SearchView in OptionsMenu not full width

Maybe I am late but thought it might be helpful for someone who faced the same issue as me.

final SearchView searchView = (SearchView) myActionMenuItem.getActionView();
searchView.setMaxWidth(Integer.MAX_VALUE);//set search menu as full width

in onCreateOptionsMenu of your class may solve this problem but one thing we need to understand. If you just want to show search menu (on clicking of search button) and not any other menu items in actionbar then you need to keep the search item as a first item in menu file. Here is an example

        <?xml version="1.0" encoding="utf-8"?>
    <menu xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:app="http://schemas.android.com/apk/res-auto">

<!--keeping searach as first item-->
        <item
            android:id="@+id/action_search"
            android:icon="@drawable/search_ab"
            android:title="@string/search"
            app:actionViewClass="android.support.v7.widget.SearchView"
            app:showAsAction="ifRoom" />

        <item
            android:id="@+id/itemFilter"
            android:icon="@drawable/filter"
            android:title="@string/filter"
            app:showAsAction="ifRoom" />
    </menu>

And it will be displayed as

on clicking search it will be shown as full width

And if you keep search as second or last item in menu then it will be show like this.

and on clicking search

Instead of creating a new layout I set the MaxWidth programmatically in onCreateOptionsMenu():

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    MenuInflater inflater = getMenuInflater();
    inflater.inflate(R.menu.menu_search, menu);
    SearchView searchView = (SearchView)menu.findItem(R.id.menu_search).getActionView();
    searchView.setMaxWidth(Integer.MAX_VALUE);
    ....

For some reason jjung's solution didn't work for me. I had to add android:maxWidth="10000dp", just a really high maxWidth, to get it to expand to the full available width.

Also, just to add, I'm using the support library (v7), and so my layout file looks like this:

<android.support.v7.widget.SearchView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:maxWidth="10000dp" />

And in my item, I have:

<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:myapp="http://schemas.android.com/apk/res-auto">
    <item
        ...
        myapp:actionLayout="@layout/searchview" />
...

Where myapp is just some arbitrary namespace for the attributes added by the support library (see Adding Action Items for more details).