refactor: Allow GsonJsonMapper to accept a Gson instance

Author: thced

src/main/java/com/retailsvc/http/internal/gson/GsonJsonMapper.java

@@ -25,6 +25,7 @@
 import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 import java.util.function.Function;
 
 /**
@@ -47,18 +48,30 @@ public final class GsonJsonMapper implements TypedTypeMapper {
   private final Gson gson;
 
   public GsonJsonMapper() {
-    this.gson =
-        new GsonBuilder()
-            .registerTypeAdapter(Instant.class, iso(Instant::toString, Instant::parse))
-            .registerTypeAdapter(
-                OffsetDateTime.class, iso(OffsetDateTime::toString, OffsetDateTime::parse))
-            .registerTypeAdapter(
-                ZonedDateTime.class, iso(ZonedDateTime::toString, ZonedDateTime::parse))
-            .registerTypeAdapter(
-                LocalDateTime.class, iso(LocalDateTime::toString, LocalDateTime::parse))
-            .registerTypeAdapter(LocalDate.class, iso(LocalDate::toString, LocalDate::parse))
-            .registerTypeAdapter(LocalTime.class, iso(LocalTime::toString, LocalTime::parse))
-            .create();
+    this(defaultGson());
+  }
+
+  public GsonJsonMapper(Gson gson) {
+    this.gson = Objects.requireNonNull(gson, "gson must not be null");
+  }
+
+  /** Returns the wrapped {@link Gson} instance. */
+  public Gson gson() {
+    return gson;
+  }
+
+  private static Gson defaultGson() {
+    return new GsonBuilder()
+        .registerTypeAdapter(Instant.class, iso(Instant::toString, Instant::parse))
+        .registerTypeAdapter(
+            OffsetDateTime.class, iso(OffsetDateTime::toString, OffsetDateTime::parse))
+        .registerTypeAdapter(
+            ZonedDateTime.class, iso(ZonedDateTime::toString, ZonedDateTime::parse))
+        .registerTypeAdapter(
+            LocalDateTime.class, iso(LocalDateTime::toString, LocalDateTime::parse))
+        .registerTypeAdapter(LocalDate.class, iso(LocalDate::toString, LocalDate::parse))
+        .registerTypeAdapter(LocalTime.class, iso(LocalTime::toString, LocalTime::parse))
+        .create();
   }
 
   @Override

src/test/java/com/retailsvc/http/internal/gson/GsonJsonMapperTest.java

@@ -2,6 +2,8 @@
 
 import static org.assertj.core.api.Assertions.assertThat;
 
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
 import java.nio.charset.StandardCharsets;
 import java.time.Instant;
 import java.time.LocalDate;
@@ -10,6 +12,7 @@
 import java.time.OffsetDateTime;
 import java.time.ZoneOffset;
 import java.time.ZonedDateTime;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import org.junit.jupiter.api.Test;
@@ -118,6 +121,24 @@ void readAsRoundTripsJsr310Fields() {
     assertThat(value.day).isEqualTo(LocalDate.of(2026, 5, 13));
   }
 
+  @Test
+  void constructorWithGsonUsesSuppliedInstance() {
+    Gson custom = new GsonBuilder().serializeNulls().create();
+    GsonJsonMapper m = new GsonJsonMapper(custom);
+
+    byte[] out = m.writeTo(Collections.singletonMap("k", null));
+
+    assertThat(new String(out, StandardCharsets.UTF_8)).isEqualTo("{\"k\":null}");
+  }
+
+  @Test
+  void gsonAccessorReturnsWrappedInstance() {
+    Gson custom = new GsonBuilder().serializeNulls().create();
+    GsonJsonMapper m = new GsonJsonMapper(custom);
+
+    assertThat(m.gson()).isSameAs(custom);
+  }
+
   static final class Item {
     String id;
     int qty;