Chapter 11 Modules

Question 1

Introducing Modules

Answer 1

• The Java Platform Module System (JPMS) was introduced in Java 9
• The main purpose of a module is to provide groups of related packages to offer a particular set of functionality to developers.

The Java Platform Module System includes the following:

• A format for module JAR files
• Partitioning of the JDK into modules
• Additional command-line options for Java tools

A module is a group of one or more packages plus a special file called module-info.java.

dependencies where one module relies on code in another.

Question 2

Java Platform Module System (JPMS)

Answer 2

• The Java Platform Module System (JPMS) was introduced in Java 9
• The Java Platform Module System includes the following:
  
  • A format for module JAR files
  • Partitioning of the JDK into modules
  • Additional command-line options for Java tools

Question 3

BENEFITS OF MODULES

Answer 3

• Better Access Control
  
  • fifth level of access control.
  • They can expose packages within the modular JAR to specific other packages.
  • This stronger form of encapsulation really does create internal packages.
• Clearer Dependency Management
  specify their dependencies in the module-info.java file.
• Custom Java Builds
  
  • allows developers to specify what modules they actually need.
  • A tool called jlink is used to create this runtime image.
• Improved Performance
  improves startup time and requires less memory to run.
• Unique Package Enforcement
  A package is only allowed to be supplied by one module.

Question 4

module-info.java

Answer 4

This is the simplest possible module-info.java file

module zoo.animal.feeding {
}

• The module-info file must be in the root directory of your module.
• The module-info file must use the keyword module.

1: module zoo.animal.care {
2: exports zoo.animal.care.medical;
3: requires zoo.animal.feeding;
4: }

• Line 1 specifies the name of the module.
• Line 2 lists the package we are exporting so it can be used by other modules.
• On line 3, we see a new keyword. The requires statement specifies that a module is needed. The zoo.animal.care module depends on the zoo.animal.feeding module.

Question 5

There are a few key differences between a module-info file and a regular Java class:

Answer 5

• The module-info file must be in the root directory of your module. Regular Java classes should be in packages.
• The module-info file must use the keyword module instead of class, interface, or enum.
• The module name follows the naming rules for package names. It often includes periods (.) in its name. Regular class and package names are not allowed to have dashes (-). Module names follow the same rule.

That’s a lot of rules for the simplest possible file. There will be many more rules when we flesh out this file later in the chapter.

Question 6

CAN A MODULE-INFO.JAVA FILE BE EMPTY?

Answer 6

Yes. As a bit of trivia,

it was legal to compile any empty file with a .java extension even before modules.

The compiler sees there isn’t a class in there and exits without creating a .class file.

Question 7

COMPILING OUR FIRST MODULE

Answer 7

> [!NOTE]
When you’re entering commands at the command line, they should be typed all on one line.

javac --module-path mods -d feeding feeding/zoo/animal/feeding/*.java feeding/module-info.java

• the -d option specifies the directory to place the class files in.
• The end of the command is a list of the .java files to compile. You can list the files individually or use a wildcard for all .java files in a subdirectory.
• --module-path option indicates the location of any custom module files.
• The syntax --module-path and -p are equivalent.

The following four commands show the -p option:

javac -p mods -d feeding feeding/zoo/animal/feeding/*.java feeding/*.java

javac -p mods -d feeding feeding/zoo/animal/feeding/*.java feeding/module-info.java

javac -p mods -d feeding feeding/zoo/animal/feeding/Task.java feeding/module-info.java

javac -p mods -d feeding feeding/zoo/animal/feeding/Task.java feeding/*.java

Question 8

WHAT HAPPENED TO THE CLASSPATH?

Answer 8

You can still use these options in Java 11. In fact, it is common to do so when writing nonmodular programs.

