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}    
</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">
${item}   
</sky:forEach>
<br />
<sky:forEach items="${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("<");
break;
case '>':
result.append(">");
break;
case '&':
result.append("&");
break;
case '"':
result.append(""");
break;
default:
result.append(content[i]);
}
}
return (result.toString());
}
}