JSP自定義標簽簡單入門教程

在sun官方文檔上有下面這樣一段話。

官方文檔聲明

public interface SimpleTag
extends JspTag
Interface for defining Simple Tag Handlers.
Simple Tag Handlers differ from Classic Tag Handlers in that instead of supporting doStartTag() and doEndTag(), the SimpleTag interface provides a simple doTag() method, which is called once and only once for any given tag invocation. All tag logic, iteration, body evaluations, etc. are to be performed in this single method. Thus, simple tag handlers have the equivalent power of BodyTag, but with a much simpler lifecycle and interface.

To support body content, the setJspBody() method is provided. The container invokes the setJspBody() method with a JspFragment object encapsulating the body of the tag. The tag handler implementation can call invoke() on that fragment to evaluate the body as many times as it needs.

A SimpleTag handler must have a public no-args constructor. Most SimpleTag handlers should extend SimpleTagSupport.

生存周期及調用流程

The following is a non-normative, brief overview of the SimpleTag lifecycle. Refer to the JSP Specification for details.

A new tag handler instance is created each time by the container by calling the provided zero-args constructor. Unlike classic tag handlers, simple tag handlers are never cached and reused by the JSP container.
The setJspContext() and setParent() methods are called by the container. The setParent() method is only called if the element is nested within another tag invocation.
The setters for each attribute defined for this tag are called by the container.
If a body exists, the setJspBody() method is called by the container to set the body of this tag, as a JspFragment. If the action element is empty in the page, this method is not called at all.
The doTag() method is called by the container. All tag logic, iteration, body evaluations, etc. occur in this method.
The doTag() method returns and all variables are synchronized.

簡單標簽使用小案例

必知必會：簡單標簽也是一個標簽，所以聲明的過程也Tag的一樣，同樣是三步。

1、建繼承SimpleTag類的實現類，重寫doTag方法
2、tld文件中進行嚴格的聲明
3、jsp頁面中taglib的命名空間及標簽前綴的聲明，然後進行調用自定義的簡單標簽

第一步：創建實現類：

package web.simpletag;
import java.io.IOException;
import java.io.StringWriter;

import javax.servlet.jsp.JspException;
import javax.servlet.jsp.PageContext;
import javax.servlet.jsp.SkipPageException;
import javax.servlet.jsp.tagext.JspFragment;
import javax.servlet.jsp.tagext.SimpleTagSupport;


/**
 * 控制標簽體是否執行
 * @author Summer
 *
 */
public class BodyController extends SimpleTagSupport {
  static{
    /*
     * 簡單標簽整體的執行流程如下：
     * 1.浏覽器向web服務器發送請求，然後web服務器調用servlet（jsp）
     * 2.complier解釋器進行初始化工作，先是調用setJspContext方法，將pageContext對象傳遞進去
     * 3.然後是看看此標簽的父標簽，即setParent方法
     * 4.再就是調用doTag方法了吧？但是要知道doTag內部會使用JspFragment對象，所以就必須先得到它，因此應該是調用setJspBody(JspFragment jspBody)方法
     * 5.最後是調用doTag 方法，執行相關的代碼邏輯
     */
  }

  /**
   * 簡單標簽可以使用這一個方法實現所有的業務邏輯
   */
  @Override
  public void doTag() throws JspException, IOException {
    //代表標簽體的對象
    JspFragment fragment = this.getJspBody();
    //fragment.invoke(null);是指將標簽中的內容寫給誰，null代表浏覽器


    //1.修改標簽體的內容
//   fragment.invoke(null);


    //2.控制標簽體內容的重復輸出
//   for(int i=1;i<=5;i++){
//     fragment.invoke(null);//設置為null，默認為向浏覽器輸出
//   }


    //3.修改標簽體的內容
    PageContext context = (PageContext) fragment.getJspContext();
    StringWriter writer = new StringWriter();
    fragment.invoke(writer);
    String content = writer.getBuffer().toString();

    this.getJspContext().getOut().write(content.toUpperCase());

    //4.控制jsp頁面的執行與否，只需要掌握一個原理即可
    /*
     * SkipPageException - If the page that (either directly or indirectly) invoked this 
     * tag is to cease evaluation. A Simple Tag Handler generated from a tag
     * file must throw this exception if an invoked Classic Tag Handler
     *  returned SKIP_PAGE or if an invoked Simple Tag Handler threw
     *  SkipPageException or if an invoked Jsp Fragment threw a 
     *  SkipPageException.
     */
//   throw new SkipPageException();
  }


}

在tld文件中進行相關約束項的配置：

<?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>JSTL 1.1 XML library</description>
  <display-name>JSTL XML</display-name>
  <tlib-version>1.1</tlib-version>
  <short-name>x</short-name>
  <uri>/simplesummer</uri>


  <!-- 控制標簽體內容的的簡單標簽的自定義標簽 -->
  <tag>
    <name>BodyController</name>
    <tag-class>web.simpletag.BodyController</tag-class>
    <body-content>scriptless</body-content>
  </tag>
</taglib>

第三步：在jsp頁面中進行聲明然後調用：

<%@ page language="java" contentType="text/html; charset=UTF-8"
  pageEncoding="UTF-8"%>
<%@taglib uri="/simplesummer" prefix="summer"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>用SimpleTag接口實現的控制標簽體內容是否執行的測試頁面</title>
</head>
<body>
  <summer:BodyController>Summer</summer:BodyController>


</body>
</html>

總結：
簡單標簽可以替代BodyTag接口完成同樣的操作，但是有更加的簡單和輕便
簡單標簽lifeCycle邏輯清晰，調用規則明確
使用相關流對象就可以完成對標簽體的操控maniplate

以上就是本文的全部內容，希望對大家的學習有所幫助。