JavaBeans

javabean是符合标准格式的Java类（就是个类，只不过把一些诸如属性、数据库连接单独拿出来作为一个类存在）。

A bean class has the same basic structure as an ordinary Java class, but should follow these rules:

  1. It must be within a named package.

  2. 每个(不是库中的)公共方法都应该以get或set开头

  3. 需要被序列化并且实现了 Serializable 接口（该接口内无方法）

    需要导包

    Import java.io.Serializable

  4. It does not usually have a main() method, but will have if some initial activity is required.

  5. Must have a zero-argument (empty) constructor.

  6. 全为私有属性，但是要public的访问方法

  7. Persistent non-boolean values should be accessed through methods called getXxx() and setXxx (). Boolean values are accessed using methods called isXxx()and setXxx().

    对于布尔类型 ， 可以按常规属性的规则编写getter/setter方法外， getter方法可以使用 is + ()的形式来代替

Steps to Create a JavaBean

为所需的bean功能编写程序代码(确保bean类实现了Serializable接口)

为每个属性（前端可能用到的所有属性）添加 accessor and mutator

编译bean，并尽可能将它(以及任何所需的资源文件)包装在JAR (Java Archive)文件中

public class BMIBean implements Serializable {
	public static String[] bmiType = {"Normal", "Overweight", "Obese", 			"Extremely Obese"};
	public static double[] bmiThreshold = {20.0,25.0,30.0,40.0};
	private double height, weight;
	public void setHeight(double h) { height = h; }
	public void setWeight(double w) { weight = w; }
	public double getHeight() { return height; }
	public double getWeight() { return weight; }
	public String calculate() throws Exception {
	if (height <= 0.0) 
        throw new Exception("Divide by zero");
		double ratio = weight/(height * height);
		for (int i = 3; i >= 0; i--)
			if (ratio > bmiThreshold[i]) 
                return bmiType[i];
			return "You are seriously underweight.Please seek medical advice.";
		}
}

Note: No constructor given（defaultconstructor has no arguments.）

JavaBean

如何使用（在JSP中如何使用JavaBean）？
1. jsp:useBean action lets you load a bean within a JSP page

用于获取 Bean 对象。jsp:useBean 首先会在scope内查找 Bean 对象，如果 Bean 对象不存在，则创建 Bean 对象。（相当于在servlet中实例化一个类）

<jsp:useBean id=“name” class=“package.class” />

1）id

表示 Bean 实例化对象的变量名，可以在指定范围内使用该变量名。

2）class

表示类的路径 package.class。

3）scope

指定 Bean 的作用域（实例化对象的作用域），取值为：

  • page(default value):：只能在当前页面使用该 Bean 对象；（最小范围）

    The bean is stored in the PageContext object for the duration of the current request

  • request：只能在一次请求范围内使用该 Bean 对象；（可以转发给别人）

    The bean is stored in the HttpServletRequest object

  • session：只能在一次会话范围内使用该 Bean 对象；

    The bean is stored in the HttpSession object.

  • application: The bean is stored in the ServletContext and can be accessed by other pages or servlets.

这通常意味着“实例化由class指定的类的对象，并将其绑定到由id指定的名称的变量”。

<jsp:useBean id=“bmi” class=“elem004.BMIBean” />等价于<% elem004.BMIBean bmi = new elem004.BMIBean(); %>

2.jsp:setProperty

设置 Bean 的属性值

<jsp:setProperty name = "beanName" property = "propname" value = "attributeValue"/>

name 指定需要修改属性的 Bean对象的名称；property 指定 Bean 对象的属性，即 Bean 类中的属性（成员变量）；value 是要设定的属性值。也可以替换为param：将后面跟着的参数的值直接赋值给这个属性

（本质上就是调用set方法）

  1. 属性名区分大小写。

  2. 值必须用单引号或双引号括起来。

  3. 非容器元素应该以/>结束标记。

       <jsp:setProperty name=“bmi” property=“height” value=“1.6”/>
   等价于
   <% bmi.setHeight(1.6); %>
   前提是这个属性以及被定义才能这么用
   
   
   <jsp:setProperty name=“beanName” property=“someProperty” 
   value=‘<% request.getParameter(”someParam”) %>’ />
   简化为：
   <jsp:setProperty name=“beanName” property=“someProperty”
param=“someParam”/>
   转换到servlet：
bean.someProperty=setSomeProperty(request.getParameter(”someParam”));

    如果bean的属性和输入参数（前端表单里传入Http中的参数）具有完全相同的名称，则可以直接将它们关联起来。（不用写 param）

如

<jsp:setProperty name=“beanName” property=“someProperty” />

设置所有属性(有参数的属性)

<jsp:setProperty name=“beanName” property=“*” />
3.jsp:getProperty
<jsp:getProperty name = "beanName" property = "attributeName">
<jsp:getProperty name=“bmi” property=“height” />
等价于
<%= bmi.getHeight() %>

HANDLING ERRORS

<%@ page errorPage=”BMIError.jsp” %>

INCLUDING FILES IN A JSP

two ways to include other files

  • Using the include directive 用includejsp指令

Used to include other JSP pages at translation time.

If file is changed, all JSP files that include it must also be updated.

引入的文件发生变化，要重新加载JSP文件

  • Using the jsp:include action
  • <%@ include file=”ContactSection.jsp” %>

Used to include static HTML or other text files at request time.

Included files can be modified without retranslating JSP page.

引入的文件发生变化，可以不用重新加载JSP文件

Useful for updating web information by non-programmers.

对于非程序员更新网页信息很有用

引入文件（include directive）：

<%@ page import="java.util.Date" %>
<%-- The following become fields in each servlet that
results from a JSP page that includes this file. --%>
<%! private int accessCount = 0;
private Date accessDate = new Date();
private String accessHost = "<I>No previous access</I>"; %>
<P><HR>This page &copy; 2000 
<A HREF="http//www.my-company.com/">my-company.com</A>.
This page has been accessed <%= ++accessCount %>
times since server reboot. It was last accessed from 
<%= accessHost %> at <%= accessDate %>.
<% accessHost = request.getRemoteHost(); %>
<% accessDate = new Date(); %>
<BODY>
<TABLE BORDER=5 ALIGN="CENTER"><TR><TH CLASS="TITLE">
Some Random Page
</TABLE>
<P> Information about our products and services.
<P> Blah, blah, blah.
<P> Yadda, yadda, yadda.
<%@ include file="ContactSection.jsp" %>
</BODY>
image-20221214174507216 
<%@ include file="ContactSection.jsp" %>

jsp:include action

<BODY>
<CENTER>
<TABLE BORDER=5>
<TR><TH CLASS="TITLE">
What's New at JspNews.com
</TABLE>
</CENTER>
<P>Here is a summary of our four most recent news stories:
<OL>
<LI> <jsp:include page="news/Item1.htm" flush="true" />
<LI> <jsp:include page="news/Item2.htm" flush="true" />
<LI> <jsp:include page="news/Item3.htm" flush="true" />
<LI> <jsp:include page="news/Item4.htm" flush="true" />
</OL>
</BODY>
<jsp:include page="news/Item1.htm" flush="true" />

Redirect and Forward

jsp:forward action 

<jsp:forward page=“Relative URL” />
<% String destination = “myDestination.jsp”; %>
<jsp:forward page=“<%= destination %>” />

Forward必须在响应提交给客户端之前调用，在主体输出缓冲区被刷新之前。

如果响应缓冲区中有IllegalStateException，则在输出之前自动清除未提交的输出

所以一般来说，如果你在servlet中访问了ServletOutputStream或PrintWriter对象，你不能使用这个方法