Deploying Functions with Spring Cloud Function: A Hands‑On Guide

Environment: SpringBoot 2.7.16 + SpringCloud 2021.0.8

1. Introduction

Spring Cloud Function provides a "deployer" library that lets you launch JAR files (or exploded archives, or a set of JARs) with an isolated classloader and expose the functions defined inside. This is powerful for adapting functions to various input/output adapters without modifying the target JAR, and it is used by many serverless platforms.

2. Practical Example

To use the library, add the following Maven dependency:

<dependency>
  <groupId>org.springframework.cloud</groupId>
  <artifactId>spring-cloud-function-deployer</artifactId>
  <version>${spring.cloud.function.version}</version>
</dependency>

With this dependency, your program can load third‑party JARs via an independent classloader and expose any Function interfaces as services.

2.1 Introductory Example

The user must provide the spring.cloud.function.location property, which points to the JAR URL (or a Maven coordinate). If the JAR starts successfully, the function is registered in the main application's FunctionCatalog and can be invoked.

@SpringBootApplication
public class SpringcloudFunctionDeployerApplication {
  public static void main(String[] args) {
    ApplicationContext context = SpringApplication.run(DeployFunctionDemo.class,
        "--spring.cloud.function.location=E:/functions/pack-function.jar",
        "--spring.cloud.function.function-class=functions.LengthFunction");
    FunctionCatalog catalog = context.getBean(FunctionCatalog.class);
    Function<String, Integer> function = catalog.lookup("lengthFunction");
    System.out.println(function.apply("hello"));
  }
}

Running the program prints 5 to the console.

The two parameters can also be supplied via the command line.

2.2 Simple JAR (no Spring dependencies)

Assume the JAR contains the following class:

public class LengthFunction implements Function<String, Integer> {
  @Override
  public Integer apply(String t) {
    return t == null ? 0 : t.length();
  }
}

Package this class into a JAR and start the Spring Cloud Function project with the JAR location and the specific function class:

--spring.cloud.function.location=E:/functions/pack-function.jar
--spring.cloud.function.function-class=functions.LengthFunction

The console shows the function name lengthFunction and it can be accessed.

If you define multiple functions, separate them with a semicolon:

--spring.cloud.function.location=E:/functions/pack-function.jar
--spring.cloud.function.function-class=functions.LengthFunction;functions.UppercaseFunction

The console then lists two recognized functions.

When many functions exist, specifying spring.cloud.function.function-class becomes cumbersome. Spring Cloud Function also supports component scanning; the package name must be functions, eliminating the need for the function-class property.

2.3 SpringBoot JAR

This packaging option includes Spring Boot dependencies, producing a Spring Boot JAR. Because the deployed JAR runs in an isolated classloader, it does not conflict with the Spring Boot version used by the deployer.

Write a simple SpringBoot program that defines the required function, for example:

@Configuration
public class FunctionConfig {
  @Bean
  Function<String, String> uppercase() {
    return value -> value.toUpperCase() + " - ";
  }
}

Package the JAR and configure the deployer as follows:

--spring.cloud.function.location=E:/functions/zzz-pack-springboot-functions-1.0.0.jar
--spring.cloud.function.definition=uppercase

After starting, two applications are launched—one using SpringBoot 2.x and the other 3.x—without any version conflict.

Testing the /uppercase endpoint returns the expected transformed string.

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