狂神说Java Web学习笔记_JSP

佚名 / 2024-02-03 / 原文

什么是JSP

JSP(Java Server Pages)：Java服务器端页面，和Servlet一样，用于动态web技术！

写JSP就像在写HTML。它们之间不同的是，JSP页面里可以嵌入Java代码，提供动态数据。

JSP原理

IDEA使用tomcat服务器，会在tomcat中产生一个work目录；

C:\Users\winner\AppData\Local\JetBrains\IntelliJIdea2022.1\tomcat\e93bc8ad-ed5a-4d64-86d6-0a6e3ce05bf1\work\Catalina\localhost\ROOT\org\apache\jsp

从这儿可以发现JSP文件被转换成为了Java程序。由此得知：浏览器向服务器发送请求，无论是访问什么资源，实际上都是在访问Servlet。JSP的本质上就是一个Servlet。

主要方法

public void _jspInit() {
}

public void _jspDestroy() {
}

public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
    throws java.io.IOException, javax.servlet.ServletException {
}

_jspService方法细节捕捉

判断请求方式

if (!javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
    final java.lang.String _jspx_method = request.getMethod();
    if ("OPTIONS".equals(_jspx_method)) {
        response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
        return;
    }
    if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method)) {
        response.setHeader("Allow","GET, HEAD, POST, OPTIONS");
        response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSP 只允许 GET、POST 或 HEAD。Jasper 还允许 OPTIONS");
        return;
    }
}

内置对象

final javax.servlet.jsp.PageContext pageContext;// 页面上下文
javax.servlet.http.HttpSession session = null;// session
final javax.servlet.ServletContext application;// applicationContext
final javax.servlet.ServletConfig config;// config
javax.servlet.jsp.JspWriter out = null;// out
final java.lang.Object page = this;// 当前页面
javax.servlet.jsp.JspWriter _jspx_out = null;
javax.servlet.jsp.PageContext _jspx_page_context = null;
HttpServletRequest request;// 请求
HttpServletResponse response;// 响应

JSP文件里面的HTML代码会被转换为out.write("\n")这种格式之后再输出

<html>
<body>
<h2>Hello World!</h2>
</body>
</html>

try {
    response.setContentType("text/html");
    pageContext = _jspxFactory.getPageContext(this, request, response,
                                              null, true, 8192, true);
    _jspx_page_context = pageContext;
    application = pageContext.getServletContext();
    config = pageContext.getServletConfig();
    session = pageContext.getSession();
    out = pageContext.getOut();
    _jspx_out = out;

    out.write("<html>\n");
    out.write("<body>\n");
    out.write("<h2>Hello World!</h2>\n");
    out.write("</body>\n");
    out.write("</html>\n");
}

JSP文件里面的Java代码不会被转换，会直接输出

<html>
<body>
<h2>Hello World!</h2>

<%
    String feeling = "happy~";
%>

i am <%=feeling%>

</body>
</html>

out.write("<html>\n");
out.write("<body>\n");
out.write("<h2>Hello World!</h2>\n");
out.write("\n");

String feeling = "happy~";//原封不动输出

out.write("\n");
out.write("\n");
out.write("i am ");
out.print(feeling);//原封不动输出
out.write("\n");
out.write("\n");
out.write("</body>\n");
out.write("</html>\n");