JVM Specialty Group December 2018 Meeting
Wednesday, December 19, 2018
Introduction to the Java Module System
Presenter: Andrej Rasevic
Wednesday, December 19, 2018
Introduction to the Java Module System
Desired outcomes:
- understand why was the module system necessary
- create and run a java module-based application
- what problems does it solve and what are its principal benefits
- get our hands dirty with the new module system
Wednesday, December 19, 2018
Introduction to the Java Module System
Why was the module system necessary - motivation & background
Improved separation of concerns
- more able to work on individual parts in isolation
- facilitate reuse of parts
- easier maintenance of overall systems
how does an application benefit from a modular system?
Wednesday, December 19, 2018
Introduction to the Java Module System
Why was the module system necessary - motivation & background
Increased encapsulation and information hiding
how does an application benefit from a modular system?
- compiler can now check that classes and packages are available only for their intended purposes
Wednesday, December 19, 2018
Introduction to the Java Module System
Why was the module system necessary - motivation & background
Modularity limitations
- Java grouped code at 3 levels (classes, packages and JARS)
- supported access modifiers and encapsulation for classes
- little encapsulation at the package and JAR level
Wednesday, December 19, 2018
Introduction to the Java Module System
Why was the module system necessary - motivation & background
Limited visibility control
- Java provides public, private, protected and package level visibility for classes
- If you want to share classes and interfaces from one package with another, you must declare them as public
- exposes your classes and interfaces to everyone allowing for increased security risks
Wednesday, December 19, 2018
Introduction to the Java Module System
Why was the module system necessary - motivation & background
CLASS PATH HELL
- distribute all compiles classes as single flat JAR which are lazily loaded by the CLASS PATH
- CLASS PATH cannot differentiate between different versions of the same class on the class path
- doesn't support explicit dependencies
- all classes of different JARs are merged into one bag of classes on the class path
- impossible to ask if anything is missing or are there any conflicts
Wednesday, December 19, 2018
Introduction to the Java Module System
Why was the module system necessary - motivation & background
Monolithic JDK
JDK is a collection of tools that lets you work with and run java programs
-
javac - lets you compile Java programs
-
java along with JDK library allow you to load and run Java program
- what does CORBA do for you?
- especially problematic for mobile or cloud based applications that don't need all the available parts of the JDK library
Wednesday, December 19, 2018
Introduction to the Java Module System
Jigsaw-puzzle style of representation of a Java system
Wednesday, December 19, 2018
Introduction to the Java Module System
module com.foo {
requires com.bar;
exports com.foo;
}In order to become a module, a project must have a module-info.java file at the root of the project's source
Wednesday, December 19, 2018
Introduction to the Java Module System
Modules are very similar to JAR's in that they are container's for types and resources
Modules differ from JAR's by having additional characteristics
- a name, preferably one that is globally unique
- declarations of dependancies on other modules
- clearly defined API that consists of exported packages
Wednesday, December 19, 2018
Introduction to the Java Module System
JDK was broken up into over 100 platform modules
If you want to see them, execute: java --list-modules
Let's run it in the terminal...
Now if we want to inspect a specific module you can run: java --describe-module ${module}
Let's see what the java.sql module API looks like
Wednesday, December 19, 2018
Introduction to the Java Module System
Let's take a look at an example of using Java modules