Hey there, let's work through your issues step by step. The root cause here is the Gradle version mismatch—Android Studio 3.1 requires Gradle 4.4 or higher, but you're still on 4.1. Fixing that will resolve most of your errors, and we'll cover the rest too.

1. 升级Gradle到要求的版本 #

First, update the Gradle wrapper to use version 4.4:

• Open your project's gradle/wrapper/gradle-wrapper.properties file
• Modify the distributionUrl line to:

  distributionUrl=https\://services.gradle.org/distributions/gradle-4.4-all.zip
  

• Save the file, then click the "Sync Now" prompt that pops up in Android Studio. It will automatically download the new Gradle version.

2. 匹配Android Gradle Plugin (AGP)版本 #

Android Studio 3.1 is designed to work with AGP 3.1.0. Update this in your project-level build.gradle:

• Open the root build.gradle file (not the module-specific one)
• Find the dependencies block and update the AGP classpath:

  dependencies {
      classpath 'com.android.tools.build:gradle:3.1.0'
      // 其他依赖保持不变
  }
  

• Sync the project again to apply this change.

3. 解决"Cannot find symbol variable"和CompileDebugJavawithJavac错误 #

These issues often stem from incomplete Gradle sync or cached data. Try these steps after updating Gradle/AGP:

• Clean & Rebuild your project: Go to Build > Clean Project, then Build > Rebuild Project. This forces a full recompile of all modules.
• Check import statements: If specific symbols are still missing, double-check that you've imported the correct classes—occasionally, AGP updates might adjust package structures, though this is rare between 3.0.1 and 3.1.0.
• Verify SDK versions: In your module's build.gradle, ensure compileSdkVersion and targetSdkVersion are set to a version supported by AGP 3.1.0 (API 27 is a safe choice, as it was the stable version when Studio 3.1 released).

4. 最后一招：清除缓存 #

If you're still seeing odd errors, invalidate Studio's caches to fix corrupted data:

• Go to File > Invalidate Caches / Restart
• Select "Invalidate and Restart" from the dialog. This will restart Studio and clear all cached build data.

内容的提问来源于stack exchange，提问作者Thrinadh Reddy