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First, what is a built-in object?

Some objects, such as ServletContext HttpSession PageContext, are frequently used in jsp development. If we need to create these objects ourselves every time we need to use these objects in jsp pages, it will be particularly tedious. Sun Company therefore, when designing jsp, automatically creates these objects for developers after the jsp page is loaded, and developers only need to use the corresponding objects to call the corresponding methods. The objects created by these systems are called built-in objects.

In the servlet program, if the developer wants to use the session object, he must get the session object through request.getSession (). In the jsp program, he can directly use session (the name of the session object created for us is session) to call the corresponding method, such as: session.getId ().

The full name of JSP is Java Server Pages, and the Chinese name is java Server Page.

A total of 9 such objects are pre-defined in JSP, namely: request, response, session, application, out, pagecontext, config, page, exception

Features of built-in objects (also known as implicit objects):

1. Provided by the JSP specification without writer instantiation.

two。 Implemented and managed through the Web container

3. All JSP pages are available

4. Can only be used in expressions or code snippets of script elements

Detailed article 1:

1. Request object

Request objects are objects of type javax.servlet.httpServletRequest. This object represents the request information of the client and is mainly used to accept the data transmitted to the server through the HTTP protocol. (including header information, system information, request method, request parameters, etc.). The scope of the request object is one request.

2. Response object

Response represents the response to the client, mainly passing the objects processed by the JSP container back to the client. The response object also has scope, which is valid only within the JSP page.

3. Session object

Session objects are objects related to user requests that are automatically created by the server. The server generates a session object for each user, which is used to save the user's information and track the user's operation status. The Map class is used inside the session object to hold the data, so the format of the saved data is "Key/value". The value of session objects can make complex object types, not just string types.

4. Application object

The application object saves the information in the server, and the information stored in the application object is valid throughout the application until the server is shut down. Compared with session objects, application objects have a longer lifetime, similar to the "global variables" of the system.

5. Out object

The out object is used to output information within a Web browser and to manage the output buffer on the application server. When using out objects to output data, you can operate on the data buffer to clear the residual data in the buffer in time and make buffer space for other outputs. After the data output is finished, the output stream should be closed in time.

6. PageContext object

The function of the pageContext object is to get any range of parameters, through which you can get the out, request, reponse, session, application and other objects of the JSP page. The creation and initialization of pageContext objects are done by containers, and pageContext objects can be used directly in JSP pages.

7. Config object

The main function of the config object is to obtain the configuration information of the server. You can get a config object through the getServletConfig () method of the pageConext object. When a Servlet is initialized, the container passes some information to the Servlet through the config object. Developers can provide initialization parameters for Servlet programs and JSP pages in the application environment in the web.xml file.

8. Page object

The page object represents the JSP itself and is legal only within the JSP page. Page implicit objects essentially contain variables referenced by the current Servlet interface, similar to this pointers in Java programming.

9. Exception object

The purpose of the exception object is to display exception information, which can only be used in pages that contain isErrorPage= "true". Using this object in a normal JSP page will not be able to compile the JSP file. Excepation objects, like all objects in Java, have system-provided inheritance structures. The exception object defines almost all exception cases. In Java programs, you can use the try/catch keyword to handle exceptions; if there are uncaught exceptions in the JSP page, the exception object is generated, and the exception object is passed to the error page set in the page directive, and then the corresponding exception object is processed in the error page.

Detailed article 2:

1. Request object javax.servlet.http.HttpServletRequest

The request object represents the request information of the client and is mainly used to accept the data transmitted to the server through the HTTP protocol. (including header information, system information, request method, request parameters, etc.). The scope of the request object is one request.

When the Request object acquires the Chinese characters submitted by the customer, the problem of garbled code will occur and must be dealt with specially. First, encode the acquired string with ISO-8859-1, store the code in a byte array, and then convert the array to a string object as follows

The common method of Request: getParameter (String strTextName) to get the information submitted by the form.

GetProtocol () gets the protocol that the customer uses.

String strProtocol=request.getProtocol ()

GetServletPath () gets the page where the customer submitted information. String strServlet=request.getServletPath ()

GetMethod () how to get the information submitted by the customer String strMethod=request.getMethod ()

GetHeader () gets the values of accept,accept-encoding and Host in the HTTP header file, String strHeader=request.getHeader ()

GetRermoteAddr () gets the customer's IP address. String strIP=request.getRemoteAddr ()

GetRemoteHost () gets the name of the client. String clientName=request.getRemoteHost ()

GetServerName () gets the server name. String serverName=request.getServerName ()

GetServerPort () gets the port number of the server. Int serverPort=request.getServerPort ()

GetParameterNames () gets the names of all parameters submitted by the client.

Enumeration enum = request.getParameterNames (); while (enum.hasMoreElements ()) {Strings (String) enum.nextElement (); out.println (s);} 123456

2. Response object javax.servlet.http.HttpServletResponse

Response represents the response to the client, mainly passing the objects processed by the JSP container back to the client. The response object also has scope, which is valid only within the JSP page.

