读源码学Servlet(5)GenericServlet中的service 方法
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2022-05-24 21:03:05
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观察GenericServlet源码中关于service()方法的实现:
//实现父类的service(ServletRequest req,ServletResponse res)方法
//通过参数的向下转型,然后调用重载的service(HttpservletRequest,HttpServletResponse)方法
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequest request;
HttpServletResponse response;
try {
request = (HttpServletRequest) req; //向下转型
response = (HttpServletResponse) res; //向下转型
} catch (ClassCastException e) {
throw new ServletException("non-HTTP request or response");
}
service(request, response); //调用重载的service(HttpServelertRequest,HttpServletResponse)方法
}
//重载的service(HttpServletRequest req, HttpServletResponse resp)方法
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
if (ifModifiedSince < (lastModified / 1000 * 1000)) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}
在抽象类中GenericServlet中service()是一个抽象方法,但在HttpServlet中对这个方法进行了实现。
servlet接口中定义的service()方法中的两个参数分别是servletRequest 和 ServletResponse 这两个类型。当前的http请求,如果需要在这个service()方法内部使用http消息特有的功能,也就是要调用HttpServletRequest 和HttpServletResponse来中定义的方法时,需要将请求和响应对象进行一个类型的转换,所以,在GenericServlet中,使用了两个方法来共同完成这个工作。
实现父类GenericServlet中的service(ServltRequest req,ServeltResponse res)抽象方法
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException
{
HttpServletRequest request;
HttpServletResponse response;
try {
request = (HttpServletRequest) req; //向下转型
response = (HttpServletResponse) res; //向下转型
} catch (ClassCastException e) {
throw new ServletException("non-HTTP request or response");
}
service(request, response); //这里是调用重载的service(HttpServelertRequest,HttpServletResponse)方法
}
service(HttpServelertRequest,HttpServletResponse)方法:
//重载的service(HttpServletRequest req, HttpServletResponse resp)方法 ,注意参数类型
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException
{
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);
} else {
long ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
if (ifModifiedSince < (lastModified / 1000 * 1000)) {
// If the servlet mod time is later, call doGet()
// Round down to the nearest second for a proper compare
// A ifModifiedSince of -1 will always be less
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
String errMsg = lStrings.getString("http.method_not_implemented");
Object[] errArgs = new Object[1];
errArgs[0] = method;
errMsg = MessageFormat.format(errMsg, errArgs);
resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
}
}
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