Add Span#unwrap

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

@@ -161,4 +161,21 @@ public interface Span {
      * @see Span#context()
      */
     void finish(long finishMicros);
+
+    /**
+     * Returns an object that is an instance of the given class to allow access to non-standard methods.
+     *
+     * <p> Using this method is highly preferred to type casts, especially because there might be a hierarchy of wrapper
+     * objects. </p>
+     *
+     * <p> If the class implementing this interface is an instance of the provided class, the <code>this</code>
+     * reference is returned. Otherwise, if the receiver is a wrapper, return the the result of calling
+     * <code>unwrap</code> recursively on the wrapped object. If the receiver is not a wrapper and is not an instance of
+     * the provided class, then <code>null</code> is returned.</p>
+     *
+     * @param clazz The Class that the result must be an instance of.
+     * @return an object that is an instance of the provided class, or <code>null</code> if there is no such class in
+     * the wrapper hierarchy.
+     */
+    <T extends Span> T unwrap(java.lang.Class<T> clazz);
 }

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

@@ -115,6 +115,15 @@ public synchronized void finish(long finishMicros) {
         this.finished = true;
     }
 
+    @Override
+    public <T extends Span> T unwrap(Class<T> clazz) {
+        if (clazz.isInstance(this)) {
+            return clazz.cast(this);
+        } else {
+            return null;
+        }
+    }
+
     @Override
     public MockSpan setTag(String key, String value) {
         return setObjectTag(key, value);

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

@@ -17,6 +17,8 @@
 import org.junit.Test;
 
 import io.opentracing.Span;
+import io.opentracing.Tracer;
+import io.opentracing.noop.NoopSpan;
 
 /**
  * @author Pavol Loffay
@@ -78,4 +80,18 @@ public void testAddBaggageAfterFinish() {
         }
         Assert.assertEquals(1, tracer.finishedSpans().get(0).generatedErrors().size());
     }
+
+    @Test
+    public void testUnwrap() throws Exception {
+        Tracer tracer = new MockTracer();
+        Span span = tracer.buildSpan("foo").startManual();
+        Assert.assertSame(span, span.unwrap(MockSpan.class));
+    }
+
+    @Test
+    public void testUnwrapWrongInstance() throws Exception {
+        Tracer tracer = new MockTracer();
+        Span span = tracer.buildSpan("foo").startManual();
+        Assert.assertNull(span.unwrap(NoopSpan.class));
+    }
 }

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

@@ -13,11 +13,11 @@
  */
 package io.opentracing.noop;
 
+import java.util.Map;
+
 import io.opentracing.Span;
 import io.opentracing.SpanContext;
 
-import java.util.Map;
-
 public interface NoopSpan extends Span {
     static final NoopSpan INSTANCE = new NoopSpanImpl();
 }
@@ -33,6 +33,15 @@ public void finish() {}
     @Override
     public void finish(long finishMicros) {}
 
+    @Override
+    public <T extends Span> T unwrap(Class<T> clazz) {
+        if (clazz.isInstance(this)) {
+            return clazz.cast(this);
+        } else {
+            return null;
+        }
+    }
+
     @Override
     public NoopSpan setTag(String key, String value) { return this; }