With the dynamic response contentType attribute, when a user visits a JSP page, if the page uses the page directive to set the contentType property of the page to text/html, then the JSP engine will respond to this property value.

If you want to dynamically change this property value in response to the customer, you need to use the setContentType (String s) method of the Response object to change the property value of contentType.

Response.setContentType (String s); parameter s should be text/html,application/x-msexcel,application/msword and so on.

In some cases, when responding to a customer, the customer needs to be redirected to another page, and the sendRedirect (URL) method of Response can be used to redirect the customer.

For example, response.sendRedirect (index.jsp)

3. Session object javax.servlet.http.HttpSession

The Session object is a JSP built-in object that is automatically created when the first JSP page is loaded, completing session management. It is called a session when a client opens a browser and connects to the server, until the client closes the browser and leaves the server. When a client accesses a server, it may switch between several pages of the server, and the server should somehow know that it is a client and requires a Session object.

When a client visits a JSP page on the server for the first time, the JSP engine generates a Session object and assigns an ID number of String type. At the same time, the JSP engine sends this ID number to the client and stores it in Cookie, so that the Session object is not canceled on the server side until the client closes the browser, and the session correspondence with the customer disappears. When a customer reopens the browser and connects to the server, the server creates a new Session object for the customer.

Session objects are objects related to user requests that are automatically created by the server. The server generates a session object for each user, which is used to save the user's information and track the user's operation status.

The Map class is used inside the session object to hold the data, so the format of the saved data is "Key/value". The value of session objects can make complex object types, not just string types.

Public String getId (): gets the Session object number.

Public void setAttribute (String key,Object obj): adds the object obj specified by the parameter Object to the Session object and specifies an index keyword for the added object.

Public Object getAttribute (String key): gets the object that contains keywords in the Session object.

Public Boolean isNew (): determines whether it is a new customer.

4. Application object javax.servlet.ServletContext

The application object saves the information in the server, and the information stored in the application object is valid throughout the application until the server is shut down. Compared with session objects, application objects have a longer lifetime, similar to the "global variables" of the system.

The Application object is generated after the server is started, and when the customer browses between the pages of the website that the customer visits, the Application object is the same until the server is shut down. But unlike the Session object, all customers' Application objects are the same, that is, all customers share this built-in Application object.

SetAttribute (String key,Object obj): adds the object obj specified by the parameter Object to the Application object and specifies an index keyword for the added object.

GetAttribute (String key): gets the object that contains keywords in the Application object.

5. Out object javax.servlet.jsp.jspWriter

The out object is used to output information within a Web browser and to manage the output buffer on the application server. When using out objects to output data, you can operate on the data buffer to clear the residual data in the buffer in time and make buffer space for other outputs. After the data output is finished, the output stream should be closed in time.

The Out object is an output stream that is used to output data to the client. The Out object is used for the output of various data. The common methods are as follows.

Out.print (): outputs various types of data.

Out.newLine (): outputs a newline character.

Out.close (): closes the stream.

6. PageContext object javax.servlet.jsp.PageContext

The function of the pageContext object is to get any range of parameters, through which you can get the out, request, reponse, session, application and other objects of the JSP page.

The creation and initialization of pageContext objects are done by containers, and pageContext objects can be used directly in JSP pages.

The page object represents the JSP itself and is legal only within the JSP page. Page implicit objects essentially contain variables referenced by the current Servlet interface, similar to this pointers in Java programming.

7. Config object javax.servlet.ServletConfig

The main function of the config object is to obtain the configuration information of the server. You can get a config object through the getServletConfig () method of the pageConext object. When a Servlet is initialized, the container passes some information to the Servlet through the config object. Developers can provide initialization parameters for Servlet programs and JSP pages in the application environment in the web.xml file.

8 cookie object

Cookie is a piece of text saved by the Web server on the user's hard drive. Cookie allows a Web site to save information on the user's computer and then retrieve it. For example, a Web site might generate a unique ID for each visitor and save it on each user's machine in the form of an Cookie file.

Create a Cookie object and call the constructor of the Cookie object to create the Cookie object. The constructor of the Cookie object takes two string arguments: the Cookie name and the cookie value.

For example: Cookie c = new Cookie (username ", john"); pass the Cookie object to the client.

In JSP, if you want to pass the encapsulated Cookie object to the client, you can use the addCookie () method of the Response object.

For example: response.addCookie (c), read the Cookie saved to the client.

Using the getCookie () method of the Request object, all Cookie objects from the client are arranged in an array at execution time, and if you want to fetch the Cookie object that meets your needs, you need to loop through the keywords of each object in the array. Set the valid time of the Cookie object, and use the setMaxAge () method of the Cookie object to set the valid time of the Cookie object

For example: Cookie c = newCookie (username "," john "); c.setMaxAge (3600)

A typical application of the Cookie object is used to count the number of visitors to a website. Due to the use of proxy servers, caches, etc., the only way to help websites count visitors accurately is to establish a unique ID for each visitor. With Cookie, a website can do the following:

Determine how many people have visited. Determine how many visitors are new (that is, first-time visitors) and how many are regular users.

