how to read the response json data from the server in sencha touch
url: 'serverurl',

    headers: { 'Content-Type': 'application/json;charset=utf-8' },

    params: {

        username: 'username',

        password: 'password'
    },

    method: 'GET',

    success: handleSuccess,

    failure: handleError

});

Ext.Ajax.request({
    url: URL,
    defaultHeaders : 'application/json',

    success : function(response, opt) {
       // this will give you the JSON
       Ext.Msg.alert('Success', response.responseText);
    },

    failure : function(response, opt) {
        Ext.Msg.alert('Failed', response.responseText);
    }
 });