The difference between return modelAndView and return "xxxx" in Springmvc

pom.xnl

<dependency>  
            <groupId>javax.servlet</groupId>  
            <artifactId>jstl</artifactId>  
            <version>1.2</version>  
            <scope>runtime</scope>  
        </dependency>  

spring-mvc.xml

<bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
<property name="viewClass">
    <value>org.springframework.web.servlet.view.JstlView</value>//This must be added
</property>         
<property name="prefix"><value>/WEB-INF/jsp/</value></property>         
<property name="suffix"><value>.jsp</value>
</property>     
</bean>

controller

@RequestMapping(value="/login", method=RequestMethod.GET)
public ModelAndView loginPage(HttpSession session) {    
  ModelAndView modelAndView = new ModelAndView("user/login");    
  return modelAndView;    
  //Access the WEB-INF/jsp/user/login.jsp file
}

@RequestMapping(value="/", method=RequestMethod.GET)
public String index(HttpSession session){    
  session.setAttribute("message", null);    
  return "redirect:/index.jsp";    
  // Access the index.jsp file in the root directory. redirect is direct url access.
}