diff --git a/extras/src/main/java/com/google/gson/graph/GraphAdapterBuilder.java b/extras/src/main/java/com/google/gson/graph/GraphAdapterBuilder.java
index cd360a4c4e..80963a68e2 100644
--- a/extras/src/main/java/com/google/gson/graph/GraphAdapterBuilder.java
+++ b/extras/src/main/java/com/google/gson/graph/GraphAdapterBuilder.java
@@ -29,6 +29,8 @@
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonToken;
 import com.google.gson.stream.JsonWriter;
+import org.jspecify.annotations.Nullable;
+
 import java.io.IOException;
 import java.lang.reflect.Type;
 import java.util.ArrayDeque;
@@ -151,7 +153,7 @@ static class Factory implements TypeAdapterFactory, InstanceCreator<Object> {
     }
 
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       if (!instanceCreators.containsKey(type.getType())) {
         return null;
       }
diff --git a/extras/src/main/java/com/google/gson/interceptors/InterceptorFactory.java b/extras/src/main/java/com/google/gson/interceptors/InterceptorFactory.java
index 7868a13352..95a60cc2f8 100644
--- a/extras/src/main/java/com/google/gson/interceptors/InterceptorFactory.java
+++ b/extras/src/main/java/com/google/gson/interceptors/InterceptorFactory.java
@@ -22,12 +22,14 @@
 import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
+import org.jspecify.annotations.Nullable;
+
 import java.io.IOException;
 
 /** A type adapter factory that implements {@code @Intercept}. */
 public final class InterceptorFactory implements TypeAdapterFactory {
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
     Intercept intercept = type.getRawType().getAnnotation(Intercept.class);
     if (intercept == null) {
       return null;
diff --git a/extras/src/main/java/com/google/gson/typeadapters/PostConstructAdapterFactory.java b/extras/src/main/java/com/google/gson/typeadapters/PostConstructAdapterFactory.java
index 924a9089ff..f6211b23f4 100644
--- a/extras/src/main/java/com/google/gson/typeadapters/PostConstructAdapterFactory.java
+++ b/extras/src/main/java/com/google/gson/typeadapters/PostConstructAdapterFactory.java
@@ -22,6 +22,8 @@
 import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
+import org.jspecify.annotations.Nullable;
+
 import java.io.IOException;
 import java.lang.reflect.InvocationTargetException;
 import java.lang.reflect.Method;
@@ -30,7 +32,7 @@
 public class PostConstructAdapterFactory implements TypeAdapterFactory {
   // copied from https://gist.github.com/swankjesse/20df26adaf639ed7fd160f145a0b661a
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
     for (Class<?> t = type.getRawType();
         (t != Object.class) && (t.getSuperclass() != null);
         t = t.getSuperclass()) {
diff --git a/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java b/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java
index 01812ad24f..e0122684c6 100644
--- a/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java
+++ b/extras/src/main/java/com/google/gson/typeadapters/RuntimeTypeAdapterFactory.java
@@ -27,6 +27,8 @@
 import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
+import org.jspecify.annotations.Nullable;
+
 import java.io.IOException;
 import java.util.LinkedHashMap;
 import java.util.Map;
@@ -241,7 +243,7 @@ public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
   }
 
   @Override
-  public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
+  public @Nullable <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
     if (type == null) {
       return null;
     }
diff --git a/gson/pom.xml b/gson/pom.xml
index 10969fb8a9..ca760b7ccc 100644
--- a/gson/pom.xml
+++ b/gson/pom.xml
@@ -52,6 +52,11 @@
       <artifactId>error_prone_annotations</artifactId>
       <version>2.43.0</version>
     </dependency>
+    <dependency>
+      <groupId>org.jspecify</groupId>
+      <artifactId>jspecify</artifactId>
+      <scope>compile</scope>
+    </dependency>
 
     <dependency>
       <groupId>junit</groupId>
diff --git a/gson/src/main/java/com/google/gson/TypeAdapter.java b/gson/src/main/java/com/google/gson/TypeAdapter.java
index e33a839dbd..c38a254cb6 100644
--- a/gson/src/main/java/com/google/gson/TypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/TypeAdapter.java
@@ -26,6 +26,7 @@
 import java.io.Reader;
 import java.io.StringReader;
 import java.io.Writer;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Converts Java objects to and from JSON.
@@ -127,7 +128,7 @@ public TypeAdapter() {}
    *
    * @param value the Java object to write. May be null.
    */
-  public abstract void write(JsonWriter out, T value) throws IOException;
+  public abstract void write(JsonWriter out, @Nullable T value) throws IOException;
 
   /**
    * Converts {@code value} to a JSON document and writes it to {@code out}.
@@ -191,7 +192,7 @@ public final JsonElement toJsonTree(T value) {
    *
    * @return the converted Java object. May be {@code null}.
    */
-  public abstract T read(JsonReader in) throws IOException;
+  public abstract @Nullable T read(JsonReader in) throws IOException;
 
   /**
    * Converts the JSON document in {@code in} to a Java object.
@@ -207,7 +208,7 @@ public final JsonElement toJsonTree(T value) {
    * @return the converted Java object. May be {@code null}.
    * @since 2.2
    */
-  public final T fromJson(Reader in) throws IOException {
+  public final @Nullable T fromJson(Reader in) throws IOException {
     JsonReader reader = new JsonReader(in);
     return read(reader);
   }
@@ -226,7 +227,7 @@ public final T fromJson(Reader in) throws IOException {
    * @return the converted Java object. May be {@code null}.
    * @since 2.2
    */
-  public final T fromJson(String json) throws IOException {
+  public final @Nullable T fromJson(String json) throws IOException {
     return fromJson(new StringReader(json));
   }
 
