`

java中JSP的面试题

阅读更多
Q: What is a output comment? (可以在客户端的页面的源码中出现的注释)

A: A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser.

JSP Syntax
<!-- comment [ <%= expression %> ] -->

Example 1
<!-- This is a commnet sent to client on
<%= (new java.util.Date()).toLocaleString() %>
-->

Displays in the page source:
<!-- This is a commnet sent to client on January 24, 2004 -->


Q: What is a Hidden Comment?(隐藏注释,就是该注释不会发送给客户端,即当在浏览器中选择页面的源码时,不会显示。)

A: A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags. A hidden comment is not sent to the client, either in the displayed JSP page or the HTML page source. The hidden comment is useful when you want to hide or "comment out" part of your JSP page.
You can use any characters in the body of the comment except the closing --%> combination. If you need to use --%> in your comment, you can escape it by typing --%\>.

JSP Syntax

<%-- comment --%>

Examples
<%@ page language="java" %>
<html>
<head><title>A Hidden Comment </title></head>
<body>
<%-- This comment will not be visible to the colent in the page source --%>
</body>
</html>



Q: What is a Expression?(什么是表达式)

A: An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. Because the value of an expression is converted to a String, you can use an expression within text in a JSP file. Like
<%= someexpression %>
<%= (new java.util.Date()).toLocaleString() %>
You cannot use a semicolon to end an expression 
不能使用分号作为表达式的终止符。

Q: What is a Declaration? (变量声明)

A: A declaration declares one or more variables or methods for use later in the JSP source file.
A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as they are separated by semicolons. The declaration must be valid in the scripting language used in the JSP file.

<%! somedeclarations %>
<%! int i = 0; %>
<%! int a, b, c; %>



Q: What is a Scriptlet? (脚本)

A: A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language.Within scriptlet tags, you can
1.Declare variables or methods to use later in the file (see also Declaration).

2.Write expressions valid in the page scripting language (see also Expression).

3.Use any of the JSP implicit objects or any object declared with a <jsp:useBean> tag.
You must write plain text, HTML-encoded text, or other JSP tags outside the scriptlet.

Scriptlets are executed at request time, when the JSP engine processes the client request. If the scriptlet produces output, the output is stored in the out object, from which you can display it.


Q: What are implicit objects? List them?(jsp的内置对象有哪些)

A: Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects re listed below
request
response
pageContext
session
application
out
config
page
exception



Q: Difference between forward and sendRedirect?(forward与sendRedirect的区别?)

A: When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward. 
 

Q: What are the different scope valiues for the <jsp:useBean>?

A: The different scope values for <jsp:useBean> are

1. page
2. request
3.session
4.application




Q: Explain the life-cycle mehtods in JSP?

A: THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three interfaces. The JspPage interface declares only two mehtods - jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the client-server protocol. However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests. This interface declares one method _jspService().
The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance.
The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects.
The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance.


Q: How do I prevent the output of my JSP or Servlet pages from being cached by the browser?

A: You will need to set the appropriate HTTP header attributes to prevent the dynamic content output by the JSP page from being cached by the browser. Just execute the following scriptlet at the beginning of your JSP pages to prevent them from being cached at the browser. You need both the statements to take care of some of the older browser versions.

<%
response.setHeader("Cache-Control","no-store"); //HTTP 1.1
response.setHeader("Pragma\","no-cache"); //HTTP 1.0
response.setDateHeader ("Expires", 0); //prevents caching at the proxy server
%>




Q: How does JSP handle run-time exceptions?

A: You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example:
<%@ page errorPage=\"error.jsp\" %> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage=\"true\" %> Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage attribute.

Q: How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it?

A: You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" %> within your JSP page. With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through the admin screen for your JSP engine. More importantly, avoid using the tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use <%! %>. You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe .




Q: How do I use a scriptlet to initialize a newly instantiated bean?

A: A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.

The following example shows the “today” property of the Foo bean initialized to the current date when it is instantiated. Note that here, we make use of a JSP expression within the jsp:setProperty action.

<jsp:useBean id="foo" class="com.Bar.Foo" > 

<jsp:setProperty name="foo" property="today" 
value="<%=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>" / >

<%-- scriptlets calling bean setter methods go here --%>

</jsp:useBean > 



Q: How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values?

A: You could make a simple wrapper function, like

<%!
String blanknull(String s) {
return (s == null) ? \"\" : s;
}
%>

then use it inside your JSP form, like
<input type="text" name="lastName" value="<%=blanknull(lastName)% >" >


 

Q: What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization?

