Servlets是Java EE的一个非常重要的主题，所有的Web应用框架，如Spring和Struts都是构建在其之上的。这使得Servlet面试问题成为面试中的热门话题。在这里，我提供了一个包含50个Servlet面试问题及答案的列表，以帮助您解决与Servlet和Java中的Web应用程序相关的大多数面试问题。
Servlet面试问题
- Web服务器和应用服务器之间的区别是什么？
- 哪种HTTP方法是非幂等的？
- GET方法和POST方法之间有什么区别？
- MIME类型是什么？
- 什么是Web应用程序，其目录结构是什么？
- Servlet是什么？
- Servlet相对于CGI的优势是什么？
- Servlet容器执行的常见任务有哪些？
- ServletConfig对象是什么？
- ServletContext对象是什么？
- ServletConfig 和 ServletContext 之间有什么区别？
- 什么是请求调度器？
- PrintWriter 和 ServletOutputStream 之间有什么区别？
- 在 servlet 中可以同时获取 PrintWriter 和 ServletOutputStream 吗？
- 如何在 servlet 中创建死锁情况？
- Servlet 包装类的作用是什么？
- SingleThreadModel 接口是什么？
- 我们需要重写 service() 方法吗？
- 创建 servlet 构造函数是个好主意吗？
- GenericServlet 和 HttpServlet 之间有什么区别？
- 什么是 servlet 间通信？
- Servlet 是否线程安全？如何在 servlet 中实现线程安全？
- Servlet 属性及其范围是什么？
- 如何从一个 servlet 调用另一个 servlet？
- 如何在不同应用程序中调用另一个 servlet？
- ServletResponse sendRedirect()和RequestDispatcher forward()方法有什么区别？
- 为什么HttpServlet类被声明为抽象的？
- Servlet生命周期有哪些阶段？
- Servlet的生命周期方法有哪些？
- 为什么我们只应该重写无参init()方法？
- 什么是URL编码？
- Servlet中会话管理的不同方法有哪些？
- 什么是URL重写？
- Servlet中的Cookie是如何工作的？
- 在会话失效或超时时，如何通知会话中的对象？
- encodeRedirectUrl和encodeURL之间的区别是什么？
- 为什么我们需要Servlet过滤器？
- 确保只有在用户有有效会话时才能访问所有Servlet的有效方法是什么？
- 为什么我们需要Servlet监听器？
- 如何使用另一个Servlet处理应用程序抛出的异常？
- 什么是部署描述符？
- 如何确保在应用程序启动时加载Servlet？
- 如何在服务器中获取Servlet的实际路径？
- 如何在Servlet中获取服务器信息？
- 编写一个Servlet在服务器上上传文件。
- 在Servlet中如何进行数据库连接和log4j集成？
- 如何在Servlet中获取客户端的IP地址？
- Servlet 3的重要特性有哪些？
- Servlet身份验证的不同方式有哪些？
- 如何为我们的Web应用程序实现传输层安全性？
Servlet面试问题与回答
-
Web服务器和应用服务器有何不同？
Web服务器的责任是处理来自客户端浏览器的HTTP请求，并以HTML响应进行回应。Web服务器理解HTTP语言，并在HTTP协议上运行。Apache Web服务器是一种Web服务器，然后我们有特定的容器可以执行Servlet和JSP，称为Servlet容器，例如Tomcat。应用服务器提供额外的功能，例如企业JavaBeans支持、JMS消息支持、事务管理等。因此，我们可以说应用服务器是具有额外功能的Web服务器，可以帮助开发人员开发企业应用程序。
-
哪种HTTP方法不是幂等的？
如果一个HTTP方法每次返回相同的结果，则称其为幂等。HTTP方法GET、PUT、DELETE、HEAD和OPTIONS都是幂等的方法，我们应该确保我们的应用程序始终使这些方法返回相同的结果。HTTP方法POST是不幂等的方法，当我们实现每个请求都会更改的功能时，应该使用POST方法。例如，要访问HTML页面或图像，我们应该使用GET，因为它总是返回相同的对象，但如果我们需要将客户信息保存到数据库中，我们应该使用POST方法。幂等方法也被称为安全方法，对于安全方法，我们不关心客户端的重复请求。
-
GET和POST方法之间有什么区别？
- GET是一种安全方法（幂等），而POST是非幂等方法。
- 我们可以使用GET方法发送有限的数据，它发送在标头请求URL中，而我们可以使用POST发送大量数据，因为它是请求主体的一部分。
- GET方法不安全，因为数据暴露在URL中，我们可以轻松地将其加为书签并再次发送类似的请求；POST方法是安全的，因为数据发送在请求主体中，我们无法将其加为书签。
- GET是默认的HTTP方法，而我们需要指定方法为POST以使用POST方法发送请求。
- 页面中的超链接使用GET方法。
-
MIME类型是什么？
“Content-Type”响应头被称为MIME类型。服务器将MIME类型发送到客户端，以让它们知道正在发送的数据类型。它帮助客户端渲染用户的数据。一些常用的MIME类型包括text/html、text/xml、application/xml等。我们可以使用ServletContext的getMimeType()方法获取文件的正确MIME类型，并用它来设置响应内容类型。在通过servlet从服务器下载文件时，这非常有用。
-
什么是Web应用程序，以及它的目录结构是什么？