Determine how often a user visits the site. When a user visits for the first time, the site creates a new ID in the database and transmits the ID to the user through Cookie. When the user visits again, the website adds 1 to the counter corresponding to the user's ID to get the number of visits by the user.

9. Exception object java.lang.Throwable

The purpose of the exception object is to display exception information, which can only be used in pages that contain isErrorPage= "true". Using this object in a normal JSP page will not be able to compile the JSP file.

Excepation objects, like all objects in Java, have system-provided inheritance structures.

The exception object defines almost all exception cases. In Java programs, you can use the try/catch keyword to handle exceptions; if there are uncaught exceptions in the JSP page, the exception object is generated, and the exception object is passed to the error page set in the page directive, and then the corresponding exception object is processed in the error page.

The object name function type scope request requests data from the client javax.servlet.ServletRequestRequestresponse encapsulates the response generated by the jsp, and then is sent to the client to respond to the customer's request javax.servlet.SrvletResponsePagepageContext wraps the context of the page for the JSP page. Manage the javax.servlet.jsp.PageContextPagesession that is used to store the information of each user that belongs to the named objects in the special visible section of the JSP in order to track the operational status of each user. The javax.servlet.http.HttpSessionSessionapplication application object javax.servlet.ServletContextApplicationout outputs data to the client that javax.servlet.jsp.JspWriterPageconfig represents the configuration of the Servlet, when a Servlet is initialized The container passes some information through this object to the instance of the Servletjavax.servlet.ServletConfigPagepageJsp implementation class, which is the jsp itself, through which you can access the javax.lang.ObjectPageexception reflection running exception javax.lang.ThrowablePage II, parsing several built-in objects 1) out object, the object type is the JspWriter class, which is equivalent to PrintWriter with cache (without cache)

PrintWriter:write ("content") outputs content directly to the browser

JspWriter:writer ("content") writes content to the jsp buffer

JspWriter the contents of the buffer are written out when one of the following conditions is met:

A, the buffer zone is full

B, flush the buffer

C, close the buffer

The execution of the dminute jsp page is complete.

2) pageContext object

The object type of pageContext is PageContext, a context object called jsp.

PageContext function: you can get eight other built-in objects

/ / example:

PageContext.getOut ()

PageContext.getServletConfig ()

Use scenarios: PageContext objects are frequently used when customizing tags, or when defining a method that requires multiple objects, passing a pageContext object can solve the problem.

The pageContext object, which represents the page context, is mainly used to access shared data between JSP.

PageContext is an instance of the PageContext class, and you can use pageContext to access variables in the range of page, request, session, and application.

GetAttribute (String name): gets the name attribute within the scope of page.

SetAttribute (String name, value, int scope): if scope is not specified, this attribute is within the page range by default, such as pageContext.setAttribute ("page", "hello")

Use pageContext to set the property in the request range pageContext.setAttribute ("request2", "hello", pageContext.REQUEST_SCOPE)

Use pageContext to set the property in the session range pageContext.setAttribute ("session2", "hello", pageContext.SESSION_SCOPE)

Use pageContext to set the property in the application range pageContext.setAttribute ("app2", "hello", pageContext.APPLICATION_SCOPE)

GetAttribute (String name,int scope): gets the name attribute within the specified range

Where scope can be the following four values:

PageContext.PAGE_SCOPE: corresponds to the page range.

PageContext.REQUEST_SCOPE: corresponds to the request range.

PageContext.SESSION_SCOPE: corresponds to the session range.

PageContext.APPLICATION_SCOPE: corresponds to the application range.

three。 Four major domain object classifications in JSP:

ServletContext context domain

HttpServletRequet request domain

HttpSession session domain-the first three can be accessed when learning Servlet

PageContext page domain-- learned by jsp

The role of domain objects: save data, obtain data, and share data.

Save data: Context.setAttribute ("content"); / / Save to page domain by default

PageContext.setAttribute ("content", domain norm constant); / / Save to the specified domain

/ / four domain constants

PageContext.PAGE_SCOPE

PageContext.REQUEST_SCOPE

PageContext..SESSION_SCOPE

PageContext.APPLICATION_SCOPE

Get data:

PageContext.getAttribute ("content")

PageContext.getAttribute ("name", domain norm constant)

/ / automatically search for data pageContext.findAttribute ("content") in four fields; / / automatically search for data in four fields in the following order: page field-> request field-> session field-> application field (context field)

Domain scope:

Page domain: can only be used on the current jsp page (current page)

Request domain: can only be used in the same request (forward)

Session domain: can only be used in the same session (session object) (private)

Context domain: can only be used in the same web application (global)

Thank you for reading! This is the end of this article on "what is a built-in object in JSP". I hope the above content can be of some help to you, so that you can learn more knowledge. if you think the article is good, you can share it for more people to see!

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