過濾器

在web容器調(diào)用Servlet的service()方法之前，Servlet并不知道請求的到來，就有了一段“請求到來空白期間”；而在調(diào)用Servlet的service()之后，web容器真正對瀏覽器響應(yīng)之前，瀏覽器也并不知道真正的Servlet的響應(yīng)，就有了一段“響應(yīng)到達(dá)空白期間”。過濾器的作用就是在兩段空白期間對請求和響應(yīng)對象進(jìn)行操作

public interface Filter {
    void init(FilterConfig var1) throws ServletException;

    void doFilter(ServletRequest var1, ServletResponse var2, FilterChain var3) throws IOException, ServletException;

    void destroy();
}

過濾器也有自身的生命周期方法和Servlet極其的相似，每一個Filter都會有一個對應(yīng)的FilterConfig，定義獲取初始化參數(shù)的方法

public interface FilterConfig {
    String getFilterName();

    ServletContext getServletContext();

    String getInitParameter(String var1);

    Enumeration<String> getInitParameterNames();
}

真正做過濾處理的方法就是doFilter()，如果調(diào)用了FilterChain的doFilter()方法，就會運行下一個過濾器，如果沒有下一個過濾器，就調(diào)用請求目標(biāo)Servlet的service()方法；不過因為某種情況（用戶驗證不過關(guān)），就不會調(diào)用FilterChain的doFilter()，當(dāng)然之后請求也不會交給相應(yīng)的Servlet來處理了

public interface FilterChain {
    void doFilter(ServletRequest var1, ServletResponse var2) throws IOException, ServletException;
}

@WebFilter(filterName = "DIYFilter", urlPatterns = "/*",
           initParams = {
                @WebInitParam(name = "filter-key", value = "filter-val")
           },
           dispatcherTypes = {
                DispatcherType.FORWARD,
                DispatcherType.INCLUDE,
                DispatcherType.ERROR,
                DispatcherType.ASYNC,
                DispatcherType.REQUEST
           })
public class DIYFilter implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        // 獲取請求開始時間，用于計算一次請求到響應(yīng)的耗時情況
        long begin = System.currentTimeMillis();
        chain.doFilter(req, resp);
        // 響應(yīng)時間
        long end = System.currentTimeMillis();
        System.out.print("請求處理時間：" + (end - begin));
    }

    public void init(FilterConfig config) throws ServletException {

    }

}

過濾器可以定義初始化參數(shù)；一般過濾器過濾的請求都是由瀏覽器直接發(fā)出的，對于請求轉(zhuǎn)發(fā)的Servlet則需要配置DispatcherType，上訴配置信息也可以在web.xml中進(jìn)行配置：

<filter>
    <filter-name>DIYFilter</filter-name>
    <filter-class>DIYFilter</filter-class>
    <!-- 配置過濾器初始化參數(shù) -->
    <init-param>
        <param-name>filter-key</param-name>
        <param-value>filter-val</param-value>
    </init-param>
</filter>
<filter-mapping>
    <filter-name>DIYFilter</filter-name>
    <url-pattern>/*</url-pattern>
    <!-- 配置通過請求轉(zhuǎn)發(fā)等其他方式也會被過濾的dispatcher -->
    <dispatcher>FORWARD</dispatcher>
    <dispatcher>REQUEST</dispatcher>
    <dispatcher>ERROR</dispatcher>
    <dispatcher>INCLUDE</dispatcher>
    <dispatcher>ASYNC</dispatcher>
</filter-mapping>

如果有多個過濾器，則會根據(jù)在web.xml中出現(xiàn)的先后順序決定過濾器的運行順序

請求裝飾器HttpServletRequestWrapper

HttpServletRequest可以通過getParameter()獲取到客戶提交的請求參數(shù)，卻沒有setParameter()方法來修改請求參數(shù)，所幸有個HttpServletRequestWrapper來幫助我們來實現(xiàn)這一功能

public class HttpServletRequestWrapper extends ServletRequestWrapper implements HttpServletRequest {
    public HttpServletRequestWrapper(HttpServletRequest request) {
        super(request);
    }

    private HttpServletRequest _getHttpServletRequest() {
        return (HttpServletRequest)super.getRequest();
    }

    public String getAuthType() {
        return this._getHttpServletRequest().getAuthType();
    }

    public Cookie[] getCookies() {
        return this._getHttpServletRequest().getCookies();
    }

    public long getDateHeader(String name) {
        return this._getHttpServletRequest().getDateHeader(name);
    }

    public String getHeader(String name) {
        return this._getHttpServletRequest().getHeader(name);
    }

    public Enumeration<String> getHeaders(String name) {
        return this._getHttpServletRequest().getHeaders(name);
    }

    public Enumeration<String> getHeaderNames() {
        return this._getHttpServletRequest().getHeaderNames();
    }

    public int getIntHeader(String name) {
        return this._getHttpServletRequest().getIntHeader(name);
    }

