Let Tracer.activeSpan() be a shorthand for ScopeManager's active Span. (

#228)

* Let Tracer.activeSpan() be a shorthand for ScopeManager's active Span.

* Document the null case for Tracer.activeSpan().

* Have Tracer.activeSpan() return null.

* Remove @link for Tracer.activeSpan().

'Nested' links do not happen to exist under javadoc,
so Tracer.scopeManager().active().span() is now
not a link.

opentracing-api/src/main/java/io/opentracing/Tracer.java

@@ -25,6 +25,12 @@ public interface Tracer {
      */
     ScopeManager scopeManager();
 
+    /**
+     * @return the active {@link Span}. This is a shorthand for Tracer.scopeManager().active().span(),
+     * and null will be returned if {@link Scope#active()} is null.
+     */
+    Span activeSpan();
+
     /**
      * Return a new SpanBuilder for a Span with the given `operationName`.
      *

opentracing-mock/src/main/java/io/opentracing/mock/MockTracer.java

@@ -182,6 +182,12 @@ public <C> SpanContext extract(Format<C> format, C carrier) {
         return this.propagator.extract(format, carrier);
     }
 
+    @Override
+    public Span activeSpan() {
+        Scope scope = this.scopeManager.active();
+        return scope == null ? null : scope.span();
+    }
+
     synchronized void appendFinishedSpan(MockSpan mockSpan) {
         this.finishedSpans.add(mockSpan);
         this.onSpanFinished(mockSpan);

opentracing-mock/src/test/java/io/opentracing/mock/MockTracerTest.java

@@ -16,6 +16,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNotEquals;
 
+import io.opentracing.Scope;
 import io.opentracing.Span;
 import io.opentracing.SpanContext;
 import io.opentracing.Tracer;
@@ -197,6 +198,22 @@ public void testTextMapPropagatorHttpHeaders() {
         Assert.assertEquals(finishedSpans.get(0).context().spanId(), finishedSpans.get(1).parentId());
     }
 
+    @Test
+    public void testActiveSpan() {
+        MockTracer mockTracer = new MockTracer();
+        Assert.assertNull(mockTracer.activeSpan());
+
+        Scope scope = null;
+        try {
+            scope = mockTracer.buildSpan("foo").startActive(true);
+            Assert.assertEquals(mockTracer.scopeManager().active().span(), mockTracer.activeSpan());
+        } finally {
+            scope.close();
+        }
+
+        Assert.assertNull(mockTracer.activeSpan());
+    }
+
     @Test
     public void testReset() {
         MockTracer mockTracer = new MockTracer();

opentracing-noop/src/main/java/io/opentracing/noop/NoopTracer.java

@@ -14,6 +14,7 @@
 package io.opentracing.noop;
 
 import io.opentracing.ScopeManager;
+import io.opentracing.Span;
 import io.opentracing.SpanContext;
 import io.opentracing.Tracer;
 import io.opentracing.propagation.Format;
@@ -29,6 +30,11 @@ public ScopeManager scopeManager() {
         return NoopScopeManager.INSTANCE;
     }
 
+    @Override
+    public Span activeSpan() {
+        return null;
+    }
+
     @Override
     public SpanBuilder buildSpan(String operationName) { return NoopSpanBuilderImpl.INSTANCE; }
 

opentracing-util/src/main/java/io/opentracing/util/GlobalTracer.java

@@ -14,6 +14,7 @@
 package io.opentracing.util;
 
 import io.opentracing.ScopeManager;
+import io.opentracing.Span;
 import io.opentracing.noop.NoopTracer;
 import io.opentracing.noop.NoopTracerFactory;
 import io.opentracing.SpanContext;
@@ -142,6 +143,11 @@ public <C> SpanContext extract(Format<C> format, C carrier) {
         return tracer.extract(format, carrier);
     }
 
+    @Override
+    public Span activeSpan() {
+        return tracer.activeSpan();
+    }
+
     @Override
     public String toString() {
         return GlobalTracer.class.getSimpleName() + '{' + tracer + '}';