A: Although the SingleThreadModel technique is easy to use, and works well for low volume sites, it does not scale well. If you anticipate your users to increase in the future, you may be better off implementing explicit synchronization for your shared data. The key however, is to effectively minimize the amount of code that is synchronzied so that you take maximum advantage of multithreading.
Also, note that SingleThreadModel is pretty resource intensive from the server\'s perspective. The most serious issue however is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the unserviced requests are queued until something becomes free - which results in poor performance. Since the usage is non-deterministic, it may not help much even if you did add more memory and increased the size of the instance pool.




Q: How can I enable session tracking for JSP pages if the browser has disabled cookies?

A: We know that session tracking uses cookies by default to associate a session identifier with a unique user. If the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using URL rewriting. URL rewriting essentially includes the session ID within the link itself as a name/value pair. However, for this to be effective, you need to append the session ID for each and every link that is part of your servlet response. Adding the session ID to a link is greatly simplified by means of of a couple of methods: response.encodeURL() associates a session ID with a given URL, and if you are using redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input. Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported by the browser; if so, the input URL is returned unchanged since the session ID will be persisted as a cookie.

Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact with each other. Basically, we create a new session within hello1.jsp and place an object within this session. The user can then traverse to hello2.jsp by clicking on the link present within the page. Within hello2.jsp, we simply extract the object that was earlier placed in the session and display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the URL, allowing hello2.jsp to still retrieve the session object. Try this example first with cookies enabled. Then disable cookie support, restart the brower, and try again. Each time you should see the maintenance of the session across pages. Do note that to get this example to work with cookies disabled at the browser, your JSP engine has to support URL rewriting.


hello1.jsp
<%@ page session=\"true\" %>
<%
Integer num = new Integer(100);
session.putValue("num",num);
String url =response.encodeURL("hello2.jsp");
%>
<a href=\'<%=url%>\'>hello2.jsp</a>

hello2.jsp
<%@ page session="true" %>
<%
Integer i= (Integer )session.getValue("num");
out.println("Num value in session is " + i.intValue());
%> 


Q: What is the difference b/w variable declared inside a declaration part and variable declared in scriplet part?

A: Variable declared inside declaration part is treated as a global variable.that means after convertion jsp file into servlet that variable will be in outside of service method or it will be declared as instance variable.And the scope is available to complete jsp and to complete in the converted servlet class.where as if u declare a variable inside a scriplet that variable will be declared inside a service method and the scope is with in the service method. 


Q: Is there a way to execute a JSP from the comandline or from my own application?

A: There is a little tool called JSPExecutor that allows you to do just that. The developers (Hendrik Schreiber <hs@webapp.de> & Peter Rossbach <pr@webapp.de>) aim was not to write a full blown servlet engine, but to provide means to use JSP for generating source code or reports. Therefore most HTTP-specific features (headers, sessions, etc) are not implemented, i.e. no reponseline or header is generated. Nevertheless you can use it to precompile JSP for your website.


分享到:
评论

相关推荐

    Java/Jsp面试题汇总

    Java/Jsp面试题汇总主要涵盖了Java编程语言及JSP(JavaServer Pages)的相关技术要点,这些知识点是面试中经常被问到的,也是开发者必备的基础技能。以下是对这些主题的详细解析: 1. **Java基础知识**: - 类与...

    最新Java面试题视频网盘,Java面试题84集、java面试专属及面试必问课程

    │ Java面试题20.jsp和Servlet的相同点和不同点?.mp4 │ Java面试题21.内置对象和四大作用域和页面传值.mp4 │ Java面试题22.Session和Cookie的区别.mp4 │ Java面试题23.mvc模式和mvc各部分的实现.mp4 │ Java面试...

    java经典面试题

    ### Java经典面试题知识点 #### Java数据结构容器 - **核心知识点**:Java集合框架,包括List、Set、Map等接口及其实现类。List接口代表有序的集合,例如ArrayList和LinkedList;Set接口代表不允许重复元素的集合,...

    java与jsp面试试题

    这份"java与jsp面试试题"集合提供了一套全面的面试问题,帮助求职者和开发者准备相关职位的面试。以下是对这些知识点的详细说明: 1. **Java基础** - 类与对象:理解面向对象编程的基本概念,包括类的定义、对象的...

    JAVA项目经理面试题.pdf

    由于提供的【部分内容】信息量非常庞大且部分内容重复、混乱,难以从中准确提取出完整的知识点。但从这些内容片段中,...在实际的面试中,这些问题将帮助面试官评估候选人是否具备带领JAVA项目成功的必要知识和经验。

    java面试题,jsp面试题集,软件工程师面试题集

    ### Java面试题概览 #### 1. Java类基础 - **`java.lang.Thread`:** 此类为线程提供了基本的功能支持。在Java中创建线程时可以直接继承此类或者实现`Runnable`接口。 - **`java.lang.Number`:** `Number`类是一个...

    java企业面试题大全(18个文档)

    java企业面试题大全(包括18个文档)。1、15个经典面试题.pdf。2、120个Java经典面试题和答案(上).pdf。3、120个Java经典面试题和答案(下).pdf。4、EJB方面.pdf。5、java web面试题.pdf。6、Java基础方面.pdf。7...

