diff --git a/container/gmap/gmap_hash_k_v_map.go b/container/gmap/gmap_hash_k_v_map.go
index 6d65d59e9..7ad5ac924 100644
--- a/container/gmap/gmap_hash_k_v_map.go
+++ b/container/gmap/gmap_hash_k_v_map.go
@@ -66,10 +66,11 @@ func NewKVMapWithCheckerFrom[K comparable, V any](data map[K]V, checker NilCheck
 // This function is used to determine if a value should be considered as nil.
 // The nil checker function takes a value of type V and returns a boolean indicating
 // whether the value should be treated as nil.
-func (m *KVMap[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) {
+func (m *KVMap[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) *KVMap[K, V] {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	m.nilChecker = nilChecker
+	return m
 }
 
 // isNil checks whether the given value is nil.
diff --git a/container/gmap/gmap_list_k_v_map.go b/container/gmap/gmap_list_k_v_map.go
index 6b37ed57c..eb273828a 100644
--- a/container/gmap/gmap_list_k_v_map.go
+++ b/container/gmap/gmap_list_k_v_map.go
@@ -85,10 +85,11 @@ func NewListKVMapWithCheckerFrom[K comparable, V any](data map[K]V, nilChecker N
 // This function is used to determine if a value should be considered as nil.
 // The nil checker function takes a value of type V and returns a boolean indicating
 // whether the value should be treated as nil.
-func (m *ListKVMap[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) {
+func (m *ListKVMap[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) *ListKVMap[K, V] {
 	m.mu.Lock()
 	defer m.mu.Unlock()
 	m.nilChecker = nilChecker
+	return m
 }
 
 // isNil checks whether the given value is nil.
diff --git a/container/gset/gset_t_set.go b/container/gset/gset_t_set.go
index 02ffbf6cd..1537f61b9 100644
--- a/container/gset/gset_t_set.go
+++ b/container/gset/gset_t_set.go
@@ -70,10 +70,11 @@ func NewTSetWithCheckerFrom[T comparable](items []T, checker NilChecker[T], safe
 // This function is used to determine if an element should be considered as nil.
 // The nil checker function takes an element of type T and returns a boolean indicating
 // whether the element should be treated as nil.
-func (set *TSet[T]) RegisterNilChecker(nilChecker NilChecker[T]) {
+func (set *TSet[T]) RegisterNilChecker(nilChecker NilChecker[T]) *TSet[T] {
 	set.mu.Lock()
 	defer set.mu.Unlock()
 	set.nilChecker = nilChecker
+	return set
 }
 
 // isNil checks whether the given value is nil.
diff --git a/container/gtree/gtree_k_v_avltree.go b/container/gtree/gtree_k_v_avltree.go
index 661364751..88a266fa1 100644
--- a/container/gtree/gtree_k_v_avltree.go
+++ b/container/gtree/gtree_k_v_avltree.go
@@ -82,10 +82,11 @@ func NewAVLKVTreeWithCheckerFrom[K comparable, V any](comparator func(v1, v2 K)
 // This function is used to determine if a value should be considered as nil.
 // The nil checker function takes a value of type V and returns a boolean indicating
 // whether the value should be treated as nil.
-func (tree *AVLKVTree[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) {
+func (tree *AVLKVTree[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) *AVLKVTree[K, V] {
 	tree.mu.Lock()
 	defer tree.mu.Unlock()
 	tree.nilChecker = nilChecker
+	return tree
 }
 
 // isNil checks whether the given value is nil.
diff --git a/container/gtree/gtree_k_v_btree.go b/container/gtree/gtree_k_v_btree.go
index 2adcede1b..eda3a1278 100644
--- a/container/gtree/gtree_k_v_btree.go
+++ b/container/gtree/gtree_k_v_btree.go
@@ -81,10 +81,11 @@ func NewBKVTreeWithCheckerFrom[K comparable, V any](m int, comparator func(v1, v
 // This function is used to determine if a value should be considered as nil.
 // The nil checker function takes a value of type V and returns a boolean indicating
 // whether the value should be treated as nil.
-func (tree *BKVTree[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) {
+func (tree *BKVTree[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) *BKVTree[K, V] {
 	tree.mu.Lock()
 	defer tree.mu.Unlock()
 	tree.nilChecker = nilChecker
+	return tree
 }
 
 // isNil checks whether the given value is nil.
diff --git a/container/gtree/gtree_k_v_redblacktree.go b/container/gtree/gtree_k_v_redblacktree.go
index 47585716a..b5b94af5b 100644
--- a/container/gtree/gtree_k_v_redblacktree.go
+++ b/container/gtree/gtree_k_v_redblacktree.go
@@ -100,10 +100,11 @@ func RedBlackKVTreeInitFrom[K comparable, V any](tree *RedBlackKVTree[K, V], com
 // This function is used to determine if a value should be considered as nil.
 // The nil checker function takes a value of type V and returns a boolean indicating
 // whether the value should be treated as nil.
-func (tree *RedBlackKVTree[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) {
+func (tree *RedBlackKVTree[K, V]) RegisterNilChecker(nilChecker NilChecker[V]) *RedBlackKVTree[K, V] {
 	tree.mu.Lock()
 	defer tree.mu.Unlock()
 	tree.nilChecker = nilChecker
+	return tree
 }
 
 // isNil checks whether the given value is nil.
diff --git a/database/gdb/gdb.go b/database/gdb/gdb.go
index 9c3bd7f81..cc99d8e5a 100644
--- a/database/gdb/gdb.go
+++ b/database/gdb/gdb.go
@@ -513,18 +513,18 @@ type StatsItem interface {
 
