Servlet JSP Tutorial -- 1.15 50 Servlet Interview Questions and Answers

谁借莪１个温暖的怀抱￠ 2022-06-01 00:44 175阅读 0赞

Servlet是Java EE非常重要的主题，所有的 Web 应用程序框架（如Spring和Struts）都建立在它之上。这使得 servlet 面试在面试中成为一个热门话题。

在这里，我提供了一个包含 50 个 servlet 面试问题的列表，其中包括解答大部分与 java 中的 servlet 和 web 应用程序有关的面试问题。

## Servlet Interview Questions and Answers ##

#### **1. What is different between web server and application server?** ####

A web server responsibility is to handler HTTP requests from client browsers and respond with HTML response. A web server understands HTTP language and runs on HTTP protocol.

Apache Web Server is kind of a web server and then we have specific containers that can execute servlets and JSPs known as servlet container, for example Tomcat.

Application Servers provide additional features such as Enterprise JavaBeans support, JMS Messaging support, Transaction Management etc. So we can say that Application server is a web server with additional functionalities to help developers with enterprise applications.

#### **1. Web 服务器和应用服务器的差别是什么?** ####

Web 服务器的职责是处理来自客户端浏览器的 HTTP request，并用 HTML response 进行响应。Web 服务器了解 HTTP 语言并运行在 HTTP 协议之上。  
Apache Web Server 是一种 Web 服务器，我们有特定的可以执行 servlet 和 JSP 的容器，就是 servlet 容器，栗如 Tomcat  
应用程序服务器提供了额外的功能，栗如 Enterprise JavaBeans 支持， JMS 消息传递支持，事务管理等。所以我们可以说 Application Server 是一个具有附加功能的 Web 服务器，可以帮助开发人员使用企业应用程序。

#### **2. Which HTTP method is non-idempotent?** ####

A HTTP method is said to be idempotent if it returns the same result every time. HTTP methods GET, PUT, DELETE, HEAD, and OPTIONS are idempotent method and we should implement our application to make sure these methods always return same result. HTTP method POST is non-idempotent method and we should use post method when implementing something that changes with every request.

#### **2. 哪个 HTTP 方法是非幂等的?** ####

如果 HTTP 方法每次返回相同的结果，就说它是幂等的。HTTP 方法 GET、PUT、DELETE、HEAD 和 OPTIONS 是幂等方法，我们应该实现我们的应用程序来确保这些方法始终返回相同的结果。HTTP 方法 POST 是非幂等方法，我们应该使用 POST 方法来实现随每个请求而改变的东西。

#### **3. What is the difference between GET and POST method?** ####

*  GET is a safe method (idempotent) where POST is non-idempotent method.
 *  We can send limited data with GET method and it’s sent in the header request URL whereas we can send large amount of data with POST because it’s part of the body.
 *  GET method is not secure because data is exposed in the URL and we can easily bookmark it and send similar request again, POST is secure because data is sent in request body and we can’t bookmark it.
 *  GET is the default HTTP method whereas we need to specify method as POST to send request with POST method.
 *  Hyperlinks in a page uses GET method.

#### **3. GET 和 POST 方法有什么区别** ####

*  GET 是一个安全的方法（幂等），POST 是非幂等方法。
 *  我们可以使用 GET 方法发送有限的数据，并将其发送到 header request URL 中，而我们可以使用 POST 发送大量数据，因为它是 body 的一部分。
 *  GET 方法是不安全的，因为数据暴露在 URL 中，我们可以很容易地将它加入书签帮再次发送类似的请求。POST 是安全的，因为数据是在 request body 发送的，我们不能将它加入书签。
 *  GET 是默认的 HTTP 方法，而我们需要使用 POST 方法指定方法作为 POST 发送请求
 *  页面中的超链接使用 GET 方法

#### **4. What is MIME Type?** ####

The “Content-Type” response header is known as MIME Type. Server sends MIME type to client to let them know the kind of data it’s sending. It helps client in rendering the data for user. Some of the mostly used mime types are text/html, text/xml, application/xml etc.

We can use ServletContext getMimeType() method to get the correct MIME type of the file and use it to set the response content type. It’s very useful in downloading file through servlet from server.

