Disallowing null values in ContextSnapshot

context-propagation/src/main/java/io/micrometer/context/DefaultContextSnapshot.java

@@ -17,6 +17,7 @@
 
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Objects;
 import java.util.function.Predicate;
 
 /**
@@ -162,6 +163,9 @@ static DefaultContextSnapshot captureFromContext(Predicate<Object> keyPredicate,
             snapshot = (snapshot != null ? snapshot : new DefaultContextSnapshot(contextRegistry));
             ((ContextAccessor<Object, ?>) accessor).readValues(context, keyPredicate, snapshot);
         }
+        if (snapshot != null) {
+            snapshot.values().removeIf(Objects::isNull);
+        }
         return (snapshot != null ? snapshot : emptyContextSnapshot);
     }
 

context-propagation/src/test/java/io/micrometer/context/DefaultContextSnapshotTests.java

@@ -53,6 +53,24 @@ void should_propagate_thread_local() {
         }
     }
 
+    @Test
+    void test_null_value_in_source_context() {
+        this.registry.registerContextAccessor(new TestContextAccessor());
+        this.registry.registerThreadLocalAccessor(new ObservationThreadLocalAccessor());
+
+        String key = ObservationThreadLocalAccessor.KEY;
+
+        String emptyValue = ObservationThreadLocalHolder.getValue();
+        Map<String, String> sourceContext = Collections.singletonMap(key, emptyValue);
+
+        ContextSnapshot snapshot = ContextSnapshot.captureAll(this.registry, sourceContext);
+
+        try (Scope scope = snapshot.setThreadLocals()) {
+            assertThat(ObservationThreadLocalHolder.getValue()).isEqualTo(emptyValue);
+        }
+        assertThat(ObservationThreadLocalHolder.getValue()).isEqualTo(emptyValue);
+    }
+
     @Test
     void should_propagate_single_thread_local_value() {
         this.registry.registerContextAccessor(new TestContextAccessor());

context-propagation/src/test/java/io/micrometer/context/ObservationThreadLocalAccessor.java

@@ -15,6 +15,8 @@
  */
 package io.micrometer.context;
 
+import java.util.Objects;
+
 /**
  * Example {@link ThreadLocalAccessor} implementation.
  */
@@ -34,6 +36,9 @@ public String getValue() {
 
     @Override
     public void setValue(String value) {
+        // ThreadLocalAccessor API is @NonNullApi by default
+        // so we don't expect null here
+        Objects.requireNonNull(value);
         ObservationThreadLocalHolder.setValue(value);
     }
 
@@ -42,4 +47,12 @@ public void reset() {
         ObservationThreadLocalHolder.reset();
     }
 
+    @Override
+    public void restore(String previousValue) {
+        // ThreadLocalAccessor API is @NonNullApi by default
+        // so we don't expect null here
+        Objects.requireNonNull(previousValue);
+        setValue(previousValue);
+    }
+
 }

context-propagation/src/test/java/io/micrometer/context/ObservationThreadLocalHolder.java

@@ -19,14 +19,11 @@ public class ObservationThreadLocalHolder {
 
     private static final ThreadLocal<String> holder = new ThreadLocal<>();
 
-    public static void resetValue() {
-        holder.remove();
-    }
-
     public static void setValue(String value) {
         holder.set(value);
     }
 
+    @Nullable
     public static String getValue() {
         return holder.get();
     }

context-propagation/src/test/java/io/micrometer/context/TestThreadLocalAccessor.java

@@ -15,6 +15,8 @@
  */
 package io.micrometer.context;
 
+import java.util.Objects;
+
 /**
  * ThreadLocalAccessor for testing purposes with a given key and {@link ThreadLocal}
  * instance.
@@ -46,6 +48,9 @@ public String getValue() {
 
     @Override
     public void setValue(String value) {
+        // ThreadLocalAccessor API is @NonNullApi by default
+        // so we don't expect null here
+        Objects.requireNonNull(value);
         this.threadLocal.set(value);
     }