Copyright©Zi10ng
Jsp和Servlet是學習JavaEE繞不過的彎，那麼為什麼Jsp就是Servlet呢？
原因
平常在測試代碼的時候，我們可能在servlet的派生類中寫上這麼一行代碼：
這樣字元串中的html語句就會通過servlet輸出到浏覽器的頁面上。
早期的程式員非常苦逼，他們不像我們做測試輸出這樣一兩行代碼，而是需要把整個html頁面都通過servlet輸出到浏覽器中，這無疑加重了程式員的負擔。
是以此時Jsp橫空出世，把需要展現成html的頁面全部封裝起來，減輕我們的負擔。
證據
不妨打開
CATALINA_BASE
的目錄，我的是
C:\Users\xxx\.IntelliJIdea2018.2\system\tomcat\Tomcat_9_0_17_Servlet4
，然後找到
work\Catalina\localhost\heima_servlet\org\apache\jsp
，就會發現有兩個檔案，一個叫
index_jsp.java
，另外一個是
index_jsp.class
對比一下index.jsp和index_jsp.java:
package org.apache.jsp;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;
public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase
implements org.apache.jasper.runtime.JspSourceDependent,
org.apache.jasper.runtime.JspSourceImports {
private static final javax.servlet.jsp.JspFactory _jspxFactory =
javax.servlet.jsp.JspFactory.getDefaultFactory();
private static java.util.Map<java.lang.String,java.lang.Long> _jspx_dependants;
private static final java.util.Set<java.lang.String> _jspx_imports_packages;
private static final java.util.Set<java.lang.String> _jspx_imports_classes;
static {
_jspx_imports_packages = new java.util.HashSet<>();
_jspx_imports_packages.add("javax.servlet");
_jspx_imports_packages.add("javax.servlet.http");
_jspx_imports_packages.add("javax.servlet.jsp");
_jspx_imports_classes = null;
}
private volatile javax.el.ExpressionFactory _el_expressionfactory;
private volatile org.apache.tomcat.InstanceManager _jsp_instancemanager;
public java.util.Map<java.lang.String,java.lang.Long> getDependants() {
return _jspx_dependants;
}
public java.util.Set<java.lang.String> getPackageImports() {
return _jspx_imports_packages;
}
public java.util.Set<java.lang.String> getClassImports() {
return _jspx_imports_classes;
}
public javax.el.ExpressionFactory _jsp_getExpressionFactory() {
if (_el_expressionfactory == null) {
synchronized (this) {
if (_el_expressionfactory == null) {
_el_expressionfactory = _jspxFactory.getJspApplicationContext(getServletConfig().getServletContext()).getExpressionFactory();
}
}
}
return _el_expressionfactory;
}
public org.apache.tomcat.InstanceManager _jsp_getInstanceManager() {
if (_jsp_instancemanager == null) {
synchronized (this) {
if (_jsp_instancemanager == null) {
_jsp_instancemanager = org.apache.jasper.runtime.InstanceManagerFactory.getInstanceManager(getServletConfig());
}
}
}
return _jsp_instancemanager;
}
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 {
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 隻允許 GET、POST 或 HEAD。Jasper 還允許 OPTIONS");
return;
}
}
final javax.servlet.jsp.PageContext pageContext;
javax.servlet.http.HttpSession session = null;
final javax.servlet.ServletContext application;
final javax.servlet.ServletConfig config;
javax.servlet.jsp.JspWriter out = null;
final java.lang.Object page = this;
javax.servlet.jsp.JspWriter _jspx_out = null;
javax.servlet.jsp.PageContext _jspx_page_context = null;
try {
response.setContentType("text/html;charset=UTF-8");
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("\n");
out.write("\n");
out.write("<html>\n");
out.write(" <head>\n");
out.write(" <title>$Title$</title>\n");
out.write(" </head>\n");
out.write(" <body>\n");
out.write(" <form action = \"/heima_servlet/demo3\" method=\"post\">\n");
out.write(" <input type=\"text\" name = \"username\" title=\"username\">\n");
out.write("\n");
out.write(" <input type=\"checkbox\" name=\"hobby\" value=\"game\">game\n");
out.write(" <input type=\"checkbox\" name=\"hobby\" value=\"study\">study\n");
out.write("\n");
out.write(" </form>\n");
out.write(" $END$\n");
out.write(" </body>\n");
out.write("</html>\n");
} catch (java.lang.Throwable t) {
if (!(t instanceof javax.servlet.jsp.SkipPageException)){
out = _jspx_out;
if (out != null && out.getBufferSize() != 0)
try {
if (response.isCommitted()) {
out.flush();
} else {
out.clearBuffer();
}
} catch (java.io.IOException e) {}
if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
else throw new ServletException(t);
}
} finally {
_jspxFactory.releasePageContext(_jspx_page_context);
}
}
}
index.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>$Title$</title>
</head>
<body>
<form action = "/heima_servlet/demo3" method="post">
<input type="text" name = "username" title="username">
<input type="checkbox" name="hobby" value="game">game
<input type="checkbox" name="hobby" value="study">study
</form>
$END$
</body>
</html>
更深一步
既然說jsp是servlet，那麼jsp的類也一定會繼承servlet類，并且一定會有service方法！
我們發現，jsp繼承了
org.apache.jasper.runtime.HttpJspBase
，查閱這個類的源碼可以發現這樣一行代碼：
public abstract class HttpJspBase extends HttpServlet implements HttpJspPage
由此我們得知，jsp通過繼承httpServlet，然後繼承了servlet類
這樣，我們就知道為什麼：
<% bala %> 在jsp編譯後的service方法中
<%! bala %> 在jsp編譯後的成員位置
<%= bala %> 會輸出到頁面上
這也是為什麼jsp内置對象的由來
說句題外話，現如今，Java的生态庫異常強大，封裝的架構也非常之多。前輩們一層層的抽象封裝使得學習成本變得越來越低，是以我們身為程式員不應該隻注重于CURD，而是多去思考面紗後面的原因，将會變得異常重要！
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