Enabling NOOP classes to work with pre-existing production code

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

@@ -36,7 +36,7 @@ public Scope activate(Span span, boolean finishOnClose) {
 
     @Override
     public Scope active() {
-        return null;
+        return NoopScope.INSTANCE;
     }
 
     static class NoopScopeImpl implements NoopScopeManager.NoopScope {

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

@@ -32,7 +32,7 @@ public ScopeManager scopeManager() {
 
     @Override
     public Span activeSpan() {
-        return null;
+        return NoopSpanImpl.INSTANCE;
     }
 
     @Override

opentracing-noop/src/test/java/io/opentracing/noop/NoopScopeManagerTest.java

@@ -0,0 +1,34 @@
+/*
+ * Copyright 2016-2018 The OpenTracing Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.opentracing.noop;
+
+import static org.junit.Assert.*;
+
+import org.junit.Test;
+
+import io.opentracing.Scope;
+
+public class NoopScopeManagerTest {
+
+    @Test
+    public void activeValueToleratesUseTest() {
+        try{
+            final Scope active = NoopScopeManager.INSTANCE.active();
+            assertNotNull(active);
+            active.close();
+        } catch (final NullPointerException e) {
+            fail("NoopScopeManagerImpl.active() should return a usable scope");
+        }
+    }
+}

opentracing-noop/src/test/java/io/opentracing/noop/NoopTracerTest.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright 2016-2018 The OpenTracing Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
+ * in compliance with the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.opentracing.noop;
+
+import static org.junit.Assert.*;
+
+import org.junit.Test;
+
+import io.opentracing.Span;
+import io.opentracing.tag.Tags;
+
+public class NoopTracerTest {
+
+    @Test
+    public void activeSpanValueToleratesUseTest() {
+        try {
+            final Span activeSpan = NoopTracerImpl.INSTANCE.activeSpan();
+            assertNotNull(activeSpan);
+            Tags.ERROR.set(activeSpan, true);
+        } catch (final NullPointerException e) {
+            fail("NoopTracer.activeSpan() should return a usable span");
+        }
+    }
+}