How can I insert an runtime environment variable in an annotation attribute value in Java?

Question

I have been trying to populate an annotation attribute value at runtime, but it is not being able to compile The error is

"The value for annotation attribute DynamoDBIndexHashKey.globalSecondaryIndexName must be a constant expression"

Here is the snippet

public static final String GLOBAL_SECONDARY_INDEX = "RENDITIONS" + System.getenv("REGION_CODE") + "-" +System.getenv("STAGE") + "-renditionStatus-Index";

@DynamoDBIndexHashKey(globalSecondaryIndexName  = GLOBAL_SECONDARY_INDEX, attributeName ="renditionStatus")

The value for GLOBAL_SECONDARY_INDEX for the attribute globalSecondaryIndexName is not able to compile as a constant despite of using the "+" operator. Am I missing something? Many Thanks in advance for trying to help

Answer 1

It is possible to dynamically resolve GSI name, although it is pretty hacky.

1. Instead of using System.getenv("REGION_CODE") in the expression just use some kind of a template string e.g. {{env.REGION_CODE}} or __REGION_CODE__, in this example I will use {{env.REGION_CODE}}.

public class MyClass {
  @DynamoDBIndexHashKey(globalSecondaryIndexName = "RENDITIONS-{{env.REGION_CODE}}-{{env.STAGE}}", attributeName = "renditionStatus")
  private String bar;
}

2. create a new method which will process the template, e.g.

package com.mycompany.myproduct.utils;

import java.util.Arrays;
import java.util.stream.Collectors;

public class TemplateProcessor {
   public static String processTemplate(String mustacheTemplate) {
       return Arrays.stream(mustacheTemplate.split("\\{\\{|}}"))
               .map(chunk -> chunk.startsWith("env.") ? System.getenv(chunk.substring(4)) : chunk)
               .collect(Collectors.joining(""));
   }
}

3. Patch all the codes where DynamoDBIndexHashKey.globalSecondaryIndexName() is used.

   i. Find and copy all the classes that use DynamoDBIndexHashKey.globalSecondaryIndexName() into your project while making sure that the package part remains the same as the original.

   ii. Everyplace where globalSecondaryIndexName() is called wrap it with calling the previously created processTemplate method.

Luckily (currently, in com.amazonaws:aws-java-sdk-dynamodb:1.12.422) the only place that calls DynamoDBIndexHashKey.globalSecondaryIndexName() is StandardAnnotationMaps.FieldMap.globalSecondaryIndexNames() so the patching won't take too much time.

The end result should look something like this:

package com.amazonaws.services.dynamodbv2.datamodeling;

...
import static com.mycompany.myproduct.utils.TemplateProcessor.processTemplate;
...

final class StandardAnnotationMaps {
  ...
  public Map<KeyType,List<String>> globalSecondaryIndexNames() {
    ...
    String processed = processTemplate(indexHashKey.globalSecondaryIndexName());
    gsis.put(HASH, Collections.singletonList(processed));
    ...
  }
  ...
}