Web Applications are modules that run on the server to provide both static and dynamic content to the client browser. Apache webserver supports PHP and we can create a web application using PHP. Java provides web application support through Servlets and JSPs that can run in a servlet container and provide dynamic content to the client browser. Java Web Applications are packaged as Web Archive (WAR) and it has a defined structure like below image. [](https://journaldev.nyc3.cdn.digitaloceanspaces.com/2013/08/WAR-directory-structure.png) Read more about web applications at [**Java Web Application**](/community/tutorials/java-web-application-tutorial-for-beginners).
Java Servlet is server side technologies to extend the capability of web servers by providing support for dynamic response and data persistence. The `javax.servlet` and `javax.servlet.http` packages provide interfaces and classes for writing our own servlets. All servlets must implement the javax.servlet.Servlet interface, which defines servlet lifecycle methods. When implementing a generic service, we can extend the GenericServlet class provided with the Java Servlet API. The HttpServlet class provides methods, such as doGet() and doPost(), for handling HTTP-specific services. Most of the times, web applications are accessed using HTTP protocol and that's why we mostly extend HttpServlet class. Servlet API hierarchy is shown in the below image. [](https://journaldev.nyc3.cdn.digitaloceanspaces.com/2013/08/Servlet-Hierarchy.png) Read more at [**Servlet Tutorial**](/community/tutorials/servlet-jsp-tutorial).
Servlet technology was introduced to overcome the shortcomings of CGI technology.
- Servlets provide better performance than CGI in terms of processing time, memory utilization because servlets use benefits of multithreading and for each request, a new thread is created, that is faster than loading creating new Object for each request with CGI.
- Servlets and platform and system independent, the web application developed with Servlet can be run on any standard web container such as Tomcat, JBoss, Glassfish servers and on operating systems such as Windows, Linux, Unix, Solaris, Mac, etc.
- Servlets are robust because container takes care of the life cycle of servlet and we don’t need to worry about memory leaks, security, garbage collection, etc.
- Servlets are maintainable and the learning curve is small because all we need to take care is business logic for our application.
Servlet containers are also known as web container, for example, Tomcat. Some of the important tasks of servlet container are:
- **Communication Support**: Servlet Container provides easy way of communication between web client (Browsers) and the servlets and JSPs. Because of the container, we don’t need to build a server socket to listen for any request from the web client, parse the request and generate a response. All these important and complex tasks are done by container and all we need to focus is on business logic for the applications.
