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

	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-web</artifactId>
		</dependency>

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

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

</project>

The following listing shows the build.gradle file created when you choose Gradle:

plugins {
	id 'org.springframework.boot' version '2.1.8.RELEASE'
	id 'io.spring.dependency-management' version '1.0.8.RELEASE'
	id 'java'
}

group = 'com.example'
version = '0.0.1-SNAPSHOT'
sourceCompatibility = '1.8'

repositories {
	mavenCentral()
}

dependencies {
	implementation 'org.springframework.boot:spring-boot-starter-web'
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
}

Create a Representation of a GitHub User

Before you can create a GitHub lookup service, you need to define a representation for the data you will retrieve through GitHub’s API.

To model the user representation, create a resource representation class. To do so, provide a plain old Java object with fields, constructors, and accessors, as the following example (from src/main/java/com/example/asyncmethod/User.java) shows:

package com.example.asyncmethod;

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

@JsonIgnoreProperties(ignoreUnknown=true)
public class User {

  private String name;
  private String blog;

  public String getName() {
    return name;
  }

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

  public String getBlog() {
    return blog;
  }

  public void setBlog(String blog) {
    this.blog = blog;
  }

  @Override
  public String toString() {
    return "User [name=" + name + ", blog=" + blog + "]";
  }

}

Spring uses the Jackson JSON library to convert GitHub’s JSON response into a User object. The @JsonIgnoreProperties annotation tells Spring to ignore any attributes not listed in the class. This makes it easy to make REST calls and produce domain objects.

In this guide, we grab only the name and the blog URL for demonstration purposes.

Create a GitHub Lookup Service

Next, you need to create a service that queries GitHub to find user information. The following listing (from src/main/java/com/example/asyncmethod/GitHubLookupService.java) shows how to do so:

package com.example.asyncmethod;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;

import java.util.concurrent.CompletableFuture;

@Service
public class GitHubLookupService {

  private static final Logger logger = LoggerFactory.getLogger(GitHubLookupService.class);

  private final RestTemplate restTemplate;

  public GitHubLookupService(RestTemplateBuilder restTemplateBuilder) {
    this.restTemplate = restTemplateBuilder.build();
  }

  @Async
  public CompletableFuture<User> findUser(String user) throws InterruptedException {
    logger.info("Looking up " + user);
    String url = String.format("https://api.github.com/users/%s", user);
    User results = restTemplate.getForObject(url, User.class);
    // Artificial delay of 1s for demonstration purposes
    Thread.sleep(1000L);
    return CompletableFuture.completedFuture(results);
  }

}

The GitHubLookupService class uses Spring’s RestTemplate to invoke a remote REST point (api.github.com/users/) and then convert the answer into a User object. Spring Boot automatically provides a RestTemplateBuilder that customizes the defaults with any auto-configuration bits (that is, MessageConverter).

The class is marked with the @Service annotation, making it a candidate for Spring’s component scanning to detect and add to the application context.

The findUser method is flagged with Spring’s @Async annotation, indicating that it should run on a separate thread. The method’s return type is CompletableFuture<User> instead of User, a requirement for any asynchronous service. This code uses the completedFuture method to return a CompletableFuture instance that is already completed with result of the GitHub query.

Creating a local instance of the GitHubLookupService class does NOT allow the findUser method to run asynchronously. It must be created inside a @Configuration class or picked up by @ComponentScan.

The timing for GitHub’s API can vary. To demonstrate the benefits later in this guide, an extra delay of one second has been added to this service.

Make the Application Executable

To run a sample, you can create an executable jar. Spring’s @Async annotation works with web applications, but you need not set up a web container to see its benefits. The following listing (from src/main/java/com/example/asyncmethod/AsyncMethodApplication.java) shows how to do so:

package com.example.asyncmethod;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.Bean;
import org.springframework.scheduling.annotation.EnableAsync;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;

import java.util.concurrent.Executor;

@SpringBootApplication
@EnableAsync
public class AsyncMethodApplication {

  public static void main(String[] args) {
    // close the application context to shut down the custom ExecutorService
    SpringApplication.run(AsyncMethodApplication.class, args).close();
  }

  @Bean
  public Executor taskExecutor() {
    ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
    executor.setCorePoolSize(2);
    executor.setMaxPoolSize(2);
    executor.setQueueCapacity(500);
    executor.setThreadNamePrefix("GithubLookup-");
    executor.initialize();
    return executor;
  }


}
The Spring Initalizr created an AsyncMethodApplication class for you. You can find it in the zip file that you downloaded from the Spring Initalizr (in src/main/java/com/example/asyncmethod/AsyncMethodApplication.java). You can either copy that class to your project and then modify it or copy the class from the preceding listing.