@@ -238,7 +239,7 @@ public final T fromJson(String json) throws IOException {
    * @throws JsonIOException wrapping {@code IOException}s thrown by {@link #read(JsonReader)}
    * @since 2.2
    */
-  public final T fromJsonTree(JsonElement jsonTree) {
+  public final @Nullable T fromJsonTree(JsonElement jsonTree) {
     try {
       JsonReader jsonReader = new JsonTreeReader(jsonTree);
       return read(jsonReader);
@@ -297,7 +298,7 @@ public final TypeAdapter<T> nullSafe() {
 
   private final class NullSafeTypeAdapter extends TypeAdapter<T> {
     @Override
-    public void write(JsonWriter out, T value) throws IOException {
+    public void write(JsonWriter out, @Nullable T value) throws IOException {
       if (value == null) {
         out.nullValue();
       } else {
@@ -306,7 +307,7 @@ public void write(JsonWriter out, T value) throws IOException {
     }
 
     @Override
-    public T read(JsonReader reader) throws IOException {
+    public @Nullable T read(JsonReader reader) throws IOException {
       if (reader.peek() == JsonToken.NULL) {
         reader.nextNull();
         return null;
diff --git a/gson/src/main/java/com/google/gson/TypeAdapterFactory.java b/gson/src/main/java/com/google/gson/TypeAdapterFactory.java
index 4473e59b7a..08141f8cb5 100644
--- a/gson/src/main/java/com/google/gson/TypeAdapterFactory.java
+++ b/gson/src/main/java/com/google/gson/TypeAdapterFactory.java
@@ -17,6 +17,7 @@
 package com.google.gson;
 
 import com.google.gson.reflect.TypeToken;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Creates type adapters for set of related types. Type adapter factories are most useful when
@@ -166,5 +167,5 @@ public interface TypeAdapterFactory {
   /**
    * Returns a type adapter for {@code type}, or null if this factory doesn't support {@code type}.
    */
-  <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type);
+  <T> @Nullable TypeAdapter<T> create(Gson gson, TypeToken<T> type);
 }
diff --git a/gson/src/main/java/com/google/gson/annotations/package-info.java b/gson/src/main/java/com/google/gson/annotations/package-info.java
index edc5304c4b..2518b81cca 100644
--- a/gson/src/main/java/com/google/gson/annotations/package-info.java
+++ b/gson/src/main/java/com/google/gson/annotations/package-info.java
@@ -20,4 +20,5 @@
  * @author Inderjeet Singh, Joel Leitch
  */
 @com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
 package com.google.gson.annotations;
diff --git a/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java b/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
index a76a0014e4..332421156b 100644
--- a/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
+++ b/gson/src/main/java/com/google/gson/internal/ConstructorConstructor.java
@@ -40,6 +40,7 @@
 import java.util.TreeSet;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentSkipListMap;
+import org.jspecify.annotations.Nullable;
 
 /** Returns a function that can construct an instance of a requested type. */
 public final class ConstructorConstructor {
@@ -63,7 +64,7 @@ public ConstructorConstructor(
    * @param c instance of the class to be checked
    * @return if instantiable {@code null}, else a non-{@code null} exception message
    */
-  static String checkInstantiable(Class<?> c) {
+  static @Nullable String checkInstantiable(Class<?> c) {
     int modifiers = c.getModifiers();
     if (Modifier.isInterface(modifiers)) {
       return "Interfaces can't be instantiated! Register an InstanceCreator"
@@ -174,7 +175,7 @@ public <T> ObjectConstructor<T> get(TypeToken<T> typeToken, boolean allowUnsafe)
    * Creates constructors for special JDK collection types which do not have a public no-args
    * constructor.
    */
-  private static <T> ObjectConstructor<T> newSpecialCollectionConstructor(
+  private static <T> @Nullable ObjectConstructor<T> newSpecialCollectionConstructor(
       Type type, Class<? super T> rawType) {
     if (EnumSet.class.isAssignableFrom(rawType)) {
       return () -> {
@@ -214,7 +215,7 @@ else if (rawType == EnumMap.class) {
     return null;
   }
 
-  private static <T> ObjectConstructor<T> newDefaultConstructor(
+  private static <T> @Nullable ObjectConstructor<T> newDefaultConstructor(
       Class<? super T> rawType, FilterResult filterResult) {
     // Cannot invoke constructor of abstract class
     if (Modifier.isAbstract(rawType.getModifiers())) {
@@ -285,7 +286,7 @@ private static <T> ObjectConstructor<T> newDefaultConstructor(
   }
 
   /** Constructors for common interface types like Map and List and their subtypes. */
-  private static <T> ObjectConstructor<T> newDefaultImplementationConstructor(
+  private static <T> @Nullable ObjectConstructor<T> newDefaultImplementationConstructor(
       Type type, Class<? super T> rawType) {
 
     /*
@@ -349,7 +350,7 @@ private static boolean hasStringKeyType(Type mapType) {
     return GsonTypes.getRawType(typeArguments[0]) == String.class;
   }
 
-  private static ObjectConstructor<? extends Map<?, Object>> newMapConstructor(
+  private static @Nullable ObjectConstructor<? extends Map<?, Object>> newMapConstructor(
       Type type, Class<?> rawType) {
     // First try Map implementation
     /*
diff --git a/gson/src/main/java/com/google/gson/internal/Excluder.java b/gson/src/main/java/com/google/gson/internal/Excluder.java
index 14ecd60fbe..537b8b10dc 100644
--- a/gson/src/main/java/com/google/gson/internal/Excluder.java
+++ b/gson/src/main/java/com/google/gson/internal/Excluder.java
@@ -34,6 +34,7 @@
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import org.jspecify.annotations.Nullable;
 
 /**
  * This class selects which fields and types to omit. It is configurable, supporting version
@@ -108,7 +109,7 @@ public Excluder withExclusionStrategy(
   }
 
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
     Class<?> rawType = type.getRawType();
 
     boolean skipSerialize = excludeClass(rawType, true);
diff --git a/gson/src/main/java/com/google/gson/internal/GsonTypes.java b/gson/src/main/java/com/google/gson/internal/GsonTypes.java
index 412c3bf98a..6dc348a761 100644
--- a/gson/src/main/java/com/google/gson/internal/GsonTypes.java
+++ b/gson/src/main/java/com/google/gson/internal/GsonTypes.java
@@ -34,6 +34,7 @@
 import java.util.NoSuchElementException;
 import java.util.Objects;
 import java.util.Properties;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Static methods for working with types.
@@ -55,7 +56,7 @@ private GsonTypes() {
    * @return a {@link java.io.Serializable serializable} parameterized type.
    */
   public static ParameterizedType newParameterizedTypeWithOwner(
-      Type ownerType, Class<?> rawType, Type... typeArguments) {
+      @Nullable Type ownerType, Class<?> rawType, Type... typeArguments) {
     return new ParameterizedTypeImpl(ownerType, rawType, typeArguments);
   }
 
