JSP自定义标签

一、为什么需要自定义标签
    1)与JSP页面整体统一
    2）还具有一定的业务逻辑功能，例如：循环、判断等

二、自定标签开发步骤
    一】第一步：写一个自定义的标签处理类

　　　　public class Demo implements SimpleTag{            private PageContext pageContext;            //PageContext是JspContext的子类            @Override            public void setJspContext(JspContext jspContext) {                System.out.println("Do setJspContext");                //得到jsp页面对象                pageContext = (PageContext) jspContext;            }            @Override            public void doTag() throws JspException, IOException {                System.out.println("Do doTag");                //取得HttpServletRequest对象                HttpServletRequest request= (HttpServletRequest) pageContext.getRequest();                //得到ip地址                String ip = request.getRemoteAddr();                System.out.println(ip);                //取得out <---> JspWriter对象                JspWriter out = pageContext.getOut();                //向浏览器输出IP地址信息                out.write("<font size='30px' color='red'>" + "ip:" + ip + "</font>");            }            @Override            public JspTag getParent() {                return null;            }            @Override            public void setJspBody(JspFragment arg0) {              }            @Override            public void setParent(JspTag arg0) {            }        }

 

    二】第二步：在/WEB-INF/目录下，写一个.tld文件，目的是让web容器知道自定义标签和标签处理类的对应关系

     　　<?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 core library</description>          <display-name>JSTL core</display-name>          <tlib-version>1.1</tlib-version>          <short-name>sky</short-name>          <uri>http://com.suse/jsp/jstl/sky</uri>                    <tag>            <name>ip</name>            <tag-class>com.suse.simpletag.Demo</tag-class>            <body-content>empty</body-content> <!--empty:此标签为空标签-->          </tag>        </taglib>

 

 

    三】第三步：在JSP页面中，通过<%@taglib%>指令引用标签库

   　　 <%@ taglib prefix="sky" uri="http://com.suse/jsp/jstl/sky"%>        <sky:ip />

 

 

三、标签处理过程
    1）SimpleTag接口中有5个方法，这5个方法容器会在适当的时候选择调用，
    2）其中，doTag()方法最为核心，该方法中封装了该标签的处理业务逻辑
    3）项目中通常都会使用SimpleTagSupport类，该类已经对SimpleTag接口实现

四、案例：
    一】控制标签中的内容是否执行( <sky:execute>标签 )
        1）执行标签体     :调用invoke(null)方法
        2）不执行标签体   :不调用invoke(null)方法

　　　　code:            public class ExecuteTag extends SimpleTagSupport {                @Override                public void doTag() throws JspException, IOException {                    //将此标签内的内容封装成JspFragment对象                    JspFragment jspFragment = this.getJspBody();                    //将标签中的内容进行输出,null表示默认输出到浏览器中，否则会输出到相应的流中                    jspFragment.invoke(null); //若没有这条语句，那么此标签的内容便不会输出到浏览器页面中                }            }        tag:             <tag>                  <name>execute</name>                  <tag-class>com.suse.simpletag.ExecuteTag</tag-class>                  <body-content>scriptless</body-content>       <!-- //scriptless表示不允许有脚本存在此标签内部 -->             </tag>

 

 

    二】控制标签后的内容是否执行
        方法：抛出 SkipPageException 异常。导致标签体后的内容不执行

    code:            public class SkipTag extends SimpleTagSupport {                @Override                public void doTag() throws JspException, IOException {                    JspFragment jspFragment = this.getJspBody();                    jspFragment.invoke(null);                    //设置此标签后的内容不再执行。方法:抛出一个 SkipPageException() 异常。                    throw new SkipPageException();                }            }      tag:              <tag>                  <name>skip</name>                  <tag-class>com.suse.simpletag.SkipTag</tag-class>                  <body-content>scriptless</body-content>              </tag>

 

 

    三】将标签体中内容转换成大写

  　　public class UpperTag extends SimpleTagSupport {        @Override        public void doTag() throws JspException, IOException {            JspFragment jspFragment = this.getJspBody();                        //创建一个缓冲区，用于存储标签中的内容            StringWriter writer = new StringWriter();            jspFragment.invoke(writer);                        //将缓冲区中的内容进行处理(小写 --> 大写)            StringBuffer buffer = writer.getBuffer();            String upString = buffer.toString().toUpperCase();                        //将转换后的内容输出到浏览器中            JspContext jspContext = this.getJspContext();            JspWriter out = jspContext.getOut();            out.write(upString);        }

