fixed: parse empty collection in kotlin (#1540)

Signed-off-by: Kraity <kraty@krait.cn>

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

@@ -5,6 +5,7 @@ import com.alibaba.fastjson2.util.Fnv;
 import com.alibaba.fastjson2.util.GuavaSupport;
 import com.alibaba.fastjson2.util.TypeUtils;
 
+import java.lang.reflect.Field;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.*;
@@ -38,6 +39,7 @@ public final class ObjectReaderImplList
     final String itemClassName;
     final long itemClassNameHash;
     final Function builder;
+    Object listSingleton;
     ObjectReader itemObjectReader;
     volatile boolean instanceError;
 
@@ -164,14 +166,31 @@ public final class ObjectReaderImplList
                     break;
                 default:
                     instanceClass = listClass;
-                    break;
             }
         }
 
-        if (type == ObjectReaderImplList.CLASS_EMPTY_SET
-                || type == ObjectReaderImplList.CLASS_EMPTY_LIST
-        ) {
-            return new ObjectReaderImplList(type, (Class) type, (Class) type, Object.class, null);
+        switch (type.getTypeName()) {
+            case "kotlin.collections.EmptySet":
+            case "kotlin.collections.EmptyList": {
+                Object empty;
+                Class<?> clazz = (Class<?>) type;
+                try {
+                    Field field = clazz.getField("INSTANCE");
+                    if (!field.isAccessible()) {
+                        field.setAccessible(true);
+                    }
+                    empty = field.get(null);
+                } catch (NoSuchFieldException | IllegalAccessException e) {
+                    throw new IllegalStateException("Failed to get singleton of " + type, e);
+                }
+                return new ObjectReaderImplList(clazz, empty);
+            }
+            case "java.util.Collections$EmptySet": {
+                return new ObjectReaderImplList((Class) type, Collections.emptySet());
+            }
+            case "java.util.Collections$EmptyList": {
+                return new ObjectReaderImplList((Class) type, Collections.emptyList());
+            }
         }
 
         if (itemType == String.class && builder == null) {
@@ -185,6 +204,11 @@ public final class ObjectReaderImplList
         return new ObjectReaderImplList(type, listClass, instanceClass, itemType, builder);
     }
 
+    ObjectReaderImplList(Class listClass, Object listSingleton) {
+        this(listClass, listClass, listClass, Object.class, null);
+        this.listSingleton = listSingleton;
+    }
+
     public ObjectReaderImplList(Type listType, Class listClass, Class instanceType, Type itemType, Function builder) {
         this.listType = listType;
         this.listClass = listClass;
@@ -306,12 +330,8 @@ public final class ObjectReaderImplList
             return new TreeSet();
         }
 
-        if (instanceType == CLASS_EMPTY_LIST) {
-            return Collections.emptyList();
-        }
-
-        if (instanceType == CLASS_EMPTY_SET) {
-            return Collections.emptySet();
+        if (listSingleton != null) {
+            return listSingleton;
         }
 
         if (instanceType != null) {

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

@@ -36,6 +36,7 @@ public final class ObjectReaderImplMap
     final Class instanceType;
     final long features;
     final Function builder;
+    Object mapSingleton;
     volatile boolean instanceError;
 
     public static ObjectReader of(Type fieldType, Class mapType, long features) {
@@ -87,9 +88,6 @@ public final class ObjectReaderImplMap
                 case "java.util.Collections$SynchronizedSortedMap":
                     instanceType = TreeMap.class;
                     builder = (Function<SortedMap, SortedMap>) Collections::synchronizedSortedMap;
-                    break;
-                default:
-                    break;
             }
         }
 
@@ -137,6 +135,22 @@ public final class ObjectReaderImplMap
                 builder = GuavaSupport.createConvertFunction(instanceType);
                 instanceType = HashMap.class;
                 break;
+            case "kotlin.collections.EmptyMap": {
+                Object mapSingleton;
+                try {
+                    Field field = instanceType.getField("INSTANCE");
+                    if (!field.isAccessible()) {
+                        field.setAccessible(true);
+                    }
+                    mapSingleton = field.get(null);
+                } catch (NoSuchFieldException | IllegalAccessException e) {
+                    throw new IllegalStateException("Failed to get singleton of " + instanceType, e);
+                }
+                return new ObjectReaderImplMap(instanceType, features, mapSingleton);
+            }
+            case "java.util.Collections$EmptyMap": {
+                return new ObjectReaderImplMap(instanceType, features, Collections.EMPTY_MAP);
+            }
             default:
                 if (instanceType == JSONObject1O.class) {
                     Class objectClass = CLASS_JSON_OBJECT_1x;
@@ -154,12 +168,16 @@ public final class ObjectReaderImplMap
                         return Collections.singletonMap(entry.getKey(), entry.getValue());
                     };
                 }
-                break;
         }
 
         return new ObjectReaderImplMap(fieldType, mapType, instanceType, features, builder);
     }
 
+    ObjectReaderImplMap(Class mapClass, long features, Object mapSingleton) {
+        this(mapClass, mapClass, mapClass, features, null);
+        this.mapSingleton = mapSingleton;
+    }
+
     ObjectReaderImplMap(Type fieldType, Class mapType, Class instanceType, long features, Function builder) {
         this.fieldType = fieldType;
         this.mapType = mapType;
@@ -193,8 +211,8 @@ public final class ObjectReaderImplMap
             return new JSONObject();
         }
 
-        if (instanceType == CLASS_EMPTY_MAP) {
-            return Collections.emptyMap();
+        if (mapSingleton != null) {
+            return mapSingleton;
         }
 
         if (instanceType == CLASS_EMPTY_SORTED_MAP) {

kotlin/src/test/kotlin/com/alibaba/fastjson2/issues/Issue1540.kt

@@ -0,0 +1,31 @@
+package com.alibaba.fastjson2.issues
+
+import com.alibaba.fastjson2.JSON
+import com.alibaba.fastjson2.toJSONString
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertInstanceOf
+
+class Issue1540 {
+
+    @Test
+    fun test_map() {
+        val map = emptyMap<Any?, Any?>()
+        assertEquals("{}", map.toJSONString())
+        assertInstanceOf(map::class.java, JSON.parseObject("{}", map::class.java))
+    }
+
+    @Test
+    fun test_set() {
+        val set = emptySet<Any?>()
+        assertEquals("[]", set.toJSONString())
+        assertInstanceOf(set::class.java, JSON.parseObject("[]", set::class.java))
+    }
+
+    @Test
+    fun test_list() {
+        val list = emptyList<Any?>()
+        assertEquals("[]", list.toJSONString())
+        assertInstanceOf(list::class.java, JSON.parseObject("[]", list::class.java))
+    }
+}