@@ -512,7 +513,7 @@ public static boolean requiresOwnerType(Type rawType) {
   // the nested Type implementations here (which are also serializable).
   private static final class ParameterizedTypeImpl implements ParameterizedType, Serializable {
     @SuppressWarnings("serial")
-    private final Type ownerType;
+    private final @Nullable Type ownerType;
 
     @SuppressWarnings("serial")
     private final Type rawType;
@@ -520,7 +521,7 @@ private static final class ParameterizedTypeImpl implements ParameterizedType, S
     @SuppressWarnings("serial")
     private final Type[] typeArguments;
 
-    ParameterizedTypeImpl(Type ownerType, Class<?> rawType, Type... typeArguments) {
+    ParameterizedTypeImpl(@Nullable Type ownerType, Class<?> rawType, Type... typeArguments) {
       requireNonNull(rawType);
 
       if (ownerType == null && requiresOwnerType(rawType)) {
@@ -548,17 +549,17 @@ public Type getRawType() {
     }
 
     @Override
-    public Type getOwnerType() {
+    public @Nullable Type getOwnerType() {
       return ownerType;
     }
 
     @Override
-    public boolean equals(Object other) {
+    public boolean equals(@Nullable Object other) {
       return other instanceof ParameterizedType
           && GsonTypes.equals(this, (ParameterizedType) other);
     }
 
-    private static int hashCodeOrZero(Object o) {
+    private static int hashCodeOrZero(@Nullable Object o) {
       return o != null ? o.hashCode() : 0;
     }
 
@@ -631,7 +632,7 @@ private static final class WildcardTypeImpl implements WildcardType, Serializabl
     private final Type upperBound;
 
     @SuppressWarnings("serial")
-    private final Type lowerBound;
+    private final @Nullable Type lowerBound;
 
     WildcardTypeImpl(Type[] upperBounds, Type[] lowerBounds) {
       if (lowerBounds.length > 1) {
diff --git a/gson/src/main/java/com/google/gson/internal/ReflectionAccessFilterHelper.java b/gson/src/main/java/com/google/gson/internal/ReflectionAccessFilterHelper.java
index ead949ae1e..efbe0a9106 100644
--- a/gson/src/main/java/com/google/gson/internal/ReflectionAccessFilterHelper.java
+++ b/gson/src/main/java/com/google/gson/internal/ReflectionAccessFilterHelper.java
@@ -21,6 +21,7 @@
 import java.lang.reflect.AccessibleObject;
 import java.lang.reflect.Method;
 import java.util.List;
+import org.jspecify.annotations.Nullable;
 
 /** Internal helper class for {@link ReflectionAccessFilter}. */
 public class ReflectionAccessFilterHelper {
@@ -73,7 +74,7 @@ public static FilterResult getFilterResult(
   }
 
   /** See {@link AccessibleObject#canAccess(Object)} (Java >= 9) */
-  public static boolean canAccess(AccessibleObject accessibleObject, Object object) {
+  public static boolean canAccess(AccessibleObject accessibleObject, @Nullable Object object) {
     return AccessChecker.INSTANCE.canAccess(accessibleObject, object);
   }
 
@@ -90,7 +91,8 @@ private abstract static class AccessChecker {
           accessChecker =
               new AccessChecker() {
                 @Override
-                public boolean canAccess(AccessibleObject accessibleObject, Object object) {
+                public boolean canAccess(
+                    AccessibleObject accessibleObject, @Nullable Object object) {
                   try {
                     return (Boolean) canAccessMethod.invoke(accessibleObject, object);
                   } catch (Exception e) {
@@ -107,7 +109,7 @@ public boolean canAccess(AccessibleObject accessibleObject, Object object) {
         accessChecker =
             new AccessChecker() {
               @Override
-              public boolean canAccess(AccessibleObject accessibleObject, Object object) {
+              public boolean canAccess(AccessibleObject accessibleObject, @Nullable Object object) {
                 // Cannot determine whether object can be accessed, so assume it can be accessed
                 return true;
               }
@@ -116,6 +118,6 @@ public boolean canAccess(AccessibleObject accessibleObject, Object object) {
       INSTANCE = accessChecker;
     }
 
-    abstract boolean canAccess(AccessibleObject accessibleObject, Object object);
+    abstract boolean canAccess(AccessibleObject accessibleObject, @Nullable Object object);
   }
 }
diff --git a/gson/src/main/java/com/google/gson/internal/bind/ArrayTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/bind/ArrayTypeAdapter.java
index 50efa418c0..da2d168926 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/ArrayTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/ArrayTypeAdapter.java
@@ -29,13 +29,14 @@
 import java.lang.reflect.GenericArrayType;
 import java.lang.reflect.Type;
 import java.util.ArrayList;
+import org.jspecify.annotations.Nullable;
 
 /** Adapter for arrays. */
 public final class ArrayTypeAdapter<E> extends TypeAdapter<Object> {
   public static final TypeAdapterFactory FACTORY =
       new TypeAdapterFactory() {
         @Override
-        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
           Type type = typeToken.getType();
           if (!(type instanceof GenericArrayType
               || (type instanceof Class && ((Class<?>) type).isArray()))) {
diff --git a/gson/src/main/java/com/google/gson/internal/bind/CollectionTypeAdapterFactory.java b/gson/src/main/java/com/google/gson/internal/bind/CollectionTypeAdapterFactory.java
index 7dd48a3ebb..d95685bf13 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/CollectionTypeAdapterFactory.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/CollectionTypeAdapterFactory.java
@@ -29,6 +29,7 @@
 import java.io.IOException;
 import java.lang.reflect.Type;
 import java.util.Collection;
+import org.jspecify.annotations.Nullable;
 
