AEM – OSGi runtime and classloading

OSGi Runtime

In AEM, OSGi runtime refers to the OSGi framework that is used to manage the lifecycle of OSGi bundles and services. AEM uses Apache Felix as the OSGi framework.

The OSGi runtime provides a modular and dynamic environment for AEM applications to run in. It allows bundles to be added, removed, or updated at runtime without affecting the stability of the entire system. This is achieved through the use of OSGi services, which are dynamic, reusable, and composable.

OSGi runtime in AEM also provides a number of services and APIs that can be used by AEM applications. These include:

  1. Configuration Admin Service: Used to manage configuration settings for OSGi bundles.
  2. Event Admin Service: Used to publish and subscribe to events between OSGi bundles.
  3. Log Service: Used to manage logging information for OSGi bundles.
  4. Service Registry: Used to register, lookup, and manage OSGi services.
  5. Bundle Context: Used to access information about the OSGi bundle, such as its state and dependencies.

In AEM, the OSGi runtime is managed by the Sling Framework, which provides a set of abstractions and APIs for developing AEM applications using the OSGi framework.

OSGi classloading

OSGi classloading is an important concept in AEM, as it is used to manage and load the classes of OSGi bundles. Here’s a detailed explanation of OSGi classloading in AEM:

  1. OSGi Bundle Classloading: Each OSGi bundle in AEM has its own classloader. This classloader is responsible for loading the classes of the bundle and its dependencies. When a class is requested, the bundle’s classloader first checks if the class is available in the bundle’s own classpath. If the class is not found, the classloader then looks for the class in the classpath of the bundle’s dependencies.
  2. OSGi Service Classloading: OSGi services are defined by Java interfaces and implemented by OSGi bundles. When a bundle registers a service, it provides an implementation of the service interface. When another bundle wants to use the service, it uses the service interface to communicate with the service.

In AEM, the OSGi service interface and implementation classes can be loaded by different classloaders. This can lead to classloading issues when the service interface and implementation classes are not loaded by the same classloader. To avoid these issues, AEM provides a mechanism called ServiceLoader, which is used to load the service interface and implementation classes using the same classloader.

  1. OSGi Dynamic Classloading: In AEM, OSGi bundles can be dynamically installed and uninstalled at runtime. This means that the classloading of OSGi bundles can also be dynamic. When a bundle is installed or uninstalled, its classes are added or removed from the OSGi framework’s classpath. This can cause classloading issues if the classes are not properly managed.

To avoid these issues, AEM provides a mechanism called DynamicImport-Package, which is used to dynamically import classes from other bundles. When a bundle dynamically imports a package, it allows the OSGi framework to load the classes of the package dynamically when needed.

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