java - What is the difference between modules and JAR files?

Question

I am learning about Java 9 from What's New in Java9 and one of the hot topics in the discussion is The Modular JDK.

Are JAR files modules?

How is a module different from a JAR file?

Answer 1

Module: A new language feature introduced in Java 9 (similar to class, interface, package, etc.) that consists of a collection of packages, similar to how a package consists of a collection of types.

JAR: An archive file format that bundles code and resources and which can be loaded by the JVM.

More specifically, a module is defined as follows:

In order to provide reliable configuration and strong encapsulation in a way that is both approachable to developers and supportable by existing tool chains we treat modules as a fundamental new kind of Java program component. A module is a named, self-describing collection of code and data. Its code is organized as a set of packages containing types, i.e., Java classes and interfaces; its data includes resources and other kinds of static information.

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A module’s self-description is expressed in its module declaration, a new construct of the Java programming language.

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A module declaration is compiled, by convention, into a file named module-info.class, placed similarly in the class-file output directory.

A module can be compiled into a Jar file, in which case the Jar file is labelled a modular Jar file:

Existing tools can already create, manipulate, and consume JAR files, so for ease of adoption and migration we define modular JAR files. A modular JAR file is like an ordinary JAR file in all possible ways, except that it also includes a module-info.class file in its root directory.

Some other differences between a module and a JAR:

1. Modules can require other modules in order to allow accessing dependent classes by the requiring module. A Jar has no such dependency concept.

2. A module can decide which classes and interfaces to export to other modules that require it. A Jar has no such encapsulation mechanism.

3. A module can be compiled into a modular Jar file, but some modules (e.g. JDK modules) are compiled into another format called JMOD.

4. The name of a JAR can be changed. As long as the JVM classloader finds the needed class on the classpath (which can be composed of a single JAR, multiple JARs, or a mix between directories or JARs), the name of the JAR file can be anything. However, the name of a module can be explicitly referenced in the declaration of other modules, and such the name defines it and cannot be changed freely.