javaweb(3)之JSP&EL&JSTL

時間 2019-12-14

標籤 javaweb jsp&el&jstl jsp jstl 欄目 Java 简体版

原文原文鏈接

JSP(Java Server Page)

介紹

什麼是 JSP ？

從用戶角度看，JSP 就是一個網頁。html

從開發者角度看，它其實就是一個繼承了 Servlet 的 java 類，因此能夠直接說 JSP 就是一個 Servlet。java

爲何會有 JSP ？

HTML 一般狀況下用來顯示一成不變的靜態內容，但實際上大部分咱們須要的網頁上是須要顯示一些靜態數據的，這些動做都涉及到了邏輯處理，這些都須要代碼輔助完成。web

HTML 中是不支持寫 java 代碼的，而JSP 裏面能夠寫 java 代碼。express

查看 JSP 翻譯後的文件示例：apache

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.92
 * Generated at: 2019-01-04 07:39:33 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 {

  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 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() {
  }

  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 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("<html>\r\n");
      out.write("<head>\r\n");
      out.write("    <title>Title</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      out.write("<h1>hhhh</h1>\r\n");
      out.write("</body>\r\n");
      out.write("</html>\r\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);
    }
  }
}

例：

指令

page
```
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
```
參數：

contentType：告知瀏覽器當前 JSP 的 MIME 類型和以何編碼來解析該 JSP 返回的內容。

pageEncoding：指定 JSP 內容的編碼。

extends：指定 JSP 翻譯成 java 文件後繼承的類。

import：導包使用。

session：可選 true 或 false，默認爲 true。指定當前 JSP 可否直接使用 session 對象。
查看 JSP 翻譯後的 java 文件能夠發現，session 若是指定爲 true，在 _jspService 方法中就會聲明一個 session 變量，並經過 pageContext.getSession() 賦值；若是 session 指定爲 false，那麼就不會聲明這個變量，即後續不能直接使用 session。

errorPage：指定當前 JSP 出錯時跳轉到的頁面。

isErrorPage：可選 true 或 false，指定當前 JSP 是否是錯誤頁，默認 false。若是指定爲 true，在當前 JSP 中就能夠直接使用 exception 對象。

include

<%@include file="page.jsp" %>

將指定 JSP 文件包含到當前 JSP。數組

示例：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>page1</title>
</head>
<body>
<% String str = "hello";%>
<%=str%>
<h1>這是 page1 的內容</h1>
</body>
</html>

page1.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>page2</title>
</head>
<body>
<%@include file="page1.jsp" %>
<h1>這是 page2 的內容</h1>
</body>
</html>

page2.jsp

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.92
 * Generated at: 2019-01-04 08:25:20 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 page2_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;

  static {
    _jspx_dependants = new java.util.HashMap<java.lang.String,java.lang.Long>(1);
    _jspx_dependants.put("/page1.jsp", Long.valueOf(1546590314039L));
  }

  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() {
  }

  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 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("<html>\r\n");
      out.write("<head>\r\n");
      out.write("    <title>page2</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      out.write("\r\n");
      out.write("<html>\r\n");
      out.write("<head>\r\n");
      out.write("    <title>page1</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
 String str = "hello";
      out.write('\r');
      out.write('\n');
      out.print(str);
      out.write("\r\n");
      out.write("<h1>這是 page1 的內容</h1>\r\n");
      out.write("</body>\r\n");
      out.write("</html>\r\n");
      out.write("\r\n");
      out.write("<h1>這是 page2 的內容</h1>\r\n");
      out.write("</body>\r\n");
      out.write("</html>\r\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);
    }
  }
}

page2_jsp.java：page2翻譯後的java文件

查看翻譯後文件能夠發現，include 指令就是將指定 JSP 文件的內容拿到本身對應的類中執行輸出。

taglib
```
<%@ taglib prefix="" uri="" %>
```
引入標籤庫。

參數：

prefix：別名。

uri：標籤庫地址。

動做標籤

include

<jsp:include page="page1.jsp"></jsp:include>

做用和 include 指令同樣，也是將指定 JSP 頁包含到當前 JSP 頁中。

示例：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>page1</title>
</head>
<body>
<% String str = "hello";%>

<h1>這是 page1 的內容</h1>

<%=str%>
</body>
</html>

page1.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>page2</title>
</head>
<body>
<jsp:include page="page1.jsp"></jsp:include>
<h1>這是 page2 的內容</h1>
</body>
</html>

page2.jsp

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.92
 * Generated at: 2019-01-04 08:49:08 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 page2_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 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() {
  }

