JSP面试问题在参加Java面试时非常重要。JSP是任何Java EE Web应用程序的重要组成部分，最近我已经写了很多关于JSP不同特性的文章。我们从JSP的基础开始，涉及JSP脚本，最后介绍了使用JSP自定义标签来避免在JSP页面中使用Java代码。您可能对一些教程感兴趣：
JSP面试问题
这里我提供了一些重要的 JSP 面试问题及答案，以帮助您准备面试。这些问题和答案适用于有经验的程序员和初学者。在查看答案之前，我建议您阅读以上教程，尝试自己回答并测试您对 JSP 的理解。
- JSP 是什么，为什么我们需要它？
- JSP 生命周期的各个阶段是什么？
- JSP 生命周期方法有哪些？
- 哪些 JSP 生命周期方法可以被重写？
- 我们如何避免客户端浏览器直接访问 JSP 页面？
- JSP 中有哪些不同类型的注释？
- 在 JSP 中，什么是脚本块、表达式和声明？
- JSP 隐含对象有哪些？
- 我们可以在 JSP 声明中定义的方法中使用 JSP 隐含对象吗？
- 在普通的 JSP 页面中，哪个隐含对象不可用？
- PageContext 隐含对象的好处是什么？
- 我们如何为 JSP 配置 init 参数？
- 为什么不建议在JSP中使用脚本元素？
- 我们能在JSP页面中定义一个类吗？
- 如何禁用JSP页面中的Java代码或脚本？
- 解释JSP动作元素或动作标签？
- include指令和jsp:include动作之间的区别是什么？
- JSP表达式语言是什么，它有什么好处？
- JSP EL隐含对象是什么，它与JSP隐含对象有何不同？
- 如何使用JSP EL获取HTTP方法名称？
- 什么是JSP标准标签库，提供一些示例用法？
- JSTL标签有哪些类型？
- 什么是JSP自定义标签，它有哪些组件？
- 举例说明何时需要使用JSP自定义标签？
- 为什么我们不需要在web.xml中配置JSP标准标签？
- 如何处理JSP服务方法抛出的异常？
- 如何使用JSTL捕获异常并处理？
- 如何在JSP中打印“<br>在HTML中创建新行”？
- jsp-config在部署描述符中是什么？
- 如何忽略JSP中的EL表达式评估？
- 容器何时初始化多个JSP/Servlet对象？
- 我们可以在JSP页面中使用JavaScript吗？
- 如何防止在JSP中隐式创建会话？
- JspWriter和Servlet PrintWriter之间有什么区别？
- 如何扩展JSP技术？
- 提供一些JSP最佳实践？
JSP面试问题和答案
-
什么是JSP，我们为什么需要它？
JSP代表JavaServer Pages。JSP是一种用于创建动态网页的Java服务器端技术。JSP是Servlet技术的扩展，帮助开发人员使用类似HTML的语法创建动态页面。我们也可以在Servlet中创建用户视图，但代码会变得非常丑陋且容易出错。此外，网页上的大多数元素都是静态的，因此JSP页面更适合网页。我们应该避免在JSP页面中包含业务逻辑，尽量仅将其用于视图目的。JSP脚本元素可用于在JSP页面中编写Java代码，但最好避免使用它们，并使用JSP动作元素、JSTL标签或自定义标签来实现相同的功能。JSP的另一个好处是大多数容器都支持JSP页面的热部署。只需在JSP页面中进行必要的更改，然后用更新后的JSP页面替换部署目录中的旧页面，容器将加载新的JSP页面。我们不需要编译项目代码或重新启动服务器，而如果我们在Servlet代码中进行更改，则需要重新构建整个项目并部署它。尽管大多数容器现在都为应用程序提供热部署支持，但与JSP页面相比，这仍然是更多的工作。
-
什么是 JSP 生命周期阶段？
如果你查看 JSP 页面代码，它看起来像 HTML，与 Java 类一点也不像。实际上，JSP 容器负责将 JSP 页面转换并创建用于 Web 应用程序的 Servlet 类。JSP 生命周期阶段包括：
- 翻译 – JSP 容器检查 JSP 页面代码并将其解析以生成 Servlet 源代码。例如，在 Tomcat 中，你会在 TOMCAT/work/Catalina/localhost/WEBAPP/org/apache/jsp 目录下找到生成的 Servlet 类文件。如果 JSP 页面名为 home.jsp，通常生成的 Servlet 类名为 home_jsp，文件名为 home_jsp.java。
- 编译 – JSP 容器在此阶段编译 jsp 类源代码并生成类文件。
- 类加载 – 容器在此阶段将类加载到内存中。
- 实例化 – 容器调用生成类的无参构造函数将其加载到内存并实例化。
- 初始化 – 容器调用 JSP 类对象的 init 方法，并使用部署描述符中配置的 init 参数初始化 Servlet 配置。在此阶段之后，JSP 准备好处理客户端请求。通常情况下，从翻译到 JSP 初始化发生在首次请求 JSP 时，但我们可以像 Servlets 一样使用 load-on-startup 元素将其配置为在部署时加载和初始化。
- 请求处理 – 这是 JSP 页面最长的生命周期，JSP 页面处理客户端请求。处理是多线程的，类似于 Servlet，并且对于每个请求，都会生成一个新线程，并创建 ServletRequest 和 ServletResponse 对象，并调用 JSP 服务方法。
- 销毁 – 这是 JSP 生命周期的最后阶段，其中 JSP 类从内存中卸载。通常情况下，当应用程序被卸载或服务器关闭时会发生这种情况。
-
JSP 生命周期方法是什么？
JSP 生命周期方法包括：
- jspInit()：此方法在
JspPage中声明，由JSP容器实现。此方法在JSP生命周期中调用一次，用于使用部署描述符中配置的配置参数对其进行初始化。我们可以使用JSP声明脚本元素覆盖此方法，以初始化在JSP页面中要使用的任何资源。 - _jspService()：这是由JSP容器为每个客户端请求调用的JSP方法，通过传递请求和响应对象。请注意，方法名称以下划线开头，以区别于其他生命周期方法，因为我们无法覆盖此方法。所有的JSP代码都放在此方法中，并且默认情况下是被覆盖的。我们不应尝试使用JSP声明脚本元素覆盖它。此方法在HttpJspPage接口中定义。
- jspDestroy()：此方法在JSP从内存中卸载时由容器调用，例如关闭应用程序或容器。此方法在JSP生命周期中仅调用一次，我们应该覆盖此方法来释放在JSP初始化方法中创建的任何资源。
- jspInit()：此方法在
-
哪些 JSP 生命周期方法可以被重写？
我们可以使用 JSP 声明脚本元素重写 jspInit() 和 jspDestroy() 方法。应该重写 jspInit() 方法来创建在 JSP 服务方法中使用的共享资源，并重写 jspDestroy() 方法来释放这些共享资源。
-
如何避免客户端浏览器直接访问 JSP 页面？
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 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.
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; %>`.
**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
详细了解它们的每一个在JSP隐含对象。19. ### 我们可以在JSP声明中定义的方法中使用JSP隐含对象吗？
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.
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.
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.
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.
