Apache HTTP Client Removed from Android 6.0 release

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'
}

Have to sync Gradle project after you insert the above code in build.gradle file.

Reference: Android 6.0 Changes - Apache HTTP Client Removal

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Related examples of using Apache HTTP client and HttpURLConnection:
- Android App to control Standalone ESP8266/ESP-12 on-board LED, using Android 6.0 removed HttpClient
- Android App to control Standalone ESP8266/ESP-12 on-board LED, using HttpURLConnection