  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 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("<html>\r\n");
      out.write("<head>\r\n");
      out.write("    <title>page2</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "page1.jsp", out, false);
      out.write("\r\n");
      out.write("<h1>這是 page2 的內容</h1>\r\n");
      out.write("</body>\r\n");
      out.write("</html>\r\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);
    }
  }
}

page2_jsp.java：page2翻譯後的java文件

查看翻譯後文件能夠發現，與 include 指令不一樣的是：該標籤是將指定 JSP 文件執行後返回的內容拿到當前 JSP 中直接輸出，而不是將代碼拿到當前 JSP 中執行。

forward

<jsp:forward page="page2.jsp"></jsp:forward>

請求轉發功能。瀏覽器

至關於執行以下代碼：tomcat

request.getRequestDispatcher("page2.jsp").forward(request, response);

還能夠搭配 param 標籤傳遞參數：cookie

<jsp:forward page="page2.jsp">
    <jsp:param name="name" value="zhangsan"></jsp:param>
    <jsp:param name="age" value="20"></jsp:param>
</jsp:forward>

示例：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>page1</title>
</head>
<body>

<jsp:forward page="page2.jsp">
    <jsp:param name="name" value="zhangsan"></jsp:param>
    <jsp:param name="age" value="20"></jsp:param>
</jsp:forward>
<h1>這是 page1 的內容</h1>
</body>
</html>

page1.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>page2</title>
</head>
<body>
<h1>這是 page2 的內容</h1>
<hr>
<%
    String name = request.getParameter("name");
    Integer age = Integer.parseInt(request.getParameter("age"));
%>
name:<%=name%> <br>
age:<%=age%>
</body>
</html>

page2.jsp

/*
 * Generated by the Jasper component of Apache Tomcat
 * Version: Apache Tomcat/7.0.92
 * Generated at: 2019-01-04 09:18:25 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 page1_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 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() {
  }

  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 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("<html>\r\n");
      out.write("<head>\r\n");
      out.write("    <title>page1</title>\r\n");
      out.write("</head>\r\n");
      out.write("<body>\r\n");
      out.write("\r\n");
      if (true) {
        _jspx_page_context.forward("page2.jsp" + "?" + org.apache.jasper.runtime.JspRuntimeLibrary.URLEncode("name", request.getCharacterEncoding())+ "=" + org.apache.jasper.runtime.JspRuntimeLibrary.URLEncode("zhangsan", request.getCharacterEncoding()) + "&" + org.apache.jasper.runtime.JspRuntimeLibrary.URLEncode("age", request.getCharacterEncoding())+ "=" + org.apache.jasper.runtime.JspRuntimeLibrary.URLEncode("20", request.getCharacterEncoding()));
        return;
      }
      out.write("\r\n");
      out.write("<h1>這是 page1 的內容</h1>\r\n");
      out.write("</body>\r\n");
      out.write("</html>\r\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);
    }
  }
}

page1_jsp.java：page1翻譯後的java文件

能夠看到，參數的攜帶其實是將咱們經過 param 標籤指定的參數先 URL 編碼，而後經過 ? 方式拼接到 URL 後面。

9個內置對象

所謂內置對象，就是咱們能夠直接在 JSP 中使用的已經建立好的對象，有九個以下：session

application
它實際上就是 javax.servlet.ServletContext 類的實例，點擊查看詳細。
request
它是 javax.servlet.http.HttpServletRequest 類的實例，點擊查看詳細。
session
它是 javax.servlet.http.HttpSession 類的實例，點擊查看詳細。
pageContext
它是 javax.servlet.jsp.PageContext 類的實例，當前 JSP 的上下文對象，其它幾個內置對象也能夠經過它獲取到。