-cp, --class-path, and -classpath

Question 9

RUNNING OUR FIRST MODULE

Answer 9

Pay special attention to the book.module/com.sybex.OCP part. It is important to remember that you specify the module name followed by a slash (/) followed by the fully qualified class name.

java --module-path mods --module book.module/com.sybex.OCP

1. Location of modules, --module-path, -p
2. Module name, --module, -m
3. Module/package separator
4. Package name
5. Class name

examples:

java --module-path feeding --module zoo.animal.feeding/zoo.animal.feeding.Task

java -p feeding -m zoo.animal.feeding/zoo.animal.feeding.Task

Question 10

PACKAGING OUR FIRST MODULE

Answer 10

Be sure to create a mods directory before running this command:

jar -cvf mods/zoo.animal.feeding.jar -C feeding/ .

• We are packaging everything under the feeding directory
• and storing it in a JAR file named zoo.animal.feeding.jar under the mods folder.

run the program again, but this time using the mods directory instead of the loose classes:

java -p mods -m zoo.animal.feeding/zoo.animal.feeding.Task

Since the module path is used, a module JAR is being run.

Question 11

> [!NOTE]
It is possible to version your module using the –module-version option.
This isn’t on the exam but is good to do when you are ready to share your module with others.

Answer 11

version your module using the –module-version option.

Question 12

ORDER MATTERS!

Answer 12

Order matters when compiling a module.

javac -p mods -d care care/module-info.java care/zoo/animal/care/details/*.java care/zoo/animal/care/medical/*.java

The compiler complains that it doesn’t know anything about the package zoo.animal.care.medical.

care/module-info.java:3: error: package is empty or does not exist:
zoo.animal.care.medical
exports zoo.animal.care.medical;

• A package must have at least one class in it in order to be exported.
• Since we haven’t yet compiled zoo.animal.care.medical.Diet, the compiler acts as if it doesn’t exist.
• If you get this error message, you can reorder the javac statement.
• Alternatively, you can compile the packages in a separate javac command, before compiling the module-info file.

jar -cvf mods/zoo.animal.care.jar -C care/ .

Question 13

Diving into the module-info File

Answer 13

These are not Java keywords they are directives, can appear in any order in the module-info file.

• exports,
• requires,
• provides,
• uses,
• and opens.

Question 14

EXPORTS

Answer 14

• exports packageName exports a package to other modules.
• It’s also possible to export a package to a specific module.

module zoo.animal.talks {
exports zoo.animal.talks.content to zoo.staff;
exports zoo.animal.talks.media;
exports zoo.animal.talks.schedule;
requires zoo.animal.feeding;
requires zoo.animal.care;
}

Question 15

EXPORTED TYPES

Answer 15

Exporting a package

• All public classes, interfaces, and enums are exported.
• Further, any public and protected fields and methods in those files are visible.
• Fields and methods that are private are not visible because they are not accessible outside the class.
• Similarly, package-private fields and methods are not visible because they are not accessible outside the package.

Question 16

REQUIRES TRANSITIVE

Answer 16

requires moduleName specifies that the current module depends on moduleName.

There’s also a requires transitive moduleName, which means that any module that requires this module will also depend on moduleName.

module zoo.animal.feeding {
exports zoo.animal.feeding;
}

module zoo.animal.care {
exports zoo.animal.care.medical;
requires transitive zoo.animal.feeding;
}

module zoo.animal.talks {
exports zoo.animal.talks.content to zoo.staff;
exports zoo.animal.talks.media;
exports zoo.animal.talks.schedule;
// no longer needed requires zoo.animal.feeding;
// no longer needed requires zoo.animal.care;
requires transitive zoo.animal.care;
}

module zoo.staff {
// no longer needed requires zoo.animal.feeding;
// no longer needed requires zoo.animal.care;
requires zoo.animal.talks;
}

Question 17

Duplicate requires Statements

module bad.module {
requires zoo.animal.talks;
requires transitive zoo.animal.talks;
}

Answer 17

Java doesn’t allow you to repeat the same module in a requires clause. It is redundant and most like an error in coding. Keep in mind that requires transitive is like requires plus some extra behavior.