- **Lifecycle and Resource Management**: Servlet Container takes care of managing the life cycle of servlet. From the loading of servlets into memory, initializing servlets, invoking servlet methods and to destroy them. The container also provides utility like JNDI for resource pooling and management.
- **Multithreading Support**: Container creates a new thread for every request to the servlet and provides them request and response objects to the processing. So servlets are not initialized for each request and save time and memory.
- **JSP Support**: JSPs doesn’t look like normal java classes but every JSP in the application is compiled by container and converted to Servlet and then container manages them like other servlets.
- **Miscellaneous Task**: Servlet container manages the resource pool, perform memory optimizations, execute garbage collector, provides security configurations, support for multiple applications, hot deployment and several other tasks behind the scene that makes a developer life easier.
`javax.servlet.ServletConfig` is used to pass configuration information to Servlet. Every servlet has it’s own **ServletConfig** object and servlet container is responsible for instantiating this object. We can provide servlet init parameters in web.xml file or through use of WebInitParam annotation. We can use getServletConfig() method to get the ServletConfig object of the servlet.
`javax.servlet.ServletContext` interface provides access to web application parameters to the servlet. The ServletContext is unique object and available to all the servlets in the web application. When we want some init parameters to be available to multiple or all of the servlets in the web application, we can use ServletContext object and define parameters in web.xml using <context-param> element. We can get the ServletContext object via the _getServletContext()_ method of ServletConfig. Servlet containers may also provide context objects that are unique to a group of servlets and which is tied to a specific portion of the URL path namespace of the host. ServletContext is enhanced in Servlet Specs 3 to introduce methods through which we can programmatically add Listeners and Filters and Servlet to the application. It also provides some utility methods such as _getMimeType()_, _getResourceAsStream()_ etc.
Some of the differences between ServletConfig and ServletContext are:
- ServletConfig is a unique object per servlet whereas ServletContext is a unique object for complete application.
- ServletConfig is used to provide init parameters to the servlet whereas ServletContext is used to provide application level init parameters that all other servlets can use.
- We can't set attributes in ServletConfig object whereas we can set attributes in ServletContext that other servlets can use in their implementation.
RequestDispatcher interface is used to forward the request to another resource that can be HTML, JSP or another servlet in the same application. We can also use this to include the content of another resource to the response. This interface is used for inter-servlet communication in the same context. There are two methods defined in this interface:
1. void forward(ServletRequest request, ServletResponse response) – forwards the request from a servlet to another resource (servlet, JSP file, or HTML file) on the server.
2. void include(ServletRequest request, ServletResponse response) – includes the content of a resource (servlet, JSP page, HTML file) in the response.
We can get RequestDispatcher in a servlet using ServletContext getRequestDispatcher(String path) method. The path must begin with a / and is interpreted as relative to the current context root.
PrintWriter is a character-stream class whereas ServletOutputStream is a byte-stream class. We can use PrintWriter to write character based information such as character array and String to the response whereas we can use ServletOutputStream to write byte array data to the response. We can use ServletResponse getWriter() to get the PrintWriter instance whereas we can use ServletResponse getOutputStream() method to get the ServletOutputStream object reference. You can read more about IO in java at [**Java IO Tutorial**](/community/tutorials/java-io-tutorial).
We can't get instances of both PrintWriter and ServletOutputStream in a single servlet method, if we invoke both the methods; getWriter() and getOutputStream() on response; we will get `java.lang.IllegalStateException` at runtime with message as other method has already been called for this response.
We can create deadlock in servlet by making a loop of method invocation, just call doPost() method from doGet() method and doGet() method to doPost() method to create deadlock situation in servlet. Read more about deadlock in multithreading at [**Java Deadlock Example**](/community/tutorials/deadlock-in-java-example).
Servlet HTTP API provides two wrapper classes - `HttpServletRequestWrapper` and `HttpServletResponseWrapper`. These wrapper classes are provided to help developers with custom implementation of servlet request and response types. We can extend these classes and override only specific methods we need to implement for custom request and response objects. These classes are not used in normal servlet programming.
