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JSP

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JSP其实就是一种模板引擎，就是一个模板，而模板需要我们填入数据，才可以变成一个页面，也就是说，我们可以直接在前端页面中直接填写数据，填写后生成一个最终的HTML页面返回给前端。

加载规则

为什么JSP页面能够在加载的时候执行Java代码呢？

将项目打包后对于index页面会多了一个index_jsp.java和index_jsp.class，反编译后：

public final class index_jsp extends org.apache.jasper.runtime.HttpJspBase  //继承自HttpServlet
    implements org.apache.jasper.runtime.JspSourceDependent,
                 org.apache.jasper.runtime.JspSourceImports {
 
 ...
 
  public void _jspService(final jakarta.servlet.http.HttpServletRequest request, final jakarta.servlet.http.HttpServletResponse response)
      throws java.io.IOException, jakarta.servlet.ServletException {
 
    if (!jakarta.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 jakarta.servlet.jsp.PageContext pageContext;
    jakarta.servlet.http.HttpSession session = null;
    final jakarta.servlet.ServletContext application;
    final jakarta.servlet.ServletConfig config;
    jakarta.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    jakarta.servlet.jsp.JspWriter _jspx_out = null;
    jakarta.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("<!DOCTYPE html>\n");
      out.write("<html>\n");
      out.write("<head>\n");
      out.write("    <title>JSP - Hello World</title>\n");
      out.write("</head>\n");
      out.write("<body>\n");
      out.write("<h1>");
      out.print( new Date() );
      out.write("</h1>\n");
 
    System.out.println("JSP页面被加载");
 
      out.write("\n");
      out.write("<br/>\n");
      out.write("<a href=\"hello-servlet\">Hello Servlet</a>\n");
      out.write("</body>\n");
      out.write("</html>");
    } catch (java.lang.Throwable t) {
      if (!(t instanceof jakarta.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);
    }
  }
}

我们发现，它是继承自HttpJspBase类，我们可以反编译一下jasper.jar（它在tomcat的lib目录中）来看看:

package org.apache.jasper.runtime;
 
import jakarta.servlet.ServletConfig;
import jakarta.servlet.ServletException;
import jakarta.servlet.http.HttpServlet;
import jakarta.servlet.http.HttpServletRequest;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.jsp.HttpJspPage;
import java.io.IOException;
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[]{"3.0"});
    }
 
    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;
}

实际上，Tomcat在加载JSP页面时，会将其动态转换为一个java类并编译为class进行加载，而生成的Java类，正是一个Servlet的子类，而页面的内容全部被编译为输出字符串，这便是JSP的加载原理，因此，JSP本质上依然是一个Servlet！

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