Enhancement: Support Executable WAR Files via `BP_EXECUTABLE_JAR_LOCATION`

[sabomichal]

Enable support for executable WAR files in the executable-jar buildpack by allowing .war files specified via BP_EXECUTABLE_JAR_LOCATION to be treated as valid runnable JVM packages.

Describe the Enhancement

The executable-jar buildpack currently does not recognize a .war file specified via the BP_EXECUTABLE_JAR_LOCATION environment variable as a valid JVM application package, even when the WAR is fully executable (i.e., runnable via java -jar). This behavior unnecessarily limits support for Spring Boot WARs, which are explicitly designed to be executable and self-contained.

Executable WARs are a first-class citizen in the Spring Boot ecosystem, using a Main-Class such as org.springframework.boot.loader.WarLauncher. Users should be able to point to a .war file using BP_EXECUTABLE_JAR_LOCATION and have it work identically to .jar files.

Motivation

Spring Boot enables users to package applications as executable WARs, combining the benefits of WAR-based servlet configuration and modern containerization:

• WARs maintain compatibility with traditional application servers and CI/CD pipelines.
• Executable WARs support standalone execution (java -jar app.war), using embedded servlet containers (Tomcat, Jetty, Undertow).
• This format is especially useful in migration scenarios from legacy WAR deployments to containerized Spring Boot apps.

According to Spring Boot documentation, executable WARs are explicitly supported:

A Spring Boot WAR can be executed using java -jar, and it will launch with an embedded servlet container.

Despite this, the executable-jar buildpack fails detection or execution if the file pointed to by BP_EXECUTABLE_JAR_LOCATION has a .war extension, even if it includes a valid Main-Class.

Expected Behavior

• The buildpack should recognize .war files as valid targets if BP_EXECUTABLE_JAR_LOCATION is explicitly set.
• Detection logic should validate the presence of Main-Class and treat .war files equivalently to .jar files if the file is executable.

Possible Solution

1. Extend the executable-jar buildpack logic to allow .war files when:
   • BP_EXECUTABLE_JAR_LOCATION is set to a .war file
   • Main-Class is present and recognizable (e.g., org.springframework.boot.loader.WarLauncher)
2. Optionally, support an env variable like BP_EXECUTABLE_WAR_LOCATION

References

Spring Boot executable WAR