What are the different compilation modes for modules in Java 9?

A module is a container for packages. Each module contains a module containing modulename, ModuleDependencies is a module descriptor, which means that the names of other modules depend on it, and the names of packages it exports can only be used by modules that depend on it.

<strong>module com.tutorialspoint.app {
   /** Modules upon which the module com.tutorialspoint.app depends on */
   requires com.tutorialspoint.services;
   /** Packages exposed by this module which can be used by other modules */
   exports com.tutorialspoint.app.util;
}</strong>

Java 9 module provides three different compilation modes: traditional mode, single module mode and multi-module mode.

Compilation mode of module:

• ##Traditional mode: When the compilation environment is changed from -- This mode can be enabled when the version defined by the source, --target, and --release options is less than or equal to 8. The compiler behaves the same as Java 8 (or before) and we can use traditional options like classpath etc. instead of any module related options like --module -path). In this mode, our code runs as an unnamed module at runtime.
• Single module mode: When the compilation environment is 9 or higher and the --module-source-path option is not used, This mode can be enabled. In this mode, code is organized in a traditional package-level directory tree structure. The code has a module-info.java file and runs on the module path, not the classpath. In this structure, we can directly place the module-info.java file in the src directory. There cannot be multiple module-info.java files in the same directory tree, so it is called Single module mode.
• Multi-module mode: When the compilation environment is 9 or higher and the --module-source-path option is used, This mode can be enabled. In this mode, we place multiple modules in the same source directory. When compiling, the main source directory can be specified using the --module-source-path option. The source tree for each individual module can be placed in a subdirectory of the main source directory.

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