Deploying an executable JAR on Azure functions without docker image?

Question

I am trying to replicate on Azure Functions the example that is in Quickstart: Create a Go or Rust function in Java. I set up a couple of methods that respond to REST requests, and I bundled them as well as a Jetty server, that manages and routes the requests, into an executable Uber JAR. When I run java -jar handler.jar, an instance of a Jetty server is started, so I can navigate to http://localhost:8080/ping. I can achieve the same by using the JRE I have installed on my computer when using func start with the following host.json:

     "customHandler": {
         "description": {
           "defaultExecutablePath": "java",
           "arguments": ["-jar", "handler.jar"]
         },
         "enableForwardingHttpRequest": true
       }

Now, I'm trying to deploy this app to Azure Functions. One possible way would be to create a custom image with the following Dockerfile:

FROM mcr.microsoft.com/azure-functions/java:4-java11-slim
ENV AzureWebJobsScriptRoot=/home/site/wwwroot
COPY ["./functions", "/home/site/wwwroot"]

and start a function by using this image - I tried this and it works ok, but it seems to be quite inefficient to create a 1GB+ image, when the JAR I want to deploy takes up just a couple of MBs.

I was wondering, if there is a way to avoid this step? For example, push only the JAR and all the .json files to functions and use the default JRE, that is included when I create a Function App with the Java runtime stack? In other words, is it possible to achieve what I achieve locally when using func start, but use the JRE that is on azure instead?

The documentation ("If your handler requires operating system or platform dependencies (such as a language runtime), you may need to use a custom container.") is not that very clear about the topic.

Thank you!

Answer 1

Have a look at the Triggers and annotations paragraph of the Azure Functions Java developer guide.

In the function.json, set the scriptFile field to the (relative) location of your jar, and set entryPoint to the location of the method the Azure Function should execute. Then using the Azure Functions Core Tools, you can do func azure functionapp publish {function_app_name_in_Azure} --java to publish too the Function App in the cloud.