Fundamentals

Table of Contents

Application Modules
Customizing the Application Modules Arrangement

Spring Modulith supports developers implementing logical modules in Spring Boot applications. It allows them to apply structural validation, document the module arrangement, run integration tests for individual modules, observe the modules' interaction at runtime, and generally implement module interaction in a loosely coupled way. This section will discuss the fundamental concepts that developers need to understand before diving into the technical support.

Application Modules

In a Spring Boot application, an application module is a unit of functionality that consists of the following parts:

An API exposed to other modules implemented by Spring bean instances and application events published by the module, usually referred to as provided interface.
Internal implementation components that are not supposed to be accessed by other modules.
References to API exposed by other modules in the form of Spring bean dependencies, application events listened to and configuration properties exposed, usually referred to as required interface.

Spring Moduliths provides different ways of expressing modules within Spring Boot applications, primarily differing in the level of complexity involved in the overall arrangement. This allows developers to start simple and naturally move to more sophisticated means as and if needed.

The `ApplicationModules` Type

Spring Moduliths allows to inspect a codebase to derive an application module model based on the given arrangement and optional configuration. The spring-modulith-core artifact contains ApplicationModules that can be pointed to a Spring Boot application class:

Creating an application module model

Java
Kotlin

var modules = ApplicationModules.of(Application.class);

val modules = ApplicationModules.of(Application::class.java)

The modules will contain an in-memory representation of the application module arrangement derived from the codebase. Which parts of that will be detected as a module depends on the Java package structure underneath the package the class pointed to resides in. Find out more about the arrangement expected by default in Simple Application Modules. Advanced arrangements and customization options are described in Advanced Application Modules and

To get an impression of what the analyzed arrangement looks like, we can just write the individual modules contained in the overall model to the console:

Writing the application module arrangement to the console

Java
Kotlin

modules.forEach(System.out::println);

modules.forEach { println(it) }

The console output of our application module arrangement

## example.inventory ##
> Logical name: inventory
> Base package: example.inventory
> Spring beans:
  + ….InventoryManagement
  o ….SomeInternalComponent

## example.order ##
> Logical name: order
> Base package: example.order
> Spring beans:
  + ….OrderManagement
  + ….internal.SomeInternalComponent

Note how each module is listed, the contained Spring components are identified, and the respective visibility is rendered, too.

Excluding Packages

In case you would like to exclude certain Java classes or full packages from the application module inspection, you can do so with:

Java
Kotlin

ApplicationModules.of(Application.class, JavaClass.Predicates.resideInAPackage("com.example.db")).verify();

ApplicationModules.of(Application::class.java, JavaClass.Predicates.resideInAPackage("com.example.db")).verify()

Additional examples of exclusions:

com.example.db — Matches all files in the given package com.example.db.
com.example.db.. — Matches all files in the given package (com.example.db) and all sub-packages (com.example.db.a or com.example.db.b.c).
..example.. — Matches a.example, a.example.b or a.b.example.c.d, but not a.exam.b

Full details about possible matchers can be found in the JavaDoc of ArchUnit PackageMatcher.

Simple Application Modules

The application’s main package is the one that the main application class resides in. That is the class, that is annotated with @SpringBootApplication and usually contains the main(…) method used to run it. By default, each direct sub-package of the main package is considered an application module package.

If this package does not contain any sub-packages, it is considered a simple one. It allows to hide code inside it by using Java’s package scope to hide types from being referred to by code residing in other packages and thus not subject to dependency injection into those. Thus, naturally, the module’s API consists of all public types in the package.

Let us have a look at an example arrangement ( denotes a public type, a package-private one).

A single inventory application module

 Example
└─  src/main/java
   ├─  example                        (1)
   |  └─  Application.java
   └─  example.inventory              (2)
      ├─  InventoryManagement.java
      └─  SomethingInventoryInternal.java

1	The application’s main package `example`.
2	An application module package `inventory`.

Advanced Application Modules

If an application module package contains sub-packages, types in those might need to be made public so that it can be referred to from code of the very same module.

