diff --git a/src/main/java/com/github/sttk/sabi/AsyncGroup.java b/src/main/java/com/github/sttk/sabi/AsyncGroup.java
index cdb8ee0..1a56b87 100644
--- a/src/main/java/com/github/sttk/sabi/AsyncGroup.java
+++ b/src/main/java/com/github/sttk/sabi/AsyncGroup.java
@@ -13,7 +13,7 @@
  * errors occurring during the execution of a {@link Runner} are stored and can be retrieved by
  * their names in a map.
  */
-public interface AsyncGroup {
+public sealed interface AsyncGroup permits com.github.sttk.sabi.internal.AsyncGroupImpl {
 
   /**
    * Represents the reason for a new {@link com.github.sttk.errs.Err} exception object when an
diff --git a/src/main/java/com/github/sttk/sabi/internal/AsyncGroupImpl.java b/src/main/java/com/github/sttk/sabi/internal/AsyncGroupImpl.java
index f598396..e0be842 100644
--- a/src/main/java/com/github/sttk/sabi/internal/AsyncGroupImpl.java
+++ b/src/main/java/com/github/sttk/sabi/internal/AsyncGroupImpl.java
@@ -9,7 +9,7 @@
 import com.github.sttk.sabi.Runner;
 import java.util.Map;
 
-public class AsyncGroupImpl implements AsyncGroup {
+public final class AsyncGroupImpl implements AsyncGroup {
   private ErrEntry errHead;
   private ErrEntry errLast;
   private VthEntry vthHead;
diff --git a/src/main/java/com/github/sttk/sabi/internal/DataConnContainer.java b/src/main/java/com/github/sttk/sabi/internal/DataConnContainer.java
index f531748..86686ec 100644
--- a/src/main/java/com/github/sttk/sabi/internal/DataConnContainer.java
+++ b/src/main/java/com/github/sttk/sabi/internal/DataConnContainer.java
@@ -6,7 +6,7 @@
 
 import com.github.sttk.sabi.DataConn;
 
-public class DataConnContainer {
+public final class DataConnContainer {
   DataConnContainer prev;
   DataConnContainer next;
   String name;
diff --git a/src/main/java/com/github/sttk/sabi/internal/DataConnList.java b/src/main/java/com/github/sttk/sabi/internal/DataConnList.java
index 8496d76..ff59acb 100644
--- a/src/main/java/com/github/sttk/sabi/internal/DataConnList.java
+++ b/src/main/java/com/github/sttk/sabi/internal/DataConnList.java
@@ -4,7 +4,7 @@
  */
 package com.github.sttk.sabi.internal;
 
-public class DataConnList {
+public final class DataConnList {
   DataConnContainer head;
   DataConnContainer last;
 
diff --git a/src/main/java/com/github/sttk/sabi/internal/DataHubInner.java b/src/main/java/com/github/sttk/sabi/internal/DataHubInner.java
index 1ed7be8..6d1063b 100644
--- a/src/main/java/com/github/sttk/sabi/internal/DataHubInner.java
+++ b/src/main/java/com/github/sttk/sabi/internal/DataHubInner.java
@@ -13,10 +13,10 @@
 import java.util.Objects;
 import java.util.concurrent.atomic.AtomicBoolean;
 
-public class DataHubInner {
+public final class DataHubInner {
 
   static final DataSrcList GLOBAL_DATA_SRC_LIST = new DataSrcList(false);
-  static AtomicBoolean GLOBAL_DATA_SRCS_FIXED = new AtomicBoolean(false);
+  static final AtomicBoolean GLOBAL_DATA_SRCS_FIXED = new AtomicBoolean(false);
 
   public static void usesGlobal(String name, DataSrc ds) {
     if (!GLOBAL_DATA_SRCS_FIXED.get()) {
diff --git a/src/main/java/com/github/sttk/sabi/internal/DataSrcContainer.java b/src/main/java/com/github/sttk/sabi/internal/DataSrcContainer.java
index 6c1a53e..b7899a6 100644
--- a/src/main/java/com/github/sttk/sabi/internal/DataSrcContainer.java
+++ b/src/main/java/com/github/sttk/sabi/internal/DataSrcContainer.java
@@ -6,7 +6,7 @@
 
 import com.github.sttk.sabi.DataSrc;
 
-public class DataSrcContainer {
+public final class DataSrcContainer {
   DataSrcContainer prev;
   DataSrcContainer next;
   boolean local;
diff --git a/src/main/java/com/github/sttk/sabi/internal/DataSrcList.java b/src/main/java/com/github/sttk/sabi/internal/DataSrcList.java
index b9f32cb..e8109ef 100644
--- a/src/main/java/com/github/sttk/sabi/internal/DataSrcList.java
+++ b/src/main/java/com/github/sttk/sabi/internal/DataSrcList.java
@@ -10,7 +10,7 @@
 import java.util.Map;
 import java.util.Objects;
 
-public class DataSrcList {
+public final class DataSrcList {
   DataSrcContainer notSetupHead;
   DataSrcContainer notSetupLast;
   DataSrcContainer didSetupHead;