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.
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 {
//这将放在方法体内部，所以不能是public
}
%>
```
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.
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).
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.
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 `${name}` and we can use EL implicit objects and EL operators to retrieve the attributes from different scopes and use them in JSP page. Read more about JSP EL with example program at [**JSP EL Tutorial**](/community/tutorials/jsp-expression-language-el-example-tutorial).
JSP Expression Language provides many implicit objects that we can use to get attributes from different scopes and parameter values. Note that these are different from JSP implicit objects and contains only the attributes in given scope. The only common implicit object in JSP EL and JSP page is pageContext object. Below table provides a list of implicit object in JSP EL.
| JSP EL Implicit Objects | Type | Description |
| --- | --- | --- |
| pageScope | Map | A map that contains the attributes set with page scope. |
| requestScope | Map | Used to get the attribute value with request scope. |
| sessionScope | Map | Used to get the attribute value with session scope. |
| applicationScope | Map | Used to get the attributes value from application scope. |
| param | Map | Used to get the request parameter value, returns a single value |
| paramValues | Map | Used to get the request param values in an array, useful when request parameter contain multiple values. |
| header | Map | Used to get request header information. |
| headerValues | Map | Used to get header values in an array. |
| cookie | Map | Used to get the cookie value in the JSP |
| initParam | Map | Used to get the context init params, we can't use it for servlet init params |
| pageContext | pageContext | Same as JSP implicit pageContext object, used to get the request, session references etc. example usage is getting request HTTP Method name. |
We can use pageContext JSP EL implicit object to get the request object reference and use dot operator to get the HTTP method name in JSP page. The JSP EL code for this will be `${pageContext.request.method}`.
JSP 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).
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.
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.
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).
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.
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).
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.
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>
```
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://journaldev.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.
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>
```
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>
```
然后，如果我们可以使用URI模式访问同一个JSP页面，并且它们都将有自己的初始化参数值。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.
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" %>
```
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.
We can extend JSP technology with custom tags to avoid scripting elements and java code in JSP pages.
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.
如果我漏掉了任何重要的 JSP 面试问题，请告诉我，我将在将来添加更多到列表中。
Source:
https://www.digitalocean.com/community/tutorials/jsp-interview-questions-and-answers