While working with a spring boot application server running on my local machine I was unable to hit the server instance with an Android emulator. The Android emulator proxy was configured for localhost:8888 (the Charles Proxy default port) using the settings GUI but I was unable to reach any instances running at localhost.

Use 10.0.2.2
Android emulators interpret the localhost address as their own localhost. In order to reach the host machine localhost use 10.0.2.2.