 /** Adapt a homogeneous collection of objects. */
 public final class CollectionTypeAdapterFactory implements TypeAdapterFactory {
@@ -39,7 +40,7 @@ public CollectionTypeAdapterFactory(ConstructorConstructor constructorConstructo
   }
 
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
     Type type = typeToken.getType();
 
     Class<? super T> rawType = typeToken.getRawType();
diff --git a/gson/src/main/java/com/google/gson/internal/bind/DefaultDateTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/bind/DefaultDateTypeAdapter.java
index b5dffe24fb..d8733e2e8f 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/DefaultDateTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/DefaultDateTypeAdapter.java
@@ -38,6 +38,7 @@
 import java.util.Locale;
 import java.util.Objects;
 import java.util.TimeZone;
+import org.jspecify.annotations.Nullable;
 
 /**
  * This type adapter supports subclasses of date by defining a {@link
@@ -65,7 +66,7 @@ public final class DefaultDateTypeAdapter<T extends Date> extends TypeAdapter<T>
       new TypeAdapterFactory() {
         @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
         @Override
-        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
           return typeToken.getRawType() == Date.class
               ? (TypeAdapter<T>)
                   new DefaultDateTypeAdapter<>(
diff --git a/gson/src/main/java/com/google/gson/internal/bind/EnumTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/bind/EnumTypeAdapter.java
index 5f24dc18d9..557740db40 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/EnumTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/EnumTypeAdapter.java
@@ -30,13 +30,14 @@
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
+import org.jspecify.annotations.Nullable;
 
 /** Adapter for enum classes (but not for the base class {@code java.lang.Enum}). */
 class EnumTypeAdapter<T extends Enum<T>> extends TypeAdapter<T> {
   static final TypeAdapterFactory FACTORY =
       new TypeAdapterFactory() {
         @Override
-        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
           Class<? super T> rawType = typeToken.getRawType();
           if (!Enum.class.isAssignableFrom(rawType) || rawType == Enum.class) {
             return null;
@@ -110,7 +111,7 @@ public T read(JsonReader in) throws IOException {
   }
 
   @Override
-  public void write(JsonWriter out, T value) throws IOException {
+  public void write(JsonWriter out, @Nullable T value) throws IOException {
     out.value(value == null ? null : constantToName.get(value));
   }
 }
diff --git a/gson/src/main/java/com/google/gson/internal/bind/JsonAdapterAnnotationTypeAdapterFactory.java b/gson/src/main/java/com/google/gson/internal/bind/JsonAdapterAnnotationTypeAdapterFactory.java
index 193ab8f7a6..571b56e29c 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/JsonAdapterAnnotationTypeAdapterFactory.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/JsonAdapterAnnotationTypeAdapterFactory.java
@@ -27,6 +27,7 @@
 import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Given a type T, looks for the annotation {@link JsonAdapter} and uses an instance of the
@@ -37,7 +38,7 @@
 public final class JsonAdapterAnnotationTypeAdapterFactory implements TypeAdapterFactory {
   private static class DummyTypeAdapterFactory implements TypeAdapterFactory {
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       throw new AssertionError("Factory should not be used");
     }
   }
@@ -74,7 +75,7 @@ private static JsonAdapter getAnnotation(Class<?> rawType) {
   // this is not safe; requires that user has specified correct adapter class for @JsonAdapter
   @SuppressWarnings("unchecked")
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> targetType) {
+  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> targetType) {
     Class<? super T> rawType = targetType.getRawType();
     JsonAdapter annotation = getAnnotation(rawType);
     if (annotation == null) {
diff --git a/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java b/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java
index 49fc731edf..72518033d2 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/MapTypeAdapterFactory.java
@@ -36,6 +36,7 @@
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Adapts maps to either JSON objects or JSON arrays.
@@ -123,7 +124,7 @@ public MapTypeAdapterFactory(
   }
 
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
     Type type = typeToken.getType();
 
     Class<? super T> rawType = typeToken.getRawType();
diff --git a/gson/src/main/java/com/google/gson/internal/bind/NumberTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/bind/NumberTypeAdapter.java
index 80eabccd93..30357f9018 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/NumberTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/NumberTypeAdapter.java
@@ -27,6 +27,7 @@
 import com.google.gson.stream.JsonToken;
 import com.google.gson.stream.JsonWriter;
 import java.io.IOException;
+import org.jspecify.annotations.Nullable;
 
 /** Type adapter for {@link Number}. */
 public final class NumberTypeAdapter extends TypeAdapter<Number> {
@@ -45,7 +46,7 @@ private static TypeAdapterFactory newFactory(ToNumberStrategy toNumberStrategy)
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked")
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         return type.getRawType() == Number.class ? (TypeAdapter<T>) adapter : null;
       }
     };
diff --git a/gson/src/main/java/com/google/gson/internal/bind/ObjectTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/bind/ObjectTypeAdapter.java
index 2fa704544f..052a5d84e4 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/ObjectTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/ObjectTypeAdapter.java
@@ -32,6 +32,7 @@
 import java.util.Deque;
 import java.util.List;
 import java.util.Map;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Adapts types whose static type is only 'Object'. Uses getClass() on serialization and a
@@ -53,7 +54,7 @@ private static TypeAdapterFactory newFactory(ToNumberStrategy toNumberStrategy)
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked")
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         if (type.getRawType() == Object.class) {
           return (TypeAdapter<T>) new ObjectTypeAdapter(gson, toNumberStrategy);
         }
diff --git a/gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java b/gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java
index ef464684aa..7eff5e43e0 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/ReflectiveTypeAdapterFactory.java
@@ -55,6 +55,7 @@
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import org.jspecify.annotations.Nullable;
 
 /** Type adapter that reflects over the fields and methods of a class. */
 public final class ReflectiveTypeAdapterFactory implements TypeAdapterFactory {
@@ -107,7 +108,7 @@ private List<String> getFieldNames(Field f) {
   }
 