Question 18

PROVIDES, USES, AND OPENS

Answer 18

The provides keyword specifies that a class provides an implementation of a service.
To use it, you supply the API and class name that implements the API:

provides zoo.staff.ZooApi with zoo.staff.ZooImpl

The uses keyword specifies that a module is relying on a service. To code it, you supply the API you want to call:

uses zoo.staff.ZooApi

Question 19

Discovering Modules

Answer 19

• “unjar” module JAR file and open the module-info file.
• java command, -d or –describe-module

java -p mods -d zoo.animal.feeding

java -p mods --describe-module zoo.animal.feeding

• jar command, -d or –describe-module

jar -f mods/zoo.animal.feeding.jar -d

jar --file mods/zoo.animal.feeding.jar --describe-module

Question 20

Discovering Modules via java command
module-info in zoo.animal.feeding:

module zoo.animal.feeding {
exports zoo.animal.feeding;
}

Answer 20

The java command now has an option to describe a module. The following two commands are equivalent:

java -p mods -d zoo.animal.feeding

java -p mods --describe-module zoo.animal.feeding

Each prints information about the module. For example, it might print this:

zoo.animal.feeding file:///absolutePath/mods/zoo.animal.feeding.jar
exports zoo.animal.feeding
requires java.base mandated

• The first line is the module we asked about:** zoo.animal.feeding**.
• The second line starts information about the module. In our case, it is the same package exports statement we had in the module-info file.
• On the third line, we see requires java.base mandated. Now wait a minute. The module-info file very clearly does not specify any modules that zoo.animal.feeding has as dependencies.
• The java.base module is special. It is automatically added as a dependency to all modules. This module has frequently used packages like java.util. That’s what the mandated is about. You get java.base whether you asked for it or not.
• In classes, the java.lang package is automatically imported whether you type it or not. The java.base module works the same way. It is automatically available to all other modules.

Question 21

MORE ABOUT DESCRIBING MODULES

module-info in zoo.animal.care:

module zoo.animal.care {
exports zoo.animal.care.medical to zoo.staff;
requires transitive zoo.animal.feeding;
}

Answer 21

Now we have the command to describe the module and the output.

java -p mods -d zoo.animal.care

zoo.animal.care file:///absolutePath/mods/zoo.animal.care.jar
requires zoo.animal.feeding transitive
requires java.base mandated
qualified exports zoo.animal.care.medical to zoo.staff
contains zoo.animal.care.details

• The first line of the output is the absolute path of the module file.
• The two requires lines should look familiar as well.
• The first is in the module-info, and the other is added to all modules.
• Next comes something new. The qualified exports is the full name of exporting to a specific module.
• Finally, the contains means that there is a package in the module that is not exported at all. This is true. Our module has two packages, and one is available only to code inside the module.

Question 22

Listing Available Modules

Answer 22

you can use the java command to list the modules that are available. The simplest form lists the modules that are part of the JDK:

java --list-modules

When we ran it, the output went on for 70 lines and looked like this:

java.base@11.0.2
java.compiler@11.0.2
java.datatransfer@11.0.2

This is a listing of all the modules that come with Java and their version numbers. You can tell that we were using Java 11.0.2 when testing this example.

java -p mods --list-modules

zoo.animal.care file:///absolutePath/mods/zoo.animal.care.jar
zoo.animal.feeding file:///absolutePath/mods/zoo.animal.feeding.jar
zoo.animal.talks file:///absolutePath/mods/zoo.animal.talks.jar
zoo.staff file:///absolutePath/mods/zoo.staff.jar

Since these are custom modules, we get a location on the file system. If the project had a module version number, it would have both the version number and the file system path.

> [!Note]
that –list-modules exits as soon as it prints the observable modules.
It does not run the program.

Question 23

Showing Module Resolution

Answer 23

• In case listing the modules didn’t give you enough output, you can also use the –show-module-resolution option.
• You can think of it as a way of debugging modules.
• It spits out a lot of output when the program starts up.
• Then it runs the program.

java --show-module-resolution -p feeding -m zoo.animal.feeding/zoo.animal.feeding.Task

Luckily you don’t need to understand this output. That said, having seen it will make it easier to remember. Here’s a snippet of the output:

root zoo.animal.feeding file:///absolutePath/feeding/
java.base binds java.desktop jrt:/java.desktop
java.base binds jdk.jartool jrt:/jdk.jartool
...
jdk.security.auth requires java.naming jrt:/java.naming
jdk.security.auth requires java.security.jgss jrt:/java.security.jgss
...

