How does jsp work?
JSP is an extension of Servlet. Before JSP appeared, Servlet technology had already appeared. Servlet uses "output stream" to dynamically generate HTML pages. Including every HTML tag and all the content that appears in the HTML page. To put it simply, it uses the output stream of the Servlet to generate an HTML page from the HTML tag. Over time, Servlets contained a large number of HTML tags, a large amount of static text and formats, etc., resulting in very low development efficiency of Servlets.
At this time, JSP was born , JSP makes up for this situation. JSP embeds JAVA code in standard THML pages so that the static parts do not need to be controlled by JAVA programs. Only those page contents that need to be read from the database or need to be dynamically generated can be controlled by JAVA scripts. .
JSP is mainly composed of two parts:
(1) Static part: standard HTML tags, static page content, these contents are the same as static HTML pages.
(2) Dynamic part: Content controlled by Java program, which is dynamically generated by Java program.
The following is the simplest piece of jsp code:
<%@ page contentType="text/html; charset=GBK" language="java" errorPage="" %> <html> <head> <title>欢迎</title> </head> <body> 欢迎学习Java JSP！现在时间是:<%out.println(new java.util.Date());%> </body> </html>
The bold code on the page is placed between <% and %>, indicating that these are Java scripts, not static Content, in this way, Java code can be embedded in the HTML page, which becomes a dynamic JSP page. From a superficial analysis, JSP pages no longer require Java classes and seem to be completely divorced from the object-oriented characteristics of Java. However, in fact, JSP is a special form of Servlet. Each JSP page is an instance of Servlet. .
Working principle
(1) When a user accesses a JSP page, a Servlet container (Tomcat) issues a request;
(2) If it is the first time to request the page, or the page has been changed, the servlet container must first convert the JSP page (assumed to be test.jsp) into Servlet code (test.java), and then convert it into (test. class file); because this process (compilation) will take a certain amount of time, the access time is a bit long when accessing for the first time or when the jsp file is modified;
(3) The JSP container is responsible for calling the JSP converted from servlets, these servlets are responsible for providing services to corresponding user requests (for example, the client sends a form and requires servlet: formprocessor.java to process it, the container will create a thread and call formprocessor.java to process the request); if the user has multiple requests, The container will create multiple threads to handle multiple requests;
(4) The container executes the bytecode file (including the called servlet: formprocessor.java bytes) and returns the results to the client; (The final way to return is to have a servlet output a file stream in html format)
Note:
The JSP page must be run in a JSP server, such as tomcat weblogic, jboss Wait. (By the way, apache and tomcat are two different things. Many people think that
apache is tomcat, which is actually a wrong understanding!~, tomcat is just a sub-project of apache, and apache is used as a WEB application server , and tomcat can be said to be a web container of JSP or Servlet);
Visitors to the JSP page do not need to install any client, or even a running environment that can run Java, because the JSP page is delivered to the client as a standard HTML page .
Related learning recommendations: java basic tutorial
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