   @Override
-  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
     Class<? super T> raw = type.getRawType();
 
     if (!Object.class.isAssignableFrom(raw)) {
diff --git a/gson/src/main/java/com/google/gson/internal/bind/TreeTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/bind/TreeTypeAdapter.java
index 62ea14d383..8c59f7bd49 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/TreeTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/TreeTypeAdapter.java
@@ -32,6 +32,7 @@
 import java.io.IOException;
 import java.lang.reflect.Type;
 import java.util.Objects;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Adapts a Gson 1.x tree-style adapter as a streaming TypeAdapter. Since the tree adapter may be
@@ -176,7 +177,7 @@ private static final class SingleTypeFactory implements TypeAdapterFactory {
 
     @SuppressWarnings("unchecked") // guarded by typeToken.equals() call
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       boolean matches =
           exactType != null
               ? exactType.equals(type) || (matchRawType && exactType.getType() == type.getRawType())
diff --git a/gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java b/gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java
index 71f98c0034..6c69dab153 100644
--- a/gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java
+++ b/gson/src/main/java/com/google/gson/internal/bind/TypeAdapters.java
@@ -48,6 +48,7 @@
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicIntegerArray;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Type adapters for basic types. More complex adapters exist as separate classes in the enclosing
@@ -819,7 +820,7 @@ public static <TT> TypeAdapterFactory newFactory(
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
         return typeToken.equals(type) ? (TypeAdapter<T>) typeAdapter : null;
       }
     };
@@ -830,7 +831,7 @@ public static <TT> TypeAdapterFactory newFactory(Class<TT> type, TypeAdapter<TT>
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
         return typeToken.getRawType() == type ? (TypeAdapter<T>) typeAdapter : null;
       }
 
@@ -847,7 +848,7 @@ public static <TT> TypeAdapterFactory newFactory(
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
         Class<? super T> rawType = typeToken.getRawType();
         return (rawType == unboxed || rawType == boxed) ? (TypeAdapter<T>) typeAdapter : null;
       }
@@ -871,7 +872,7 @@ public static <TT> TypeAdapterFactory newFactoryForMultipleTypes(
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
         Class<? super T> rawType = typeToken.getRawType();
         return (rawType == base || rawType == sub) ? (TypeAdapter<T>) typeAdapter : null;
       }
@@ -898,7 +899,7 @@ public static <T1> TypeAdapterFactory newTypeHierarchyFactory(
     return new TypeAdapterFactory() {
       @SuppressWarnings("unchecked")
       @Override
-      public <T2> TypeAdapter<T2> create(Gson gson, TypeToken<T2> typeToken) {
+      public @Nullable <T2> TypeAdapter<T2> create(Gson gson, TypeToken<T2> typeToken) {
         Class<? super T2> requestedType = typeToken.getRawType();
         if (!clazz.isAssignableFrom(requestedType)) {
           return null;
diff --git a/gson/src/main/java/com/google/gson/internal/bind/package-info.java b/gson/src/main/java/com/google/gson/internal/bind/package-info.java
new file mode 100644
index 0000000000..5341e33820
--- /dev/null
+++ b/gson/src/main/java/com/google/gson/internal/bind/package-info.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2011 Google Inc.
+ *
+ * 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.
+ */
+
+/**
+ * Do NOT use any class in this package as they are meant for internal use in Gson. These classes
+ * will very likely change incompatibly in future versions. You have been warned.
+ *
+ * @author Inderjeet Singh, Joel Leitch, Jesse Wilson
+ */
+@com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
+package com.google.gson.internal.bind;
diff --git a/gson/src/main/java/com/google/gson/internal/bind/util/package-info.java b/gson/src/main/java/com/google/gson/internal/bind/util/package-info.java
new file mode 100644
index 0000000000..50226806ac
--- /dev/null
+++ b/gson/src/main/java/com/google/gson/internal/bind/util/package-info.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2011 Google Inc.
+ *
+ * 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.
+ */
+
+/**
+ * Do NOT use any class in this package as they are meant for internal use in Gson. These classes
+ * will very likely change incompatibly in future versions. You have been warned.
+ *
+ * @author Inderjeet Singh, Joel Leitch, Jesse Wilson
+ */
+@com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
+package com.google.gson.internal.bind.util;
diff --git a/gson/src/main/java/com/google/gson/internal/package-info.java b/gson/src/main/java/com/google/gson/internal/package-info.java
index c92c71de63..a0577c0162 100644
--- a/gson/src/main/java/com/google/gson/internal/package-info.java
+++ b/gson/src/main/java/com/google/gson/internal/package-info.java
@@ -21,4 +21,5 @@
  * @author Inderjeet Singh, Joel Leitch, Jesse Wilson
  */
 @com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
 package com.google.gson.internal;
diff --git a/gson/src/main/java/com/google/gson/internal/reflect/package-info.java b/gson/src/main/java/com/google/gson/internal/reflect/package-info.java
new file mode 100644
index 0000000000..f7552e0d83
--- /dev/null
+++ b/gson/src/main/java/com/google/gson/internal/reflect/package-info.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2011 Google Inc.
+ *
+ * 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.
+ */
+
+/**
+ * Do NOT use any class in this package as they are meant for internal use in Gson. These classes
+ * will very likely change incompatibly in future versions. You have been warned.
+ *
+ * @author Inderjeet Singh, Joel Leitch, Jesse Wilson
+ */
+@com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
+package com.google.gson.internal.reflect;
diff --git a/gson/src/main/java/com/google/gson/internal/sql/SqlDateTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/sql/SqlDateTypeAdapter.java
index 1991daefb0..7bbbfaba5a 100644
--- a/gson/src/main/java/com/google/gson/internal/sql/SqlDateTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/sql/SqlDateTypeAdapter.java
@@ -30,6 +30,7 @@
 import java.text.SimpleDateFormat;
 import java.util.Date;
 import java.util.TimeZone;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Adapter for java.sql.Date. Although this class appears stateless, it is not. DateFormat captures
@@ -42,7 +43,7 @@ final class SqlDateTypeAdapter extends TypeAdapter<java.sql.Date> {
       new TypeAdapterFactory() {
         @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
         @Override
-        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
           return typeToken.getRawType() == java.sql.Date.class
               ? (TypeAdapter<T>) new SqlDateTypeAdapter()
               : null;
diff --git a/gson/src/main/java/com/google/gson/internal/sql/SqlTimeTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/sql/SqlTimeTypeAdapter.java
index d63ae0677e..84a5463d9f 100644
--- a/gson/src/main/java/com/google/gson/internal/sql/SqlTimeTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/sql/SqlTimeTypeAdapter.java
@@ -31,6 +31,7 @@
 import java.text.SimpleDateFormat;
 import java.util.Date;
 import java.util.TimeZone;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Adapter for java.sql.Time. Although this class appears stateless, it is not. DateFormat captures
@@ -43,7 +44,7 @@ final class SqlTimeTypeAdapter extends TypeAdapter<Time> {
       new TypeAdapterFactory() {
         @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
         @Override
-        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
           return typeToken.getRawType() == Time.class
               ? (TypeAdapter<T>) new SqlTimeTypeAdapter()
               : null;
diff --git a/gson/src/main/java/com/google/gson/internal/sql/SqlTimestampTypeAdapter.java b/gson/src/main/java/com/google/gson/internal/sql/SqlTimestampTypeAdapter.java
index ab1cefe544..fb2f3e89c5 100644
--- a/gson/src/main/java/com/google/gson/internal/sql/SqlTimestampTypeAdapter.java
+++ b/gson/src/main/java/com/google/gson/internal/sql/SqlTimestampTypeAdapter.java
@@ -25,6 +25,7 @@
 import java.io.IOException;
 import java.sql.Timestamp;
 import java.util.Date;
+import org.jspecify.annotations.Nullable;
 