    public String getMethod() {
        return this._getHttpServletRequest().getMethod();
    }

    public String getPathInfo() {
        return this._getHttpServletRequest().getPathInfo();
    }

    public String getPathTranslated() {
        return this._getHttpServletRequest().getPathTranslated();
    }

    public String getContextPath() {
        return this._getHttpServletRequest().getContextPath();
    }

    public String getQueryString() {
        return this._getHttpServletRequest().getQueryString();
    }

    public String getRemoteUser() {
        return this._getHttpServletRequest().getRemoteUser();
    }

    public boolean isUserInRole(String role) {
        return this._getHttpServletRequest().isUserInRole(role);
    }

    public Principal getUserPrincipal() {
        return this._getHttpServletRequest().getUserPrincipal();
    }

    public String getRequestedSessionId() {
        return this._getHttpServletRequest().getRequestedSessionId();
    }

    public String getRequestURI() {
        return this._getHttpServletRequest().getRequestURI();
    }

    public StringBuffer getRequestURL() {
        return this._getHttpServletRequest().getRequestURL();
    }

    public String getServletPath() {
        return this._getHttpServletRequest().getServletPath();
    }

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

    public HttpSession getSession() {
        return this._getHttpServletRequest().getSession();
    }

    public String changeSessionId() {
        return this._getHttpServletRequest().changeSessionId();
    }

    public boolean isRequestedSessionIdValid() {
        return this._getHttpServletRequest().isRequestedSessionIdValid();
    }

    public boolean isRequestedSessionIdFromCookie() {
        return this._getHttpServletRequest().isRequestedSessionIdFromCookie();
    }

    public boolean isRequestedSessionIdFromURL() {
        return this._getHttpServletRequest().isRequestedSessionIdFromURL();
    }

    /** @deprecated */
    public boolean isRequestedSessionIdFromUrl() {
        return this._getHttpServletRequest().isRequestedSessionIdFromUrl();
    }

    public boolean authenticate(HttpServletResponse response) throws IOException, ServletException {
        return this._getHttpServletRequest().authenticate(response);
    }

    public void login(String username, String password) throws ServletException {
        this._getHttpServletRequest().login(username, password);
    }

    public void logout() throws ServletException {
        this._getHttpServletRequest().logout();
    }

    public Collection<Part> getParts() throws IOException, ServletException {
        return this._getHttpServletRequest().getParts();
    }

    public Part getPart(String name) throws IOException, ServletException {
        return this._getHttpServletRequest().getPart(name);
    }

    public <T extends HttpUpgradeHandler> T upgrade(Class<T> httpUpgradeHandlerClass) throws IOException, ServletException {
        return this._getHttpServletRequest().upgrade(httpUpgradeHandlerClass);
    }
}

/**
 * HttpServletRequest包裝類，用于處理請求中的參數(shù)
 */
public class DIYRequestWrapper extends HttpServletRequestWrapper {

    public DIYRequestWrapper(HttpServletRequest request) {
        super(request);
    }

    public String getParameter(String name) {
        String oldPara = getRequest().getParameter(name);
        // 當(dāng)請求參數(shù)中包含><等字符，將其轉(zhuǎn)義為&gt;&lt；防止腳本注入
        String newPara = oldPara.replace("<","&lt;").replace(">","&gt;");
        return newPara;
    }

}

然后將該Wrapper類用于過濾器中，這樣子每次請求中的請求參數(shù)都能被Wrapper中定義的處理機制處理

@WebFilter("/*")
public class RequestWrapperFilter implements Filter {
    public void destroy() {
    }

    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws ServletException, IOException {
        // 在過濾器中，通過requestWrapper類來處理請求參數(shù)
        HttpServletRequest reqWrp = new DIYRequestWrapper((HttpServletRequest) req);
        HttpServletResponse response = (HttpServletResponse)resp;
        // 解決中文字符以 錛氱幇鍦 的亂碼格式
        response.setHeader("Content-type", "text/html;charset=UTF-8");
        // 解決中文字符以 ???? 的亂碼格式
        response.setCharacterEncoding("utf-8");
        chain.doFilter(reqWrp, response);
    }

    public void init(FilterConfig config) throws ServletException {

    }

}

響應(yīng)裝飾器HttpServletResponseWrapper

若要對瀏覽器進(jìn)行輸出響應(yīng)，必須通過getWriter()獲取到PrintWriter對象或者通過getOutputStream()取得ServletOutputStream對象，而響應(yīng)裝飾器的主要做法就是重新定義這兩個方法；不過在Servlet規(guī)范中，同一個請求期間，getWriter()和getOutputStream()只能擇一調(diào)用，否則拋出IllegalStateException，因此在響應(yīng)裝飾器中，也應(yīng)該遵循這個規(guī)范。
HttpServletResponseWrapper和Filter工作原理與HttpServletRequestWrapper和Filter工作原理一致
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