fix issue #2856, improve fastjson 1.x compatibility

fastjson1-compatible/src/main/java/com/alibaba/fastjson/JSONArray.java

@@ -592,7 +592,17 @@ public boolean containsAll(Collection c) {
 
     @Override
     public Object get(int index) {
-        return list.get(index);
+        Object value = list.get(index);
+
+        if (value instanceof com.alibaba.fastjson2.JSONObject) {
+            return new JSONObject((com.alibaba.fastjson2.JSONObject) value);
+        }
+
+        if (value instanceof com.alibaba.fastjson2.JSONArray) {
+            return new JSONArray((com.alibaba.fastjson2.JSONArray) value);
+        }
+
+        return value;
     }
 
     /**

fastjson1-compatible/src/main/java/com/alibaba/fastjson/JSONObject.java

@@ -131,6 +131,10 @@ public Object get(Object key) {
             val = new com.alibaba.fastjson.JSONObject((com.alibaba.fastjson2.JSONObject) val);
         }
 
+        if (val instanceof com.alibaba.fastjson2.JSONArray) {
+            val = new com.alibaba.fastjson.JSONArray((com.alibaba.fastjson2.JSONArray) val);
+        }
+
         return val;
     }
 

fastjson1-compatible/src/test/java/com/alibaba/fastjson/v2issues/Issue2856.java

@@ -0,0 +1,79 @@
+package com.alibaba.fastjson.v2issues;
+
+import com.alibaba.fastjson.JSON;
+import com.alibaba.fastjson.JSONArray;
+import com.alibaba.fastjson.JSONObject;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+import java.util.LinkedList;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * JSON 处理方法
+ *
+ * @author zhangrong
+ * @date 2024/08/07
+ */
+public class Issue2856 {
+    @Test
+    public void test1() {
+        Map map1 = new HashMap();
+
+        List list1 = new LinkedList();
+        map1.put("list1", list1);
+
+        Map map2 = new HashMap();
+        list1.add(map2);
+
+        List list2 = new LinkedList();
+        map2.put("list2", list2);
+
+        Map map3 = new HashMap();
+        list2.add(map3);
+
+        List list3 = new LinkedList();
+        map3.put("list3", list3);
+
+        list3.add("str1");
+        list3.add("str2");
+        list3.add("str3");
+
+        Object o1 = JSON.toJSON(map1);
+        Object o2 = ((JSONObject) o1).get("list1");
+        Object o3 = ((JSONArray) o2).get(0);
+        Object o4 = ((JSONObject) o3).get("list2");
+        Object o5 = ((JSONArray) o4).get(0);
+        Object o6 = ((JSONObject) o5).get("list3");
+    }
+
+    @Test
+    public void test2() {
+        List list1 = new LinkedList();
+        List list2 = new LinkedList();
+        list2.add("str1");
+        list2.add("str2");
+
+        list1.add(list2);
+
+        JSONArray o1 = (JSONArray) JSON.toJSON(list1);
+        JSONArray inner = (JSONArray) o1.get(0);
+
+        System.out.println(inner);
+    }
+
+    @Test
+    public void test3() {
+        List list1 = new LinkedList();
+        Map map1 = new HashMap();
+
+        map1.put("key", "val");
+        list1.add(map1);
+
+        JSONArray o1 = (JSONArray) JSON.toJSON(list1);
+        JSONObject inner = (JSONObject) o1.get(0);
+
+        System.out.println(inner);
+    }
+}