上面四個對象爲四大域對象，由於它們均可以經過 setAttribute 方法存值， getAttribute 方法取值，只是做用的範圍不一樣。
application 即 ServletContext 實例以前就說過，它在整個應用程序內被共享。
request 僅在一次請求內有效。
session 在一次會話內有效。
而新引入的 pageContext 則只是在當前 JSP 範圍內有效，即每一個 JSP 都有它獨立的 pageContext 對象。

config
它是 javax.servlet.ServletConfig 類的實例，點擊查看詳細。
exception
它是 java.lang.Throwable 類的實例，JSP 報錯時拋出的異常對象。
當 JSP 中 page 指令的 isErrorPage 參數值指定爲 true 時，纔可直接使用 exception 對象。
out
它是 javax.servlet.jsp.JspWriter 類的實例。
示例：
<%@ page contentType="text/html;charset=UTF-8" isErrorPage="true" language="java" %> <html> <head> <title>Title</title> </head> <body> <% out.write("這是 out 輸出的內容"); %> <% response.getWriter().write("這是 response.getWriter() 輸出的內容"); %> </body> </html>

page.jsp
能夠看到在代碼中 response.getWriter() 是後輸出的，可是實際訪問時它倒是先輸出的。這是由於 out 對象輸出的內容會放到 response 的緩衝區中， response.getWriter() 的內容輸出後纔會接着輸出 out 對象輸出的內容。而經過查看 JSP 對應 java 代碼咱們知道，JSP 頁面內容就是使用 out 對象輸出的，因此得出結論：當咱們在 JSP 中使用 response.getWriter() 輸出內容時，在瀏覽器渲染後 response.getWriter() 輸出的內容會出如今頁面的最頂部。
page
它是當前 JSP 翻譯成 java 類的實例。
response
它是 javax.servlet.http.HttpServletResponse 類的實例，點擊查看詳細。

EL(Expression Language)

介紹

EL 表達式能夠幫助咱們簡化 JSP 中的 java代碼。

取值

在四大做用域中取值

<%@ page contentType="text/html;charset=UTF-8" isErrorPage="true" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%
    pageContext.setAttribute("str", "from pageContext");
    request.setAttribute("str", "from request");
    session.setAttribute("str", "from session");
    application.setAttribute("str", "from application");
%>

${pageScope.str} <br>
${requestScope.str} <br>
${sessionScope.str} <br>
${applicationScope.str} <br>
<%--在 el 表達式中不指定 scope 直接使用 name，會依次在 pageContext->request->session->application --%>
${str}
</body>
</html>

page.jsp

取array和list中的值

<%@ page import="java.util.List" %>
<%@ page import="java.util.ArrayList" %>
<%@ page contentType="text/html;charset=UTF-8" isErrorPage="true" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%
    String[] array = {"a", "b", "c", "d"};
    pageContext.setAttribute("array", array);

    List list = new ArrayList();
    list.add(11);
    list.add(22);
    list.add(33);
    list.add(44);
    pageContext.setAttribute("list", list);
%>

${array[0]},${array[1]},${array[2]},${array[3]}
<hr/>
${list[0]},${list[1]},${list[2]},${list[3]}
</body>
</html>

page.jsp

取map中的值

<%@ page import="java.util.HashMap" %>
<%@ page import="java.util.Map" %>
<%@ page contentType="text/html;charset=UTF-8" isErrorPage="true" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%
    String[] array = {"aa", "bb"};

    Map map = new HashMap();
    map.put("name1", "zhangsan");
    map.put("name2", "lisi");
    map.put("name3", "wangwu");
    map.put("name4", "zhaoliu");
    pageContext.setAttribute("map", map);
%>
${map["name1"]},${map["name2"]},${map.name3},${map.name4}
</body>
</html>

page.jsp

取對象屬性的值

package com.zze.bean;

public class User {
    private String name;
    private Integer age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }
}

com.zze.bean.User

<%@ page import="com.zze.bean.User" %>
<%@ page contentType="text/html;charset=UTF-8" isErrorPage="true" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<%
    User user = new User();
    user.setName("張三");
    user.setAge(18);

    pageContext.setAttribute("user", user);
%>
姓名：${pageScope.user.name} <br>
年齡：${user.age}
</body>
</html>

page.jsp

11個隱式對象

隱式對象即在 EL 表達式中能直接使用的對象，有 11 個以下：

pageContext
就是 JSP 內置對象的 pageContext。
pageScope
當前 JSP 頁的 Map 域。
requestScope
當前請求的 Map 域。
sessionScope
當前會話的 Map 域。
applicationScope
當前應用程序的 Map 域。

這裏說的 Map 域指的是使用 setAttribute 存放值存放到的 Map 實例。

header
請求頭鍵和值的 Map。
headerValues
請求頭鍵和值的 Map，值爲數組結構。
param
請求參數的參數名和值的 Map。
paramValues
請求參數的參數名名和值的 Map，值爲數組結構。
initparam
全部初始化參數的 Map（這裏的初始化參數指的是全局初始化參數）。
cookie
全部 Cookie 的鍵和值 Map。

