Add content for Java's Tracers.

content/guides/java/tracers.md

@@ -2,7 +2,67 @@
 title: "Java: Tracers"
 ---
 
-* Setting up your tracer
-* Using the Global Tracer
-* Accessing the Active Span
-* Accessing tracer specific features.
+#### Setting up your Tracer
+
+Different **Tracer** implementations vary in how and what parameters they receive at initialization time, such as:
+
+* Component name for this application's traces.
+* Tracing endpoint.
+* Tracing credentials.
+* Sampling strategy.
+
+
+For example, initializing the **Tracer** implementation of `Jaeger` looks like this:
+
+```java
+  import io.opentracing.Tracer;
+
+  SamplerConfiguration samplerConfig = new SamplerConfiguration("const", 1);
+  ReporterConfiguration reporterConfig = new ReporterConfiguration(true, null, null, null, null);
+  Configuration config = new Configuration(service, samplerConfig, reporterConfig);
+
+  // Get the actual OpenTracing-compatible Tracer.
+  Tracer tracer = config.getTracer();
+```
+
+One a **Tracer** instance is obtained, it can be used to manually create **Span**, or pass it to existing instrumentation for frameworks and libraries:
+
+```java
+  // OpenTracing Redis client. It can be *any* OT-compatible tracer.
+  Tracer tracer = ...;
+  new TracingJedis(tracer);
+```
+
+#### Global Tracer
+
+In order to not force the user to keep around a **Tracer**, the **io.opentracing.util** artifact includes a helper **GlobalTracer** class implementing the **io.opentracing.Tracer**, which, as the name implies, acts as as a global instance that can be used from anywhere. It works by forwarding all operations to another underlying **Tracer**, that will get registered at some future point.
+
+By default, the underlying **Tracer** is a no-nop implementation.
+
+```java
+  import io.opentracing.util.GlobalTracer;
+
+  // As part of initialization, pass it as an actual Tracer
+  // to code that will create Spans in the future.
+  new TracingJedis(GlobalTracer.get());
+
+  // Eventually register it, so all the calls to GlobalTracer.get()
+  // are forwarded to this object. Registration can happen only once.
+  Tracer tracer = new CustomTracer(...);
+  GlobalTracer.register(tracer);
+  ...
+
+  // Create a Span as usually. This Span creation will happen
+  // using your CustomTracer.
+  Span span = GlobalTracer.get().buildSpan("foo").start();
+```
+
+#### Using Tracer specific features
+
+For using **Tracer**-specific features, the instance needs to be casted back to the original type:
+
+```java
+  ((CustomTracer)tracer).customFeature(100);
+```
+
+**GlobalTracer** does not expose the original **Tracer**, and thus is not possible to use **Tracer**-specific features through it.