ShiroFilterFactoryBean分析

创建核心Filter

同其他框架一样，都有个切入点，这个核心Filter就是拦截所有请求的。

通过web.xml中配置的Filer进入，执行init方法获取这个instance，调用下面的createInstance方法创建核心Filter:

protected AbstractShiroFilter createInstance() throws Exception {

    log.debug("Creating Shiro Filter instance.");

    SecurityManager securityManager = getSecurityManager();
    if (securityManager == null) {
        String msg = "SecurityManager property must be set.";
        throw new BeanInitializationException(msg);
    }

    if (!(securityManager instanceof WebSecurityManager)) {
        String msg = "The security manager does not implement the WebSecurityManager interface.";
        throw new BeanInitializationException(msg);
    }

    FilterChainManager manager = createFilterChainManager();

    //Expose the constructed FilterChainManager by first wrapping it in a
    // FilterChainResolver implementation. The AbstractShiroFilter implementations
    // do not know about FilterChainManagers - only resolvers:
    PathMatchingFilterChainResolver chainResolver = new PathMatchingFilterChainResolver();
    chainResolver.setFilterChainManager(manager);

    //Now create a concrete ShiroFilter instance and apply the acquired SecurityManager and built
    //FilterChainResolver.  It doesn't matter that the instance is an anonymous inner class
    //here - we're just using it because it is a concrete AbstractShiroFilter instance that accepts
    //injection of the SecurityManager and FilterChainResolver:
    return new SpringShiroFilter((WebSecurityManager) securityManager, chainResolver);
}

createFilterChainManager方法比较重要，包含以下必要操作：

1. DefaultFilterChainManager对象的创建

2. defaultFilters的获取和相关url的填充

3. 自定义filters的获取和相关url的填充

4. 获取FilterChainDefinitionMap (这个就是配置文件中的filterChainDefinitions的映射关系)

5. 对url和权限的映射关系作处理

默认的DefaultFilter枚举中包含以下filter类：

anon(AnonymousFilter.class),
authc(FormAuthenticationFilter.class),
authcBasic(BasicHttpAuthenticationFilter.class),
logout(LogoutFilter.class),
noSessionCreation(NoSessionCreationFilter.class),
perms(PermissionsAuthorizationFilter.class),
port(PortFilter.class),
rest(HttpMethodPermissionFilter.class),
roles(RolesAuthorizationFilter.class),
ssl(SslFilter.class),
user(UserFilter.class);

执行doFilter方法

看下内部类 SpringShiroFilter：

private static final class SpringShiroFilter extends AbstractShiroFilter {

    protected SpringShiroFilter(WebSecurityManager webSecurityManager, FilterChainResolver resolver) {
        super();
        if (webSecurityManager == null) {
            throw new IllegalArgumentException("WebSecurityManager property cannot be null.");
        }
        setSecurityManager(webSecurityManager);
        if (resolver != null) {
            setFilterChainResolver(resolver);
        }
    }
}

在org.apache.shiro.web.servlet.OncePerRequestFilter中实现了doFilter方法：

public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)
        throws ServletException, IOException {
    String alreadyFilteredAttributeName = getAlreadyFilteredAttributeName();
    if ( request.getAttribute(alreadyFilteredAttributeName) != null ) {
        log.trace("Filter '{}' already executed.  Proceeding without invoking this filter.", getName());
        filterChain.doFilter(request, response);
    } else //noinspection deprecation
        if (/* added in 1.2: */ !isEnabled(request, response) ||
            /* retain backwards compatibility: */ shouldNotFilter(request) ) {
        log.debug("Filter '{}' is not enabled for the current request.  Proceeding without invoking this filter.",
                getName());
        filterChain.doFilter(request, response);
    } else {
        // Do invoke this filter...
        log.trace("Filter '{}' not yet executed.  Executing now.", getName());
        request.setAttribute(alreadyFilteredAttributeName, Boolean.TRUE);

        try {
            doFilterInternal(request, response, filterChain);
        } finally {
            // Once the request has finished, we're done and we don't
            // need to mark as 'already filtered' any more.
            request.removeAttribute(alreadyFilteredAttributeName);
        }
    }
}

三种情况，分为两条路线：
1. filterChain.doFilter(request, response); 直接放行
2. doFilterInternal(request, response, filterChain); 走子类实现

在org.apache.shiro.web.servlet.AbstractShiroFilter实现doFilterInternal：

protected void doFilterInternal(ServletRequest servletRequest, ServletResponse servletResponse, final FilterChain chain)
        throws ServletException, IOException {

    Throwable t = null;

    try {
        final ServletRequest request = prepareServletRequest(servletRequest, servletResponse, chain);
        final ServletResponse response = prepareServletResponse(request, servletResponse, chain);

        final Subject subject = createSubject(request, response);

        //noinspection unchecked
        subject.execute(new Callable() {
            public Object call() throws Exception {
                updateSessionLastAccessTime(request, response);
                executeChain(request, response, chain);
                return null;
            }
        });
    } catch (ExecutionException ex) {
        t = ex.getCause();
    } catch (Throwable throwable) {
        t = throwable;
    }

    if (t != null) {
        if (t instanceof ServletException) {
            throw (ServletException) t;
        }
        if (t instanceof IOException) {
            throw (IOException) t;
        }
        //otherwise it's not one of the two exceptions expected by the filter method signature - wrap it in one:
        String msg = "Filtered request failed.";
        throw new ServletException(msg, t);
    }
}

将HttpServletRequest和HttpServletResponse包装成shiro自己的ShiroHttpServletRequest和ShiroHttpServletResponse。

利用包装后的对象创建Subject。

然后是执行subject的execute的方法。

protected void executeChain(ServletRequest request, ServletResponse response, FilterChain origChain)
        throws IOException, ServletException {
    FilterChain chain = getExecutionChain(request, response, origChain);
    chain.doFilter(request, response);
}

获取的FilterChain是一个代理的ProxiedFilterChain，持有自己的Filter集合，重写doFilter方法：取自己持有的Filter集合，一个个执行其doFilter方法，当所有的Filter都执行完后，再执行servlet的FilterChain。

后面的过程就是一个原始的FilterChain的执行过程。

可以想象，原生的Filter链似乎也是这样一个执行过程，遍历web.xml中配置的所有Filter。

整个过程就是代理模式的运用。