    JAVA笔试面试资料JDBC HTTP、JSP、Servlet、Struts面试题汇总资料.zip

    Java重点面试题 – 针对java面试过程中经常遇到的一些试题进行总结.docx JAVA面试基础知识点总结.docx Java面试笔记.docx 写出正则表达式,从一个字符串中提取链接地址.docx 出现几率最高和覆盖范围最广的一套经典...

    知名企业java高级工程师面试题附答案

    ### Java高级工程师面试知识点解析 #### JSP与Servlet的异同及联系 - **相同点**:JSP和Servlet都属于Java Web开发的核心技术之一,主要用于动态网页的生成。 - **不同点**: - **本质区别**:JSP本质上是简化版...

    Java面试题,jsp&servlet面试题

    Java是企业级应用开发的...这些面试题涵盖了Java基础、集合框架、多线程、网络编程、Servlet和JSP等多个核心领域,全面展示了开发者应该具备的技能和知识。理解和掌握这些内容对于成为一名优秀的Java开发人员至关重要。

    个人面试题总结(java,数据库,前端).zip

    文件中包含了本人最近在网上总结的面试题,有java面试题,jq面试题,jsp、servlet、ajax面试题,mysql面试题,oracle面试题,redis教案,也有最近时间总结的公司面试题,涉及的层面虽然不是很多,但是应对面试 应该...

    最全java面试题及答案(208道).pdf

    "Java面试题及答案详解" Java是目前最流行的编程语言之一,在软件开发行业中广泛应用。以下是Java面试题及答案的详解: Java基础 1. JDK 和 JRE 的区别是什么? JDK(Java Development Kit)是Java开发工具包,...

    jsp页经典面试题java程序员面试经常考到的面试题总结.pdf

    jsp页经典面试题java程序员面试经常考到的面试题总结 jsp是一种基于Java技术的服务器端编程语言,广泛应用于Web开发中。以下是jsp页经典面试题总结,涵盖了jsp的基本概念、jsp与ssi的混合使用、线程安全、html表单...

    jsp面试题题库

    在选择题 1 中,我们看到 JSP 本身已加载的基本类包括 `java.lang.*`、`java.io.*`、`javax.servlet.*` 和 `javax.servlet.jsp.*`。这些类都是 JSP 的基础类,提供了基本的功能和工具。 在选择题 2 中,我们看到 ...

    java程序员经典面试题

    Java 程序员经典面试题 本资源为Java程序员面试必备资料,涵盖了Java基础、线程编程、JSP、Servlet、JDBC、JDO、EJB、应用服务器、J2EE、MVC、设计模式等多个方面的知识点,旨在帮助Java程序员更好地准备面试,获得...

    jsp面试题及答案

    jsp面试题及答案 JSP(Java Server Pages)是一种服务器端脚本语言,用于创建动态网页。下面是关于JSP的一些知识点: 1.JSP与HTML的关系 JSP与HTML之间的关系是,JSP在服务器端执行,返回给客户端的是一个HTML...

    JAVA编程百例+JAVA面试题集十套(经典)

    面试题(很多企业都常常从这些面试题库中选题作为笔试题): Java就业面试题大全.pdf Java最全的面试题.pdf java面试100题目.pdf SQL面试题大全.pdf 开发面试75条.pdf java 面试题 数据库方面.pdf 如何写出高性能的...

    java面试大全视频版

    Java面试题01....Java面试题20.jsp和Servlet的相同点和不同点 Java面试题21.内置对象和四大作用域和页面传值 Java面试题22.Session和Cookie的区别和使用场景 Java面试题23.mvc模式和mvc各部分的实现

    java,jsp面试题

    根据给定文件的信息,我们可以提炼出一系列与Java和JSP相关的面试知识点,下面将逐一进行详细解释。 ### JSP指令标签解析 1. **`jsp:include`**:该指令用于在请求处理阶段动态地包含另一个页面。它可以分为两种...

    java程序员面试题——863面试集

    - **JSP 面试题**: 覆盖 JSP 基础、生命周期、内置对象等。 - **Servlet 面试题**: 重点在于 Servlet 生命周期、请求处理机制等。 - **Struts 面试题**: 涉及 MVC 架构、配置文件解析、拦截器等。 - **Spring 面试题...

Global site tag (gtag.js) - Google Analytics