All fed!

• It starts out by listing the root module. That’s the one we are running:
  zoo.animal.feeding.
• Then it lists many lines of packages included by the mandatory java.base module.
• After a while, it lists modules that have dependencies.
• Finally, it outputs the result of the program All fed!.
• The total output of this command is 66 lines.

Question 24

THE JAR COMMAND

Answer 24

the jar command can describe a module. Both of these commands are equivalent:

jar -f mods/zoo.animal.feeding.jar -d

jar --file mods/zoo.animal.feeding.jar --describe-module

The output is slightly different from when we used the java command to describe the module. With jar, it outputs the following:

zoo.animal.feeding jar:file:///absolutePath/mods/zoo.animal.feeding.jar
/!module-info.class
exports zoo.animal.feeding
requires java.base mandated

The JAR version includes the module-info in the filename, which is not a particularly significant difference in the scheme of things. You don’t need to know this difference. You do need to know that both commands can describe a module.

Question 25

THE JDEPS COMMAND

Answer 25

The jdeps command gives you information about dependencies within a module.

it looks at the code in addition to the module-info file.

This tells you what dependencies are actually used rather than simply declared.

Let’s start with a simple example and ask for a summary of the dependencies in zoo.animal.feeding. Both of these commands give the same output:

jdeps -s mods/zoo.animal.feeding.jar
jdeps -summary mods/zoo.animal.feeding.jar

Notice that there is one dash (-) before -summary rather than two.

Regardless, the output tells you that there is only one package and it depends on the built-in java.base module.

zoo.animal.feeding -> java.base

Alternatively, you can call jdeps without the summary option and get the long form:

jdeps mods/zoo.animal.feeding.jar
[file:///absolutePath/mods/zoo.animal.feeding.jar]
requires mandated java.base (@11.0.2)
zoo.animal.feeding -> java.base
zoo.animal.feeding -> java.io
java.base
zoo.animal.feeding -> java.lang
java.base

• The first part of the output shows the module filename and path.
• The second part lists the required java.base dependency and version number.
• This has the high-level summary that matches the previous example.
• Finally, the last four lines of the output list the specific packages within the java.base modules that are used by zoo.animal.feeding.

Question 26

There is not a short form of --module-path in the jdeps command. The output is only two lines:

jdeps -s --module-path mods mods/zoo.animal.care.jar

jdeps -summary --module-path mods mods/zoo.animal.care.jar

Answer 26

There is not a short form of –module-path in the jdeps command. The output is only two lines:

zoo.animal.care -> java.base
zoo.animal.care -> zoo.animal.feeding

We can see that the zoo.animal.care module depends on our custom zoo.animal.feeding module along with the built-in java.base.

In case you were worried the output was too short, we can run it in full mode:

jdeps --module-path mods mods/zoo.animal.care.jar

This time we get lots of output:

zoo.animal.care
[file:///absolutePath/mods/zoo.animal.care.jar]
requires mandated java.base (@11.0.2)
requires transitive zoo.animal.feeding
zoo.animal.care -> java.base
zoo.animal.care -> zoo.animal.feeding
zoo.animal.care.details -> java.lang
java.base
zoo.animal.care.details -> zoo.animal.feeding
zoo.animal.feeding
zoo.animal.care.medical -> java.lang
java.base

As before, there are three sections.
* The first section is the filename and required dependencies.
* The second section is the summary showing the two module dependencies with an arrow.
* The last six lines show the package-level dependencies.

Question 27

THE JMOD COMMAND

Answer 27

jmod command

JMOD files are recommended only when you have native libraries or something that can’t go inside a JAR file. This is unlikely to affect you in the real world.

The most important thing to remember is that jmod is only for working with the JMOD files.