java module-info example

Cherry 146 Published: 12/12/2024

java module-info example

Here's a Java example for creating a simple module-info file:

In Java 9 and later versions, modules are the new way to structure your Java programs. A module is like a JAR file that contains all your classes and other resources that belong together.

The main difference between a JAR file and a module is that a module can be used by other modules, and it has its own module-info.class file that describes its contents.

Here's an example of what the module-info.class file might look like:

// In the following line "org.example.mymodule" is the name of your module. module org.example.mymodule { // This is a list of all the packages that are part of this module. requires java; requires org.example.library; }

In this example, org.example.mymodule is the name of the module. The requires keyword means that the classes in this module depend on (i.e., use) some other modules.

Here's how you can create a module-info.class file:

Start by creating a new directory for your module and add to it a file called module-info.java. In this file, write the following code:

// The name of our module.
public class module_info {
// This is where you define what packages are part of this module.
}

Then you can require some other modules with the requires keyword.

Here's how you can do that:

public class module_info {
// This is a list of all the packages that are part of this module.
requires java;
// You can also specify specific classes or resources that your module uses.
requires org.example.library;
}

Next, you'll need to create an org/example/library/module-info.class file for any other modules that you're using. This is because when you require a module in your own module-info.class, Java will look at the required module's module-info.class file to make sure everything is correct.

Here's what that would look like:

// org.example.library is our module. module org.example.library { // We're using some other libraries here, so we need them. requires java; requires another.library; }

Once you have all your module-info.class files created, then you can compile and run your Java program.

Remember that when you require a module in your own module-info.class, Java will look at the required module's module-info.class file to make sure everything is correct. This is because modules are not just packages with some metadata attached; they're actually a new kind of thing called "modules".

What is the difference between Java 8 and 9 modules?

I'll respond in English as per your request!

Java 8 introduced a new module system, which allows developers to create modularized applications that are more maintainable, reusable, and efficient. This change was brought about by the introduction of Java Platform Modules (JPMS) and Project Jigsaw.

The primary goal of JPMS is to enhance the modularity and scalability of the Java platform. To achieve this, modules were introduced as a way to group related classes, interfaces, and other resources together. Each module has its own manifest file that specifies the module's name, dependencies on other modules, and which packages it exports or opens.

In Java 9, several significant changes were made to the module system:

Module declarations: In Java 8, modules were declared using the module-info.class file. This was a manual process that required developers to create this file themselves. In Java 9, module declarations are generated automatically when you compile a source directory or use the -m option with the javac compiler. Automatic modules: In Java 8, any Java package could be used as a module by creating a module-info.class file. In Java 9, this is no longer necessary, and you can create modules without explicitly declaring them in a manifest file. This change simplifies the process of creating new modules. Read-only modules: Java 8 allowed modules to modify their own content at runtime. Java 9 introduces read-only modules (RPMs), which cannot be modified or deleted once they're created. RPMs ensure that modules can't alter their own behavior, preventing unintended changes that could affect the stability of your application. Module system APIs: The java.lang.Module, java.lang.ModuleProvides, and java.lang.ModuleRequires classes were introduced in Java 9 to provide direct access to module information. This allows developers to programmatically query module details, such as dependencies or exported packages, from within their Java code. JAR files no longer imply a module: In Java 8, JAR files automatically became modules if they contained a module-info.class file. Starting in Java 9, this is no longer the case. A JAR file will not be treated as a module unless it contains an explicit module-info.class file or is created using the -m option with the jar tool. Enhanced debugging tools: Java 9's new modules system also includes improved debugging features, such as better error reporting and enhanced inspection capabilities within the Eclipse Java Debugger (JDB).

In summary, Java 9 builds upon the foundational work of Java 8 by simplifying module declarations, introducing read-only modules for increased stability, providing APIs for programmatically accessing module information, relaxing JAR file dependencies on modules, and enhancing debugging tools. These changes improve the overall maintainability, scalability, and usability of the Java platform, making it an attractive choice for modern software development projects.

I hope this helps!