SingleThreadModel interface was provided for thread safety and it guarantees that no two threads will execute concurrently in the servlet's service method. However, SingleThreadModel does not solve all thread-safety issues. For example, session attributes and static variables can still be accessed by multiple requests on multiple threads at the same time, even when SingleThreadModel servlets are used. Also, it takes out all the benefits of multithreading support of servlets, that's why this interface is Deprecated in Servlet 2.4.
When servlet container receives client request, it invokes the service() method which in turn invokes the doGet(), doPost() methods based on the HTTP method of request. I don't see any use case where we would like to override the service() method. The whole purpose of service() method is to forward to request to corresponding HTTP method implementations. If we have to do some pre-processing of request, we can always use servlet filters and listeners.
We can define a constructor for servlet but I don't think it's of any use because we won't be having access to the ServletConfig object until unless servlet is initialized by the container. Ideally, if we have to initialize any resource for the servlet, we should override init() method where we can access servlet init parameters using ServletConfig object.
GenericServlet is protocol independent implementation of Servlet interface whereas HttpServlet is HTTP protocol specific implementation. Most of the times we use servlet for creating web application and that's why we extend HttpServlet class. HttpServlet class extends GenericServlet and also provide some other methods specific to HTTP protocol.
When we want to invoke another servlet from a servlet service methods, we use inter-servlet communication mechanisms. We can invoke another servlet using RequestDispatcher forward() and include() methods and provide additional attributes in request for other servlet use.
HttpServlet init() method and destroy() method are called only once in the servlet life cycle, so we don't need to worry about their synchronization. But service methods such as doGet() or doPost() are getting called in every client request and since servlet uses multithreading, we should provide thread safety in these methods. If there are any local variables in service methods, we don't need to worry about their thread-safety because they are specific to each thread but if we have a shared resource then we can use synchronization to achieve thread-safety in servlets when working with shared resources. The thread safety mechanisms are similar to thread safety in standalone java application, read more about them at [**Thread Safety in Java**](/community/tutorials/thread-safety-in-java).
Servlet attributes are used for inter-servlet communication, we can set, get and remove attributes in web application. There are three scopes for servlet attributes – request scope, session scope and application scope. ServletRequest, HttpSession, and ServletContext interfaces provide methods to get/set/remove attributes from request, session and application scope respectively. Servlet attributes are different from init parameters defined in web.xml for ServletConfig or ServletContext.
We can use RequestDispatcher forward() method to forward the processing of a request to another servlet. If we want to include the another servlet output to the response, we can use RequestDispatcher include() method.
We can't use RequestDispatcher to invoke servlet from another application because it's specific for the application. If we have to forward the request to a resource in another application, we can use the ServletResponse sendRedirect() method and provide the complete URL of another servlet. This sends the response to the client with the response code as 302 to forward the request to another URL. If we have to send some data also, we can use cookies that will be part of the servlet response and sent in the request to another servlet.
1. RequestDispatcher forward() is used to forward the same request to another resource whereas ServletResponse sendRedirect() is a two step process. In sendRedirect(), web application returns the response to client with status code 302 (redirect) with URL to send the request. The request sent is a completely new request.
2. forward() is handled internally by the container whereas sednRedirect() is handled by browser.
3. We should use forward() when accessing resources in the same application because it's faster than sendRedirect() method that required an extra network call.
4. In forward() browser is unaware of the actual processing resource and the URL in address bar remains same whereas in sendRedirect() URL in address bar change to the forwarded resource.
5. forward() can't be used to invoke a servlet in another context, we can only use sendRedirect() in this case.
HttpServlet class provide HTTP protocol implementation of servlet but it's left abstract because there is no implementation logic in service methods such as doGet() and doPost() and we should override at least one of the service methods. That's why there is no point in having an instance of HttpServlet and is declared abstract class. Read more about [**abstract class**](/community/tutorials/abstract-class-in-java).
