How to group priority in TestNG for each class?

When I execute my testNG project, it is mixing up all the Tests from different classes which i add under the same <test> based on the priority. How do i code so that, I need to run all the methods on first class in order of there priority before moving to the next class?

<test name="test1">
    <classes>


        <class name="class1" />
        <class name="class2" />

    </classes>

</test>