 @SuppressWarnings("JavaUtilDate")
 class SqlTimestampTypeAdapter extends TypeAdapter<Timestamp> {
@@ -32,7 +33,7 @@ class SqlTimestampTypeAdapter extends TypeAdapter<Timestamp> {
       new TypeAdapterFactory() {
         @SuppressWarnings("unchecked") // we use a runtime check to make sure the 'T's equal
         @Override
-        public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
+        public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> typeToken) {
           if (typeToken.getRawType() == Timestamp.class) {
             TypeAdapter<Date> dateTypeAdapter = gson.getAdapter(Date.class);
             return (TypeAdapter<T>) new SqlTimestampTypeAdapter(dateTypeAdapter);
diff --git a/gson/src/main/java/com/google/gson/internal/sql/package-info.java b/gson/src/main/java/com/google/gson/internal/sql/package-info.java
new file mode 100644
index 0000000000..42481c665b
--- /dev/null
+++ b/gson/src/main/java/com/google/gson/internal/sql/package-info.java
@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2011 Google Inc.
+ *
+ * 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.
+ */
+
+/**
+ * Do NOT use any class in this package as they are meant for internal use in Gson. These classes
+ * will very likely change incompatibly in future versions. You have been warned.
+ *
+ * @author Inderjeet Singh, Joel Leitch, Jesse Wilson
+ */
+@com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
+package com.google.gson.internal.sql;
diff --git a/gson/src/main/java/com/google/gson/package-info.java b/gson/src/main/java/com/google/gson/package-info.java
index 04d56b2f14..5f2c816bc9 100644
--- a/gson/src/main/java/com/google/gson/package-info.java
+++ b/gson/src/main/java/com/google/gson/package-info.java
@@ -25,4 +25,5 @@
  * @author Inderjeet Singh, Joel Leitch
  */
 @com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
 package com.google.gson;
diff --git a/gson/src/main/java/com/google/gson/reflect/TypeToken.java b/gson/src/main/java/com/google/gson/reflect/TypeToken.java
index 7e228de716..6fac3a72a8 100644
--- a/gson/src/main/java/com/google/gson/reflect/TypeToken.java
+++ b/gson/src/main/java/com/google/gson/reflect/TypeToken.java
@@ -26,6 +26,7 @@
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Objects;
+import org.jspecify.annotations.Nullable;
 
 /**
  * Represents a generic type {@code T}. Java doesn't yet provide a way to represent generic types,
@@ -118,7 +119,7 @@ else if (superclass == TypeToken.class) {
     throw new IllegalStateException("Must only create direct subclasses of TypeToken");
   }
 
-  private static void verifyNoTypeVariable(Type type) {
+  private static void verifyNoTypeVariable(@Nullable Type type) {
     if (type instanceof TypeVariable) {
       TypeVariable<?> typeVariable = (TypeVariable<?>) type;
       throw new IllegalArgumentException(
@@ -183,7 +184,7 @@ public boolean isAssignableFrom(Class<?> cls) {
    * @deprecated this implementation may be inconsistent with javac for types with wildcards.
    */
   @Deprecated
-  public boolean isAssignableFrom(Type from) {
+  public boolean isAssignableFrom(@Nullable Type from) {
     if (from == null) {
       return false;
     }
@@ -242,7 +243,7 @@ private static boolean isAssignableFrom(Type from, GenericArrayType to) {
 
   /** Private recursive helper function to actually do the type-safe checking of assignability. */
   private static boolean isAssignableFrom(
-      Type from, ParameterizedType to, Map<String, Type> typeVarMap) {
+      @Nullable Type from, ParameterizedType to, Map<String, Type> typeVarMap) {
 
     if (from == null) {
       return false;
diff --git a/gson/src/main/java/com/google/gson/reflect/package-info.java b/gson/src/main/java/com/google/gson/reflect/package-info.java
index ddfafdba08..0a9c914590 100644
--- a/gson/src/main/java/com/google/gson/reflect/package-info.java
+++ b/gson/src/main/java/com/google/gson/reflect/package-info.java
@@ -20,4 +20,5 @@
  * @author Inderjeet Singh, Joel Leitch
  */
 @com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
 package com.google.gson.reflect;
diff --git a/gson/src/main/java/com/google/gson/stream/JsonWriter.java b/gson/src/main/java/com/google/gson/stream/JsonWriter.java
index f08a698240..9d870ef25b 100644
--- a/gson/src/main/java/com/google/gson/stream/JsonWriter.java
+++ b/gson/src/main/java/com/google/gson/stream/JsonWriter.java
@@ -27,6 +27,8 @@
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
 import com.google.gson.FormattingStyle;
 import com.google.gson.Strictness;
+import org.jspecify.annotations.Nullable;
+
 import java.io.Closeable;
 import java.io.Flushable;
 import java.io.IOException;
@@ -175,8 +177,8 @@ public class JsonWriter implements Closeable, Flushable {
    * newline characters. This prevents eval() from failing with a syntax
    * error. http://code.google.com/p/google-gson/issues/detail?id=341
    */
-  private static final String[] REPLACEMENT_CHARS;
-  private static final String[] HTML_SAFE_REPLACEMENT_CHARS;
+  private static final @Nullable String[] REPLACEMENT_CHARS;
+  private static final @Nullable String[] HTML_SAFE_REPLACEMENT_CHARS;
 
   static {
     REPLACEMENT_CHARS = new String[128];
@@ -219,7 +221,7 @@ public class JsonWriter implements Closeable, Flushable {
 
   private boolean htmlSafe;
 