We know that Servlet Container manages the life cycle of Servlet, there are four phases of servlet life cycle.
1. Servlet Class Loading - When container receives a request for a servlet, it first loads the class into memory and calls it's default no-args constructor.
2. Servlet Class Initialization - Once the servlet class is loaded, container initializes the ServletContext object for the servlet and then invoke its init method by passing the servlet config object. This is the place where a servlet class transforms from normal class to servlet.
3. Request Handling - Once the servlet is initialized, it's ready to handle the client requests. For every client request, servlet container spawns a new thread and invokes the service() method by passing the request and response object reference.
4. Removal from Service - When container stops or we stop the application, servlet container destroys the servlet class by invoking the destroy() method.
Servlet Life Cycle consists of three methods:
1. public void init(ServletConfig config) - This method is used by container to initialize the servlet, this method is invoked only once in the lifecycle of servlet.
2. public void service(ServletRequest request, ServletResponse response) - This method is called once for every request, container can't invoke service() method until unless init() method is executed.
3. public void destroy() - This method is invoked once when servlet is unloaded from memory.
If we have to initialize some resource before we want our servlet to process client requests, we should override the init() method. If we override init(ServletConfig config) method, then the first statement should be super(config) to make sure superclass init(ServletConfig config) method is invoked first. That's why GenericServlet provides another helper init() method without argument that get's called at the end of init(ServletConfig config) method. We should always utilize this method for overriding init() method to avoid any issues as we may forget to add super() call in overriding init method with ServletConfig argument.
URL Encoding is the process of converting data into CGI form so that it can travel across the network without any issues. URL Encoding strips the white spaces and replaces special characters with escape characters. We can use java.net.URLEncoder.encode(String str, String unicode) to encode a String. URL Decoding is the reverse process of encoding and we can use java.net.URLDecoder.decode(String str, String unicode) to decode the encoded string. For example "Pankaj's Data" is encoded to "Pankaj%27s+Data".
The session is a conversational state between client and server and it can consist of multiple request and response between client and server. Since HTTP and Web Server both are stateless, the only way to maintain a session is when some unique information about the session (session-id) is passed between server and client in every request and response. Some of the common ways of session management in servlets are:
1. User Authentication
2. HTML Hidden Field
3. Cookies
4. URL Rewriting
5. Session Management API
Read more about these session management approaches in detail at **Servlet Session Management Tutorial**.
We can use HttpSession for session management in servlets but it works with Cookies and we can disable the cookie in client browser. Servlet API provides support for URL rewriting that we can use to manage session in this case. The best part is that from a coding point of view, it’s very easy to use and involves one step – encoding the URL. Another good thing with Servlet URL Encoding is that it’s a fallback approach and it kicks in only if browser cookies are disabled. We can encode URL with HttpServletResponse encodeURL() method and if we have to redirect the request to another resource and we want to provide session information, we can use encodeRedirectURL() method. Read More at [**Servlet URL Rewriting**](/community/tutorials/java-session-management-servlet-httpsession-url-rewriting#servlet-url-rewriting).
Cookies are used a lot in web client-server communication, it’s not something specific to java. Cookies are text data sent by server to the client and it gets saved at the client local machine. Servlet API provides cookies support through javax.servlet.http.Cookie class that implements Serializable and Cloneable interfaces. HttpServletRequest getCookies() method is provided to get the array of Cookies from the request, since there is no point of adding Cookie to request, there are no methods to set or add a cookie to request. Similarly, HttpServletResponse addCookie(Cookie c) method is provided to attach cookie in the response header, there are no getter methods for a cookie. Read more at [**Cookies in Servlets**](/community/tutorials/java-servlet-cookies-example).
If we have to make sure an object gets notified when session is destroyed, the object should implement `javax.servlet.http.HttpSessionBindingListener` interface. This interface defines two callback methods - valueBound() and valueUnbound() that we can define to implement processing logic when the object is added as attribute to the session and when session is destroyed. Recommended reading [**Servlet Listener**](/community/tutorials/servletcontextlistener-servlet-listener-example).
