JSP面试问题与回答

如果你要参加Java面试，JSP面试问题非常重要。JSP是任何Java EE网络应用的重要组成部分，最近我写了很多关于JSP不同特性的内容。我们从JSP的基础开始，涵盖了JSP脚本和使用JSP自定义标签避免在JSP页面中编写Java代码。以下是你可能感兴趣的一些教程。

1. 初学者的JSP教程

 

1. JSP隐式对象

 

1. JSP指令

 

1. JSP异常处理

 

1. JSP动作标签

 

1. JSP表达式语言

 

1. JSTL核心标签

 

JSP自定义标签

JSP面试问题

在这里，我提供了一些重要的JSP面试问题及其答案，以帮助你准备面试。无论你是经验丰富的程序员还是初学者，这些都是很好的JSP面试问题和答案。在阅读答案之前，我建议你先阅读上面的教程，并尝试自己回答问题，测试你对JSP的理解。

1. JSP是什么，为什么我们需要它？

 

1. JSP生命周期阶段有哪些？

 

1. JSP生命周期方法有哪些？

 

1. 哪些JSP生命周期方法可以被重写？

 

1. 如何避免客户端浏览器直接访问JSP页面？

 

1. JSP中有哪些不同类型的注释？

 

1. JSP中的Scriptlet、Expression和Declaration是什么意思？

 

1. JSP中有哪些隐式对象？

 

1. 我们可以在JSP声明中使用JSP隐式对象吗？

 

1. 在普通的JSP页面中哪个隐式对象不可用？

 

1. PageContext隐式对象的好处是什么？

 

1. 如何为JSP配置init参数？

 

1. 为什么不鼓励在JSP中使用脚本元素？

 

1. 我们可以在JSP页面中定义一个类吗？

 

1. 如何禁用JSP页面中的Java代码或脚本？

 

1. 解释一下JSP的Action元素或Action标签？

 

1. include指令和jsp:include动作的区别是什么？

 

1. JSP表达式语言是什么，它有什么好处？

 

1. JSP EL隐式对象是什么，它与JSP隐式对象有何区别？

 

1. 如何使用JSP EL获取HTTP方法名？

 

1. 什么是JSP标准标签库，举个例子使用的场景？

 

1. JSTL标签有哪些类型？

 

1. JSP自定义标签是什么，它的组成部分有哪些？

 

1. 举个你需要使用JSP自定义标签的例子？

 

1. 为什么我们不需要在web.xml中配置JSP标准标签？

 

1. 如何处理JSP服务方法抛出的异常？

 

1. 如何使用JSTL捕获异常并处理它？

 

1. 如何在JSP中打印“

 

1. 在HTML中创建一个新行”？

 

1. 部署描述符中的jsp-config是什么？

 

1. 如何在JSP中忽略EL表达式的评估？

 

1. 何时会初始化多个JSP/Servlet对象？

 

1. 我们可以在JSP页面中使用JavaScript吗？

 

1. 如何防止在JSP中隐式创建会话？

 

1. JspWriter和Servlet PrintWriter的区别是什么？

 

1. 如何扩展JSP技术？

 

提供一些JSP的最佳实践。

JSP面试问题与答案

1. 我们如何避免客户端浏览器直接访问JSP页面？

我们可以通过以下方法来避免客户端浏览器直接访问JSP页面：

1.将JSP页面放在WEB-INF目录下。在这个目录下的文件只能通过服务器来访问，而不能直接通过浏览器访问。这样可以防止用户直接访问JSP页面。

2.使用Servlet来作为中间人。可以创建一个Servlet作为JSP页面的代理，通过Servlet来处理客户端请求，并在Servlet中控制JSP页面的访问。

3.在web.xml文件中进行配置。可以在web.xml文件中配置过滤器，拦截掉对JSP页面的直接访问。只允许通过指定的路径或URL访问JSP页面。

以上这些方法都可以有效地避免客户端浏览器直接访问JSP页面，增加系统的安全性。

We know that anything inside the WEB-INF directory can't be accessed directly in a web application, so we can place our JSP pages in WEB-INF directory to avoid direct access to JSP page from the client browser. But in this case, we will have to configure it in deployment descriptor just like Servlets. The sample configuration is given below code snippet of web.xml file.

```
<servlet>
  <servlet-name>Test</servlet-name>
  <jsp-file>/WEB-INF/test.jsp</jsp-file>
  <init-param>
    <param-name>test</param-name>
    <param-value>Test Value</param-value>
  </init-param>
</servlet>
   
<servlet-mapping>
  <servlet-name>Test</servlet-name>
  <url-pattern>/Test.do</url-pattern>
</servlet-mapping>
```