An inventory and order application module

 Example
└─  src/main/java
   ├─  example
   |  └─  Application.java
   ├─  example.inventory
   |  ├─  InventoryManagement.java
   |  └─  SomethingInventoryInternal.java
   ├─  example.order
   |  └─  OrderManagement.java
   └─  example.order.internal
      └─  SomethingOrderInternal.java

In such an arrangement, the order package is considered an API package. Code from other application modules is allowed to refer to types within that. order.internal, just as any other sub-package of the application module base package, is considered an internal one. Code within those must not be referred to from other modules. Note how SomethingOrderInternal is a public type, likely because OrderManagement depends on it. This unfortunately means that it can also be referred to from other packages such as the inventory one. In this case, the Java compiler is not of much use to prevent these illegal references.

Open Application Modules

The arrangement described above are considered closed as they only expose types to other modules that are actively selected for exposure. When applying Spring Modulith to legacy applications, hiding all types located in nested packages from other modules might be inadequate or require marking all those packages for exposure, too.

To turn an application module into an open one, use the @ApplicationModule annotation on the package-info.java type.

Declaring an Application Modules as Open

Java
Kotlin

@org.springframework.modulith.ApplicationModule(
  type = Type.OPEN
)
package example.inventory;

@org.springframework.modulith.ApplicationModule(
  type = Type.OPEN
)
package example.inventory

Declaring an application module as open will cause the following changes to the verification:

Access to application module internal types from other modules is generally allowed.
All types, also ones residing in sub-packages of the application module base package are added to the unnamed named interface, unless explicitly assigned to a named interface.

This feature is intended to be primarily used with code bases of existing projects gradually moving to the Spring Modulith recommended packaging structure. In a fully-modularized application, using open application modules usually hints at sub-optimal modularization and packaging structures.

Explicit Application Module Dependencies

A module can opt into declaring its allowed dependencies by using the @ApplicationModule annotation on the package, represented through the package-info.java file. As, for example, Kotlin lacks support for that file, you can also use the annotation on a single type located in the application module’s root package.

Inventory explicitly configuring module dependencies

Java
Kotlin

@org.springframework.modulith.ApplicationModule(
  allowedDependencies = "order"
)
package example.inventory;

package example.inventory

import org.springframework.modulith.ApplicationModule

@ApplicationModule(allowedDependencies = "order")
class ModuleMetadata {}

In this case code within the inventory module was only allowed to refer to code in the order module (and code not assigned to any module in the first place). Find out about how to monitor that in Verifying Application Module Structure.

Named Interfaces

By default and as described in Advanced Application Modules, an application module’s base package is considered the API package and thus is the only package to allow incoming dependencies from other modules. In case you would like to expose additional packages to other modules, you need to use named interfaces. You achieve that by annotating the package-info.java file of those packages with @NamedInterface or a type explicitly annotated with @org.springframework.modulith.PackageInfo.

A package arrangement to encapsulate an SPI named interface

 Example
└─  src/main/java
   ├─  example
   |  └─  Application.java
   ├─ …
   ├─  example.order
   |  └─  OrderManagement.java
   ├─  example.order.spi
   |  ├—  package-info.java
   |  └─  SomeSpiInterface.java
   └─  example.order.internal
      └─  SomethingOrderInternal.java

package-info.java in example.order.spi

Java
Kotlin

@org.springframework.modulith.NamedInterface("spi")
package example.order.spi;

package example.order.spi

import org.springframework.modulith.PackageInfo
import org.springframework.modulith.NamedInterface

@PackageInfo
@NamedInterface("spi")
class ModuleMetadata {}

The effect of that declaration is twofold: first, code in other application modules is allowed to refer to SomeSpiInterface. Application modules are able to refer to the named interface in explicit dependency declarations. Assume the inventory module was making use of that, it could refer to the above declared named interface like this:

Defining allowed dependencies to dedicated named interfaces

Java
Kotlin

@org.springframework.modulith.ApplicationModule(
  allowedDependencies = "order :: spi"
)
package example.inventory;

@org.springframework.modulith.ApplicationModule(
  allowedDependencies = "order :: spi"
)
package example.inventory

Note how we concatenate the named interface’s name spi via the double colon ::. In this setup, code in inventory would be allowed to depend on SomeSpiInterface and other code residing in the order.spi interface, but not on OrderManagement for example. For modules without explicitly described dependencies, both the application module root package and the SPI one are accessible.

