What I researched about Java 9

Overview

It seems that Java 10 (Is it 18.3?) Will appear soon, so I have described what I investigated and tried with the function added in 9.

What I looked up

JShell JShell was introduced. You can execute the code interactively from the command line. When you want to execute a simple code, you can save the trouble of executing it by using a class file.

List.of、Set.of、Map.of A factory method has been added to the collection class.

List<String> list = List.of("coffee milk", "Strawberry milk");
Set<String> set = Set.of("Lemon milk", "Melon milk");
Map<String, String> map = Map.of("coffee milk", "100 yen", "Strawberry milk", "120 yen");

The collection created by the of method is immutable.

Module function

With the introduction of the module function, it has become possible to clarify the scope of disclosure and dependencies of modules. Public methods such as libraries were referenced in classes that were not supposed to be used ... You can also prevent such things. Reference: Project Jigsaw

When not using the module function

Folder structure of the called side (jar) (1)

gyunyu/
　┗━　src/
　　　　┗━　jp/gr/java_conf/masakado/gyunyu/
　　　　　　　┣━　Coffee.java
　　　　　　　┗━　util/
　　　　　　　　　　┗━　Utility.java

Coffee.java

public class Coffee {
	public void print() {
		Utility.output("coffee");
	}
}

java.Utility.java

public class Utility {
	public static void output(String name) {
		System.out.println("this is," + name + "It's milk.");
	}
}

ʻUtility.output (...)` is supposed to be used only in the gyunyu project, This method is not supposed to be executed from the outside.

Compile the above two files to create gyunyu.jar.

javac -d bin -encoding UTF-8 src\jp\gr\java_conf\masakado\gyunyu\Coffee.java src\jp\gr\java_conf\masakado\gyunyu\util\Utility.java
jar -cvf gyunyu.jar -C bin .

Caller's folder structure (1)

sample/
　┣━　src/
　┃　　┗━　jp/gr/java_conf/masakado/sample/
　┃　　　　　┗━　Main.java
　┗━　lib/
　　　　┗━　gyunyu.jar

Main.java

import jp.gr.java_conf.masakado.gyunyu.Coffee;
import jp.gr.java_conf.masakado.gyunyu.util.Utility;
public class Main {
	public static void main(String[] args) {
		(new Main()).execute();
	}

	public void execute() {
		Coffee coffee = new Coffee();
		coffee.print();
		Utility.output("Unexpected call: Strawberry"); //Refer to a method that is not supposed to be executed from the outside
	}
}

It refers to ʻUtility.output (...)ofgyunyu.jar` which is not supposed to be executed from the outside. If you compile and run this

javac -d bin -cp lib\gyunyu.jar -encoding UTF-8  src\jp\gr\java_conf\masakado\sample\Main.java
java -cp lib\gyunyu.jar;bin jp.gr.java_conf.masakado.sample.Main

It will be displayed as below.

When using the module function

Create module-info.java on the called side (jar).

Folder structure of the called side (jar) (2)

gyunyu/
　┗━　src/
　　　　┣━　jp/gr/java_conf/masakado/gyunyu/
　　　　┃　　┣━　Coffee.java
　　　　┃　　┗━　util/
　　　　┃　　　　　┗━　Utility.java
　　　　┗━　module-info.java

module-info.java

module jp.gr.java_conf.masakado.gyunyu {
	exports jp.gr.java_conf.masakado.gyunyu;
}

ʻExports`: Specify the package to be published to the outside.

The module name is the same as the package name, but you can name it freely without having to match it with the package name. Compile this to create a jar file.

javac -d bin -encoding UTF-8 src\module-info.java src\jp\gr\java_conf\masakado\gyunyu\Coffee.java src\jp\gr\java_conf\masakado\gyunyu\util\Utility.java
jar -cvf gyunyu.jar -C bin .

Place the created gyunyu.jar in sample / lib and create module-info.java.

Caller's folder structure (1)

sample/
　┣━　src/
　┃　　┣━　jp/gr/java_conf/masakado/sample/
　┃　　┃　　┗━　Main.java
　┃　　┗━　module-info.java
　┗━　lib/
　　　　┗━　gyunyu.jar

module-info.java

module jp.gr.java_conf.masakado.sample {
	requires jp.gr.java_conf.masakado.gyunyu;
}

requires: Specify the dependent (used) module.

Compile by specifying gyunyu.jar with the -p option.

javac -d bin -p lib\gyunyu.jar -encoding UTF-8 src\module-info.java src\jp\gr\java_conf\masakado\sample\Main.java

The following error message is output and compilation fails.

Package jp.gr.java_conf.masakado.gyunyu.util cannot be displayed
(Package jp.gr.java_conf.masakado.gyunyu.util is a module jp.gr.java_conf.masakado.Declared by gyunyu but not exported)

If you delete ʻUtility.output (...)described inMain.java`, the compilation will pass.

Display module information

You can refer to the module information by executing the jar command with the -d option specified as shown below.

jar -d -f lib\gyunyu.jar

exports jp.gr.java_conf.masakado.gyunyu
requires java.base mandated
contains jp.gr.java_conf.masakado.gyunyu.util

ʻExports: Published package requires: Dependent modules contains`: Packages that are not listed in exports but are included in the jar

Finally

The introduction of the module function made it easier to understand the dependencies, so when I tried to execute it because the compilation went well, the JAR was not enough and an error occurred ... I felt that the possibility of something like that happening would decrease. I haven't dealt with module-enabled JARs yet, so it may be a long time ago that I can benefit from module functionality.