JAR & layertools

JAR

JAR stands for Java Archive. It is a file format used to package Java classes, resources, and metadata into a single file for distribution. JAR files are similar to ZIP files, but they have additional features that make them useful for Java applications.

JAR files can be used for a variety of purposes, such as:

• Distributing Java libraries: JAR files can be used to package Java libraries and distribute them to other developers or users.

• Deploying Java applications: JAR files can be used to package Java applications and deploy them to servers or other environments.

• Executing Java applications: JAR files can be used to package standalone Java applications that can be executed using the Java Virtual Machine (JVM).

• JAR files can be created using the jar command-line tool that comes with the Java Development Kit (JDK). JAR files can also be created using build tools such as Apache Maven or Gradle.

• To run a Java application packaged in a JAR file, you can use the java command with the -jar option, followed by the name of the JAR file. For example, to run a JAR file named myapp.jar, you can use the following command:

java -jar myapp.jar

• JAR files can also be signed using digital certificates to ensure their authenticity and integrity. This is useful for distributing Java applications and libraries over the internet.

-Djarmode=layertools

-Djarmode=layertools is a command-line option used with Spring Boot applications. It enables the “layered JAR” mode, which allows the application to be packaged as a single JAR file with separate layers for dependencies, resources, and classes.

When the application is run with this option, the JAR file is exploded into a directory structure with separate directories for each layer. This allows for more efficient deployment and startup times, as only the necessary layers need to be loaded.

The layertools command can be used to manipulate the layers of the JAR file. For example, you can use layertools list to list the layers, layertools extract to extract a layer, or layertools dependencies to list the dependencies of a layer.