Hey there, let's work through this frustrating dependency error together — I've dealt with this exact issue a few times, so here are the most reliable fixes to try step by step:

1. Ensure Google's Maven Repository is Configured #

Android Support Library versions 26 and above are hosted exclusively on Google's Maven repo. If your project isn't pointing to it, Gradle can't locate the appcompat-v7:27.+ dependency.

• Open the project-level build.gradle (the file in your root project folder, not the app module's one)
• Verify that the repositories blocks in both buildscript and allprojects include google():

buildscript {
    repositories {
        google() // This is critical for accessing support libraries
        jcenter() // Optional, but can remain for other dependencies
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.0+' // Keep this version compatible
    }
}

allprojects {
    repositories {
        google()
        jcenter()
    }
}

2. Install the Matching Android Support Repository #

Even with the repo set up, you need the actual support library files downloaded locally.

• Go to Tools > SDK Manager in Android Studio
• Switch to the SDK Tools tab, then check "Show Package Details" at the bottom
• Locate Android Support Repository, and confirm the 27.x.x version (e.g., 27.0.2) is installed
• If missing, click "Apply" to download it, then restart Android Studio

3. Replace Dynamic Version + with a Specific Stable Version #

Using 27.+ lets Gradle auto-pick the latest version, but this can cause resolution issues due to sync mismatches. The final stable version in the 27.x line is 27.1.1.

• Open your module-level build.gradle (usually app/build.gradle)
• Update the appcompat dependency to use the specific version:

dependencies {
    implementation 'com.android.support:appcompat-v7:27.1.1'
    // ... other project dependencies
}

4. Fix Network/Proxy Issues #

If you're in a region with restricted access to Google's services, try adding a mirror like Aliyun's to your project-level build.gradle:

allprojects {
    repositories {
        google()
        jcenter()
        maven { url 'https://maven.aliyun.com/repository/google' }
        maven { url 'https://maven.aliyun.com/repository/jcenter' }
    }
}

Also, check Android Studio's proxy settings (File > Settings > Appearance & Behavior > System Settings > HTTP Proxy) — ensure it's configured correctly, or try disabling it temporarily if you don't need a proxy.

5. Clean, Rebuild, and Invalidate Caches #

Stuck cache data often causes weird dependency issues:

• Click Build > Clean Project, then Build > Rebuild Project
• If that doesn't work, go to File > Invalidate Caches / Restart and select "Invalidate and Restart" to clear Gradle's cached data

6. Verify Gradle Plugin Compatibility #

Support library 27.x requires at least Gradle plugin 3.0.0. Double-check your project-level build.gradle:

buildscript {
    dependencies {
        classpath 'com.android.tools.build:gradle:3.2.1' // 3.0.0+ works; this is a stable option
    }
}

Make sure your gradle-wrapper.properties has a matching Gradle version (e.g., distributionUrl=https\://services.gradle.org/distributions/gradle-4.6-all.zip for plugin 3.2.1)

Give these steps a shot — one of them should resolve your dependency issue!

内容的提问来源于stack exchange，提问作者mao