 

 

    四】开发带属性的标签：
        1）在标签处理器中编写每个属性对应的setter方法
        2）在TLD文件中描述标签属性

 　　code:        public class ForSkip extends SimpleTagSupport{            private String var;            private int begin;            private int end;            private int step = 1; //增量默认为1                        public void setVar(String var) {                this.var = var;            }            public void setBegin(int begin) {                this.begin = begin;            }            public void setEnd(int end) {                this.end = end;            }            public void setStep(int step) {                this.step = step;            }                        @Override            public void doTag() throws JspException, IOException {                for (int i = this.begin; i <= this.end; i += this.step ) {                    this.getJspContext().setAttribute(this.var, i);                    this.getJspBody().invoke(null);                }            }        }    tld:        <tag>             <name>forskip</name>             <tag-class>com.suse.simpletag.ForSkip</tag-class>             <body-content>scriptless</body-content>             <attribute>                 <name>var</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>             <attribute>                 <name>begin</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>             <attribute>                 <name>end</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>             <attribute>                 <name>step</name>                 <required>false</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>        </tag>    jsp:        <sky:forskip var="item" begin="1" end="25" step="3">            ${item} &nbsp;&nbsp;&nbsp;        </sky:forskip>

 

 

    五】开发带有父标签的自定义标签
        思路：当某些标签是排它执行，此时：
          1) 可以在这些标签外，嵌入一个父标签，
          2) 并在父标签中做一个标志位，判断是否执行过。

　　code:        //1, <sky:choose> content </sky:choose>        public class ChooseTag extends SimpleTagSupport {            private boolean done = false;            public boolean isDone() {                return done;            }                        public void setDone(boolean done) {                this.done = done;            }            @Override            public void doTag() throws JspException, IOException {                JspFragment jspFragment =  this.getJspBody();                jspFragment.invoke(null);            }                    }        //2,<sky:while test="${expr}"> content </sky:while>        public class WhileTag extends SimpleTagSupport {                        private boolean test;            public void setTest(boolean test) {                this.test = test;            }                        @Override            public void doTag() throws JspException, IOException {                //得到父标签后获取是否已经做过了                ChooseTag chooseTag = (ChooseTag) this.getParent();                boolean done = chooseTag.isDone();                                //如果没有做过,并且表达式成立                if (!done && test) {                    this.getJspBody().invoke(null);                    chooseTag.setDone(true);                }            }        }        //3,<sky:otherwise>content</sky:otherwise>        public class OtherWiseTag extends SimpleTagSupport {            @Override            public void doTag() throws JspException, IOException {                //得到父类标签对象，获取到Done的状态，进行相应的处理                ChooseTag chooseTag = (ChooseTag) this.getParent();                boolean done = chooseTag.isDone();                if (!done) {                    this.getJspBody().invoke(null);                    chooseTag.setDone(true);                }            }        }    tld:         <tag>             <name>choose</name>             <tag-class>com.suse.simpletag.ChooseTag</tag-class>             <body-content>scriptless</body-content>         </tag>         <tag>             <name>while</name>             <tag-class>com.suse.simpletag.WhileTag</tag-class>             <body-content>scriptless</body-content>             <attribute>                 <name>test</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>         </tag>                 <tag>             <name>otherwise</name>             <tag-class>com.suse.simpletag.OtherWiseTag</tag-class>             <body-content>scriptless</body-content>         </tag>    jsp:        <%        pageContext.setAttribute("age", 25);        %>        <sky:choose>            <sky:while test="${age > 16}">                你成年了<br />            </sky:while>            <sky:otherwise>                你未成年 <br />            </sky:otherwise>        </sky:choose>

 

 

