Add Checker Framework stub file for gRPC Context.Key. (#1357)

The stub file adds Nullable annotations to indicate that Context.Key.get can
return null when the key has no default or the key has a nullable value type.

2 files changed, 8 insertions, 1 deletions

diff --git a/api/src/main/java/io/opencensus/trace/CurrentSpanUtils.java b/api/src/main/java/io/opencensus/trace/CurrentSpanUtils.java
index 21511142..aa2f055a 100644
--- a/api/src/main/java/io/opencensus/trace/CurrentSpanUtils.java
+++ b/api/src/main/java/io/opencensus/trace/CurrentSpanUtils.java
@@ -20,6 +20,7 @@ import io.grpc.Context;
 import io.opencensus.common.Scope;
 import io.opencensus.trace.unsafe.ContextUtils;
 import java.util.concurrent.Callable;
+import javax.annotation.Nullable;
 
 /** Util methods/functionality to interact with the {@link Span} in the {@link io.grpc.Context}. */
 final class CurrentSpanUtils {
@@ -31,6 +32,7 @@ final class CurrentSpanUtils {
    *
    * @return The {@code Span} from the current context.
    */
+  @Nullable
   static Span getCurrentSpan() {
     return ContextUtils.CONTEXT_SPAN_KEY.get();
   }
diff --git a/api/src/main/java/io/opencensus/trace/unsafe/ContextUtils.java b/api/src/main/java/io/opencensus/trace/unsafe/ContextUtils.java
index f7905c25..3f4b9889 100644
--- a/api/src/main/java/io/opencensus/trace/unsafe/ContextUtils.java
+++ b/api/src/main/java/io/opencensus/trace/unsafe/ContextUtils.java
@@ -19,6 +19,10 @@ package io.opencensus.trace.unsafe;
 import io.grpc.Context;
 import io.opencensus.trace.Span;
 
+/*>>>
+import org.checkerframework.checker.nullness.qual.Nullable;
+*/
+
 /**
  * Util methods/functionality to interact with the {@link io.grpc.Context}.
  *
@@ -36,5 +40,6 @@ public final class ContextUtils {
    *
    * @since 0.5
    */
-  public static final Context.Key<Span> CONTEXT_SPAN_KEY = Context.key("opencensus-trace-span-key");
+  public static final Context.Key</*@Nullable*/ Span> CONTEXT_SPAN_KEY =
+      Context.key("opencensus-trace-span-key");
 }