@SpringBootApplication is a convenience annotation that adds all of the following:

  • @Configuration: Tags the class as a source of bean definitions for the application context.

  • @EnableAutoConfiguration: Tells Spring Boot to start adding beans based on classpath settings, other beans, and various property settings. For example, if spring-webmvc is on the classpath, this annotation flags the application as a web application and activates key behaviors, such as setting up a DispatcherServlet.

  • @ComponentScan: Tells Spring to look for other components, configurations, and services in the com/example package, letting it find the controllers.

The main() method uses Spring Boot’s SpringApplication.run() method to launch an application. Did you notice that there was not a single line of XML? There is no web.xml file, either. This web application is 100% pure Java and you did not have to deal with configuring any plumbing or infrastructure.

The @EnableAsync annotation switches on Spring’s ability to run @Async methods in a background thread pool. This class also customizes the Executor by defining a new bean. Here, the method is named taskExecutor, since this is the specific method name for which Spring searches. In our case, we want to limit the number of concurrent threads to two and limit the size of the queue to 500. There are many more things you can tune. If you do not define an Executor bean, Spring creates a SimpleAsyncTaskExecutor and uses that.

There is also a CommandLineRunner that injects the GitHubLookupService and calls that service three times to demonstrate the method is executed asynchronously.

You also need a class to run the application. You can find that in src/main/java/com/example/asyncmethod/AppRunner.java. The following listing shows that class:

Unresolved directive in <stdin> - include::complete/src/main/java/hello/AppRunner.java[]

Build an executable JAR

You can run the application from the command line with Gradle or 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 so 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 use Gradle, you can run the application by using ./gradlew bootRun. Alternatively, you can build the JAR file by using ./gradlew build and then run the JAR file, as follows:

java -jar build/libs/gs-async-method-0.1.0.jar

If you 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-async-method-0.1.0.jar
The steps described here create a runnable JAR. You can also build a classic WAR file.

The application shows logging output, showing each query to GitHub. With the help of the allOf factory method, we create an array of CompletableFuture objects. By calling the join method, it is possible to wait for the completion of all of the CompletableFuture objects.

The following listing shows typical output from this sample application:

2016-09-01 10:25:21.295  INFO 17893 --- [ GithubLookup-2] hello.GitHubLookupService                : Looking up CloudFoundry
2016-09-01 10:25:21.295  INFO 17893 --- [ GithubLookup-1] hello.GitHubLookupService                : Looking up PivotalSoftware
2016-09-01 10:25:23.142  INFO 17893 --- [ GithubLookup-1] hello.GitHubLookupService                : Looking up Spring-Projects
2016-09-01 10:25:24.281  INFO 17893 --- [           main] hello.AppRunner                          : Elapsed time: 2994
2016-09-01 10:25:24.282  INFO 17893 --- [           main] hello.AppRunner                          : --> User [name=Pivotal Software, Inc., blog=http://pivotal.io]
2016-09-01 10:25:24.282  INFO 17893 --- [           main] hello.AppRunner                          : --> User [name=Cloud Foundry, blog=https://www.cloudfoundry.org/]
2016-09-01 10:25:24.282  INFO 17893 --- [           main] hello.AppRunner                          : --> User [name=Spring, blog=http://spring.io/projects]

Note that the first two calls happen in separate threads (GithubLookup-2, GithubLookup-1) and the third one is parked until one of the two threads became available. To compare how long this takes without the asynchronous feature, try commenting out the @Async annotation and runing the service again. The total elapsed time should increase noticeably, because each query takes at least a second. You can also tune the Executor to increase the corePoolSize attribute for instance.

Essentially, the longer the task takes and the more tasks are invoked simultaneously, the more benefit you see from making things asynchronous. The trade off is handling the CompletableFuture interface. It adds a layer of indirection, because you are no longer dealing directly with the results.

Summary

Congratulations! You have just developed an asynchronous service that lets you scale multiple calls at once.

See Also

The following guides may also be helpful:

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All guides are released with an ASLv2 license for the code, and an Attribution, NoDerivatives creative commons license for the writing.