EL語言(減少JSP頁面中的Java代碼)

String password = request.getParameter("password");

%>

username:

password:

username: ${param.username }

password: ${param.password }

屬性范圍->在EL中的名稱

Page->pageScope；Request->requestScope；Session->sessionScope；Application->applicationScope.

此外，啟動tomcat服務(wù)器，在瀏覽器中訪問localhost:8080/examples，選擇JSP Examples，其中給出了若干實例，也可以進(jìn)行相關(guān)的學(xué)習(xí)。

首先創(chuàng)建標(biāo)簽處理類HelloTag.java：

package com.jsp.tag;

import javax.servlet.jsp.JspException;

import javax.servlet.jsp.JspTagException;

import javax.servlet.jsp.tagext.TagSupport;

public class HelloTag extends TagSupport

{

public HelloTag()

{

}

public int doStartTag() throws JspException

{

try

{

this.pageContext.getOut().print("nihao");

}

catch (Exception ex)

{

throw new JspTagException(ex.getMessage());

}

return EVAL_BODY_INCLUDE;

}

// Method called when the closing hello tag is encountered

public int doEndTag() throws JspException

{

try

{

// We use the pageContext to get a Writer

// We then print the text string Hello

this.pageContext.getOut().print("Hello");

}

catch (Exception e)

{

throw new JspTagException(e.getMessage());

}

return EVAL_PAGE;

}

public void release()

{

// Call the parent's release to release any resources

// used by the parent tag.

// This is just good practice for when you start creating

// hierarchies of tags.

super.release();

}

}

然后，創(chuàng)建標(biāo)簽庫描述文件(在WEB-INF下新建一個tld(taglib description?)文件，這里命名為mytaglib.tld)：

/p>

PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN"

"http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd">

1.0

1.1

mytaglib

/mytaglib

hello

com.jsp.tag.HelloTag

empty

然后，在hellowithtag1.jsp中引入標(biāo)簽庫，然后插入標(biāo)簽：

tag library example

訪問http://localhost:8080/test/hellowithtag1.jsp，輸出nihaoHello

范例2：

創(chuàng)建一個能替換test應(yīng)用中JSP網(wǎng)頁的靜態(tài)文本的標(biāo)簽，這個標(biāo)簽名為message，它放在mytaglib標(biāo)簽庫中。

首先在WEB-INF下面放置一個靜態(tài)文本messageresource.properties

hello.title = Tile of hello.jsp

hello.hello = Hello

然后通過一個DispatcherServlet裝載：

package com.test.servlet;

import java.io.InputStream;

import java.util.Properties;

import javax.servlet.ServletConfig;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

public class DispatcherServlet extends HttpServlet

{

public void init(ServletConfig config) throws ServletException

{

super.init(config);

Properties ps = new Properties();

try

{

ServletContext context = config.getServletContext();

InputStream in = context

.getResourceAsStream("/WEB-INF/messageresource.properties");

ps.load(in);

in.close();

context.setAttribute("ps", ps);

}

catch (Exception e)

{

e.printStackTrace();

}

}

public void destroy()

{

}

}

接下來，是標(biāo)簽庫的處理類MessageTag.java：

package com.jsp.tag;

import java.util.Properties;

import javax.servlet.jsp.JspException;

import javax.servlet.jsp.JspTagException;

import javax.servlet.jsp.PageContext;

import javax.servlet.jsp.tagext.TagSupport;

public class MessageTag extends TagSupport

{

private String key = null;

public MessageTag()

{

}

public String getKey()

{

return this.key;

}

public void setKey(String key)

{

this.key = key;

}

// Method called when the closing hello tag is encountered

public int doEndTag() throws JspException

{

try

{

Properties ps = (Properties) pageContext.getAttribute("ps",

PageContext.APPLICATION_SCOPE);

String message = (String) ps.get(key);

pageContext.getOut().print(message);

}

catch (Exception e)

{

throw new JspTagException(e.getMessage());

}

return EVAL_PAGE;

}

public void release()

{

super.release();

}

}

添加相關(guān)信息到標(biāo)簽庫描述文件mytaglib.tld中：

/p>

PUBLIC "-//Sun Microsystems, Inc.//DTD JSP Tag Library 1.1//EN"

"http://java.sun.com/j2ee/dtds/web-jsptaglibrary_1_1.dtd">

1.0

1.1

mytaglib

/mytaglib

hello

com.jsp.tag.HelloTag

empty

message

com.jsp.tag.MessageTag

empty

key

true

最后，在hellowithtag2.jsp文件中引入標(biāo)簽庫，然后插入標(biāo)簽：

為了在web應(yīng)用啟動時通過DispatcherServlet裝載靜態(tài)文本，應(yīng)該在web.xml中配置這個Servlet時設(shè)置load-on-startup屬性：

DispatcherServlet

com.test.servlet.DispatcherServlet

5

在MessageTag的doEndTag方法中，首先從pageContext中讀取包含靜態(tài)文本的Properties對象：

public int doEndTag() throws JspException

{

try

{

Properties ps = (Properties) pageContext.getAttribute("ps",

PageContext.APPLICATION_SCOPE);

String message = (String) ps.get(key);

pageContext.getOut().print(message);

}

catch (Exception e)

{

throw new JspTagException(e.getMessage());

}

return EVAL_PAGE;

}

然后從Properties對象中讀取key對應(yīng)的靜態(tài)文本，最后輸出該文本。

最后，訪問http://localhost:8080/test/hellowithtag2.jsp，輸出hello。

總結(jié)

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