-  private String deferredName;
+  private @Nullable String deferredName;
 
   private boolean serializeNulls = true;
 
@@ -524,7 +526,7 @@ private void writeDeferredName() throws IOException {
    * @return this writer.
    */
   @CanIgnoreReturnValue
-  public JsonWriter value(String value) throws IOException {
+  public JsonWriter value(@Nullable String value) throws IOException {
     if (value == null) {
       return nullValue();
     }
@@ -554,7 +556,7 @@ public JsonWriter value(boolean value) throws IOException {
    * @since 2.7
    */
   @CanIgnoreReturnValue
-  public JsonWriter value(Boolean value) throws IOException {
+  public JsonWriter value(@Nullable Boolean value) throws IOException {
     if (value == null) {
       return nullValue();
     }
@@ -630,7 +632,7 @@ public JsonWriter value(long value) throws IOException {
    *     JSON number.
    */
   @CanIgnoreReturnValue
-  public JsonWriter value(Number value) throws IOException {
+  public JsonWriter value(@Nullable Number value) throws IOException {
     if (value == null) {
       return nullValue();
     }
@@ -689,7 +691,7 @@ public JsonWriter nullValue() throws IOException {
    * @since 2.4
    */
   @CanIgnoreReturnValue
-  public JsonWriter jsonValue(String value) throws IOException {
+  public JsonWriter jsonValue(@Nullable String value) throws IOException {
     if (value == null) {
       return nullValue();
     }
@@ -741,7 +743,7 @@ private static boolean alwaysCreatesValidJsonNumber(Class<? extends Number> c) {
   }
 
   private void string(String value) throws IOException {
-    String[] replacements = htmlSafe ? HTML_SAFE_REPLACEMENT_CHARS : REPLACEMENT_CHARS;
+    @Nullable String[] replacements = htmlSafe ? HTML_SAFE_REPLACEMENT_CHARS : REPLACEMENT_CHARS;
     out.write('\"');
     int last = 0;
     int length = value.length();
diff --git a/gson/src/main/java/com/google/gson/stream/package-info.java b/gson/src/main/java/com/google/gson/stream/package-info.java
index 84fbdbba58..111dc3fb04 100644
--- a/gson/src/main/java/com/google/gson/stream/package-info.java
+++ b/gson/src/main/java/com/google/gson/stream/package-info.java
@@ -16,4 +16,5 @@
 
 /** This package provides classes for processing JSON in an efficient streaming way. */
 @com.google.errorprone.annotations.CheckReturnValue
+@org.jspecify.annotations.NullMarked
 package com.google.gson.stream;
diff --git a/gson/src/test/java/com/google/gson/GsonTest.java b/gson/src/test/java/com/google/gson/GsonTest.java
index 4798168984..4991f32980 100644
--- a/gson/src/test/java/com/google/gson/GsonTest.java
+++ b/gson/src/test/java/com/google/gson/GsonTest.java
@@ -35,6 +35,7 @@
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicReference;
+import org.jspecify.annotations.Nullable;
 import org.junit.Test;
 
 /**
@@ -222,7 +223,7 @@ public T read(JsonReader in) throws IOException {
                   private volatile boolean isFirstCall = true;
 
                   @Override
-                  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+                  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
                     if (isFirstCall) {
                       isFirstCall = false;
 
@@ -312,7 +313,7 @@ public T read(JsonReader in) throws IOException {
                   volatile boolean isFirstCaller = true;
 
                   @Override
-                  public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+                  public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
                     Class<?> raw = type.getRawType();
 
                     if (raw == CustomClass1.class) {
@@ -402,7 +403,7 @@ class DummyFactory implements TypeAdapterFactory {
 
       @SuppressWarnings("unchecked")
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         return (TypeAdapter<T>) adapter;
       }
 
diff --git a/gson/src/test/java/com/google/gson/functional/DelegateTypeAdapterTest.java b/gson/src/test/java/com/google/gson/functional/DelegateTypeAdapterTest.java
index 86d0a75b6e..b8faef7d2f 100644
--- a/gson/src/test/java/com/google/gson/functional/DelegateTypeAdapterTest.java
+++ b/gson/src/test/java/com/google/gson/functional/DelegateTypeAdapterTest.java
@@ -28,6 +28,7 @@
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import org.jspecify.annotations.Nullable;
 import org.junit.Before;
 import org.junit.Test;
 
@@ -76,7 +77,7 @@ private static class StatsTypeAdapterFactory implements TypeAdapterFactory {
     int numWrites = 0;
 
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
       return new TypeAdapter<>() {
         @Override
diff --git a/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnClassesTest.java b/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnClassesTest.java
index 9e8077569e..fe617d733e 100644
--- a/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnClassesTest.java
+++ b/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnClassesTest.java
@@ -40,6 +40,7 @@
 import java.lang.reflect.Type;
 import java.util.List;
 import java.util.Locale;
+import org.jspecify.annotations.Nullable;
 import org.junit.Test;
 