 // Core is the base struct for database management.
 type Core struct {
-	db            DB              // DB interface object.
-	ctx           context.Context // Context for chaining operation only. Do not set a default value in Core initialization.
-	group         string          // Configuration group name.
-	schema        string          // Custom schema for this object.
-	debug         *gtype.Bool     // Enable debug mode for the database, which can be changed in runtime.
-	cache         *gcache.Cache   // Cache manager, SQL result cache only.
-	links         *gmap.Map       // links caches all created links by node.
-	logger        glog.ILogger    // Logger for logging functionality.
-	config        *ConfigNode     // Current config node.
-	localTypeMap  *gmap.StrAnyMap // Local type map for database field type conversion.
-	dynamicConfig dynamicConfig   // Dynamic configurations, which can be changed in runtime.
-	innerMemCache *gcache.Cache   // Internal memory cache for storing temporary data.
+	db            DB                               // DB interface object.
+	ctx           context.Context                  // Context for chaining operation only. Do not set a default value in Core initialization.
+	group         string                           // Configuration group name.
+	schema        string                           // Custom schema for this object.
+	debug         *gtype.Bool                      // Enable debug mode for the database, which can be changed in runtime.
+	cache         *gcache.Cache                    // Cache manager, SQL result cache only.
+	links         *gmap.KVMap[ConfigNode, *sql.DB] // links caches all created links by node.
+	logger        glog.ILogger                     // Logger for logging functionality.
+	config        *ConfigNode                      // Current config node.
+	localTypeMap  *gmap.StrAnyMap                  // Local type map for database field type conversion.
+	dynamicConfig dynamicConfig                    // Dynamic configurations, which can be changed in runtime.
+	innerMemCache *gcache.Cache                    // Internal memory cache for storing temporary data.
 }
 
 type dynamicConfig struct {
@@ -944,6 +944,9 @@ func NewByGroup(group ...string) (db DB, err error) {
 	)
 }
 
+// linksChecker is the checker function for links map.
+var linksChecker = func(v *sql.DB) bool { return v == nil }
+
 // newDBByConfigNode creates and returns an ORM object with given configuration node and group name.
 //
 // Very Note:
@@ -960,7 +963,7 @@ func newDBByConfigNode(node *ConfigNode, group string) (db DB, err error) {
 		group:         group,
 		debug:         gtype.NewBool(),
 		cache:         gcache.New(),
-		links:         gmap.New(true),
+		links:         gmap.NewKVMapWithChecker[ConfigNode, *sql.DB](linksChecker, true),
 		logger:        glog.New(),
 		config:        node,
 		localTypeMap:  gmap.NewStrAnyMap(true),
@@ -1127,7 +1130,7 @@ func (c *Core) getSqlDb(master bool, schema ...string) (sqlDb *sql.DB, err error
 
 	// Cache the underlying connection pool object by node.
 	var (
-		instanceCacheFunc = func() any {
+		instanceCacheFunc = func() *sql.DB {
 			if sqlDb, err = c.db.Open(node); err != nil {
 				return nil
 			}
@@ -1159,7 +1162,7 @@ func (c *Core) getSqlDb(master bool, schema ...string) (sqlDb *sql.DB, err error
 	)
 	if instanceValue != nil && sqlDb == nil {
 		// It reads from instance map.
-		sqlDb = instanceValue.(*sql.DB)
+		sqlDb = instanceValue
 	}
 	if node.Debug {
 		c.db.SetDebug(node.Debug)
diff --git a/database/gdb/gdb_core.go b/database/gdb/gdb_core.go
index 4bb712c02..6cca52c7e 100644
--- a/database/gdb/gdb_core.go
+++ b/database/gdb/gdb_core.go
@@ -113,19 +113,17 @@ func (c *Core) Close(ctx context.Context) (err error) {
 	if err = c.cache.Close(ctx); err != nil {
 		return err
 	}
-	c.links.LockFunc(func(m map[any]any) {
+	c.links.LockFunc(func(m map[ConfigNode]*sql.DB) {
 		for k, v := range m {
-			if db, ok := v.(*sql.DB); ok {
-				err = db.Close()
-				if err != nil {
-					err = gerror.WrapCode(gcode.CodeDbOperationError, err, `db.Close failed`)
-				}
-				intlog.Printf(ctx, `close link: %s, err: %v`, k, err)
-				if err != nil {
-					return
-				}
-				delete(m, k)
+			err = v.Close()
+			if err != nil {
+				err = gerror.WrapCode(gcode.CodeDbOperationError, err, `db.Close failed`)
 			}
+			intlog.Printf(ctx, `close link: %s, err: %v`, k, err)
+			if err != nil {
+				return
+			}
+			delete(m, k)
 		}
 	})
 	return
diff --git a/database/gdb/gdb_core_stats.go b/database/gdb/gdb_core_stats.go
index 8b64f14e8..de7febdbd 100644
--- a/database/gdb/gdb_core_stats.go
+++ b/database/gdb/gdb_core_stats.go
@@ -30,14 +30,10 @@ func (item *localStatsItem) Stats() sql.DBStats {
 // Stats retrieves and returns the pool stat for all nodes that have been established.
 func (c *Core) Stats(ctx context.Context) []StatsItem {
 	var items = make([]StatsItem, 0)
-	c.links.Iterator(func(k, v any) bool {
-		var (
-			node  = k.(ConfigNode)
-			sqlDB = v.(*sql.DB)
-		)
+	c.links.Iterator(func(k ConfigNode, v *sql.DB) bool {
 		items = append(items, &localStatsItem{
-			node:  &node,
-			stats: sqlDB.Stats(),
+			node:  &k,
+			stats: v.Stats(),
 		})
 		return true
 	})