Fat JAR

A fat-jar (or uber-jar) archive is a normal jar file single archive packing all the dependencies togetherso it can be run a standalone application directly using Java:

java -jar yourapplication.jar

This is the preferred way for running it in a container like docker, when deploying to herokuor when being reverse-proxied with nginx.

Gradle

When using Gradle, you can use the shadow gradle plugin to generate it. For example,to generate a fat JAR using netty as an engine:

build.gradle

  1. buildscript {
  2.     repositories {
  3.         jcenter()
  4.     }
  5.     dependencies {
  6.         classpath 'com.github.jengelman.gradle.plugins:shadow:2.0.4'
  7.     }
  8. }
  10. apply plugin: 'com.github.johnrengelman.shadow'
  11. apply plugin: 'kotlin'
  12. apply plugin: 'application'
  14. //mainClassName = 'io.ktor.server.netty.DevelopmentEngine' // For versions < 1.0.0-beta-3
  15. mainClassName = 'io.ktor.server.netty.EngineMain' // Starting with 1.0.0-beta-3
  17. // This task will generate your fat JAR and put it in the ./build/libs/ directory
  18. shadowJar {
  19.     manifest {
  20.         attributes 'Main-Class': mainClassName
  21.     }
  22. }

build.gradle.kts

  1. plugins {
  2.     application
  3.     kotlin("jvm") version "1.3.21"
  4.     // Shadow 5.0.0 requires Gradle 5+. Check the shadow plugin manual if you're using an older version of Gradle.
  5.     id("com.github.johnrengelman.shadow") version "5.0.0"
  6. }
  8. application {
  9.     mainClassName = "io.ktor.server.netty.EngineMain"
  10. }
  12. tasks.withType<Jar> {
  13.     manifest {
  14.         attributes(
  15.             mapOf(
  16.                 "Main-Class" to application.mainClassName
  17.             )
  18.         )
  19.     }
  20. }

Maven

When using Maven, you can generate a fat JAR archive with the maven-assembly-plugin. For example, to generatea fat JAR using netty as an engine:

pom.xml

  1. <plugin>
  2.     <groupId>org.apache.maven.plugins</groupId>
  3.     <artifactId>maven-assembly-plugin</artifactId>
  4.     <version>3.1.0</version>
  5.     <configuration>
  6.         <descriptorRefs>
  7.             <descriptorRef>jar-with-dependencies</descriptorRef>
  8.         </descriptorRefs>
  9.         <archive>
  10.             <manifest>
  11.                 <addClasspath>true</addClasspath>
  12.                 <mainClass>io.ktor.server.netty.EngineMain</mainClass>
  13.             </manifest>
  14.         </archive>
  15.     </configuration>
  16.     <executions>
  17.         <execution>
  18.             <id>assemble-all</id>
  19.             <phase>package</phase>
  20.             <goals>
  21.                 <goal>single</goal>
  22.             </goals>
  23.         </execution>
  24.     </executions>
  25. </plugin>