Let's keep this in mind What's new in Java 9

This article is the first day article of SRA Advent Calendar 2017.

Java 9 release content

Hello! This is Sasaki from the Kansai Division. In September 2017, Java 9 was released. Java 8 got a lot of attention because of the contents that have a big influence on coding, such as Lambda expressions, default methods of interface, method references and addition of Optional. Thanks to that, even if you don't watch the latest Java information, it seems that many people know some new features about Java 8. However, regarding Java 9, people who are not actively collecting information do not seem to be aware of what kind of release was made, and for those who do not have time for the time being, one of the contents of Java 9 release that "Keep this alone" I would like to introduce the department. "This is all" here is my personal judgment, so there may be some disagreements, but in that case I would be grateful if you could supplement it with comments. (Other power application)

jshell You can now use the REPL tool jshell. You can omit the semicolon at the end of the sentence, and you don't have to create a class or method to execute Java code. (Of course, you can create classes and methods) Also, in the above example, after creating a Random object, type " r.` `` "and press the TAB key to complete the method. In addition, there are many commands in jshell that start with "/ (slash)". Below are just a few

• / list Display the entered contents
• Save to / save file
• / open Enter the contents of the file
• /! Re-execute the last instruction

Javadoc search function

You can now use the search function in Javadoc. You can perform an incremental search such as package name and class name by entering a keyword in the search window on the upper right. For package names and class names, I think that the search function of the browser has been able to do it to some extent, but you can also search for method names suddenly.

Modular system

In fact, the main feature of Java 9 is the "module system". The modular system is quite esoteric when trying to understand it in depth, but I'll keep it to a minimum here. First, before the modular system, I will briefly explain the following two problems.

• JAR hell
• The problem of being too visible

JAR hell JAR hell is a problem caused by Java applications relying on many JARs. It depends too many JARs to manage and you get a ClassNotFoundException at runtime, or you get an error because you have the same JAR file in different versions.

The problem of being overlooked

This "problem of having trouble seeing too much" was arbitrarily named by me, but in Java, the classes included in the JAR can be "seen". You can control access to classes such as private and public, but you need to make them public in order to be able to access them from other packages, and as a result, you will be able to access them from other JARs as well. Also, the package did not have class-like access control.

module-info.java The module system solves the above problems. To use the module system, place a file called "module-info.java" in the top directory of src.

src -- kazsharp.lambda_logging.jcl
     |                          |
     |                          -- LambdaLog4jLog.java
     |
     -- module-info.java

And in module-info.java, describe the dependent modules, the packages to be published, etc.

module-info.java

module <Module name> {
  requires <Dependent module name>;
  exports <Package name to publish>;
}

You can still access packages that are not exported within your module, but other modules can only access packages that are exported. Also, if multiple modules have the same package, an error will occur.

Transitional measures

I'm worried about the module system when using Java 9

• Can I use other JARs without writing requires?
• Can I use it from other JARs without writing exports?

That being said, there is a period called "transitional measures". Transitional measures include "anonymous modules" and "automatic modules", even if module-info.java is not placed

• Requires all modules on the module path
• Export all your packages

However, since it is a transitional measure, if you use Java 9, you should use module-info.java to make the appropriate settings.

So far, we have introduced the content that Java 9 should hold down at least this amount. You'll need to know more about module systems and other content to actually develop, but I hope this article will give you a chance to find out more about Java 9.