fix: fix time instant

core/src/main/java/com/alibaba/fastjson2/writer/ObjectWriterImplInstant.java

@@ -5,14 +5,20 @@
 import com.alibaba.fastjson2.util.DateUtils;
 
 import java.lang.reflect.Type;
-import java.time.*;
+import java.time.DateTimeException;
+import java.time.Instant;
+import java.time.Year;
+import java.time.ZoneId;
+import java.time.ZonedDateTime;
 import java.time.format.DateTimeFormatter;
 import java.util.Locale;
 
 final class ObjectWriterImplInstant
         extends DateTimeCodec
         implements ObjectWriter {
-    static final ObjectWriterImplInstant INSTANCE = new ObjectWriterImplInstant(null, null);
+    static final ObjectWriterImplInstant INSTANCE =
+            new ObjectWriterImplInstant(null, null);
+
     public ObjectWriterImplInstant(String format, Locale locale) {
         super(format, locale);
     }
@@ -36,8 +42,15 @@ public void write(JSONWriter jsonWriter, Object object, Object fieldName, Type f
                 : context.getDateFormat();
 
         Instant instant = (Instant) object;
-        if (dateFormat == null) {
-            jsonWriter.writeInstant(instant);
+
+        if (formatUnixTime || (format == null && context.isDateFormatUnixTime())) {
+            long millis = instant.toEpochMilli();
+            jsonWriter.writeInt64(millis / 1000);
+            return;
+        }
+
+        if (formatMillis || (format == null && context.isDateFormatMillis())) {
+            jsonWriter.writeInt64(instant.toEpochMilli());
             return;
         }
 
@@ -63,7 +76,7 @@ public void write(JSONWriter jsonWriter, Object object, Object fieldName, Type f
 
                 long zeroDay = localEpochDay + DAYS_0000_TO_1970;
                 // find the march-based year
-                zeroDay -= 60;  // adjust to 0000-03-01 so leap day is at end of four year cycle
+                zeroDay -= 60; // adjust to 0000-03-01 so leap day is at end of four year cycle
                 long adjust = 0;
                 if (zeroDay < 0) {
                     // adjust negative years to positive for calculation
@@ -78,7 +91,7 @@ public void write(JSONWriter jsonWriter, Object object, Object fieldName, Type f
                     yearEst--;
                     doyEst = zeroDay - (365 * yearEst + yearEst / 4 - yearEst / 100 + yearEst / 400);
                 }
-                yearEst += adjust;  // reset any negative year
+                yearEst += adjust; // reset any negative year
                 int marchDoy0 = (int) doyEst;
 
                 // convert march-based values back to january-based
@@ -158,19 +171,6 @@ public void write(JSONWriter jsonWriter, Object object, Object fieldName, Type f
 
         ZonedDateTime zdt = ZonedDateTime.ofInstant(instant, context.getZoneId());
 
-        if (formatUnixTime || (format == null && context.isDateFormatUnixTime())) {
-            long millis = zdt.toInstant().toEpochMilli();
-            jsonWriter.writeInt64(millis / 1000);
-            return;
-        }
-
-        if (formatMillis || (format == null && context.isDateFormatMillis())) {
-            jsonWriter.writeInt64(zdt
-                    .toInstant()
-                    .toEpochMilli());
-            return;
-        }
-
         int year = zdt.getYear();
         if (year >= 0 && year <= 9999) {
             if (formatISO8601 || (format == null && context.isDateFormatISO8601())) {
@@ -195,7 +195,7 @@ public void write(JSONWriter jsonWriter, Object object, Object fieldName, Type f
         }
 
         if (formatter == null) {
-            jsonWriter.writeZonedDateTime(zdt);
+            jsonWriter.writeInstant(instant);
         } else {
             String str = formatter.format(zdt);
             jsonWriter.writeString(str);

core/src/test/java/com/alibaba/fastjson2/issues_4000/Issue4003.java

@@ -0,0 +1,47 @@
+package com.alibaba.fastjson2.issues_4000;
+
+import com.alibaba.fastjson2.JSONFactory;
+import com.alibaba.fastjson2.JSONWriter;
+import org.junit.jupiter.api.Test;
+
+import java.time.Instant;
+import java.time.ZoneId;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue4003 {
+    private static String toJSONString(Instant instant, String format) {
+        JSONWriter.Context context = JSONFactory.createWriteContext();
+        context.setZoneId(ZoneId.of("UTC"));
+        if (format != null) {
+            context.setDateFormat(format);
+        }
+        try (JSONWriter writer = JSONWriter.of(context)) {
+            writer.writeAny(instant);
+            return writer.toString();
+        }
+    }
+
+    @Test
+    public void testInstantWithISO8601Format() {
+        Instant instant = Instant.parse("2026-02-24T14:50:00Z");
+        String json = toJSONString(instant, "iso8601");
+
+        assertEquals("\"2026-02-24T14:50:00Z\"", json);
+    }
+
+    @Test
+    public void testInstantWithISO8601FormatWithMillis() {
+        Instant instant = Instant.parse("2026-02-24T14:50:00.123Z");
+        String json = toJSONString(instant, "iso8601");
+        assertEquals("\"2026-02-24T14:50:00.123Z\"", json);
+    }
+
+    @Test
+    public void testInstantWithoutISO8601Format() {
+        Instant instant = Instant.parse("2026-02-24T14:50:00Z");
+        String json = toJSONString(instant, null);
+
+        assertEquals("\"2026-02-24T14:50:00Z\"", json);
+    }
+}