JSP的原理

Tomcat的lib目录下的jasper.jar包，这个包里面有一个HttpJspBase类，这个类我们看一下源码：

//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by FernFlower decompiler)
//

package org.apache.jasper.runtime;

import java.io.IOException;
import javax.servlet.ServletConfig;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.jsp.HttpJspPage;
import org.apache.jasper.compiler.Localizer;

public abstract class HttpJspBase extends HttpServlet implements HttpJspPage {
    private static final long serialVersionUID = 1L;

    protected HttpJspBase() {
    }

    public final void init(ServletConfig config) throws ServletException {
        super.init(config);
        this.jspInit();
        this._jspInit();
    }

    public String getServletInfo() {
        return Localizer.getMessage("jsp.engine.info", new Object[]{"2.3"});
    }

    public final void destroy() {
        this.jspDestroy();
        this._jspDestroy();
    }

    public final void service(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        this._jspService(request, response);
    }

    public void jspInit() {
    }

    public void _jspInit() {
    }

    public void jspDestroy() {
    }

    protected void _jspDestroy() {
    }

    public abstract void _jspService(HttpServletRequest var1, HttpServletResponse var2) throws ServletException, IOException;
}

index_jsp.java

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/9.0.53
 * Generated at: 2022-01-16 01:01:45 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 {

    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;
      }
    }

    final javax.servlet.jsp.PageContext pageContext;
    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, false, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      out = pageContext.getOut();
      _jspx_out = out;

     
      out.write("	</div>\r\n");
      out.write("</div>\r\n");
      out.write("</body></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);
    }
  }
}

可以发现jsp本质上就是一个servlet，实际上我们原理是在servlet中打印html的代码，实际上这个底层也是这样实现的，只不过这个不是我们自己做了，而是服务器帮助我们做了。
它生成的java类叫index_jsp.java；这个

访问http://localhost:8080/hello/index.jsp 如何显示效果？
1.访问到index.jsp页面，tomcat扫描到jsp文件，在tomcat/work目录下把jsp文件翻译成java源文件
index.jsp-->_index.java(翻译)
2.tomcat服务器把java源文件编译成class字节码文件（编译）
_index.java-->_index.class
3.tomcat服务器构造_index_jsp类对象
4.tomcat服务器调用_index_jsp类里面的方法，返回内容显示到浏览器
如果是第一次访问当前jsp文件时，执行步骤是：1-->2-->3-->4
第N次访问jsp时，执行步骤是：4，不再执行其他步骤
注意：jsp文件被修改或者jsp的临时文件被删除了，要重新走翻译（1）和编译（2）的过程

当jsp文件被翻译成java的源码时，可以在翻译出来的源码文件里看到当前访问的jsp文件继承HttpJspBase类，再通过查询JDK，可以发现HttpJspBase又是继承于HttpServlet的。所以，可以得到结论，jsp既是一个servlet的程序，而servlet的技术可以用在jsp程序中，但是并不是所有的servlet技术全部适用于jsp程序

Servlet的生命周期：
1.构造方法（第一次访问时执行）
2.init方法（第一次访问时执行）
3.service方法（每次访问都会执行）
4.destory方法（退出时执行）

Jsp的生命周期：
1.翻译：Jsp-->java源文件
2.编译：java文件-->class文件（servlet程序）
3.构造方法（第一次访问时执行）
4.init方法（第一次访问）：JspInit（）
5.service方法：JspService()
6.destory方法：jspDestory()
1.Http服务器将JSP文件内容【编辑】为一个Servlet接口实现类（.java）
    2.Http服务器将Servlet接口实现类【编译】为class文件(.class)
    3.Http服务器负责创建这个class的实例对象，这个实例对象就是Servlet实例对象
    4.Http服务器通过Servlet实例对象调用_jspService方法，将jsp文件内容写入到响应体
Http服务器【编辑】与【编译】JSP文件位置：    
C:\Users\[登录windows系统用户角色名]\.IntelliJIdea【版本号】\system\tomcat\[网站工作空间]\work\Catalina\localhost\【网站别名】\org\apache\jsp

关于jsp的详细原理解读推荐一篇文章，觉得写的非常详细特别棒，这里就不班门弄斧了，直接给出原文链接：
引用地址：https://www.cnblogs.com/cenyu/p/6170495.html?
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