#### **4. 什么是 MIME 类型？** ####

“Content-Type”response header 被称为MIME类型。服务器发送MIME类型给客户端，让他们知道它发送的数据的种类。 它帮助客户为用户呈现数据。一些最常用的MIME类型有`text/html`、`text/xml`和`application/xml`等。

我们可以使用`ServletContext`的`getMimeType()`方法获取文件的正确的MIME类型，并使用它来 response content 类型。从服务器通过 servlet 下载文件时非常有用。

#### **5. What is a web application and what is it’s directory structure?** ####

Web Applications are modules that run on server to provide both static and dynamic content to the client browser. Apache web server supports PHP and we can create 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 client browser.

Java Web Applications are packaged as Web Archive (WAR) and it has a defined structure like below image.  
![WAR-directory-structure][]

Read more about web applications at [Java Web Application][].

#### **5. 什么是 Web 应用程序，它的目录结构是什么** ####

Web 应用程序是服务器上用于向客户端提供静态和动态内容的模块。Apache Web 服务器支持 PHP，我们可以使用 PHP 创建 Web 应用程序。Java 通过在 servlet 容器中运行 Servlet 和 JSP 提供 Web 应用程序支持，并向客户端浏览器提供动态内容。

Java Web 应用程序打包为 Web Archive（WAR），并具有如下图所示的定义的结构  
![WAR-directory-structure][]  
在[Java Web Application][] 上阅读更多关于 Web 应用程序的信息。

#### **6. What is a servlet?** ####

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 thats why we mostly extend HttpServlet class. Servlet API hierarchy is shown in below image.

![这里写图片描述][SouthEast]

Read more at [Servlet Tutorial][].

#### **6. 什么是 Servlet？** ####

Java Servlet 是服务器端技术，通过提供动态响应和护具持久的支持来扩展 Web 服务器的功能。

`java.servlet` 和 `javax.servlet.http` 包提供了编写我们自己的 servlet 的接口和类。  
所有的 servlet 必须实现 `java.servlet.Servlet`接口，该接口定义了 servlet 的生命周期方法。在实现通用服务时，我们可以扩展 Java Servlet API 提供的 `GenericServlet` 类。`HttpServlet`类提供了用于处理特定 HTTP 服务的方法，如`doGet()`和`doPost()`。

大多数情况下，Web 应用程序是使用 HTTP 协议访问的，这就是为什么我们能主要扩展 HttpServlet 类的原因。Servlet API层次结构如下图所示。

![这里写图片描述][SouthEast]

阅读更多：[Servlet Tutorial][].

#### **7. What are the advantages of Servlet over CGI?** ####

Servlet technology was introduced to overcome the shortcomings of CGI technology.

*  Servlets provide better performance that CGI in terms of processing time, memory utilization because servlets uses 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 life cycle of servlet and we don’t need to worry about memory leaks, security, garbage collection etc.
 *  Servlets are maintainable and learning curve is small because all we need to take care is business logic for our application.

#### **7. Servlet 对比 CGI 有什么优势？** ####

引入了 Servlet 技术来克服 CGI 技术的缺点。

*  Servlet 在处理时间、内存利用率方面提供了更好的性能，这就是 servlet 使用多线程并为每个请求创建一个新线程的好处，这比使用 CGI 为每个请求创建新对象要快。
 *  Servlet 于平台和系统无关，使用 Servlet 开发的 Web 应用程序可以运行在任何标准 Web 容器上，如Tomcat、JBoss、Glassfish 服务器以及 Windows、Linux、Unix、Solaris、Mac 等操作系统。
 *  Servlet 是健壮的，因为容器负责 servlet 的生命周期，沃恩不需要担心内存泄漏、安全性、垃圾回收等。
 *  Servlet 是可维护的，学习曲线很小，因为我们只需要注意的是我们的应用程序的业务逻辑。

#### **8. What are common tasks performed by Servlet Container?** ####

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 container, we don’t need to build a server socket to listen for any request from web client, parse the request and generate 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. Container also provides utility like JNDI for resource pooling and management.
 *  `Multithreading Support`: Container creates new thread for every request to the servlet and provide them request and response objects to process. So servlets are not initialized for each request and saves 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.

