Q:

Android Studio can't resolve jUnit 4.12 dependency

I am having a problem that seems to be quite popular on this plattform,
yet no answer seems to work for me. I am facing a similar problem to this:
Error:(23, 17) Failed to resolve: junit:junit:4.12
But none of the 43 answers are applicaple to my problem. 
(I hope this won't be marked as duplicate)
I have tried importing the org.junit package to the test class manually, but it couldn't resolve it. I figured that the reason for this is that Gradle can't load the JUnit library, but I have no clue how to change that.
Even upon creating a new android studio project, I have problems setting up JUnit. Not even the example tests can be ran:
package de.ckc.azubis.fleetparkmanagement;

import static junit.framework.Assert.assertEquals;

/**
 * Example local unit test, which will execute on the development machine 
(host).
 *
 * @see <a href="http://d.android.com/tools/testing">Testing documentation</a>
 */
public class ExampleUnitTest {

    @Test
    public void addition_isCorrect() throws Exception {
        assertEquals(4, 2 + 2);
    }
}

When I try to import the Test annotation, the JUnit dependency gets added to the gradle file, but does not get imported to the test class.
here my module level build.gradle:
apply plugin: 'com.android.application'
android {
    compileSdkVersion 27
    defaultConfig {
        applicationId "de.ckc.azubis.fleetparkmanagement"
        minSdkVersion 21
        targetSdkVersion 27
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
        }
    }
}

dependencies {
    implementation fileTree(dir: 'libs', include: ['*.jar'])
    implementation 'com.android.support:appcompat-v7:27.0.2'
    implementation 'com.android.support:design:27.0.2'
    implementation 'com.android.support.constraint:constraint-layout:1.0.2'
    testImplementation 'junit:junit:4.12'
    androidTestImplementation 'com.android.support.test:runner:1.0.1'
    androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.1'
    implementation 'com.android.support:cardview-v7:27.0.2'
}

and my project level build.gradle
// Top-level build file where you can add configuration options common to all sub-projects/modules.
buildscript {

    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.1'

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

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

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

I have tried everything i can think of, including reconfiguring the dependencies, manually adding every repository i could think of (mavenCentral, jcenter...), checked my proxy settings and much more.

A:

There was a problem with caching.
I fixed it by clicking on file -> Invalidate Caches and then restart