JSP有哪些不同类型的评论？

JSP pages provide two types of comments that we can use:
-   **HTML Comments**: Since JSP pages are like HTML, we can use HTML comments like `<!-- HTML Comment -->`. These comments are sent to client also and we can see it in HTML source. So we should avoid any code level comments or debugging comments using HTML comments.
-   **JSP Comments**: JSP Comments are written using scriptlets like `<%-- JSP Comment --%>`. These comments are present in the generated servlet source code and doesn't sent to client. For any code level or debugging information comments we should use JSP comments.

JSP中Scriptlet、Expression和Declaration是什么？

Scriptlets, Expression and Declaration are scripting elements in JSP page using which we can add java code in the JSP pages. A scriptlet tag starts with `<%` and ends with `%>`. Any code written inside the scriptlet tags go into the `_jspService()` method. For example;

```
<%
Date d = new Date();
System.out.println("Current Date="+d);
%>
```

Since most of the times we print dynamic data in JSP page using _out.print()_ method, there is a shortcut to do this through JSP Expressions. JSP Expression starts with `<%=` and ends with `%>`. `<% out.print("Pankaj"); %>` can be written using JSP Expression as `<%= "Pankaj" %>` Notice that anything between `<%= %>` is sent as parameter to `out.print()` method. Also notice that scriptlets can contain multiple java statements and always ends with semicolon (;) but expression doesn't end with semicolon. JSP Declarations are used to declare member methods and variables of servlet class. JSP Declarations starts with `<%!` and ends with `%>`. For example we can create an int variable in JSP at class level as `<%! public static int count=0; %>`.

隐式对象是什么？

**JSP implicit objects** are created by container while translating JSP page to Servlet source to help developers. We can use these objects directly in **scriptlets** that goes in service method, however we can't use them in JSP Declaration because that code will go at class level. We have 9 implicit objects that we can directly use in JSP page. Seven of them are declared as local variable at the start of _\_jspService()_ method whereas two of them are part of _\_jspService()_ method argument that we can use.
1.  out Object
2.  request Object
3.  response Object
4.  config Object
5.  application Object
6.  session Object
7.  pageContext Object
8.  page Object
9.  exception Object
10.  JSP Implicit Objects Example

可以在 JSP 声明中定义的方法中使用 JSP 隐式对象吗？请在 JSP 隐式对象页面详细阅读每个对象的介绍。

No we can't because JSP implicit objects are local to service method and added by JSP Container while translating JSP page to servlet source code. JSP Declarations code goes outside the service method and used to create class level variables and methods and hence can't use JSP implicit objects.

在普通的JSP页面中，哪个隐含对象不可用？

JSP exception implicit object is not available in normal JSP pages and it's used in JSP error pages only to catch the exception thrown by the JSP pages and provide useful message to the client.

PageContext隐式对象的好处有哪些？

JSP pageContext implicit object is instance of javax.servlet.jsp.PageContext abstract class implementation. We can use pageContext to get and set attributes with different scopes and to forward request to other resources. pageContext object also hold reference to other implicit object. This is the only object that is common in both JSP implicit objects and in JSP EL implicit objects.

我们如何为JSP配置初始化参数？

We can configure init params for JSP similar to servlet in web.xml file, we need to configure JSP init params with servlet and servlet-mapping element. The only thing differs from servlet is jsp-file element where we need to provide the JSP page location.

为什么不鼓励在JSP中使用脚本元素？

JSP pages are mostly used for view purposes and all the business logic should be in the servlet or model classes. We should pass parameters to JSP page through attributes and then use them to create the HTML response in JSP page. Most part of the JSP page contains HTML code and to help web designers to easily understand JSP page and develop them, JSP technology provides action elements, JSP EL, JSP Standard Tag Library and custom tags that we should use rather than scripting elements to bridge the gap between JSP HTML part and JSP java part.