#### **8. Servlet 容器执行的常见任务有那些？** ####

Servlet 容器也被称为 Web 容器，例如Tomcat。一些 servlet 容器的重要任务有：

*  `Communication Support（通信支持）`：Servlet 容器提供了简单的 Web 客户端（浏览器）与 servlet 和 JSP 之间的通信方式。由于容器的原因，我们不需要构建 server socket（服务器套接字）来监听来自 Web 客户端的请求，解析请求并生成响应。所有这些重要而复杂的任务都是通过容器完成的，我们需要关注的只是应用程序的业务逻辑。
 *  `Lifecycle and Resource Management（生命周期和资源管理）`：Servlet Container 负责管理 servlet 的生命周期。从将servlet 加载到内存中，初始化 servlet，调用 servlet 方法并销毁它们。容器也提供像 JNDI 这样的工具来进行资源池和管理。
 *  `Multithreading Support（多线程支持）`：Container 为 servlet 的每个请求创建一个新线程，并为它们提供 request 和response 对象进行处理。所以 servlet 不会为每个请求初始化，并节省时间和内存。
 *  `JSP Support（JSP支持）`：JSP 看起来不像普通的 Java 类，但应用程序中的每个 JSP 都是由容器编译并转换为 Servlet，然后容器像其他 servlet 一样管理它们。
 *  `Miscellaneous Task（杂项任务）`：Servlet 容器管理资源池，执行内存优化，执行垃圾回收器，提供安全配置，支持多个应用程序，热部署和场景后面的几个其他任务，使开发人员生活变得更容易。

#### **9. What is ServletConfig object?** ####

`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.

#### **9. 什么是 ServletConfig 对象？** ####

`javax.servlet.ServletConfig`被用于将配置信息传递给 Servle。每个 servlet 都有自己的 ServletConfig 对象，servlet 容器负责实例化这个对象。我们可以在`web.xml`文件中提供 Servlet 初始化参数，也可以使用 `WebInitParam` 注解。我们可以使用`getServletConfig()`方法类获取 Servlet 的`ServletConfig`对象。

#### **10. What is ServletContext object?** ####

`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.

#### **10. 什么是 ServletContext 对象？** ####

`javax.servlet.ServletContext`接口提供对 servlet 的 Web 应用程序参数的访问。`ServletContext`是可用于 Web 应用程序中所有的 servlet 的唯一的对象。当我们想要一些初始化参数可用于 Web 应用程序中的多个或所有 servlet 时，我们可以使用`ServletContext`对象并使用`<context-param>`元素在 `web.xml`中定义参数。我们可以通过`ServletConfig`的`getServletContext()`方法获取`ServletContext`对象。Servlet 容器还提供 context 对象，这些 context 对象对于一组 servlet 时唯一的。并且绑定到主机的 URL 路径名称空间的特定部分。

Servlet Specs 3 中增强了`ServletContext`，以引入方法，同故宫这些方法我们可以以编程方式将`Listener`和`Filters`以及`Servlet`添加到应用程序中。它还提供了一些实用的方法，如`getMimeType()`、`getResourceAsStream()`等。

#### **11. What is difference between ServletConfig and ServletContext?** ####

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.

#### **11. ServletConfig 和 ServletContext 之间的区别是什么？** ####

ServletConfig 和 ServletContext 之间的区别有：

*  `ServletConfig`是每个 servlet 的唯一对象。`ServletContext`是整个应用程序的唯一对象。
 *  `ServletConfig`用于给 servlet 提供 init parameters。`ServletContext`用于提供应用程序级别的可供所有其他 servlet使用的 init parameters。

#### **12. What is Request Dispatcher?** ####

`RequestDispatcher` interface is used to forward the request to another resource that can be HTML, JSP or another servlet in 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.  
3. 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.

#### **12. 什么是Request Dispatcher？** ####

`RequestDispatcher`接口是用于将请求转发到另一个资源的，该资源可以是同一应用程序中的 HTML、JSP 或其他 servlet。我们也可以使用这个来将其他资源的内容包含到 response 中。该接口用于在相同上下文中进行 内部 servlet 通信。

这个接口定义了两个方法：  
1. `void forward(ServletRequest request, ServletResponse response)`– 将请求从 servlet 转发到服务器上的另一个资源。  
2. `void include(ServletRequest request, ServletResponse response)` – 在响应中包含资源（servlet、JSP 页面、HTML 文件）  
3. 我们在 servlet 中可以使用`ServletContext`、`getRequestDispatcher(String path)`方法来获得`RequestDispatcher`。该路径必须以“/”开始，并解释为相对于当前上下文跟。

#### **13. What is difference between PrintWriter and ServletOutputStream?** ####

`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.