If you wanted to express that an application module is allowed to refer to all explicitly declared named interfaces, you can use the asterisk (*) as follows:

Using the asterisk to declare allowed dependencies to all declared named interfaces

Java
Kotlin

@org.springframework.modulith.ApplicationModule(
  allowedDependencies = "order :: *"
)
package example.inventory;

@org.springframework.modulith.ApplicationModule(
  allowedDependencies = "order :: *"
)
package example.inventory

Customizing Module Detection

By default, application modules will be expected to be located in direct sub-packages of the package the Spring Boot application class resides in. An alternative detection strategy can be activated to only consider package explicitly annotated, either via Spring Modulith’s @ApplicationModule or jMolecules @Module annotation. That strategy can be activated by configuring the spring.modulith.detection-strategy to explicitly-annotated.

Switching the application module detection strategy to only consider annotated packages

spring.modulith.detection-strategy=explicitly-annotated

If the neither default application module detection strategy nor the manually annotated one does not work for your application, the detection of the modules can be customized by providing an implementation of ApplicationModuleDetectionStrategy. That interface exposes a single method Stream<JavaPackage> getModuleBasePackages(JavaPackage) and will be called with the package the Spring Boot application class resides in. You can then inspect the packages residing within that and select the ones to be considered application module base packages based on a naming convention or the like.

Assume you declare a custom ApplicationModuleDetectionStrategy implementation like this:

Implementing a custom ApplicationModuleDetectionStrategy

Java
Kotlin

package example;

class CustomApplicationModuleDetectionStrategy implements ApplicationModuleDetectionStrategy {

  @Override
  public Stream<JavaPackage> getModuleBasePackages(JavaPackage basePackage) {
    // Your module detection goes here
  }
}

package example

class CustomApplicationModuleDetectionStrategy : ApplicationModuleDetectionStrategy {

  override fun getModuleBasePackages(basePackage: JavaPackage): Stream<JavaPackage> {
    // Your module detection goes here
  }
}

This class can now be registered as spring.modulith.detection-strategy as follows:

spring.modulith.detection-strategy=example.CustomApplicationModuleDetectionStrategy

Customizing the Application Modules Arrangement

Spring Moduliths allows to configure some core aspects around the application module arrangement you create via the @Modulithic annotation to be used on the main Spring Boot application class.

Java
Kotlin

package example;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.modulith.Modulithic;

@Modulithic
@SpringBootApplication
class MyApplication {

  public static void main(String... args) {
    SpringApplication.run(DemoApplication.class, args);
  }
}

package example

import org.springframework.boot.autoconfigure.SpringBootApplication
import org.springframework.boot.runApplication
import org.springframework.modulith.Modulithic

@Modulithic
@SpringBootApplication
class DemoApplication

fun main(args: Array<String>) {
  runApplication<DemoApplication>(*args)
}

The annotation exposes the following attributes to customize:

Annotation attribute Description

Annotation attribute	Description
`systemName`	The human readable name of the application to be used in generated documentation.
`sharedModules`	Declares the application modules with the given names as shared modules, which means that they will always be included in application module integration tests.
`additionalPackages`	Instructs Spring Modulith to treat the configured packages as additional root application packages. In other words, application module detection will be triggered for those as well.

systemName

The human readable name of the application to be used in generated documentation.

sharedModules

Declares the application modules with the given names as shared modules, which means that they will always be included in application module integration tests.

additionalPackages

Instructs Spring Modulith to treat the configured packages as additional root application packages. In other words, application module detection will be triggered for those as well.