Why this line xmlns:android="http://schemas.android.com/apk/res/android" must be the first in the layout xml file?

Question

In XML, xmlns declares a Namespace. In fact, when you do:

<LinearLayout android:id>
</LinearLayout>

Instead of calling android:id, the xml will use http://schemas.android.com/apk/res/android:id to be unique. Generally this page doesn't exist (it's a URI, not a URL), but sometimes it is a URL that explains the used namespace.

The namespace has pretty much the same uses as the package name in a Java application.

Here is an explanation.

Uniform Resource Identifier (URI)

A Uniform Resource Identifier (URI) is a string of characters which identifies an Internet Resource.

The most common URI is the Uniform Resource Locator (URL) which identifies an Internet domain address. Another, not so common type of URI is the Universal Resource Name (URN).

In our examples we will only use URLs.

To understand why xmlns:android=“http://schemas.android.com/apk/res/android” must be the first in the layout xml file We shall understand the components using an example

Sample::

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:id="@+id/container" >    
</FrameLayout>

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Uniform Resource Indicator(URI):

• In computing, a uniform resource identifier (URI) is a string of characters used to identify a name of a resource.
• Such identification enables interaction with representations of the resource over a network, typically the World Wide Web, using specific protocols.

Ex:http://schemas.android.com/apk/res/android:id is the URI here

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XML Namespace:

• XML namespaces are used for providing uniquely named elements and attributes in an XML document. xmlns:android describes the android namespace.
• Its used like this because this is a design choice by google to handle the errors at compile time.
• Also suppose we write our own textview widget with different features compared to android textview, android namespace helps to distinguish between our custom textview widget and android textview widget

xmlns refers to the XML namespace

When using prefixes in XML, a so-called namespace for the prefix must be defined. The namespace is defined by the xmlns attribute in the start tag of an element. The namespace declaration has the following syntax. xmlns:prefix="URI".

Note: The namespace URI is not used by the parser to look up information.

The purpose is to give the namespace a unique name. However, often companies use the namespace as a pointer to a web page containing namespace information.

This is just the XML Name Space declaration. We use this Name Space in order to specify that the attributes listed below, belongs to Android. Thus they starts with "android:"

You can actually create your own custom attributes. So to prevent the name conflicts where 2 attributes are named the same thing, but behave differently, we add the prefix "android:" to signify that these are Android attributes.

Thus, this Name Space declaration must be included in the opening tag of the root view of your XML file.

xmlns:android

Defines the Android namespace. This attribute should always be set to "http://schemas.android.com/apk/res/android".

refer https://developer.android.com/guide/topics/manifest/manifest-element#nspace

• Xmlns means xml namespace.
• It is created to avoid naming conflicts in the xml’s.
• In order to avoid naming conflicts by any other way we need to provide each element with a prefix.
• to avoid repetitive usage of the prefix in each xml tag we use xmlns at the root of the xml. Hence we have the tag xmlns:android=”http://schemas.android.com/apk/res/android”
• Now android here simply means we are assigning the namespace “http://schemas.android.com/apk/res/android” to it.
• This namespace is not a URL but a URI also known as URN(universal resource name) which is rarely used in place of URI.
• Due to this android will be responsible to identify the android related elements in the xml document which would be android:xxxxxxx etc. Without this namespace android:xxxxxxx will be not recognized.

To put in layman’s term :

without xmlns:android=”http://schemas.android.com/apk/res/android” android related tags wont be recognized in the xml document of our layout.