#### **13. PrintWriter 和 ServletOutputStream 之间的区别？** ####

`PrintWriter`是字符流类，而`ServletOutputStream`是字节流类。我们可以使用`PrintWriter`将基于字符的信息（如字符数组和字符串）写入 response，而我们使用`ServletOutputStream`将字节数组数据写入 response。

可以使用`ServletResponse`的`getWriter()`获取`PrintWriter`实例。使用`ServletResponse`的`getOutputStream()`方法获取`ServletOutputStream`对象的引用。

你可以在Java IO Tutorial上阅读更多关于 IO 的信息。

#### **14. Can we get PrintWriter and ServletOutputStream both in a servlet?** ####

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.

#### **14. 我们能在一个 servlet 中获取 PrintWriter 和 ServletOutputStream 吗？** ####

如果我们调用两个方法，不能在一个 servlet 中同时获取`PrintWriter`和`ServletOutputStream`的实例；`getWriter()`和`getOutputStream()`作为响应；我们将在运行时得到`java.lang.IllegalStateException`消息，其他方法已经为响应被调用。

#### **15. How can we create deadlock situation in servlet?** ####

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.

#### **15. 在 servlet 中我们如何去创建一个死锁状态?** ####

我们可以通过创建一个方法调用的循环来创建 servlet 死锁，只要从`doGet()`方法调用`doPost()`方法，`doPost()`方法调用`doGet()`方法在 servlet 中创建死锁情况。

在 Java Deadlock Example 中阅读更多有关多线程死锁的信息。

#### **16. What is the use of servlet wrapper classes?** ####

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.

#### **16. servlet 包装类?** ####

Servlet HTTP API 提供两个包装类 - `HttpServletRequestWrapper`和`HttpServletResponseWrapper`。提供这些包装类是为了帮助开发人员定制实现 servlet 请求和响应类型。我们可以扩展这些类并重写我们需要为自定义请求和响应对象实现的特定方法。这些类不用于通常的 servlet 编程。

#### **17.What is SingleThreadModel interface?** ####

`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, thats why this interface is Deprecated in Servlet 2.4.

#### **17. 什么是 SingleThreadModel 接口？** ####

`SingleThreadModel`接口是为了线程安全而提供的，它保证 servlet 的服务方法中不会同时执行两个线程。但是`SingleThreadModel`并不能解决所有的线程安全问题。栗如，即使在使用`SingleThreadModel`servlet 的时候，会话属性和静态变量依旧可以被多个线程上的多个请求访问。同时它抵消了 servlet 的多线程支持的所有好处，这就是这个接口在 Servlet 2.4 被弃用的原因。

#### **18. Do we need to override service() method?** ####

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 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.

#### **18, 我们需要去重写 service() 方法吗？** ####

当 servlet 容器收到客户端请求时，它会调用`service()`方法，然后调用基于请求的 HTTP 方法的`doGet()`、`doPost()`方法。我没有看到任何我们想要重写`service()`方法的用例。`service()`方法的全部用途是转发请求到相应的 HTTP 方法实现。如果我们需要做一些预处理请求，我们可以使用 servlet 过滤器和监听器。

#### **19. Is it good idea to create servlet constructor?** ####

We can define a constructor for servlet but I don’t think its of any use because we won’t be having access to the `ServletConfig` object until unless servlet is initialized by container. Ideally if we have to initialize any resource for servlet, we should override `init()` method where we can access servlet init parameters using `ServletConfig` object.

#### **19. 创建 servlet 构造器是个好主意吗?** ####

我们为 servlet 可以定义一个构造器，但是我不认为它有任何用处，除非 servlet 被容器初始化，否则不能将不能访问`ServletConfig`对象。理想情况下，如果我们必须初始化 servlet 的任何资源，我们应该重写`init()`方法，在那里我们可以使用`ServletConfig`对象来访问 Servlet 初始化参数。

#### **20. What is difference between GenericServlet and HttpServlet?** ####

`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.