JSTL(Java Standard Tag Library)

介紹

JSTL 是一個不斷完善的開放源代碼的 JSP 標籤庫。

它的做用是簡化 JSP 中代碼中 java 代碼的編寫，即替換 <% %> 寫法，通常與 EL 表達式配合使用。

使用

一、導包： jstl.jar、standard.jar 。

二、在 JSP 中使用 taglib 指令引入標籤庫。

若是想要支持 EL 表達式，那麼引入的標籤庫版本要在 1.1 以上。下面列出幾個比較經常使用的標籤：

set

將指定 key-value 值存放到域對象。

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>set</title>
</head>
<body>
<%--
參數：
    var：名稱
    value：值
    scope：默認爲 page ,還有 request、session、application 選項
        爲 page 時，至關於 <% pageContext.setAttribute("名稱","值"); %>
        爲 request 時，至關於 <% request.setAttribute("名稱","值"); %>
        爲 session 時，至關於 <% session.setAttribute("名稱","值"); %>
        爲 application 時，至關於 <% application.setAttribute("名稱","值"); %>
--%>
<c:set var="age" value="18" scope="page"></c:set>
${age}
</body>
</html>

set.jsp

if

簡單的 if 條件判斷。

<%@ page contentType="text/html;charset=UTF-8" isErrorPage="true" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<html>
<head>
    <title>if</title>
</head>
<body>
<c:set var="age" value="18" scope="page"></c:set>
<%--
參數：
    test：條件表達式
    var：指定變量接收條件表達式結果
    scope：標識條件表達式中的變量從哪一個域中取
下面代碼至關於：
--%>
<%
    Integer age = Integer.parseInt(pageContext.getAttribute("age").toString());
    boolean flag = age >= 18;
    pageContext.setAttribute("flag", flag);
    if (flag) {
%>
成年人
<%
    }
%>

<c:if test="${age>=18}" var="flag" scope="page">
成年人
</c:if>
${flag}

if.jsp

forEach

循環。

<%@ page import="com.zze.bean.User" %>
<%@ page import="java.util.ArrayList" %>
<%@ page import="java.util.List" %>
<%@ page contentType="text/html;charset=UTF-8" isErrorPage="true" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>

<html>
<head>
    <title>forEach</title>
</head>
<body>
<%--
for 循環
參數：
    begin：起始數字
    end：結束數字
    var：定義一個變量代指當前遍歷對象
    step：增幅
--%>
<c:forEach begin="1" end="10" var="item" step="2">
    ${item}
</c:forEach>
<%
    List<User> userList = new ArrayList<User>();
    userList.add(new User("張三", 18));
    userList.add(new User("李四", 19));
    userList.add(new User("王五", 20));
    pageContext.setAttribute("userList", userList);
%>
<hr>
<%--
foreach 循環
參數：
    begin：起始下標從 0 開始
    end：結束下標
    step：下標增幅
    var：定義一個變量代指當前遍歷對象
    items：指定要遍歷的可迭代對象
    varStatus：定義一個保存了當前遍歷信息的對象，有以下屬性：
        begin：當前遍歷的起始下標從 0 開始
        end：當前遍歷的結束下標
        step：當前遍歷的下標增幅
        count：當前遍歷的是第幾個，從 1 開始
        current：代指當前遍歷對象，和 var 定義的變量指向同一個地址
        first：當前遍歷對象是不是第一個
        last：當前遍歷對象是不是最後一個
        index：當前遍歷對象的下標
--%>
<c:forEach begin="0" end="${userList.size()}" step="2" var="user" items="${userList}" varStatus="s">
    user.name：${user.name}，user.age：${user.age}，s.step：${s.step}，s.begin：${s.begin}，s.end：${s.end}，s.count：${s.count}，s.current：${s.current}，s.current == user：${s.current == user}，s.first：${s.first}，s.last：${s.last}<br>
</c:forEach>
</body>
</html>

forEach.jsp

補充

CRUD實例下載

https://pan.baidu.com/s/122hKpiKr3vplZ0uTS5lGPQ

自定義標籤庫

最簡單的標籤庫

一、建立一個標籤處理類，該類須要繼承 SimpleTagSupport 類，重寫 doTag 方法：

package com.zze.tag;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.SimpleTagSupport;
import java.io.IOException;

