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what is 'enable=true' attribute of @Component in OSGi Component?

I had came through an attribute of @Component in OSGi which I dont understand through docs and didn't get any detailed information i.e enabled.

What I got from Felix documentation about scr annotation is:

enabled

enabled 
Default: true 
SCR Descriptor: component.enabled 
Metatype Descriptor: --

Whether the component is enabled when the bundle starts

immediate

immediate 
Default: -- 
SCR Descriptor: component.immediate 
Metatype Descriptor: --

Whether the component is immediately activated

Though I know "Activating a component", but I don't understand what the term "Enabling a component" means?

Any answers with the help of use-cases or example, when to use what, are more appreciated.

Thank you in Advance.

Solution

  • Components are enabled=true by default, which means they will be available for activation as soon as their dependencies -- e.g. required configuration and/or mandatory service references -- have been satisfied.

    A component that is enabled=false will NOT be available for activation even if all its dependencies are satisfied. It is completely disabled and will not start.

    So... what use is this?? Well, a disabled component can be programmatically enabled by another component in the same bundle.

    The primary use-case for this is shared initialization. Suppose you have a bundle that contains several components that all need to wait for some initialization steps to occur, like setting up a bunch of files. You can make all of the components except one enabled=false. The single enabled component does the initialization in its activation method and then calls ComponentContext.enableComponent(null) to enable all of the other components in the bundle.

    immediate is a completely separate and orthogonal lifecycle concept. A component that provides a service is, by default, "delayed", meaning the component is only loaded and activated when some other bundle actually uses the service. This is a really useful lazy-loading optimization. However sometimes you want your component to start as soon as possible even if nobody is using its service. In those cases you set immediate=true.