On Substrate VM URL protocols can be divided in three classes:
These are protocols that are enabled by default and added to every built image.
Currently file and resource are the only supported URL protocols enabled by default.
These are protocols that are supported but are not enabled by default when building an image.
They must be enabled during image building by adding the --enable-url-protocols=<protocols> option to the native-image command.
The option accepts a list of comma separated protocols.
The rationale behind enabling protocols on demand is that you can start with a minimal image and add features as you need them.
This way your image will only include the features that you use which helps keeping the overall size small.
Currently http and https are the only URL protocols that are supported and can be enabled on demand.
Alternatively http and https can be enabled using the --enable-http and --enable-https options.
Support for the https URL protocol relies on the Java Cryptography Architecture (JCA) framework.
Thus when https is enabled --enable-all-security-services is set by default.
This adds to the generated image the code required by the JCA, including statically linking native libraries that the JCA may depend on.
See the documentation on security services for more details.
No other URL protocols are currently tested.
They can still be enabled using --enable-url-protocols=<protocols>, however they might not work as expected.