fix toJSON date field error, for issue #2550

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

@@ -616,7 +616,11 @@ public JSONObject toJSONObject(T object, long features) {
             Class fieldClass = fieldWriter.fieldClass;
             if (format != null) {
                 if (fieldClass == Date.class) {
-                    fieldValue = DateUtils.format((Date) fieldValue, format);
+                    if ("millis".equals(format)) {
+                        fieldValue = ((Date) fieldValue).getTime();
+                    } else {
+                        fieldValue = DateUtils.format((Date) fieldValue, format);
+                    }
                 } else if (fieldClass == LocalDate.class) {
                     fieldValue = DateUtils.format((LocalDate) fieldValue, format);
                 } else if (fieldClass == LocalDateTime.class) {

fastjson1-compatible/src/test/java/com/alibaba/fastjson/v2issues/Issue2550.java

@@ -0,0 +1,83 @@
+package com.alibaba.fastjson.v2issues;
+
+import com.alibaba.fastjson2.JSON;
+import com.alibaba.fastjson2.annotation.JSONField;
+import org.junit.jupiter.api.Test;
+
+import java.util.Date;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class Issue2550 {
+    @Test
+    public void mutatedTest1() {
+        Bean1 bean = new Bean1();
+        bean.date = new Date(1655097829796L);
+        String str = JSON.toJSON(bean).toString();
+        assertEquals("{\"date\":1655097829796}", str);
+
+        Bean1 bean1 = JSON.parseObject(str, Bean1.class);
+        assertEquals(bean.date.getTime(), bean1.date.getTime());
+        Bean1 bean2 = JSON.parseObject(str).toJavaObject(Bean1.class);
+        assertEquals(bean.date.getTime(), bean2.date.getTime());
+    }
+
+    public static class Bean1 {
+        @JSONField(format = "millis")
+        public Date date;
+    }
+
+    @Test
+    public void mutatedTest2() {
+        Bean2 bean = new Bean2();
+        bean.date = new Date(1655097829796L);
+        String str = JSON.toJSON(bean).toString();
+        assertEquals("{\"date\":1655097829796}", str);
+
+        Bean2 bean1 = JSON.parseObject(str, Bean2.class);
+        assertEquals(bean.date.getTime(), bean1.date.getTime());
+        Bean2 bean2 = JSON.parseObject(str).toJavaObject(Bean2.class);
+        assertEquals(bean.date.getTime(), bean2.date.getTime());
+    }
+
+    public static class Bean2 {
+        @JSONField(format = "millis")
+        public Date date;
+    }
+
+    @Test
+    public void mutatedTest1fj() {
+        Bean1fj bean = new Bean1fj();
+        bean.date = new Date(1655097829796L);
+        String str = com.alibaba.fastjson.JSON.toJSON(bean).toString();
+        assertEquals("{\"date\":1655097829796}", str);
+
+        Bean1fj bean1 = com.alibaba.fastjson.JSON.parseObject(str, Bean1fj.class);
+        assertEquals(bean.date.getTime(), bean1.date.getTime());
+        Bean1fj bean2 = com.alibaba.fastjson.JSON.parseObject(str).toJavaObject(Bean1fj.class);
+        assertEquals(bean.date.getTime(), bean2.date.getTime());
+    }
+
+    public static class Bean1fj {
+        @com.alibaba.fastjson.annotation.JSONField(format = "millis")
+        public Date date;
+    }
+
+    @Test
+    public void mutatedTest2fj() {
+        Bean2fj bean = new Bean2fj();
+        bean.date = new Date(1655097829796L);
+        String str = com.alibaba.fastjson.JSON.toJSON(bean).toString();
+        assertEquals("{\"date\":1655097829796}", str);
+
+        Bean2fj bean1 = com.alibaba.fastjson.JSON.parseObject(str, Bean2fj.class);
+        assertEquals(bean.date.getTime(), bean1.date.getTime());
+        Bean2fj bean2 = com.alibaba.fastjson.JSON.parseObject(str).toJavaObject(Bean2fj.class);
+        assertEquals(bean.date.getTime(), bean2.date.getTime());
+    }
+
+    public static class Bean2fj {
+        @com.alibaba.fastjson.annotation.JSONField(format = "millis")
+        public Date date;
+    }
+}