Usage of spring application index

Question

Can someone please clarify what is the usage of spring.application.index property and why do we need it?

Application.yml:

spring:
    application:
        name: ServiceName
        index: 

Answer 1

As far as I see, the spring.application.index has been considered deprecated since the version 2.0.0.RC1. I judge from comparing the following these appendices:

• Spring Boot 2.0.0.M7: Common application properties where the spring.application.index is mentioned.
• Spring Boot 2.0.0.RC1: Common application properties where the spring.application.index is not mentioned.

The previous statement is proved by inspecting the source codes of the ContextIdApplicationContextInitializer across these versions:

• Version 2.0.0.M7 ContextIdApplicationContextInitializer. These versions provide more variability in customization the application index used for ApplicationContextID creation.

  /**
   * Placeholder pattern to resolve for application index. The following order is used
   * to find the name:
   * <ul>
   * <li>{@code vcap.application.instance_index}</li>
   * <li>{@code spring.application.index}</li>
   * <li>{@code server.port}</li>
   * <li>{@code PORT}</li>
   * </ul>
   * This order favors a platform defined index over any user defined value.
   */`"${vcap.application.instance_index:${spring.application.index:${server.port:${PORT:null}}}}"`
  

• Version 2.0.0.RC1 ContextIdApplicationContextInitializer. There could be find the index is incremented using automatically using AtomicLong, which also assures its uniqueness. See the inner ContextIdApplicationContextInitializer$ContextId class for more detail in the source. The key method is its constructor:

  ContextId createChildId() {
      return new ContextId(this.id + "-" + this.children.incrementAndGet());
  }
  

Answer 2

You can find this property in Spring sources:

private static final String INDEX_PATTERN = 
"${vcap.application.instance_index:${spring.application.index:${server.port:${PORT:null}}}}";

As the Javadoc of this class says, it is used to create ApplicationContextID which is a unique id of an application context.