How do I get the Session Object in Spring?

Since you're using Spring, stick with Spring, don't hack it yourself like the other post posits.

The Spring manual says:

You shouldn't interact directly with the HttpSession for security purposes. There is simply no justification for doing so - always use the SecurityContextHolder instead.

The suggested best practice for accessing the session is:

Object principal = SecurityContextHolder.getContext().getAuthentication().getPrincipal();

if (principal instanceof UserDetails) {
  String username = ((UserDetails)principal).getUsername();
} else {
  String username = principal.toString();
}

The key here is that Spring and Spring Security do all sorts of great stuff for you like Session Fixation Prevention. These things assume that you're using the Spring framework as it was designed to be used. So, in your servlet, make it context aware and access the session like the above example.

If you just need to stash some data in the session scope, try creating some session scoped bean like this example and let autowire do its magic. :)

i made my own utils. it is handy. :)

package samples.utils;

import java.util.Arrays;
import java.util.Collection;
import java.util.Locale;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import javax.sql.DataSource;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.NoUniqueBeanDefinitionException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.MessageSource;
import org.springframework.core.convert.ConversionService;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.io.support.ResourcePatternResolver;
import org.springframework.ui.context.Theme;
import org.springframework.util.ClassUtils;
import org.springframework.web.context.request.RequestContextHolder;
import org.springframework.web.context.request.ServletRequestAttributes;
import org.springframework.web.context.support.WebApplicationContextUtils;
import org.springframework.web.servlet.LocaleResolver;
import org.springframework.web.servlet.ThemeResolver;
import org.springframework.web.servlet.support.RequestContextUtils;


/**
 * SpringMVC通用工具
 * 
 * @author 应卓([email protected])
 *
 */
public final class WebContextHolder {

    private static final Logger LOGGER = LoggerFactory.getLogger(WebContextHolder.class);

    private static WebContextHolder INSTANCE = new WebContextHolder();

    public WebContextHolder get() {
        return INSTANCE;
    }

    private WebContextHolder() {
        super();
    }

    // --------------------------------------------------------------------------------------------------------------

    public HttpServletRequest getRequest() {
        ServletRequestAttributes attributes = (ServletRequestAttributes) RequestContextHolder.currentRequestAttributes();
        return attributes.getRequest();
    }

    public HttpSession getSession() {
        return getSession(true);
    }

    public HttpSession getSession(boolean create) {
        return getRequest().getSession(create);
    }

    public String getSessionId() {
        return getSession().getId();
    }

    public ServletContext getServletContext() {
        return getSession().getServletContext();    // servlet2.3
    }

    public Locale getLocale() {
        return RequestContextUtils.getLocale(getRequest());
    }

    public Theme getTheme() {
        return RequestContextUtils.getTheme(getRequest());
    }

    public ApplicationContext getApplicationContext() {
        return WebApplicationContextUtils.getWebApplicationContext(getServletContext());
    }

    public ApplicationEventPublisher getApplicationEventPublisher() {
        return (ApplicationEventPublisher) getApplicationContext();
    }

    public LocaleResolver getLocaleResolver() {
        return RequestContextUtils.getLocaleResolver(getRequest());
    }

    public ThemeResolver getThemeResolver() {
        return RequestContextUtils.getThemeResolver(getRequest());
    }

    public ResourceLoader getResourceLoader() {
        return (ResourceLoader) getApplicationContext();
    }

    public ResourcePatternResolver getResourcePatternResolver() {
        return (ResourcePatternResolver) getApplicationContext();
    }

    public MessageSource getMessageSource() {
        return (MessageSource) getApplicationContext();
    }

    public ConversionService getConversionService() {
        return getBeanFromApplicationContext(ConversionService.class);
    }

    public DataSource getDataSource() {
        return getBeanFromApplicationContext(DataSource.class);
    }

    public Collection<String> getActiveProfiles() {
        return Arrays.asList(getApplicationContext().getEnvironment().getActiveProfiles());
    }

    public ClassLoader getBeanClassLoader() {
        return ClassUtils.getDefaultClassLoader();
    }

    private <T> T getBeanFromApplicationContext(Class<T> requiredType) {
        try {
            return getApplicationContext().getBean(requiredType);
        } catch (NoUniqueBeanDefinitionException e) {
            LOGGER.error(e.getMessage(), e);
            throw e;
        } catch (NoSuchBeanDefinitionException e) {
            LOGGER.warn(e.getMessage());
            return null;
        }
    }

}

Your friend here is org.springframework.web.context.request.RequestContextHolder

// example usage
public static HttpSession session() {
    ServletRequestAttributes attr = (ServletRequestAttributes) RequestContextHolder.currentRequestAttributes();
    return attr.getRequest().getSession(true); // true == allow create
}

This will be populated by the standard spring mvc dispatch servlet, but if you are using a different web framework you have add org.springframework.web.filter.RequestContextFilter as a filter in your web.xml to manage the holder.

EDIT: just as a side issue what are you actually trying to do, I'm not sure you should need access to the HttpSession in the retieveUser method of a UserDetailsService. Spring security will put the UserDetails object in the session for you any how. It can be retrieved by accessing the SecurityContextHolder:

public static UserDetails currentUserDetails(){
    SecurityContext securityContext = SecurityContextHolder.getContext();
    Authentication authentication = securityContext.getAuthentication();
    if (authentication != null) {
        Object principal = authentication.getPrincipal();
        return principal instanceof UserDetails ? (UserDetails) principal : null;
    }
    return null;
}