 /** Functional tests for the {@link JsonAdapter} annotation on classes. */
@@ -186,7 +187,7 @@ private static class WithNullReturningFactory<T> {
 
     static class NullReturningFactory implements TypeAdapterFactory {
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         // Don't handle raw (non-parameterized) type
         if (type.getType() instanceof Class) {
           return null;
@@ -250,7 +251,7 @@ private static class C {
 
     static final class JsonAdapterFactory implements TypeAdapterFactory {
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         return new TypeAdapter<>() {
           @Override
           public void write(JsonWriter out, T value) throws IOException {
@@ -406,7 +407,7 @@ private static class WithDelegatingFactory<T> {
 
     static class Factory implements TypeAdapterFactory {
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         if (type.getRawType() != WithDelegatingFactory.class) {
           return null;
         }
@@ -462,7 +463,7 @@ private static class WithDelayedDelegatingFactory {
 
     static class Factory implements TypeAdapterFactory {
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         return new TypeAdapter<>() {
           // suppress Error Prone warning; should be clear that `Factory` refers to enclosing class
           @SuppressWarnings("SameNameButDifferent")
@@ -652,7 +653,7 @@ private static class WithDelegatingFactoryOnClassAndField {
 
     static class Factory implements TypeAdapterFactory {
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
 
         return new TypeAdapter<>() {
diff --git a/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnFieldsTest.java b/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnFieldsTest.java
index 459877d68f..51d35f429e 100644
--- a/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnFieldsTest.java
+++ b/gson/src/test/java/com/google/gson/functional/JsonAdapterAnnotationOnFieldsTest.java
@@ -40,6 +40,7 @@
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.List;
+import org.jspecify.annotations.Nullable;
 import org.junit.Test;
 
 /** Functional tests for the {@link JsonAdapter} annotation on fields. */
@@ -146,7 +147,7 @@ public Part read(JsonReader in) throws IOException {
 
   private static class GizmoPartTypeAdapterFactory implements TypeAdapterFactory {
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       return new TypeAdapter<>() {
         @Override
         public void write(JsonWriter out, T value) throws IOException {
@@ -317,7 +318,7 @@ public Long read(JsonReader in) throws IOException {
 
     @SuppressWarnings("unchecked")
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       Class<?> cls = type.getRawType();
       if (Long.class.isAssignableFrom(cls)) {
         return (TypeAdapter<T>) ADAPTER;
@@ -351,7 +352,7 @@ private static final class Gizmo2 {
 
   private static class Gizmo2PartTypeAdapterFactory implements TypeAdapterFactory {
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       return new TypeAdapter<>() {
         @Override
         public void write(JsonWriter out, T value) throws IOException {
@@ -532,7 +533,7 @@ private static class DelegatingAndOverwriting {
 
     static class DelegatingAdapterFactory implements TypeAdapterFactory {
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         return gson.getDelegateAdapter(this, type);
       }
     }
@@ -568,7 +569,7 @@ private static class WithDelegatingFactory<T> {
     static class Factory implements TypeAdapterFactory {
       @SuppressWarnings("unchecked")
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         TypeAdapter<String> delegate = (TypeAdapter<String>) gson.getDelegateAdapter(this, type);
 
         return (TypeAdapter<T>)
@@ -613,7 +614,7 @@ private static class WithDelayedDelegatingFactory {
     static class Factory implements TypeAdapterFactory {
       @SuppressWarnings("unchecked")
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         return (TypeAdapter<T>)
             new TypeAdapter<String>() {
               private TypeAdapter<String> delegate() {
@@ -665,7 +666,7 @@ private static class GetAdapterDelegation {
     static class Factory implements TypeAdapterFactory {
       @SuppressWarnings("unchecked")
       @Override
-      public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+      public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
         // Uses `Gson.getAdapter` instead of `Gson.getDelegateAdapter`
         TypeAdapter<String> delegate = (TypeAdapter<String>) gson.getAdapter(type);
 
diff --git a/gson/src/test/java/com/google/gson/functional/RuntimeTypeAdapterFactoryFunctionalTest.java b/gson/src/test/java/com/google/gson/functional/RuntimeTypeAdapterFactoryFunctionalTest.java
index d115c0df53..f81c0daea3 100644
--- a/gson/src/test/java/com/google/gson/functional/RuntimeTypeAdapterFactoryFunctionalTest.java
+++ b/gson/src/test/java/com/google/gson/functional/RuntimeTypeAdapterFactoryFunctionalTest.java
@@ -33,6 +33,7 @@
 import java.io.IOException;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import org.jspecify.annotations.Nullable;
 import org.junit.Test;
 
 /** Functional tests for the RuntimeTypeAdapterFactory feature in extras. */
@@ -161,7 +162,7 @@ public RuntimeTypeAdapterFactory<T> registerSubtype(Class<? extends T> type) {
     }
 
     @Override
-    public <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
+    public @Nullable <R> TypeAdapter<R> create(Gson gson, TypeToken<R> type) {
       if (type.getRawType() != baseType) {
         return null;
       }
diff --git a/pom.xml b/pom.xml
index f25a4110ce..722656a503 100644
--- a/pom.xml
+++ b/pom.xml
@@ -92,6 +92,12 @@
         <artifactId>truth</artifactId>
         <version>1.4.5</version>
       </dependency>
+
+      <dependency>
+        <groupId>org.jspecify</groupId>
+        <artifactId>jspecify</artifactId>
+        <version>1.0.0</version>
+      </dependency>
     </dependencies>
   </dependencyManagement>
 
diff --git a/test-shrinker/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java b/test-shrinker/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java
index aab1ef6112..ff471cba63 100644
--- a/test-shrinker/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java
+++ b/test-shrinker/src/main/java/com/example/ClassWithJsonAdapterAnnotation.java
@@ -15,6 +15,8 @@
 import com.google.gson.reflect.TypeToken;
 import com.google.gson.stream.JsonReader;
 import com.google.gson.stream.JsonWriter;
+import org.jspecify.annotations.Nullable;
+
 import java.io.IOException;
 import java.lang.reflect.Type;
 
@@ -82,7 +84,7 @@ public void write(JsonWriter out, DummyClass value) throws IOException {
 
   static class Factory implements TypeAdapterFactory {
     @Override
-    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+    public @Nullable <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
       // the code below is not type-safe, but does not matter for this test
       @SuppressWarnings("unchecked")
       TypeAdapter<T> r =