Apache HTTP Client Removal
Android 6.0 release removes support for the Apache HTTP client. If your app is using this client and targets Android 2.3 (API level 9) or higher, use the HttpURLConnection class instead. This API is more efficient because it reduces network use through transparent compression and response caching, and minimizes power consumption. To continue using the Apache HTTP APIs, you must first declare the following compile-time dependency in your build.gradle file:
android {
useLibrary 'org.apache.http.legacy'
}
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原文地址：http://developer.android.com/intl/zh-cn/about/versions/marshmallow/android-6.0-changes.html#behavior-apache-http-client