Services

A Java implementation/copy/derivative of .NET Core's dependency and service description framework. Probably terrible. Coded late in the night.

Key Terms

Thing	Purpose
IServiceDescriptor<T>	Basic interface for all service resolution.
IServiceScope	A service scope. Holds a bunch of service instances and can create more.

Reference Example: C# Singleton

IServiceCollection services;
IServiceCollection services = services.AddSingleton<MyService>( /* OPTIONS */ );

IServiceProvider resolvableServices = services.BuildServiceProvider(); // construct once, typically

// resolve
MyService hooray = resolvableServices.GetService<MyService>(); // get the singleton back

Singletons in particular can be registered multiple ways, like so:

Type	Description
INSTANCE (MyService)	pass instance directly, resolve directly
Func<MyService>	a function that returns MyService
Func<in IServiceProvider, out MyService>	resolver function

Basically all of these in .NET turn into a ServiceDescriptor which has the following STUFF in it:

• Service Type (Actual type info)
• Implementation Factory (How to construct given a ServiceProvider and known details)
• Service Lifetime (Scoped, Transient, Singleton)

Note: .NET and C# get to "cheat" here

C# has full type information, Java has type erasure. So, we need to get creative when storing our resolver info with Java since we need that type information during runtime to make sure we construct everything correctly.

Difference: Service Scopes

Difference: C# IDisposable vs Java AutoClosable

Due to these two having a pile of subtle but nonetheless implementation differences, this implementation opts to ignore .NET's Transient lifetime in favor of mapping Scoped as close to 1:1 as possible.

This ends up looking like this in practice:

void CSharpDisposable() {
    IServiceProvider services; // given

    using var myDisposable = services.GetRequiredService<MyDisposableService>();
    // do stuff w/ myDisposable
    myDisposable.DoThings();
    
    // closed @ logical scope release
}

void JavaClosable() {
    IServiceScope scope; // given
    
    try(var myCloseable = scope.service(DEFINITION)) {
        // use myCloseable, Java cleans up after itself
        myCloseable.doThings();
    }
}