    六】防盗链

　　code:         public class Reference extends SimpleTagSupport{            private String url;            private String error;                        public void setUrl(String url) {                this.url = url;            }            public void setError(String error) {                this.error = error;            }            @Override            public void doTag() throws JspException, IOException {                //得到页面内置对象                PageContext pageContext = (PageContext) this.getJspContext();                HttpServletRequest request = (HttpServletRequest) pageContext.getRequest();                HttpServletResponse response = (HttpServletResponse) pageContext.getResponse();                                //获取标签体中的内容并进行处理                JspFragment jspFragment = this.getJspBody();                String refer = request.getHeader("referer");                System.out.println(refer);                if (this.url.equals(refer)) { //referer头表明来自于哪里                    jspFragment.invoke(null);                } else {                    try {                        request.getRequestDispatcher(this.error).forward(request, response);                    } catch (ServletException e) {                        e.printStackTrace();                    }                }                            }        }    tld:       <tag>               <name>refer</name>             <tag-class>com.suse.simpletag.Reference</tag-class>             <body-content>scriptless</body-content>             <attribute>                 <name>url</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>             <attribute>                 <name>error</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>         </tag>    jsp:        <sky:refer error="/ad.jsp" url="http://localhost:8080/day_16/index.jsp">        <a href="#">下载</a>        </sky:refer>

 

 

   七】仿写forEach自定义标签

　　 code：        //Foreach自定义标签        //Connection类族：  <sky:forEach var="item" items="${list}">${item}</sky:forEach>   <sky:forEach var="item" items="${set}">${item}</sky:forEach>        //Map类族：<sky:forEach var="en" items="map">${en.key} ----- ${en.value}</sky:forEach>        public class ForEachTag extends SimpleTagSupport {                        /*用一个Collection类来进行转换*/            private Collection coll;                        private Object items;            private String var;            public void setItems(Object items) {                /*设置coll的值：当为Collection类族时直接转换，否则保存Set<Entry<K,V>>形式的Connection*/                if (items instanceof Collection) {                    coll = (Collection) items;                } else if(items instanceof Map) {                    Map map = (Map) items;                    coll = map.entrySet();                }            }            public void setVar(String var) {                this.var = var;            }                                    @Override            public void doTag() throws JspException, IOException {                for (Iterator it = coll.iterator(); it.hasNext(); ) {                    //得到单个对象                    Object obj = (Object) it.next();                                        //将单个对象放入域对象中                    PageContext pageContext = (PageContext) this.getJspContext();                    pageContext.getRequest().setAttribute(this.var, obj);                                        //将标签封装的表达式输出到页面中                    this.getJspBody().invoke(null);                }            }            tld:         <tag>             <name>forEach</name>             <tag-class>com.suse.simpletag.ForEachTag</tag-class>             <body-content>scriptless</body-content>             <attribute>                 <name>var</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>             <attribute>                 <name>items</name>                 <required>true</required>                 <rtexprvalue>true</rtexprvalue>             </attribute>         </tag>      jsp:              <%                List<String> list = new ArrayList<String>();                list.add("jack");                list.add("merry");                list.add("berry");                list.add("sky");                                pageContext.setAttribute("LIST", list);                                Map<String, String> map = new HashMap<String, String>();                map.put("id", "121010");                map.put("username", "jack");                map.put("age", "15");                                pageContext.setAttribute("MAP", map);            %>            <sky:forEach items="${LIST}" var="item">                ${item} &nbsp;&nbsp;            </sky:forEach>            <br />            <sky:forEach items="${MAP}" var="en">                ${en.key}:${en.value} <br />            </sky:forEach>

 

 

    八】过滤器(防止恶意JS代码)

　　code：        //<simple:filter>标签处理类        public class FilterTag extends SimpleTagSupport {            public void doTag() throws JspException, IOException {                JspFragment jspFragment = this.getJspBody();                StringWriter writer = new StringWriter();                jspFragment.invoke(writer);                String temp = writer.getBuffer().toString();                //结果必定是转义后的字符串                temp = filter(temp);                PageContext pageContext = (PageContext) this.getJspContext();                pageContext.getOut().write(temp);            }            public String filter(String message) {                if (message == null)                    return (null);                char content[] = new char[message.length()];                message.getChars(0, message.length(), content, 0);                StringBuffer result = new StringBuffer(content.length + 50);                for (int i = 0; i < content.length; i++) {                    switch (content[i]) {                    case '<':                        result.append("&lt;");                        break;                    case '>':                        result.append("&gt;");                        break;                    case '&':                        result.append("&amp;");                        break;                    case '"':                        result.append("&quot;");                        break;                    default:                        result.append(content[i]);                    }                }                return (result.toString());            }        }