Chapter 17 Modular Applications Flashcards

Question

CREATING A SERVICE LOCATOR

Answer 1

A

Common module directives

exports <package>
Allows all modules to access the package
exports <package> to <module>
Allows a specific module to access the package
requires <module>
Indicates module is dependent on another module
requires transitive <module>
Indicates the module and that all modules that use this module are dependent on another module
uses <interface>
Indicates that a module uses a service
provides <interface> with <class>
Indicates that a module provides an implementation of a service

Answer 2

A

Three types of modules:

Answer 3

A

the Java runtime is capable of using class and interface types from both the classpath and the module path, although the rules for each are a bit different.
An application can access any type in the classpath that is exposed via standard Java access modifiers, such as a public class.
public types in the module path are not automatically available. While Java access modifiers must still be used, the type must also be in a package that is exported by the module in which it is defined. In addition, the module making use of the type must contain a dependency on the module.

Answer 4

A

A named module is one containing a module‐info file.
this file appears in the root of the JAR alongside one or more packages.
Unless otherwise specified, a module is a named module.
Named modules appear on the module path rather than the classpath.
You’ll learn what happens if a JAR containing a module‐info file is on the classpath.
For now, just know it is not considered a named module because it is not on the module path.

Answer 5

A

An automatic module appears on the module path but does not contain a module‐info file.
It is simply a regular JAR file that is placed on the module path and gets treated as a module.
The code referencing an automatic module treats it as if there is a module‐info file present. It automatically exports all packages.
It also determines the module name.
When Java 9 was released, authors of Java libraries were encouraged to declare the name they intended to use for the module in the future. All they had to do was set a property called Automatic‐Module‐Name in the MANIFEST.MF file.

Answer 6

A

A JAR file is a zip file with a special directory named META‐INF.
The MANIFEST.MF file is always present.
The manifest contains extra information about the JAR file.
For example, it often contains the version of Java used to build the JAR file. For command‐line programs, the class with the main() method is commonly specified.
Each line in the manifest is a key/value pair separated by a colon.
The default manifest in Java 11 looks like this:

Manifest-Version: 1.0
Created-By: 11.0.2 (Oracle Corporation)

Answer 7

A

If the MANIFEST.MF specifies an Automatic‐Module‐Name, use that. Otherwise, proceed with the remaining rules.
Remove the file extension from the JAR name.
Remove any version information from the end of the name. A version is digits and dots with possible extra information at the end, for example, ‐1.0.0 or ‐1.0‐RC.
Replace any remaining characters other than letters and numbers with dots.
Replace any sequences of dots with a single dot.
Remove the dot if it is the first or last character of the result.

Answer 8

A

Beginning JAR name
commons2‐x‐1.0.0‐SNAPSHOT.jar
Remove file extenstion
commons2‐x‐1.0.0‐SNAPSHOT
Remove version information
commons2‐x
Replace special characters
commons2.x
Replace sequence of dots
commons2.x
Remove leading/trailing dots (results in the automatic module name)
commons2.x

Answer 9

A

Answer 10

A

An unnamed module appears on the classpath.
Like an automatic module, it is a regular JAR.
Unlike an automatic module, it is on the classpath rather than the module path.
This means an unnamed module is treated like old code and a second‐class citizen to modules.
An unnamed module does not usually contain a module‐info file.
If it happens to contain one, that file will be ignored since it is on the classpath.
Unnamed modules do not export any packages to named or automatic modules.
The unnamed module can read from any JARs on the classpath or module path.
You can think of an unnamed module as code that works the way Java worked before modules. Yes, we know it is confusing to have something that isn’t really a module having the word module in its name.