Building a Spring Boot server with Bazel

Spring is a very popular Java Framework that is often used to build servers. It’s widely used by many companies because it provides a rich toolbox that can help achieve a great variety of tasks.

In this article, we’re going to learn how to create a simple Spring server using Bazel as build system.

Building the server

If you have never used Bazel, I recommend you take a look at my introduction to Bazel to get familiar with it.

Spring has a tool called spring intializr that helps create a Spring Boot project from scratch. The problem is that at the time of this writing it only supports Maven and Gradle.

Since we can’t use that tool, we’re going to create a folder for our application and add a WORKSPACE file:

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load('@bazel_tools//tools/build_defs/repo:http.bzl', 'http_archive')

RULES_JVM_EXTERNAL_TAG = '4.1'
RULES_JVM_EXTERNAL_SHA = 'f36441aa876c4f6427bfb2d1f2d723b48e9d930b62662bf723ddfb8fc80f0140'

http_archive(
  name = 'rules_jvm_external',
  strip_prefix = 'rules_jvm_external-%s' % RULES_JVM_EXTERNAL_TAG,
  sha256 = RULES_JVM_EXTERNAL_SHA,
  url = 'https://github.com/bazelbuild/rules_jvm_external/archive/%s.zip' % RULES_JVM_EXTERNAL_TAG,
)

load('@rules_jvm_external//:defs.bzl', 'maven_install')

maven_install(
  artifacts = [
    'org.springframework.boot:spring-boot-autoconfigure:2.1.3.RELEASE',
    'org.springframework.boot:spring-boot-starter-web:2.1.3.RELEASE',
    'org.springframework.boot:spring-boot:2.1.3.RELEASE',
    'org.springframework:spring-web:5.1.5.RELEASE',
  ],
  repositories = [
    'https://repo1.maven.org/maven2',
  ],
  fetch_sources = True,
)

In this file we can see how we load our spring dependencies. The next thing we need is a BUILD file:

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java_binary(
  name = 'app',
  main_class = 'com.example.demo.DemoApplication',
  srcs = glob(['src/**/*.java']),
  deps = [
    '@maven//:org_springframework_boot_spring_boot',
    '@maven//:org_springframework_boot_spring_boot_autoconfigure',
    '@maven//:org_springframework_boot_spring_boot_starter_web',
    '@maven//:org_springframework_spring_web'
  ],
)

Here we create a java binary using the files in the src folder. Let’s create the file src/main/java/com/example/demo/DemoApplication.java with this content:

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package com.example.demo;

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

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

And we can now run our Spring Boot application:

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bazel run :app

Our app doesn’t yet do anything, but we have built and ran our app with Bazel.

We can modify our java file so the server listens to an endpoint:

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package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

@RestController
@SpringBootApplication
public class DemoApplication {
  public static void main(String[] args) {
    SpringApplication.run(DemoApplication.class, args);
  }

  @GetMapping("/greet")
  public String hello(@RequestParam(value = "name", defaultValue = "World") String name) {
    return String.format("Hello %s!", name);
  }
}

We now have a greet endpoint that we can hit:

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curl http://localhost:8080/greet?name=adrian

Conclusion

In this article we learned how to build a simple Spring Boot server with Bazel. We didn’t touch much into all the features that Spring has, but this can serve as the foundation we use to explore the framework.