如何从Ext_js中的J_security_check获取用户名？

Question

其实我正在使用extjs框架，并希望获得用户名以供进一步使用。任何人都可以帮我解决这个问题。 我的extjs代码就是这个....

var username= new Ext.form.field.Text({
    width:300,
    margin:'0 0 10 0',
    name:'j_username',
    fieldLabel:'Email',
    allowBlank:false,
    minLength:6,
    maxLength:100
});

var password= new Ext.form.field.Text({
    width:300,
    margin:'0 0 10 0',
    name:'j_password',
    minLength:6,
    maxLength:20,
    inputType:'password',
    allowBlank:false,
    fieldLabel:'Password'
});

var signin = new Ext.form.Panel({
    width:350,
    renderTo:'signinform',
    url:'j_security_check',
    method:'POST',
    title:'Sign In',
    bodyPadding:20,
    layout:'anchor',
    items:[username,password,
    {
        xtype:'checkboxfield',
        fieldName:'accept',
        margin:'0 0 10 0',
        boxLabel:'I Accept <a class="cont" id="tousignin" href="#"><u>Terms Of Use</u></a>',
        hideLabel:true,
        checked:true,
        readOnly:true,
        labelAlign:'left'
    }],
    standardSubmit:true,
    buttons:[{
        text:'Reset Your Password',
        handler:function(){return false;
            window.location.href=forgoturl;
        }
    },{
        text:'Sign In',
        handler:function(){                 
            signin.submit();
        }
    }]
});

if(window.location.href.indexOf('j_security_check')>-1){
    Ext.MessageBox.alert('Error','Invalid e-mail or password specified.');
}

并将此代码导入login.jsp页面，如下所示：

那么如何从中获取用户名？