How to compile a jar from the command line

On a Debian GNU/Linux version 11.0 Bullseye it is required to install the default-jdk package, to have the javac compiler.

Finding the reqired libraries

A Java project generally uses several libraries; you can find lines like this into the .java sources:

import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

If you don't download and install the proper .jar archives, you will get errors like the following when executing the javac compiler:

./org/vmax/amba/cfg/FirmwareConfig.java:3:
    error: package com.fasterxml.jackson.annotation does not exist
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;

Each library may exists on the internet in several versions: it may be an hard work to guess which exact version was used by the developer for each one. You can use several internet repositories to search and download the archives you need, we suggest at least jar-download.com, repo1.maven.org and www.java2s.com.

Download the included libraries

It is advisable to create a directory which will contains all the libraries (downloaded as .jar archives) required by your project, that directory should be placed at the root level of your Java project.

└─ java-project/
   ├─ lib/
   │  ├─ apache-commons-net.jar
   │  ├─ commons-math3-3.0.jar
   │  ├─ ...
   │  └─ slf4j.jar
   └─ org/
      └─ vmax/
         ├─ amba/
         │  ├─ bitrate/
         │  ├─ ...
         │  ├─ AppMain.java
         │  ├─ ...
         │  └─ Utils.java
         └─ midrive/

Compile the Java code

To be on the safe side, we delete all the previously compiled files which have extension .class, then we create a list of all the .java files which are to be compiled. The CLASSPATH environment variable is intialized with the name of all the jar libraries, separated by a colon:

#!/bin/sh
CLASSPATH="$(echo lib/*.jar | sed 's/\.jar /\.jar:/g')"
find . -name '*.class' | xargs -r rm
find . -name '*.java' > sources.txt
javac --class-path "$CLASSPATH" @sources.txt

Pack the Jar archive

#!/bin/sh
CLASSES="$(echo lib/*.jar)"
echo 'Main-Class: org.vmax.amba.AppMain' > manifest.txt
echo "Class-Path: $CLASSES" >> manifest.txt
jar --create --verbose \
    --file='firmware-editor-tool.jar' \
    --manifest='manifest.txt' \
    $(find . -type f -name '*.class')