错误详情:

Error:Execution failed for task ':app:transformResourcesWithMergeJavaResForDebug'.
> com.android.build.api.transform.TransformException: com.android.builder.packaging.DuplicateFileException: Duplicate files copied in APK META-INF/rxjava.properties
    File1: C:\Users\\.gradle\caches\modules-2\files-2.1\io.reactivex.rxjava2\rxjava\2.0.4\cbcf91e47e43607fb628b11b3571f212f11a6f84\rxjava-2.0.4.jar
    File2: C:\Users\\.gradle\caches\modules-2\files-2.1\io.reactivex\rxjava\1.1.5\ece7b5d0870e66d8226dab6dcf47a2b12afff061\rxjava-1.1.5.jar  

解读:

编译时出现了重复的文件,编译工具不知道应该编译哪个文件,
这种错误一般是因为导入jar包,jar包中有重复的文件导致的

正确的解决方式:

进入as工程报错的这个module的build.gradle文件,
在android{ }中增加这么几行代码:

    packagingOptions {
        exclude 'META-INF/rxjava.properties'
    }

exclude后面跟的东西就是报错里面APK 后面的东西:
com.android.build.api.transform.TransformException: com.android.builder.packaging.DuplicateFileException: Duplicate files copied in APK META-INF/rxjava.properties

于是,修复后的gradle文件变为:

apply plugin: 'com.android.application'

android {
    compileSdkVersion 25
    buildToolsVersion "25.0.3"
    defaultConfig {
        applicationId "com.heima.testdemo"
        minSdkVersion 16
        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 'META-INF/rxjava.properties'
    }
}


dependencies {
    compile fileTree(include: ['*.jar'], dir: 'libs')
    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:1.0.0-alpha7'
    testCompile 'junit:junit:4.12'

    compile 'io.reactivex:rxjava:1.1.0'
    compile 'io.reactivex:rxandroid:1.1.0'
    
    compile 'com.squareup.retrofit2:retrofit:2.0.0-beta4'
    compile 'com.squareup.retrofit2:converter-gson:2.0.0-beta4'
    compile 'com.squareup.retrofit2:adapter-rxjava:2.0.0-beta4'
    compile 'com.google.code.gson:gson:2.6.2'

}

重新编译运行一下错误就解决啦~~~