我们能在JSP页面中定义一个类吗？

It's not a good practice though, but we can define a class inside a JSP Page. Below is the sample code for this:

```
<%!
private static class NestedClass { //static is better because Servlet is multi-threaded
  private final int num = 0;
  public int getNum() {
    return num;
  }
}
%>
```

Or

```
<%      
    class Person { 
        //this will go inside method body, so can't be public
    }
%>
```

我们如何在JSP页面中禁用Java代码或脚本?

We can disable scripting elements in JSP pages through deployment descriptor configuration like below.

```
<jsp-config>
	<jsp-property-group>
		<url-pattern>*.jsp</url-pattern>
		<scripting-invalid>true</scripting-invalid>
	</jsp-property-group>
</jsp-config>
```

Above URL-pattern will disable scripting for all the JSP pages but if you want to disable it only for a specific page, you can give the JSP file name itself.

解释JSP Action Elements 或者 Action Tags。

JSP action elements or action tags are HTML like tags that provide useful functionalities such as working with Java Bean, including a resource, forwarding the request and to generate dynamic XML elements. JSP action elements always starts with jsp: and we can use them in JSP page directly without the need to import any tag libraries or any other configuration changes. Some of the important action elements are jsp:useBean, jsp:getProperty, jsp:setProperty, jsp:include and jsp:forward. Read more in details about these at [**JSP Action Elements**](/community/tutorials/jsp-action-tags-jsp-usebean-include-forward).

包含指令和jsp:include动作有何区别？

The difference between JSP include directive and include action is that in include directive the content to other resource is added to the generated servlet code at the time of translation whereas with include action it happens at runtime. Another difference is that in JSP include action, we can pass params to be used in the included resource with `jsp:param` action element but in JSP include directive we can't pass any params. When the included resource is static such as header, footer, image files then we should use include directive for faster performance but if the included resource is dynamic and requires some parameters for processing then we should use include action tag.

JSP表达式语言是什么，它有哪些好处？

Most of the times we use JSP for view purposes and all the business logic is present in servlet code or model classes. When we receive client request in the servlet, we process it and then add attributes in request/session/context scope to be retrieved in JSP code. We also use request params, headers, cookies and init params in JSP to create response views. We can use scriptlets and JSP expressions to retrieve attributes and parameters in JSP with java code and use it for view purpose. But for web designers, java code is hard to understand and that’s why JSP Specs 2.0 introduced Expression Language (EL) through which we can get attributes and parameters easily using HTML like tags. Expression language syntax is `${name}` and we can use EL implicit objects and EL operators to retrieve the attributes from different scopes and use them in JSP page. Read more about JSP EL with example program at [**JSP EL Tutorial**](/community/tutorials/jsp-expression-language-el-example-tutorial).

JSP EL隐式对象是什么，它与JSP隐式对象有什么不同？

JSP Expression Language provides many implicit objects that we can use to get attributes from different scopes and parameter values. Note that these are different from JSP implicit objects and contains only the attributes in given scope. The only common implicit object in JSP EL and JSP page is pageContext object. Below table provides a list of implicit object in JSP EL.

| JSP EL Implicit Objects | Type | Description |
| --- | --- | --- |
| pageScope | Map | A map that contains the attributes set with page scope. |
| requestScope | Map | Used to get the attribute value with request scope. |
| sessionScope | Map | Used to get the attribute value with session scope. |
| applicationScope | Map | Used to get the attributes value from application scope. |
| param | Map | Used to get the request parameter value, returns a single value |
| paramValues | Map | Used to get the request param values in an array, useful when request parameter contain multiple values. |
| header | Map | Used to get request header information. |
| headerValues | Map | Used to get header values in an array. |
| cookie | Map | Used to get the cookie value in the JSP |
| initParam | Map | Used to get the context init params, we can't use it for servlet init params |
| pageContext | pageContext | Same as JSP implicit pageContext object, used to get the request, session references etc. example usage is getting request HTTP Method name. |

如何使用JSP EL获取HTTP方法名称？

We can use pageContext JSP EL implicit object to get the request object reference and use dot operator to get the HTTP method name in JSP page. The JSP EL code for this will be `${pageContext.request.method}`.

