Handling jar compressed files

Handling jar compressed files

Quick notes about handling jars on the CLI in maven projects with java. Dependencies:

generate jar passing the directory the pom.xml is located:

; mvn package -f ./server/

path to the jar:

; file ./server/target/server-1.0-SNAPSHOT.jar
./server/target/server-1.0-SNAPSHOT.jar: Java archive data (JAR)

list contents with index of the jar

; jar tf ./server/target/server-1.0-SNAPSHOT.jar

to find the mainClass of the generated jar, you can decompress the META-INF/MANIFEST.MF file to stdout:

; unzip -p ./server/target/server-1.0-SNAPSHOT.jar META-INF/MANIFEST.MF
Manifest-Version: 1.0
Created-By: Maven JAR Plugin 3.3.0
Build-Jdk-Spec: 17
Implementation-Title: server
Implementation-Version: 1.0-SNAPSHOT
Main-Class: org.springframework.boot.loader.launch.JarLauncher
Start-Class: com.organization.app.App
Spring-Boot-Version: 3.2.4
Spring-Boot-Classes: BOOT-INF/classes/
Spring-Boot-Lib: BOOT-INF/lib/
Spring-Boot-Classpath-Index: BOOT-INF/classpath.idx
Spring-Boot-Layers-Index: BOOT-INF/layers.idx

;

Above, the mainClass is “Main-Class: org.springframework.boot.loader.launch.JarLauncher”. Now, you can execute the app in these two ways:

  1. directly execute the mainClass app in the jar: 
    ; java -cp ./server/target/server-1.0-SNAPSHOT.jar org.springframework.boot.loader.launch.JarLauncher
Hello World!
;
  2. …or just pass the jar file as parameter using the jar flag: 
    ; java -jar ./server/target/server-1.0-SNAPSHOT.jar
Hello World!
;

Layered jars

Spring boot provides the default 4 layers on jars:

to examine the layers of any layered jar (example using Spring Boot):

; java -Djarmode=layertools -jar ./server/target/server-1.0-SNAPSHOT.jar list
dependencies
spring-boot-loader
snapshot-dependencies
application
;

to extract the layers of any layered jar:

; java -Djarmode=layertools -jar ./server/target/server-1.0-SNAPSHOT.jar extract