解决flutter doctor出现Android toolchain: Android license status unknown.

• 问题描述
• 解决方案

使用flutter doctor检查开发环境，出现以下问题

wangyangyangdeMacBook-Pro:~ wangyang$ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 2.2.1, on Mac OS X 10.15.7 19H2 darwin-x64, locale
    zh-Hans-US)
[!] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
    ✗ Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/macos#android-setup for
      more details.
[✓] Xcode - develop for iOS and macOS
[✓] Chrome - develop for the web
[✓] Android Studio (version 4.2)
[✓] VS Code (version 1.61.0)
[✓] Connected device (2 available)

! Doctor found issues in 1 category.

根据错误提示运行flutter doctor --android-licenses，出现以下Java异常。

wangyangyangdeMacBook-Pro:~ wangyang$ flutter doctor --android-licenses
Exception in thread "main" java.lang.NoClassDefFoundError: javax/xml/bind/annotation/XmlSchema
	at com.android.repository.api.SchemaModule$SchemaModuleVersion.(SchemaModule.java:156)
	at com.android.repository.api.SchemaModule.(SchemaModule.java:75)
	at com.android.sdklib.repository.AndroidSdkHandler.(AndroidSdkHandler.java:81)
	at com.android.sdklib.tool.sdkmanager.SdkManagerCli.main(SdkManagerCli.java:73)
	at com.android.sdklib.tool.sdkmanager.SdkManagerCli.main(SdkManagerCli.java:48)
Caused by: java.lang.ClassNotFoundException: javax.xml.bind.annotation.XmlSchema
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:581)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:178)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:522)
	... 5 more

虽然这个错误不影响在Android上的编译和运行，但是本人是个强迫症，必须消灭掉！！！

根据上面的错误显示，猜测难道是Android SDK的问题？

打开Android Studio, Preferences-Appearance & Behavior- System Settings- Android SDK-SDK Tools

发现SDK的命令行工具没有安装。

勾选，然后选择Apply，会出现安装提示.

点击OK，会出现安装进度。

安装完成后，再次运行flutter doctor，已经没有错误了。OK 完美！

wangyangyangdeMacBook-Pro:~ wangyang$ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 2.2.1, on Mac OS X 10.15.7 19H2 darwin-x64, locale
    zh-Hans-US)
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.3)
[✓] Xcode - develop for iOS and macOS
[✓] Chrome - develop for the web
[✓] Android Studio (version 4.2)
[✓] VS Code (version 1.61.0)
[✓] Connected device (2 available)

• No issues found!