Java - (Jar|Java ARchive) File

About

JAR stands for Java ARchive and is file format born in 1996 as a simple extension of the popular ZIP archive format with class and other required resource files:

• manifest
• signature files
• images, sounds
• …

The primary motivation for its development was so that Java applets and their requisite components (files, images and sounds) can be downloaded to a browser in a single HTTP transaction, rather than opening a new connection for each piece.

With the advent of the Main-Class manifest header, entire applications can be delivered.

Hell

Jar hell is a term similar to describe all the various ways in which the class loading process can end up not working.

To avoid a Jar hell, the shading technique can be used.

Management

Name

Jar Archive name follows the coordinate schema.

Create

You can create JAR files, either:

• by using the JAR utility directly,
• through an IDE such as Oracle JDeveloper, Eclipse, …
• or by using a build tool such as the ant utility, maven or gradle.

With the jar command-line utility.

jar cf myPackage.jar *.class

where:

• 'c': create new archive.
• 'f': create a file.

Create an Executable Jar

Creating a executable jar file needs more steps:

• create a manifest-file containing the start class,
• creating the target directory
• and archiving the files

echo Main-Class: com.gerardnico.HelloWorld>myManifest
md build\jar
jar cfm build\jar\HelloWorld.jar myManifest -C build\classes

Execute

• an executable JAR

java -jar myJAR.jar

• an class with a main class

java -jar myJAR.jar com.example.main arg1 arg2

Verify / Sign

• jarsigner: You use the jarsigner tool to sign and verify Java Archive (JAR) files.

Verification

jar tf build/libs/building-java-libraries.jar

META-INF/
META-INF/MANIFEST.MF
Library.class

You should see the required manifest file —MANIFEST.MF— and the compiled class.

Search class

Windows

for /R %G in (*.jar) do @jar -tvf "%G" | find "ClassName" > NUL && echo %G

where:

• <wrap>for /R %G in (*.jar) do</note>

  loop over all JAR files in the directory tree, store the file name in %G. See Dos - For Statement

• <wrap>@jar -tvf %G |</note>

  - run the Java Archive command to list all file names within the given archive, and write the results to standard output; the @ symbol suppresses the echoing of the command to standard output.

• <wrap>find “ClassName” > NUL</note>

  - search standard input, piped from the output of the jar command, for the given class name. It will set ERRORLEVEL to 1 if there's a match (otherwise 0). See DOS - Find Command.

• <wrap>&& echo %G</note>

  - if ERRORLEVEL is non-zero, write the Java archive file name to standard output (the console). See the AND control opertaor (&&)

Linux

• below a certain location

find path/to/jars -name "*.jar" -exec grep -Hsli ClassName  {} \;

• anywhere

for i in $(locate "*.jar");
  do echo $i; jar -tvf $i | grep -Hsi ClassName;
done