fix deserialize HashMap subclass, for issue alibaba#1686

core/src/main/java/com/alibaba/fastjson2/reader/ObjectReaderImplMap.java

@@ -156,6 +156,20 @@ public static ObjectReader of(Type fieldType, Class mapType, long features) {
                 return new ObjectReaderImplMap(instanceType, features, Collections.EMPTY_MAP);
             }
             default:
+                Type genericSuperclass = instanceType.getGenericSuperclass();
+                if (mapType != JSONObject.class && genericSuperclass instanceof ParameterizedType) {
+                    ParameterizedType parameterizedType = (ParameterizedType) genericSuperclass;
+                    Type[] arguments = parameterizedType.getActualTypeArguments();
+                    if (arguments.length == 2) {
+                        Type arg0 = arguments[0];
+                        Type arg1 = arguments[1];
+                        boolean typed = !(arg0 instanceof TypeVariable || arg1 instanceof TypeVariable);
+                        if (typed) {
+                            return new ObjectReaderImplMapTyped(mapType, instanceType, arg0, arg1, 0, builder);
+                        }
+                    }
+                }
+
                 if (instanceType == JSONObject1O.class) {
                     builder = createObjectSupplier(CLASS_JSON_OBJECT_1x);
                     instanceType = LinkedHashMap.class;

core/src/test/java/com/alibaba/fastjson2/issues_1600/Issue1686.java

@@ -0,0 +1,30 @@
+package com.alibaba.fastjson2.issues_1600;
+
+import com.alibaba.fastjson2.JSON;
+import org.junit.jupiter.api.Test;
+
+import java.util.HashMap;
+
+public class Issue1686 {
+    @Test
+    public void test_nest_map_subclass() {
+        A a = new A();
+        a.put("a", new B());
+        a.get("a").put("b", "1");
+
+        String jsonString = JSON.toJSONString(a);
+        assert jsonString.equals("{\"a\":{\"b\":\"1\"}}");
+
+        A a1 = JSON.parseObject(jsonString, A.class);
+        // java.lang.ClassCastException: com.alibaba.fastjson2.JSONObject cannot be cast to B
+        assert a1.get("a").get("b").equals("1");
+    }
+
+    public static class A
+            extends HashMap<String, B> {
+    }
+
+    public static class B
+            extends HashMap<String, String> {
+    }
+}