#### **20. GenericServlet 和 HttpServlet 之间的区别是什么？** ####

`GenericServlet`是 Servlet 接口的独立协议实现，而`HttpServlet`是 HTTP 协议特定的实现。大多数情况下，我们使用 servlet 里创建 Web 应用程序，这就是我们扩展`HttpServlet`类的原因。`HttpServlet`类扩展了`GenericServlet`，还提供了一些特定于 HTTP 协议的其他方法。

#### **21. What is the inter-servlet communication?** ####

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.

#### **21. 什么是 servlet 间的通信？** ####

当我们想要从一个 servlet 服务方法中调用另一个 servlet 时，我们使用了 servlet 间通信机制。我们可以使用`RequestDispatcher` `forward()`和`include()`方法调用另一个 servlet，并在请求其他 servlet 时提供其他属性。

#### **22. Are Servlets Thread Safe? How to achieve thread safety in servlets?** ####

`HttpServlet` `init()` method and `destroy()` method are called only once in 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.

#### **22. Servlet 是线程安全的吗?如何在 servlet 中实现线程安全** ####

`HttpServlet`的`init()`方法和`destroy()`方法在 servlet 声明周期中只会被调用一次。所以我们不要去担心它们的同步。但是诸如`doGet()`或`doPost()`等服务方法在每个客户端请求中都会被调用，并且由于 servlet 使用多线程，所以我们应该在这些方法中提供线程安全性。

如果服务方法中有任何局部变量，我们不必担心它们的线程安全性，因为它们是针对每个线程的，但是如果我们有一个共享的资源，那么在使用共享时，我们可以使用同步来实现 servlet 中的线程安全资源。

线程安全机制类似于独立 Java 应用程序中的线程安全性，在 Thread Safety in Java 中阅读更多内容。

#### **23. What is servlet attributes and their scope?** ####

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`.

#### **23. 什么是 servlet 属性和它们的范围?** ####

Servlet 属性用于 servlet 间的通信，我们可以在 Web 应用程序中set、get 和 remove 属性。servlet 属性有三个范围 - request scope、session scope 和 application scope。

#### **24. How do we call one servlet from another servlet?** ####

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.

#### **24. 我们如何在一个 servlet 中调用另一个 servlet?** ####

我们可以使用`RequestDispatcher`的`forward()`方法来将请求处理转发给另一个 servlet。如果我们想要将另一个 servlet 输出包含到 response 中，我们可以使用`RequestDispatcher`的`include()`方法。

#### **25. How can we invoke another servlet in a different application?** ####

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 `ServletResponse` `sendRedirect()` method and provide complete URL of another servlet. This sends the response to client with 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.

#### **25. 我们如何在一个应用程序中调用另一个应用程序中的 servlet** ####

我们不能使用`RequestDispatcher`从另一个程序中调用 servlet，因为它是特定于应用程序的。如果我们必须将请求转发到另一个应用程序中的资源，我们可以使用`ServletResponse` 的`sendRedirect()`方法，并提供另一个 servlet 的完整 URL。这将 响应代码为 302 的 response 发送给客户端，将请求转发到另一个 URL。如果我们还要发送一些数据，我们可以使用 cookie 作为 servlet response 的一部分，并将请求发送到另一个 servlet。

#### **26. What is difference between ServletResponse sendRedirect() and RequestDispatcher forward() method?** ####

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 `sendRedirect()` 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.

#### **26. ServletResponse 的 sendRedirect() 方法和RequestDispatcher 的 forward() 方法有什么区别？** ####

1.  `RequestDispatcher`的`forward()`方法用来转发相同的请求到另一个资源，而`ServletResponse`的`sendRedirect()`方法则是一个两步的过程。在`sendRedirect()`中，Web 应用程序使用 URL 向客户端返回具有 302 状态码的发送请求。这个请求将发送一个全新的请求。
2.  `forward()`由容器内部处理，而`sendRedirect()`由浏览器处理。
3.  当访问统一应用程序中的资源时，我们应该使用`forward()`，因为它比需要额外网络调用的`sendRedirect()`方法要快。
4.  在`forward()`中，浏览器并不知道实际处理资源，地址栏的 URL 保持不变，而在`sendRedirect()`中，URL 转换为转发的资源。
5.  `forward()`不能用于在一个上下文中调用一个 servlet，这种情况下我梦只能使用`sendRedirect()`。

#### **27.Why HttpServlet class is declared abstract?** ####

`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.