JSP标准标签库是什么，可以提供一些使用示例吗？

JSP Standard Tag Library or JSTL is more versatile than JSP EL or Action elements because we can loop through a collection or escape HTML tags to show them like text in response. JSTL is part of the Java EE API and included in most servlet containers. But to use JSTL in our JSP pages, we need to download the JSTL jars for your servlet container. Most of the times, you can find them in the example projects and you can use them. You need to include these libraries in the project WEB-INF/lib directory. These jars are container specific, for example in Tomcat, we need to include jstl.jar and standard.jar jar files in the project build path. Read more about JSTL tags with example program at [**JSTL Tutorial**](/community/tutorials/jstl-tutorial-jstl-tags-example).

JSTL标签有哪些类型？

Based on the JSTL functions, they are categorized into five types.
1.  Core Tags - Core tags provide support for iteration, conditional logic, catch exception, url, forward or redirect response etc.
2.  Formatting and Localization Tags - These tags are provided for formatting of Numbers, Dates and i18n support through locales and resource bundles.
3.  SQL Tags - JSTL SQL Tags provide support for interaction with relational databases such as Oracle, MySql etc.
4.  XML Tags - XML tags are used to work with XML documents such as parsing XML, transforming XML data and XPath expressions evaluation.
5.  JSTL Functions Tags - JSTL tags provide a number of functions that we can use to perform common operation, most of them are for String manipulation such as String Concatenation, Split String etc.

JSP 自定义标签是什么，它包括哪些组件？

Sometimes JSP EL, Action Tags and JSTL tags are not enough and we might get tempted to write java code to perform some operations in JSP page. Fortunately JSP is extendable and we can create our own custom tags to perform certain operations. We can create JSP Custom Tags with following components:
-   JSP Custom Tag Handler
-   Creating Tag Library Descriptor (TLD) File
-   Deployment Descriptor Configuration for TLDWe can add custom tag library in JSP page using taglib directive and then use it.

给出一个你需要使用JSP自定义标签的例子。

Let’s say we want to show a number with formatting with commas and spaces. This can be very useful for user when the number is really long. So we want some custom tags like below: `<mytags:formatNumber number="123456.789" format="#,###.00"/>` Based on the number and format passed, it should write the formatted number in JSP page, for the above example it should print 123,456.79 We know that JSTL doesn’t provide any inbuilt tags to achieve this, so we will create our own custom tag implementation and use it in the JSP page. Read above example implementation at [**JSP Custom Tag**](/community/tutorials/jsp-custom-tags-example-tutorial).

为什么我们不需要在web.xml中配置JSP标准标签？

We don't need to configure JSP standard tags in web.xml because the TLD files are inside the META-INF directory of the JSTL jar files. When container loads the web application and finds TLD files inside the META-INF directory of the JAR file, it automatically configures them to be used directly in the application JSP pages. All we need to do it to include it in the JSP page using taglib directive.

我们如何处理JSP服务方法抛出的异常？

To handle exceptions thrown by the JSP page, all we need is an error page and define the error page in JSP using page directive. To create a JSP error page, we need to set the page directive attribute isErrorPage value to true, then we can access exception implicit object in the JSP and use it to send a customized error message to the client. We need to define exception and error handler JSP pages in the deployment descriptor like below.

```
<error-page>
	<error-code>404</error-code>
	<location>/error.jsp</location>
</error-page>

<error-page>
	<exception-type>java.lang.Throwable</exception-type>
	<location>/error.jsp</location>
</error-page>
```

Read more with example program at [**JSP Exception Handling**](/community/tutorials/jsp-exception-handling-jsp-error-page).

我们如何使用JSTL捕获并处理异常？

We can use JSTL Core tags c:catch and c:if to catch exception inside the JSP service method and process it. c:catch tag catches the exception and wraps it into the exception variable and we can use c:if condition tag to process it. Below code snippet provide sample usage.

```
<c:catch var ="exception">
   <% int x = 5/0;%>
</c:catch>
 
<c:if test = "${exception ne null}">
   <p>Exception is : ${exception} <br />
   Exception Message: ${exception.message}</p>
</c:if>
```

Notice the use of JSP EL in the `c:if` condition.

1. 在JSP中，我们如何打印出“

 

在HTML中创建一个新行”的内容？

We can use c:out escapeXml attribute to escape the HTML elements so that it get's shown as text in the browser, for this scenario we will write code like below.

```
<c:out value="<br> creates a new line in HTML" escapeXml="true"></c:out>
```

部署描述中的jsp-config是什么？

jsp-config element is used to configure different parameters for JSP pages. Some of it's usage are:
-   Configuring tag libraries for the web application like below.
    
