JSP的原理

　JSP其实就是Servlet，在运行时，JSP将由JSP编译器进行转换，将JSP转换为Java代码，由JSP所创建的类都实现了Servlet接口，所以JSP就是Servlet，它也有声明声明方法，其实请求该JSP页面实际上也是在调用该JSP转换过了的Servlet的service方法，我们到tomcat目录下查看对应的Servlet代码

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>第一个web程序</title>
  </head>
  <body>
    Hello,World!
  </body>
</html>

　　该JSP的名字是index.jsp，我们看到它对应的Servlet的名字是index_jsp.java，由tomcat生成的源码如下:

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/8.0.52
 * Generated at: 2018-10-09 11:42:02 UTC
 * Note: The last modified time of this file was set to
 *       the last modified time of the source file after
 *       generation to assist with modification tracking.
 */
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 {

final java.lang.String _jspx_method = request.getMethod();
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
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>第一个web程序</title>\n");
      out.write("  </head>\n");
      out.write("  <body>\n");
      out.write("    Hello,World!\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);
    }
  }
}

　　这个类继承了org.apache.jasper.runtime.HttpJspBase，而该抽象类继承了HTTPServlet，而且类中还有一个 _jspService方法，参数的类型是HttpRequest和HttpResponse，可以猜测在HttpJspBase类中，将在声明周期方法service中调用这个 _jspService方法，再来看方法的具体实现，方法首先获取了请求方式，如果请求方式不是GET或者POST或者HEAD，那么客户端将收到一个错误状态码，"JSPs only permit GET POST or HEAD"，可见JSP只允许GET、POST和HEAD的请求方式，我们一开始在JSP页面中写的HTML文本，都使用了JspWriter的write方法发送到客户端了，我们使用IDE中在JSP页面的HTML文本部分打一个断点，然后浏览器请求该jsp，我们发现断点被命中了，并且调试器的栈堆将显示代码停留在_jspService方法中，这也证明了JSP其实就是Servlet，

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