From the course: Advanced Java Programming

Understanding modules in Java - Java Tutorial

From the course: Advanced Java Programming

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Understanding modules in Java

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- [Instructor] In Java 9, a new module system was introduced. Modularity makes it easier to write well encapsulated code by breaking up large code bases into small sections. So why was a module system introduced? This can be answered by looking at the JDK itself. Before Java 9, the JDK was one huge monolithic structure. Even if you only wanted to write a small application, you still had to have the whole huge JDK on your machine. That was even if you weren't using a lot of it. Another problem was that old unused code or old code with a security vulnerabilities was very difficult to remove. Removing it would lead to backwards compatibility problems. The solution to this problem was to introduce modules. This meant that the JDK could be broken up into small sections. Old legacy code could be removed and so could codes that isn't used often. Anything extra can be added in if needed. If there is a need to make use of lots of the available modules, these can easily be imported into the application. So the module system has been used to break up the JDK itself. But it can also be used for creating Java applications. Existing Java applications can be made modular too. Java is often used for building large applications. And huge code bases are not unusual. Structuring large applications can be very difficult. But modularity is a useful tool for reducing their complexity. So what is a module? A module is a group of related code. So it contains some code and it might contain some other resources too. It also has to have a name. Like packages in Java, module names live in the global namespace and they need to be unique. That means that every single Java module in existence should have a different name. With packages, this is often achieved by using a company domain name, for example, com.mycompany.mypackage. A similar thing can be done with modules. For example, you can call it, com.mycompany.mymodule. A module also contains some information about the module itself. By default, everything in a module is hidden to the outside world. Even if a class is public, it's actually not accessible outside of the module. But each module has a module descriptor. This can be used to specify if code in the module should be accessible to other modules. The descriptor can also be used to state if code from other modules is going to be used. This is a useful way to make sure code is encapsulated. Only the code that is needed can be made visible to the outside world. And it makes structuring large code bases much more manageable.

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