Modules

Question 1

What types of modules are there?

Answer 1

Named, unnamed, automatic

Question 2

What are named modules?

Answer 2

They have module-info.java file.
They are on modules path.

Question 3

What are unnamed modules?

Answer 3

They are on classpath.
They ignore module-info.java.
Modules cannot import them, they don’t export anything.
They can read everything from classpath and other modules.

Question 4

What are automatic modules?

Answer 4

They don’t have module-info.java file.
They are on modules path.
Name of the module is determined by Java based on the name of the module’s jar file.
All packages are public.

Question 5

What is typical workflow of defining modules?

Answer 5

Definition
Compiling
Running or packing

Question 6

How are modules defined?

Answer 6

They are based on file structure. Inside src folder we define a folder and name it something. It can be package. Inside that folder or package we create “module-info.java” file. Everything that is inside that folder is now module.

Question 7

Do modules need to have main method?

Answer 7

No

Question 8

Can module-info.java file be empty?

Answer 8

No, it must at least have definition

Question 9

How do we define module in module-info.java

Answer 9

module full.name.of.the.module {}

Question 10

How are modules compiled?

Answer 10

javac -d out –module-source-path ./path/to/FirstClass.java ./path/to/SecondClass.java ./path/to/module-info.java

javac -d out -p ./path/to/FirstClass.java ./path/to/SecondClass.java ./path/to/module-info.java

Question 11

How do we run module?

Answer 11

It must have main class if we want to run it separately.

java –module-path out –module module.name/path.to.Main
java –p out -m module.name/path.to.Main

Question 12

How do we pack module?

Answer 12

jar -cvf /path/to/module.name.jar -C out/module.name .

Question 13

What is Jmod?

Answer 13

Command to work with jmod files.
It is made for native libs and for things that don’t go into jars.

Question 14

What is jlink?

Answer 14

It is a tool that creates smallest possible runnable module, leaving only dependencies module needs to run.

jdeps -summary module.name.jar
jdeps –jdk-internals module.name.jar
jdeps module.name.jar

Question 15

How do we list available modules?

Answer 15

java -p mods –list-modules
java –list-modules

Question 16

How do we show module resolution?

Answer 16

Showing resolution means to get information what modules are needed or used with some module. We do it as a part of module startup.

java –show-module-resolution -p path/to/module -m module.name/path.to.Main

Question 17

How do we describe module with CLI?

Answer 17

java -p path/to/module –describe-module module.name
jar –file path/to/module.jar –describe-module

Question 18

How can we do migration to modules?

Answer 18

Bottom-up method
Top-down method
Splitting big projects into module

Question 19

What is Bottom-up method of migration?

Answer 19

Process where:
- main project stays as unnamed module until all other dependencies become module
- dependencies are converted by adding module-info.java

Question 20

What is Top-down method of migration?

Answer 20

Process where:
- main project is put onto module path and becomes auto. module, and then it is turned into named module
- dependencies are turned into auto. modules and then to named modules

Question 21

Splitting big projects into module?

Answer 21

We split packages into groups and then we determine relationships

Question 22

How do we implement services trough modules?

Answer 22

1. Defining service interface
2. Defining service provider
3. Defining service locator
4. Defining service consumer

Question 23

How do we define service interface in modules?

Answer 23

We create module interface with interface methods in certain package.
We define module file in that package, which definition must contain:
“exports full.package.path”

Question 24

How do we define service provider?

Answer 24

Service provider implements service interface.
We create class in certain package, where we implement service interface defined before.
We create module file in that package, where we must define:
requires full.path.to.interface.module
provides full.path.to.interface.module.InterfaceName with full.path.to.interface.implementation.ClassName

Question 25

How do we define service locator?

Answer 25

We create class in certain package, where we use ServiceLoader.load() or ServiceLoader.stream():

public void useService() {
    ServiceLoader<MyService> loader = ServiceLoader.load(MyService.class);
    for (MyService service : loader) {
        service.doSomething();
    }
}

In same package we define module file, where we must define:
requires full.path.to.interface.implementation.ClassName
uses full.path.to.interface.module

Question 26

How do we define service consumer?

Answer 26

We can create class in some package, where we instantiate service locator and run wanted methods.
In same package we define module file, where we must define:
requires full.path.to.service.locator

Question 27

What does “requires” keyword mean?

Answer 27

It is used in module-info files to define dependecy that certaing module needs

Question 28

What does “requires transitive” keywords mean?

Answer 28

If some module B requires another module C, that module C will be required by every module that wants to require B. B can use “requires transitive” to enable every other module to implicitly extend some module.

Question 29

What does “exports” keyword mean?

Answer 29

It is used to define what package from module will be available to other modules, but only public methods.

Question 30

What does “exports … to” keyword mean?

Answer 30

It is used to define what package from module will be available to other specific module, but only public methods.

Question 31

What does “opens” keyword mean?

Answer 31

It is used to define what package methods and variables from module will be available to other modules via reflection.

Question 32

What does “opens .. to “ keyword mean?

Answer 32

It is used to define what package methods and variables from module will be available to other specific module via reflection.