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How to understand the concept of Servlet and Servlet containers and the use of attention points, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain in detail for you, people with this need can come to learn, I hope you can get something.

Servlet

Servlet is a server-side programming component, the main ability to generate dynamic data content, like any java component, Servlet is also platform-independent. The operation of Servlet depends on a container (or servlet engine). At run time, the container loads Servlet bytecode into the virtual machine to survive a servlet instance and is responsible for managing the life cycle of the servlet instance.

The opposite side of the interaction with servlet can be a web browser or any client program that supports http protocol. The interaction between client and Servlet follows request/response mode. In fact, Servlet is designed not only to be limited to http protocol, but also to support any request/response-based protocol Servlet.

Servlet container

The Servlet container provides a running environment for servlet, and the servlet container is usually implemented as part of a Web server or application server.

The core function of the Servlet container is to accept the request, decode the request content, and then distribute the parsed request content to a relevant Servlet,Servlet for processing, the container will encode the data content returned by Servlet and send it to the client. During this process, the container can modify the request content before distributing the request to the servlet, or modify the response content before sending the return content of the servlet to the client.

The Servlet container must provide support for http 1.0 amp 1.1 or https.

Servlet has a complete life cycle.

The life cycle phase of a servlet mainly includes: loading and creating servlet instances, initializing servlet instances, processing requests, and destroying.

The life cycle phase of servlet is completely managed by the servlet container. From a programming point of view, the life cycle is mainly reflected in the three methods defined in the Servlet interface:

Init method, which gives the application the opportunity to perform initialization

Service method, where the application processes the request and responds to the content

Destroy method, which gives the application the opportunity to release all kinds of resources held by servlet

Any servlet must implement these three methods directly or indirectly. The Servlet instance is instantiated by the container without the involvement of the application, and the container can start loading the servlet after startup, or load the servlet the first time the servlet is selected to process the request.

The servlet container initializes the servlet after it has instantiated it. The container calls the init method and passes a reference to the ServletConfig object. Through the ServletConfig reference, you can sense some detailed configuration parameters of Servlet and ServletContext references representing the application in Servlet. The init method is called by the container only once during the entire life cycle of a servlet instance.

Only the initialized servlet can further provide the request service. If a ServletException exception is thrown in the init method or is not returned within the time defined by the container, the servlet container considers initialization failed and the servlet cannot provide service. The init method may throw a UnavailableException exception, which indicates that the container servlet is currently unavailable: permanently or temporarily unavailable. If it is temporarily unavailable, the UnavailableException should bring an estimated number of unavailable seconds. In this case, the container will not directly fail, but will block the estimated length of time and then recreate a servlet instance and perform initialization again.

After servlet is initialized successfully, the request can be processed. The container will call the service method and pass two references, ServletRequest and ServletResponse. ServletRequest encapsulates the client request information, and the user application is responsible for populating the ServletResponse object. When the service method returns, the process is taken over by the container.

If the service method throws a ServletException exception, the container needs to take the appropriate next step. The sevlet specification defines the following process: if the service method throws an actual UnavailableException exception, it means that the servlet is currently unavailable. There are also two cases: permanent unavailable and temporary unavailable. If it is permanently unavailable, the container will call the destroy method of servlet to clean up and then release the servlet. And respond to the SC_NOT_FOUND status to the client. If it is temporarily unavailable, the servlet container will directly reject all requests matching to this servlet within the estimated unavailable time and respond directly to the SC_SERVICE_UNAVAILABLE (503) state and respond to a Retry-After header. The container can also choose to ignore the difference between the two unavailable, directly treat it as permanent unavailable and clean up and release servlet objects.

Normally, a servlet instance should reside permanently in the container, and the servlet instance in the container will not be destroyed until the container is closed. Before cleaning a servlet instance, the container always calls the destroy method. Users can usually do some resource cleaning, persistence and other work in the destroy method. Once the destroy method of a servlet is called, the container will completely release it for garbage collector collection.

Programming with Servlet

Usually if we want to implement a Servlet class in the application, we do not need to implement the Servlet interface directly, but by extending GenericServlet, in the http environment, we can extend HttpServlet,GenericServlet to do the basic implementation of the Servlet interface, but reserved service methods need to be implemented by users, the implementation of GenericServlet is not bound to any specific application layer protocols, it is universal.

In the HTTP protocol environment, the platform also provides an implementation class of HttpServlet. HttpServlet implements the service method from GenericServlet,HttpServlet. The implementation of the service method is to make some basic judgments according to different http verbs, and then delegate the normal process to several specific http methods: doGet,doPost,doPut,..., in the vast majority of cases our own Servlet implementation class should choose to extend HttpServlet And instead of directly overriding the service method, you should override the doXXX method.

Public class RequestParameterServlet extends HttpServlet {@ Override protected void doGet (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {super.doGet (req, resp); / /... }} the parameterless init method should be overridden

The init method defined in the Servlet interface takes a ServletConfig parameter:

Public void init (ServletConfig config) throws ServletException

The init method makes a basic implementation in GenericServlet:

Public void init (ServletConfig config) throws ServletException {this.config = config; this.init ();}

In the init implementation of the GenericServlet version, we first save the ServletConfig reference passed by the container, and then call a parameterless init method, which is reserved for the user to implement. Therefore, whether our Servlet class extends GenericServlet or HttpServlet, if there is a need for initialization, we should override the parameterless init method in GenericServlet.

Servlet in multithreaded environment

In general, the container creates only one instance of each servlet declared in web descriptor, which means that in the case of multithreading, its service method runs concurrently, so we have to ensure its thread safety when implementing the service method by not using shared fields in the Servlet implementation class or locking the relevant code snippets in the service method.

Another way is that our Servlet class implements the SingleThreadModel interface. SingleThreadModel is an identification interface. The servlet container ensures that the service method of the servlet instance of SingleThreadModel is accessed by only one thread at a time. The container may lock the servlet instance, or the container may create multiple instances of the same Servlet class and cache them in the container, and then distribute each request to one of the idle servlet instances. However, the SingleThreadModel interface is not recommended because SingleThreadModel does not completely solve the thread safety problem, for example, the data in session does not guarantee thread safety and has been discarded in servlet version 2.4.

How to handle the character encoding of the request

All the request data of the client, including the request path, protocol, request header and request content, are all encapsulated in a ServletRequest object. When reading the request content from the ServletRequest object, you must know how the data is encoded, otherwise it is impossible to parse the request data correctly.

Since most browser clients do not provide character-encoded tags in the Content-Type domain when sending requests, the servlet specification explicitly requires that all servlet containers use "ISO-8859-1" to parse the request data by default when reading the request data. If the client does not have the encoding format for sending the request data, the specification defines that calling the getCharacterEncoding method should return null.

If the client does not send the encoding format of the request content, and if the default encoding set by the container is inconsistent with the actual request content encoding, the wrong data will be parsed. In order to correct this situation, ServletRequest provides another setCharacterEncoding method, programmers can call this method to override the encoding format provided by the container to read the data. The call of this method must be before reading any request content. Otherwise it won't work.

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