diff --git a/CHANGELOG.md b/CHANGELOG.md
index 256a93a..1541093 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+## [4.0.0] - 2021.04.07
+
+* Add null safety support;
+* Upgrade dependencies.
+
 ## [3.0.2] - 2019.9.19
 
 * A dio transformer especially for flutter, by which the json decoding will be in background with [compute] function.
diff --git a/README.md b/README.md
index a0c7cf3..140dc0a 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ A [dio](https://github.com/flutterchina/dio) transformer especially for flutter,
 
 ```yaml
 dependencies:
-  dio_flutter_transformer: ^3.0.2 # latest version
+  dio_flutter_transformer: ^4.0.0 # latest version
 ```
 
 ## Usage
diff --git a/pubspec.lock b/pubspec.lock
new file mode 100644
index 0000000..fecb557
--- /dev/null
+++ b/pubspec.lock
@@ -0,0 +1,160 @@
+# Generated by pub
+# See https://dart.dev/tools/pub/glossary#lockfile
+packages:
+  async:
+    dependency: transitive
+    description:
+      name: async
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.5.0"
+  boolean_selector:
+    dependency: transitive
+    description:
+      name: boolean_selector
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.1.0"
+  characters:
+    dependency: transitive
+    description:
+      name: characters
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.1.0"
+  charcode:
+    dependency: transitive
+    description:
+      name: charcode
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.2.0"
+  clock:
+    dependency: transitive
+    description:
+      name: clock
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.1.0"
+  collection:
+    dependency: transitive
+    description:
+      name: collection
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.15.0"
+  dio:
+    dependency: "direct main"
+    description:
+      name: dio
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "4.0.0"
+  fake_async:
+    dependency: transitive
+    description:
+      name: fake_async
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.2.0"
+  flutter:
+    dependency: "direct main"
+    description: flutter
+    source: sdk
+    version: "0.0.0"
+  flutter_test:
+    dependency: "direct dev"
+    description: flutter
+    source: sdk
+    version: "0.0.0"
+  http_parser:
+    dependency: transitive
+    description:
+      name: http_parser
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "4.0.0"
+  matcher:
+    dependency: transitive
+    description:
+      name: matcher
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.12.10"
+  meta:
+    dependency: transitive
+    description:
+      name: meta
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.3.0"
+  path:
+    dependency: transitive
+    description:
+      name: path
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.8.0"
+  sky_engine:
+    dependency: transitive
+    description: flutter
+    source: sdk
+    version: "0.0.99"
+  source_span:
+    dependency: transitive
+    description:
+      name: source_span
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.8.0"
+  stack_trace:
+    dependency: transitive
+    description:
+      name: stack_trace
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.10.0"
+  stream_channel:
+    dependency: transitive
+    description:
+      name: stream_channel
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.1.0"
+  string_scanner:
+    dependency: transitive
+    description:
+      name: string_scanner
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.1.0"
+  term_glyph:
+    dependency: transitive
+    description:
+      name: term_glyph
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.2.0"
+  test_api:
+    dependency: transitive
+    description:
+      name: test_api
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.2.19"
+  typed_data:
+    dependency: transitive
+    description:
+      name: typed_data
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "1.3.0"
+  vector_math:
+    dependency: transitive
+    description:
+      name: vector_math
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.1.0"
+sdks:
+  dart: ">=2.12.0 <3.0.0"
diff --git a/pubspec.yaml b/pubspec.yaml
index 2da1b47..fa8be97 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,17 +1,17 @@
 name: dio_flutter_transformer
 description:  A dio transformer espically for flutter, by which the json decoding will be in background with [compute] function in flutter sdk.
 
-version: 3.0.2
+version: 4.0.0
 author: wendux <824783146@qq.com>
 homepage: https://github.com/flutterchina/dio_flutter_transformer
 
 environment:
-  sdk: ">2.4.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 dependencies:
   flutter:
     sdk: flutter
-  dio: ^3.0.0
+  dio: ^4.0.0
 
 dev_dependencies:
   flutter_test: