Google app engine 无法在android studio中导入后端模块的cloudendpoint库

我创建了一个包含端点类的后端模块。它还有包含endpointapi类的库。在编译期间，我能够在应用程序的MainActivity文件中导入该模块的类及其库。但是，在运行时，它无法检测模块库中存在的类。可能是什么问题

运行代码时出现如下错误：

Error:(10, 66) error: package com.example.shashank_pc.myapplication.backend.tempClassApi does not exist

项目内部后端模块的文件夹结构如下：

Error:(10, 66) error: package com.example.shashank_pc.myapplication.backend.tempClassApi does not exist

由于某些原因，后端库中的类（如tempClassApi和myApi文件夹中的类）未被检测到

设置渐变文件：

include ':app', ':backend'

应用程序模块渐变生成文件：

apply plugin: 'com.android.application'

android {
compileSdkVersion 25
buildToolsVersion "26.0.1"
defaultConfig {
    applicationId "com.example.shashank_pc.testcloudendpoints"
    minSdkVersion 22
    targetSdkVersion 25
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
    }
}

packagingOptions {
    exclude 'com/google/appengine/repackaged/org/apache/commons/codec/language/bm/*'
    exclude 'com/google/appengine/repackaged/org/codehaus/jackson/impl/*'
    exclude 'com/google/appengine/repackaged/org/apache/commons/codec/language/*'
}
}

dependencies {
compile fileTree(dir: 'libs', include: ['*.jar'])
androidTestCompile('com.android.support.test.espresso:espresso-core:2.2.2', 
{
    exclude group: 'com.android.support', module: 'support-annotations'
})
compile 'com.android.support:appcompat-v7:25.3.1'
compile 'com.android.support.constraint:constraint-layout:1.0.2'
compile 'com.google.code.findbugs:jsr305:2.0.1'
testCompile 'junit:junit:4.12'
compile project(':backend')
}