最有效的解决方案：Unable to resolve dependency for ':app@debug/compileClasspath': Could not download xx.jar

网上百度和google了很久，无非就是关闭androidStudio的settings里的gradle的offline。但是在我这并不好使。
根本原因是jcenter 不稳定造成的下载中断

解决方案

1.修改项目下的build.gradle

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    
    repositories {
        google()
//       1 jcenter()
        maven{ url 'http://maven.aliyun.com/nexus/content/groups/public/'}
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.1.3'
        

        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        google()
//      2  jcenter()
        maven{url 'http://maven.aliyun.com/nexus/content/groups/public/'}
    }
}

task clean(type: Delete) {
    delete rootProject.buildDir
}