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

Answer 1

Are JAR files Modules? How Module is different from JAR file?

No, a Java Archive is not a Module.

Just for an example, while classes of the same package could have been spread across JARs, the same package now can not be read from multiple modules.

A JAR is a file format that enables you to bundle multiple files into a single archive file. Typically this contains the class files and auxiliary resources associated with applets and applications.

on the other hand (I'd tried describing this here ~> java-module as well)

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.

This also consists of the module declaration as specified with the help of module-info.java.

Each module definition is either

• A module artifact, i.e., a modular JAR file or a JMOD file containing a compiled module definition, or else

• An exploded-module directory whose name is, by convention, the module's name and whose content is an "exploded" directory tree corresponding to a package hierarchy.

As introduced with the module system, a modular image is composed of modules rather than JAR files. Modularity is foreseen for with dynamic configuration in terms of Modular WAR file as well.

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But for the ease of adoption of Modules a Modular JAR file, was introduced in JDK9, such that lets say for a module consisting of a module-info.java such that

module com.foo.bar { }

and other java classes as

com/foo/bar/alpha/AlphaFactory.java
com/foo/bar/alpha/Alpha.java

Existing tools can already create, manipulate, and consume 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. A modular JAR file for the above com.foo.bar module, e.g., might have the content:

META-INF/
META-INF/MANIFEST.MF
module-info.class
com/foo/bar/alpha/AlphaFactory.class
com/foo/bar/alpha/Alpha.class
...

A modular JAR file can be used as a module, in which case its module-info.class file is taken to contain the module’s declaration. It can, alternatively, be placed on the ordinary class path, in which case its module-info.class file is ignored.