Document using the Shadow plugin as an alternative to Boot's fat jars when using Gradle

I found the following is required in Gradle to get a fat jar working when cannot use the spring-boot-gradle-plugin plugin:

import com.github.jengelman.gradle.plugins.shadow.transformers.*

// Can't use Boot's nested jar packaging due to environment constrained classloader
apply plugin: 'com.github.johnrengelman.shadow'

shadowJar {
  zip64 true

  exclude 'META-INF/*.SF'
  exclude 'META-INF/*.DSA'
  exclude 'META-INF/*.RSA'

  // Required for Spring
  mergeServiceFiles()
  transform(AppendingTransformer) { resource = 'reference.conf'  }
  transform(AppendingTransformer) { resource = 'META-INF/spring.handlers'  }
  transform(AppendingTransformer) { resource = 'META-INF/spring.schemas'  }
  transform(AppendingTransformer) { resource = 'META-INF/spring.tooling'  }
  transform(PropertiesFileTransformer) { paths = ['META-INF/spring.factories' ] }

  // ...
}

The docs should strongly recommend the shadow / shade plugins for environmentally constrained classloaders¹ because resource transformation is a requirement. Without it, essential beans such as converters, property placeholder configurers, etc. go missing from Spring Boot’s autoconfiguration (i.e. spring.factories). The result is very difficult to diagnose.

¹environmentally constrained classloaders: A system classloader not under your control.

[unilama]

Final working example should be like that, strategy is required if you are using several components that contains spring.factories:

import com.github.jengelman.gradle.plugins.shadow.transformers.*
shadowJar {

    // Required for Spring
    mergeServiceFiles()
    append 'META-INF/spring.handlers'
    append 'META-INF/spring.schemas'
    append 'META-INF/spring.tooling'
    transform(PropertiesFileTransformer) {
        paths = ['META-INF/spring.factories' ]
        mergeStrategy = "append"
    }

}

I hope it will save someone’s lot of time 😃

[raj-saxena]

@unilama thanks for your comment. I was looking for this as Google’s Dataflow also doesn’t work with Spring’s Fat-jar. For KotlinDSL, here’s what I had to do to make it work -

plugins {
    id("com.github.johnrengelman.shadow") version "5.2.0"
}

...

tasks.withType<ShadowJar> {
    isZip64 = true
    // Required for Spring
    mergeServiceFiles()
    append("META-INF/spring.handlers")
    append("META-INF/spring.schemas")
    append("META-INF/spring.tooling")
    transform(PropertiesFileTransformer().apply {
        paths = listOf("META-INF/spring.factories")
        mergeStrategy = "append"
    })
}