Spring Security3源码分析-SecurityContextPersistenceFilter分析

通过观察Filter的名字，就能大概猜出来这个过滤器的作用，是的，持久化SecurityContext实例。这个过滤器位置是；
org.springframework.security.web.context.SecurityContextPersistenceFilter
废话不说，看源码

    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        if (request.getAttribute(FILTER_APPLIED) != null) {
            // ensure that filter is only applied once per request
            chain.doFilter(request, response);
            return;
        }

        final boolean debug = logger.isDebugEnabled();

        request.setAttribute(FILTER_APPLIED, Boolean.TRUE);

        if (forceEagerSessionCreation) {
            HttpSession session = request.getSession();

            if (debug && session.isNew()) {
                logger.debug("Eagerly created session: " + session.getId());
            }
        }
        //将request、response对象交给HttpRequestResponseHolder维持
        HttpRequestResponseHolder holder = new HttpRequestResponseHolder(request, response);
        //通过SecurityContextRepository接口的实现类装载SecurityContext实例
        //HttpSessionSecurityContextRepository将产生SecurityContext实例的任务交给SecurityContextHolder.createEmptyContext()完成
        //SecurityContextHolder再根据策略模式的不同，
        //把任务再交给相应策略类完成SecurityContext的创建
        //如果没有配置策略名称，则默认为
        //ThreadLocalSecurityContextHolderStrategy，
        //该类直接通过new SecurityContextImpl()创建实例
        SecurityContext contextBeforeChainExecution = repo.loadContext(holder);

        try {
        	//将产生的SecurityContext再通过SecurityContextHolder->
          //ThreadLocalSecurityContextHolderStrategy设置到ThreadLocal中
            SecurityContextHolder.setContext(contextBeforeChainExecution);
            //继续把请求流向下一个过滤器执行
            chain.doFilter(holder.getRequest(), holder.getResponse());

        } finally {
        	//先从SecurityContextHolder获取SecurityContext实例
            SecurityContext contextAfterChainExecution = SecurityContextHolder.getContext();
            // Crucial removal of SecurityContextHolder contents - do this before anything else.
            //再把SecurityContext实例从SecurityContextHolder中清空
            SecurityContextHolder.clearContext();
            //将SecurityContext实例持久化到session中
            repo.saveContext(contextAfterChainExecution, holder.getRequest(), holder.getResponse());
            request.removeAttribute(FILTER_APPLIED);
            if (debug) {
                logger.debug("SecurityContextHolder now cleared, as request processing completed");
            }
        }
    }

通过源码中的注释，应该可以看出来，这个Filter的作用主要是创建一个空的SecurityContext（如果session中没有SecurityContext实例），然后持久化到session中。
接下来看看repo.loadContext(holder);代码：

    public SecurityContext loadContext(HttpRequestResponseHolder requestResponseHolder) {
        HttpServletRequest request = requestResponseHolder.getRequest();
        HttpServletResponse response = requestResponseHolder.getResponse();
        HttpSession httpSession = request.getSession(false);
        //从session中获取SecurityContext
         SecurityContext context = readSecurityContextFromSession(httpSession);
        //如果获取不到SecurityContext，新建一个空的SecurityContext实例
        if (context == null) {
            if (logger.isDebugEnabled()) {
                logger.debug("No SecurityContext was available from the HttpSession: " + httpSession +". " +
                        "A new one will be created.");
            }
            context = generateNewContext();

        }
        //这里需要注意一下，response装饰器类重新包装了response
        requestResponseHolder.setResponse(new SaveToSessionResponseWrapper(response, request,
                httpSession != null, context.hashCode()));

        return context;
    }

进一步分析generateNewContext方法

    SecurityContext generateNewContext() {
        SecurityContext context = null;
        //创建SecurityContext实例并返回
        if (securityContextClass == null) {
            context = SecurityContextHolder.createEmptyContext();

            return context;
        }

        try {
            context = securityContextClass.newInstance();
        } catch (Exception e) {
            ReflectionUtils.handleReflectionException(e);
        }
        return context;
    }

