Resolver API for OpenTracing Tracer implementations.

NOTE: The Tracer Resolver mechanism is only intended to be used at application startup/initialization time. This responsibility should be handled by the application, possibly using some runtime specific support (e.g. providing a Tracer @Bean in Spring Boot, or a CDI producer). Framework integrations used to instrument specific technologies should not use this library, but should allow a Tracer to be injected instead, with fallback to the GlobalTracer.

This is a utility class providing a static resolveTracer() method using the JDK ServiceLoader to find declared TracerFactory implementations providing a Tracer.

A tracer factory implements a getTracer() method and is used by the TracerResolver to look up tracer implementations via a JDK ServiceLoader.

If no TracerFactory is found or no Tracer is returned, a ServiceLoader lookup for a declared concrete TracerResolver class is used. This supports the 'legacy' lookup for TracerResolver subclasses providing a resolve() implementation. Finally, if this also fails, the Tracer class is used as last-resort lookup.

A resolved tracer is passed to all TracerConverter instances that were found.

Tracer converters can be useful for automatically wrapping the resolved Tracer:

public final class FooWrapperConverter implements TracerConverter {
    public Tracer convert(Tracer existingTracer) {
        return new FooTracerWrapper(existingTracer);
    }
}

If multiple TracerResolver, TracerConverter or Tracer implementations are found, they are checked for presence of the @Priority annotation on their class or superclasses. The priority is applied as follows:

1. First, non-negative priority is applied in natural order (e.g. 0, 1, 2, ...).
2. Next, objects without @Priority annotation are applied by assigning a default priority of Integer.MAX_VALUE.
3. Finally, negative priority is applied in reverse-natural order (e.g. -1, -2, -3, ...).

The order of objects with equal (implicit) priority is undefined.

If the opentracing-util library is detected and a GlobalTracer is already-registered, the resolving mechanism will be disabled. In this case the GlobalTracer is always returned as-is, without applying any converters.