HttpServletResponse provide method to encode URL in HTML hyperlinks so that the special characters and white spaces are escaped and append session id to the URL. It behaves similar to URLEncoder encode method with additional process to append jsessionid parameter at the end of the URL. However HttpServletResponse encodeRedirectUrl() method is used specially for encode the redirect URL in response. So when we are providing URL rewriting support, for hyperlinks in HTML response, we should use encodeURL() method whereas for redirect URL we should use encodeRedirectUrl() method.
Servlet Filters are pluggable java components that we can use to intercept and process requests before they are sent to servlets and response after servlet code is finished and before container sends the response back to the client. Some common tasks that we can do with filters are:
- Logging request parameters to log files.
- Authentication and authorization of request for resources.
- Formatting of request body or header before sending it to servlet.
- Compressing the response data sent to the client.
- Alter response by adding some cookies, header information etc.
Read more about filters at [**Servlet Filter**](/community/tutorials/java-servlet-filter-example-tutorial).
We know that servlet filters can be used to intercept request between a servlet container and servlet, we can utilize it to create an authentication filter and check if the request contains a valid session or not. Check out Authentication Filter example at [**Servlet Filter Example**](/community/tutorials/java-servlet-filter-example-tutorial#servlet-filter-example).
We know that using ServletContext, we can create an attribute with application scope that all other servlets can access but we can initialize ServletContext init parameters as String only in the deployment descriptor (web.xml). What if our application is database-oriented and we want to set an attribute in ServletContext for Database Connection. If your application has a single entry point (user login), then you can do it in the first servlet request but if we have multiple entry points then doing it everywhere will result in a lot of code redundancy. Also if the database is down or not configured properly, we won’t know until the first client request comes to the server. To handle these scenarios, servlet API provides Listener interfaces that we can implement and configure to listen to an event and do certain operations. Read more about different types of listeners and example at [**Servlet Listener**](/community/tutorials/servletcontextlistener-servlet-listener-example).
If you notice, doGet() and doPost() methods throw ServletException and IOException. Since browser understand only HTML, when our application throw exception, servlet container processes the exception and generate a HTML response. Same goes with other error codes like 404, 403 etc. Servlet API provides support for custom Exception and Error Handler servlets that we can configure in the deployment descriptor, the whole purpose of these servlets are to handle the Exception or Error raised by application and send HTML response that is useful for the user. We can provide a link to the application home page or some details to let the user know what went wrong. We can configure them in web.xml like below:
```
<error-page>
<error-code>404</error-code>
<location>/AppExceptionHandler</location>
</error-page>
<error-page>
<exception-type>javax.servlet.ServletException</exception-type>
<location>/AppExceptionHandler</location>
</error-page>
```
Read more at [**Servlet Exception Handling**](/community/tutorials/servlet-exception-and-error-handling-example-tutorial).
The deployment descriptor is a configuration file for the web application and its name is web.xml and it resides in WEB-INF directory. Servlet container uses this file to configure web application servlets, servlet config params, context init params, filters, listeners, welcome pages and error handlers. With servlet 3.0 annotations, we can remove a lot of clutter from web.xml by configuring servlets, filters, and listeners using annotations.
Usually, servlet container loads a servlet on the first client request. Sometimes the servlet is heavy and takes time to loads, we might want to load it on application startup. We can use a load-on-startup element with servlet configuration in the web.xml file or use WebServlet annotation loadOnStartup variable to tell the container to load the servlet on system startup.
```
<servlet>
<servlet-name>foo</servlet-name>
<servlet-class>com.foo.servlets.Foo</servlet-class>
<load-on-startup>5</load-on-startup>
</servlet>
```
The load-on-startup value should be int, if it's a negative integer then servlet container will load the servlet based on client requests and requirement but if it's 0 or positive, then the container will load it on application startup. If there are multiple servlets with load-on-startup value such as 0,1,2,3 then lower integer value servlet will be loaded first.