实际上，SecurityContextHolder类也是把创建SecurityContext任务交给具体的SecurityContextHolderStrategy实现类处理，SecurityContextHolder类有一个静态初始化过程

    static {
        initialize();
    }

…………
    private static void initialize() {
        if ((strategyName == null) || "".equals(strategyName)) {
            // Set default
            strategyName = MODE_THREADLOCAL;
        }
        //默认的SecurityContextHolderStrategy实现类为
         //ThreadLocalSecurityContextHolderStrategy
        if (strategyName.equals(MODE_THREADLOCAL)) {
            strategy = new ThreadLocalSecurityContextHolderStrategy();
        } else if (strategyName.equals(MODE_INHERITABLETHREADLOCAL)) {
            strategy = new InheritableThreadLocalSecurityContextHolderStrategy();
        } else if (strategyName.equals(MODE_GLOBAL)) {
            strategy = new GlobalSecurityContextHolderStrategy();
        } else {
            // Try to load a custom strategy
            try {
                Class<?> clazz = Class.forName(strategyName);
                Constructor<?> customStrategy = clazz.getConstructor();
                strategy = (SecurityContextHolderStrategy) customStrategy.newInstance();
            } catch (Exception ex) {
                ReflectionUtils.handleReflectionException(ex);
            }
        }

        initializeCount++;
    }

现在来看ThreadLocalSecurityContextHolderStrategy源码

final class ThreadLocalSecurityContextHolderStrategy implements SecurityContextHolderStrategy {
    //~ Static fields/initializers =====================================================================================

    private static final ThreadLocal<SecurityContext> contextHolder = new ThreadLocal<SecurityContext>();

    //~ Methods ========================================================================================================

    public void clearContext() {
        contextHolder.set(null);
    }

    public SecurityContext getContext() {
        SecurityContext ctx = contextHolder.get();

        if (ctx == null) {
            ctx = createEmptyContext();
            contextHolder.set(ctx);
        }

        return ctx;
    }

    public void setContext(SecurityContext context) {
        Assert.notNull(context, "Only non-null SecurityContext instances are permitted");
        contextHolder.set(context);
    }
    //直接new一个SecurityContextImpl对象，
     //SecurityContextImpl类实现SecurityContext接口
    public SecurityContext createEmptyContext() {
        return new SecurityContextImpl();
    }
}

分析到这里，整个过程也清楚了。不过在filter原路返回时，还需要保存这个SecurityContext实例到session中，并且通过SecurityContextHolder将ThreadLocalSecurityContextHolderStrategy中ThreadLocal维持的SecurityContext实例清空。

          //将SecurityContext实例持久化到session中
            repo.saveContext(contextAfterChainExecution, holder.getRequest(), holder.getResponse());

    public void saveContext(SecurityContext context, HttpServletRequest request, HttpServletResponse response) {
        //由于之前response装饰器类SaveToSessionResponseWrapper
        //重新装饰了response
        SaveContextOnUpdateOrErrorResponseWrapper responseWrapper = (SaveContextOnUpdateOrErrorResponseWrapper)response;
        // saveContext() might already be called by the response wrapper
        // if something in the chain called sendError() or sendRedirect(). This ensures we only call it
        // once per request.
        if (!responseWrapper.isContextSaved() ) {
            //SaveToSessionResponseWrapper保存SecurityContext实例
            responseWrapper.saveContext(context);
        }
    }

SaveToSessionResponseWrapper的saveContext方法源码：

        protected void saveContext(SecurityContext context) {
            // See SEC-776
            if (authenticationTrustResolver.isAnonymous(context.getAuthentication())) {
                if (logger.isDebugEnabled()) {
                    logger.debug("SecurityContext contents are anonymous - context will not be stored in HttpSession. ");
                }
                return;
            }

            HttpSession httpSession = request.getSession(false);

            if (httpSession == null) {
                httpSession = createNewSessionIfAllowed(context);
            }

            // If HttpSession exists, store current SecurityContextHolder contents but only if
            // the SecurityContext has actually changed (see JIRA SEC-37)
            if (httpSession != null && context.hashCode() != contextHashBeforeChainExecution) {
                //保存SecurityContext到session中
                httpSession.setAttribute(SPRING_SECURITY_CONTEXT_KEY, context);

                if (logger.isDebugEnabled()) {
                    logger.debug("SecurityContext stored to HttpSession: '" + context + "'");
                }
            }
        }