二、Servlet中的注释
Servlet 3.0中的重大革新之一是支持注释。通过使用注释定义Servlet和过滤器，就无需在web部署描述符(web.xml)中建立Servlet/过滤器条目了。
@WebServlet
为了在web应用中定义Servlet组件，我们可以使用@WebServlet。我们可以将其用于继承类javax.servlet.http.HttpServlet的类。注释@WebServlet具有许多属性。例如name、urlPatterns和initParams，我们可以通过它们来定义Servlet的行为。对于url模式，我们必须规定注释本身，或者规定注释的属性。
我们可以利用@WebServlet定义一个简单的Servlet，如下所示：
public class GetQuoteServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
PrintWriter out = response.getWriter();
try {
String symbol = request.getParameter("symbol");
out.println("<h1>Stock Price is</h1>" + StockQuoteBean.getPrice(symbol);
} finally {
out.close();
}
}
}
public class StockQuoteBean {
private StockQuoteServiceEntity serviceEntity = new StockQuoteServiceEntity();
public double getPrice(String symbol) {
if(symbol !=null ) {
return serviceEntity.getPrice(symbol);
} else {
return 0.0;
}
}
}
我们可以修改这个Servlet，让它使用注释属性处理来自多个url的请求。
public class GetQuoteServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
PrintWriter out = response.getWriter();
try {
String symbol = request.getParameter("symbol");
out.println("<h1>Stock Price is</h1>" + StockQuoteBean.getPrice(symbol);
} finally {
out.close();
}
}
}
@WebFilter
我们可以使用注释@WebFilter来定义过滤器。这个注释还具有可选参数。我们可以在任何实现了javax.servlet.Filter接口的类上使用@WebFilter。类似于@WebServlet注释，我们也必须为这个注释指定url模式。
public class AuthenticateFilter implements Filter {
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
String username = ((HttpServletRequest) request).getParameter("uname");
String password = ((HttpServletRequest) request).getParameter("password");
if (username == null || password == null) {
((HttpServletResponse) response).sendRedirect("index.jsp"); }
if (username.equals("admin") && password.equals("admin")) {
chain.doFilter(request, response); }
else {
((HttpServletResponse) response).sendRedirect("index.jsp"); }
}
public void destroy() {
}
public void init(FilterConfig filterConfig) {
}
}
@WebInitParam
我们可以使用注释@WebInitParam把init参数指定为Servlet或者过滤器。另外，我们还可以使用注释@WebFilter和@WebServlet 的initParam属性来规定init参数。
@WebInitParam(name = "default_market", value = "NASDAQ")
public class GetQuoteServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
try {
String market = getInitParameter("default_market");
String symbol = request.getParameter("symbol");
out.println("<h1>Stock Price in " + market + " is</h1>" + StockQuoteBean.getPrice(symbol, market));
} finally {
out.close();
}
}
}
下面是一个将注释@WebInitParam用作@WebServlet和@WebFilter initParams 属性的一部分的例子：
urlPatterns = {"/getquote"},
initParams={@WebInitParam(name="default_market", value="NASDAQ")}
)
public class GetQuoteServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
response.setContentType("text/html;charset=UTF-8");
PrintWriter out = response.getWriter();
try {
String market = getInitParameter("default_market");
String symbol = request.getParameter("symbol");
out.println("<h1>Stock Price in " + market + " is</h1>" + StockQuoteBean.getPrice(symbol, market));
} finally {
out.close();
}
}
}
@WebListener
我们可以将注释@WebListener用于充当给定web应用上下文中各种web应用事件的监听器的类。我们可以使用@WebListener来标注一个实现ServletContextListener、ServletContextAttributeListener、ServletRequestListener、ServletRequestAttributeListener、HttpSessionListener和HttpSessionAttributeListener的类。下面是一个使用ServletContextListener的例子：
public class QuoteServletContextListener implements ServletContextListener {
public void contextInitialized(ServletContextEvent sce) {
ServletContext context = sce.getServletContext();
context.setInitParameter(“default_market”, “NASDAQ”);
}
public void contextDestroyed(ServletContextEvent sce) {
}
}
@MultipartConfig
使用注释@MultipartConfig可以为Servlet指定多部分组成的MIME类型请求。MIME附件是从该请求对象中读取的。
