【Head First Servlets and JSP】笔记17：JSP所生成的servlet相关问题

1、容器根据你所写的JSP生成一个类，

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.77
 * Generated at: 2017-06-11 15:47:28 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 HelloWorld_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  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 org.apache.jasper.runtime.TagHandlerPool _005fjspx_005ftagPool_005fs_005fproperty_0026_005fvalue_005fnobody;

  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 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() {
    _005fjspx_005ftagPool_005fs_005fproperty_0026_005fvalue_005fnobody = org.apache.jasper.runtime.TagHandlerPool.getTagHandlerPool(getServletConfig());
  }

  public void _jspDestroy() {
    _005fjspx_005ftagPool_005fs_005fproperty_0026_005fvalue_005fnobody.release();
  }

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

    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("\r\n");
      out.write("\r\n");
      out.write("<html>\r\n");
      out.write("<head>\r\n");
      out.write("<title>Hello World</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      out.write("   Hello World, ");
      if (_jspx_meth_s_005fproperty_005f0(_jspx_page_context))
        return;
      out.write("\r\n");
      out.write("</body>\r\n");
      out.write("</html>");
    } 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);
    }
  }

  private boolean _jspx_meth_s_005fproperty_005f0(javax.servlet.jsp.PageContext _jspx_page_context)
          throws java.lang.Throwable {
    javax.servlet.jsp.PageContext pageContext = _jspx_page_context;
    javax.servlet.jsp.JspWriter out = _jspx_page_context.getOut();
    //  s:property
    org.apache.struts2.views.jsp.PropertyTag _jspx_th_s_005fproperty_005f0 = (org.apache.struts2.views.jsp.PropertyTag) _005fjspx_005ftagPool_005fs_005fproperty_0026_005fvalue_005fnobody.get(org.apache.struts2.views.jsp.PropertyTag.class);
    boolean _jspx_th_s_005fproperty_005f0_reused = false;
    try {
      _jspx_th_s_005fproperty_005f0.setPageContext(_jspx_page_context);
      _jspx_th_s_005fproperty_005f0.setParent(null);
      // /HelloWorld.jsp(8,16) name = value type = java.lang.String reqTime = false required = false fragment = false deferredValue = false expectedTypeName = null deferredMethod = false methodSignature = null
      _jspx_th_s_005fproperty_005f0.setValue("name");
      int _jspx_eval_s_005fproperty_005f0 = _jspx_th_s_005fproperty_005f0.doStartTag();
      if (_jspx_th_s_005fproperty_005f0.doEndTag() == javax.servlet.jsp.tagext.Tag.SKIP_PAGE) {
        return true;
      }
      _005fjspx_005ftagPool_005fs_005fproperty_0026_005fvalue_005fnobody.reuse(_jspx_th_s_005fproperty_005f0);
      _jspx_th_s_005fproperty_005f0_reused = true;
    } finally {
      org.apache.jasper.runtime.JspRuntimeLibrary.releaseTag(_jspx_th_s_005fproperty_005f0, _jsp_getInstanceManager(), _jspx_th_s_005fproperty_005f0_reused);
    }
    return false;
  }
}

2、需要知道的一些问题

对于问题1：JSP有一个内置对象叫config，它相当于servlet中的ServletConfig，在servlet中我们需要通过getServletConfig来获得它，然后再通过例如getInitParameter("name");获得参数，整个过程类似这样

而在JSP，由于它本身具有这样一个内置对象，因此只需要像下面这样就可以了

对于问题2：我猜想也是在web.xml中配置，因为JSP说到底不过是servlet的另外一种“书写方式”罢了。

事实证明这是正确的，

对于问题3：直接在<%!%>中进行覆盖就可以了

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