    ```
    <jsp-config>
    		<taglib>
    			<taglib-uri>https://scdev.com/jsp/tlds/mytags</taglib-uri>
    			<taglib-location>/WEB-INF/numberformatter.tld</taglib-location>
    		</taglib>
    </jsp-config>
    ```
    
-   We can control scripting elements in JSP pages.
-   We can control JSP Expression Language (EL) evaluation in JSP pages.
-   We can define the page encoding for URL pattern.
-   To define the buffer size to be used in JSP page out object.
-   To denote that the group of resources that match the URL pattern are JSP documents, and thus must be interpreted as XML documents.

如何在JSP中忽略EL表达式的评估？

We can ignore EL evaluation in JSP page by two ways.
1.  Using page directive as `<%@ page isELIgnored="true" %>`
2.  Configuring in web.xml - better approach when you want to disable EL evaluation for many JSP pages.
    
    ```
    <jsp-config>
    	<jsp-property-group>
    		<url-pattern>*.jsp</url-pattern>
    		<el-ignored>true</el-ignored>
    	</jsp-property-group>
    </jsp-config>
    ```

容器什么时候会初始化多个JSP/Servlet对象？

If we have multiple servlet and servlet-mapping elements in deployment descriptor for a single servlet or JSP page, then container will initialize an object for each of the element and all of these instances will have their own ServletConfig object and init params. For example, if we configure a single JSP page in web.xml like below.

```
<servlet>
  <servlet-name>Test</servlet-name>
  <jsp-file>/WEB-INF/test.jsp</jsp-file>
  <init-param>
    <param-name>test</param-name>
    <param-value>Test Value</param-value>
  </init-param>
</servlet>
   
<servlet-mapping>
  <servlet-name>Test</servlet-name>
  <url-pattern>/Test.do</url-pattern>
</servlet-mapping>
   
<servlet>
  <servlet-name>Test1</servlet-name>
  <jsp-file>/WEB-INF/test.jsp</jsp-file>
</servlet>
   
<servlet-mapping>
  <servlet-name>Test1</servlet-name>
  <url-pattern>/Test1.do</url-pattern>
</servlet-mapping>
```

那么如果我们可以使用URI模式访问相同的JSP页面，那么两种方式将拥有各自的初始化参数值。65. ### 我们可以在JSP页面中使用JavaScript吗？

Yes why not, I have seen some developers getting confused with this. Even though JSP is a server-side technology, it's used to generate a client-side response and we can add javascript or CSS code like any other HTML page.

我们如何防止在JSP中隐式创建会话？

By default JSP page creates a session but sometimes we don’t need session in JSP page. We can use JSP page directive session attribute to indicate compiler to not create session by default. It’s default value is true and session is created. To disable the session creation, we can use it like below.

```
<%@ page session="false" %>
```

JspWriter和Servlet PrintWriter之间的区别是什么？

PrintWriter is the actual object responsible for writing the content in response. JspWriter uses the PrintWriter object behind the scene and provide buffer support. When the buffer is full or flushed, JspWriter uses the PrintWriter object to write the content into response.

我们如何扩展JSP技术?

We can extend JSP technology with custom tags to avoid scripting elements and java code in JSP pages.

提供一些JSP的最佳实践？

Some of the JSP best practices are:
1.  Avoid scripting elements in JSP pages. If JSP EL, action elements and JSTL not serve your needs then create custom tags.
2.  Use comment properly, use JSP comments for code level or debugging purpose so that it's not sent to client.
3.  Avoid any business logic in JSP page, JSP pages should be used only for response generation for client.
4.  Disable session creation in JSP page where you don't need it for better performance.
5.  Use page, taglib directives at the start of JSP page for better readability.
6.  Proper use of jsp include directive or include action based on your requirements, include directive is good for static content whereas include action is good for dynamic content and including resource at runtime.
7.  Proper exception handling using JSP error pages to avoid sending container generated response incase JSP pages throw exception in service method.
8.  If you are having CSS and JavaScript code in JSP pages, it's best to place them in separate files and include them in JSP page.
9.  Most of the times JSTL is enough for our needs, if you find a scenario where it's not then check your application design and try to put the logic in a servlet that will do the processing and then set attributes to be used in JSP pages.

请您告知我是否遗漏了任何重要的JSP面试问题，将来我会继续完善这个列表。