We can use following code snippet to get the actual path of the servlet in file system. `getServletContext().getRealPath(request.getServletPath())`
We can use below code snippet to get the servlet information in a servlet through servlet context object. `getServletContext().getServerInfo()`
File Upload and Download and common tasks in a java web application. Unfortunately Servlet API doesn't provide easy methods to upload file on server, so we can use Apache FileUpload jar to make our life easier. Please read [**File Upload Servlet**](/community/tutorials/servlet-upload-file-download-example) post that provide all the necessary details with example program to upload and download file using servlets.
If you work with database connection a lot in your web application, its best to initialize it in a servlet context listener and set it as a context attribute for other servlets to use. Integrating Log4j is also very easy in web applications, all we need is a log4j configuration XML or property file and then configure it in a servlet context listener. For complete example, please look into [**Servlet Database and Log4j Example**](/community/tutorials/servlet-jdbc-database-connection-example).
We can use `request.getRemoteAddr()` to get the client IP address in servlet.
Servlet Specs 3.0 was a major release and some of the important features are:
1. **Servlet Annotations**: Prior to Servlet 3, all the servlet mapping and its init parameters were used to defined in web.xml, this was not convenient and more error prone when number of servlets are huge in an application. Servlet 3 introduced the use of Java annotations to define a servlet, filter and listener servlets and init parameters. Some of the important Servlet API annotations are WebServlet, WebInitParam, WebFilter, and WebListener. Read more about them at [Servlet 3 annotations](/community/tutorials/servlet-tutorial-java#servlet-3-annotations).
2. **Web Fragments**: Prior to servlet specs 3.0, all the web application configurations are required to be present in the web.xml that makes it cluttered with a lot of elements and chances of error increases. So servlet 3 specs introduced web fragments where we can have multiple modules in a single web application, all these modules should have a web-fragment.xml file in META-INF directory. We can include all the elements of web.xml inside the web-fragment.xml too. This helps us in dividing our web application into separate modules that are included as a JAR file in the web application lib directory.
3. **Adding Web Components dynamically**: We can use ServletContext object to add servlets, filters and listeners programmatically. This helps us in building a dynamic system where we are loading a component only if we need it. These methods are addServlet(), addFilter() and addListener() defined in the servlet context object.
4. **Asynchronous Processing**: Asynchronous support was added to delegate the request processing to another thread rather than keeping the servlet thread busy. It can increase the throughput performance of the application. This is an advance topic and I recommend to read [**Async Servlet**](/community/tutorials/async-servlet-example) tutorial.
Servlet Container provides different ways of login based servlet authentication:
1. **HTTP Basic Authentication**
2. **HTTP Digest Authentication**
3. **HTTPS Authentication**
4. **Form Based Login**: A standard HTML form for authentication, advantage is that we can change the login page layout as our application requirements rather than using HTTP built-in login mechanisms.
We can configure our servlet container to use SSL for message communication over the network. To configure SSL on Tomcat, we need a digital certificate that can be created using Java keytool for a development environment. For the production environment, you should get the digital certificate from SSL certificate providers, for example, Verisign or Entrust. Read more at [**Configure SSL on Tomcat**](/community/tutorials/steps-to-configure-ssl-on-tomcat-and-setup-auto-redirect-from-http-to-https) article.
这就是关于Servlet面试问题和答案的全部内容。我会继续在将来添加更多基于Servlet的面试问题到列表中，所以别忘了收藏以备将来参考。请在评论中分享您的想法，并通过在Google Plus、Facebook或Twitter上分享您的爱来分享。:) 更新: 如果您喜欢这些问题，我相信您也会喜欢JSP面试问题。参考资料：
Source:
https://www.digitalocean.com/community/tutorials/servlet-interview-questions-and-answers