public class HelloTag extends SimpleTagSupport {

    @Override
    public void doTag() throws JspException, IOException {
        getJspContext().getOut().print("<h1>Hello My Tag!!!</h1>");
    }
}

com.zze.tag.HelloTag

二、在 WEB-INF 下建立標籤庫配置文件：

<?xml version="1.0" encoding="UTF-8" ?>

<taglib xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
    version="2.0">
    <description>tag created by zze</description>
    <tlib-version>1.0</tlib-version>
    <short-name>myTag</short-name>
    <!--標籤庫惟一標識-->
    <uri>zze.test</uri>
    <tag>
        <description>Outputs Hello</description>
        <!--標籤名稱-->
        <name>helloTag</name>
        <tag-class>com.zze.tag.HelloTag</tag-class>
        <body-content>empty</body-content>
    </tag>
</taglib>

WEB-INF/tld/mytag.tld

三、在 jsp 中使用：

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="mytag" uri="zze.test"%>
<html>
  <head>
    <title>自定義標籤庫測試</title>
  </head>
  <body>
  <mytag:helloTag/>
  </body>
</html>

index.jsp

帶屬性的標籤庫

一、建立一個標籤處理類，聲明一個屬性並給它 setter ，重寫 doTag 方法：

package com.zze.tag;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.tagext.SimpleTagSupport;
import java.io.IOException;
import java.util.HashMap;

public class TableTag extends SimpleTagSupport {
    private String map;

    public void setMap(String map) {
        this.map = map;
    }

    @Override
    public void doTag() throws JspException, IOException {
        HashMap<String, Integer> maps = (HashMap<String, Integer>) (getJspContext().getAttribute(map));
        Object[] array = maps.keySet().toArray();

        for (String str : maps.keySet()) {
            getJspContext().getOut().write("<tr>");
            getJspContext().getOut().write("<td>");
            getJspContext().getOut().write(str);
            getJspContext().getOut().write("</td>");
            getJspContext().getOut().write("<td>");
            getJspContext().getOut().write("" + maps.get(str));
            getJspContext().getOut().write("</td>");
            getJspContext().getOut().write("</tr>");
        }
    }
}

com.zze.tag.TableTag

二、在 WEB-INF 下建立標籤庫配置文件：

<?xml version="1.0" encoding="UTF-8" ?>

<taglib xmlns="http://java.sun.com/xml/ns/j2ee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-jsptaglibrary_2_0.xsd"
    version="2.0">
    <description>tag created by zze</description>
    <tlib-version>1.0</tlib-version>
    <short-name>myTag</short-name>
    <uri>zze.test</uri>

    <tag>
        <name>tr</name>
        <tag-class>com.zze.tag.TableTag</tag-class>
        <body-content>empty</body-content>
        <attribute>
            <!--聲明一個屬性-->
            <name>map</name>
            <required>true</required>
            <fragment>true</fragment>
        </attribute>
    </tag>
</taglib>

WEB-INF/tld/mytag.tld

三、在 jsp 中使用：

<%@ page import="java.util.HashMap" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="mytag" uri="zze.test" %>
<html>
<head>
    <title>自定義標籤庫測試</title>
</head>
<body>
<%
    HashMap<String, Integer> maps = new HashMap<String, Integer>();
    maps.put("張三", 18);
    maps.put("李四", 23);
    maps.put("王五", 23);
    pageContext.setAttribute("map", maps);
%>
<table border="1">
    <mytag:tr map="map"/>
</table>
</body>
</html>

index.jsp

相關標籤/搜索

每日一句

每一个你不满意的现在，都有一个你没有努力的曾经。

javaweb(3)之JSP&EL&JSTL

JSP(Java Server Page)

介紹

什麼是 JSP ？

爲何會有 JSP ？

指令

page

include

taglib

動做標籤

include

forward

9個內置對象

application

request

session

pageContext

config

exception

out

page

response

EL(Expression Language)

介紹

取值

在四大做用域中取值

取array和list中的值

取map中的值

取對象屬性的值

11個隱式對象

pageContext

pageScope

requestScope

sessionScope

applicationScope

header

headerValues

param

paramValues

initparam

cookie

JSTL(Java Standard Tag Library)

介紹

使用

set

if

forEach

補充

CRUD實例下載

自定義標籤庫

最簡單的標籤庫

帶屬性的標籤庫