![\"JSP](\"https:\/\/data-flair.training\/blogs\/wp-content\/uploads\/sites\/2\/2020\/08\/JSP-Page-Directive.jpg\")
<\/a><\/p>\nJSP Page Directive<\/h2>\n
The page directive is an important part of directives. Page directive has instructions that set the attributes of that JSP page. They determine the interpretation and execution of that JSP page. It gives instructions for that JSP page in which it is physically present to the JSP web container.<\/p>\n Page directive has following attributes:<\/strong><\/p>\n More than one page directive tag can be in a program as well as the order of the attributes is not important. However, one attribute can only be defined once except the import attribute.<\/p>\n **The page directive attributes won\u2019t show any output as they just set off the start for our JSP code. Thus there won\u2019t be any outputs for page directive.<\/p>\n JSP architecture allows it to be extended as a framework for server-side scripting only. For this reason, page directive supports language attribute.<\/p>\n By default, the value for this attribute is Java. It applies to all declarations, expressions, and scriptlets present under the files mentioned in an include directive.<\/p>\n Newer versions allow the other languages only with some restrictions. The language used as the value for language attribute must support the JRE so that it allows access to the implicit objects, to access get and set methods of JavaBeans, and to use public methods of Java classes. When Java is the value of language attribute—the code gets copied to the auto generated servlet the way it is.<\/p>\n then the generated servlet includes the statements,<\/p>\n which treats k as a Java variable,<\/p>\n When the language is javascript—the code is not as it is. Instead, a servlet engine gets initialized that reads and interprets<\/p>\n A scripting engine specific to JRE will generate following code:<\/strong><\/p>\n For Example:<\/strong><\/p>\n Explanation:<\/strong><\/p>\n The language here is Java. Java Compiler will compile all the code under expression tags.<\/p>\n The page directive allows extending the functionalities of a class (parent) to subclass. Specify the full name of the class under the extends attribute that you want to extend. Without explicitly coding one can add an additional behavior of another class in a JSP page.<\/p>\n However, take some cautions while extending a class because it may affect factors like vendor-specific performance and reliability of a JSP page.<\/p>\n Syntax:<\/strong><\/p>\n For Example:<\/strong><\/p>\n Explanation:<\/strong><\/p>\n The extend tag in this example extends the first class of pack package that makes the first class available to this JSP page.<\/p>\n The import describes the qualified names of classes, the JSP page will use. You can also import a package, but import makes that class. Import the classes by their names rather than package names. Imports can be done for interfaces, enumerations, etc. It is one of the frequently used attributes under the page directive.<\/p>\n Import statements will convert to import statements of Java.<\/p>\n The syntax is as follows:<\/strong><\/p>\n or<\/p>\n In the auto generated servlet it converts to following –<\/p>\n Apart from the import statement, four packages get directly imported. We don\u2019t need to mention them. The list of default import is:<\/p>\n a. java.lang For Example:<\/strong><\/p>\n Explanation:<\/strong> The work of session directive is to see if the HTTP sessions are the requirement of your page or not. It can have two values:<\/p>\n a. If the value of session=”true” b. If the value of session=”false” Syntax:<\/strong><\/p>\n For Example:<\/strong><\/p>\n Explanation:<\/strong> These attributes describe the way the output will be processed. If buffer is set as none, the output gets written to the response directly, whereas the output is written to buffer of the specified size. 8kb is the default size of the buffer. Syntax:<\/strong><\/p>\n The following are the possible combinations:<\/strong><\/p>\n \u00a0<\/span><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n For Example:<\/strong><\/p>\n Explanation:<\/strong> A servlet engine, by default, uses a lot of threads and loads a solo instance of generated servlet to service one request. This implies that two or even more threads might be executing the same method of servlet simultaneously. The threads thus can collide. This can lead to interference of threads with each other towards the access of the variable.<\/p>\n Thus, to go a way around with this SingleThreadModel, the isThreadSafe attribute of the page directive helps. If the value is “false”, the JSP container implements SingleThreadModel. By default value is taken as \u201ctrue\u201d. The Syntax for isThreadSafe is as follows:<\/p>\n Class signature is generated as follows :<\/p>\n Class signature is:<\/p>\n For Example:<\/strong><\/p>\n Explanation:<\/strong> The info attribute provides the functionality to provide the descriptive info of the JSP page\/servlet. A description is useful for an interactive page.The Syntax for info is as follows:<\/p>\n This value gets compiled to a class. This is made available using the getServletInfo() method.<\/p>\n For Example:<\/strong><\/p>\n Explanation:<\/strong> A JSP page normally generates output in HTML output. But this is not always the case. Using contentType, you can specify the type of content according to you. It can be a jpg image or a pdf. The Syntax for contentType can be:<\/p>\n The character set can be specified in addition to the content type. The default value for contentType is “text\/html; charset=ISO-8859-1” until and unless the user defines it. For example:<\/p>\n For Example:<\/strong><\/p>\n Explanation:<\/strong> While a JSP page is being evaluated and an exception occurs, the servlet engine dumps a stack track. This is not desirable for commercial Web applications. JSP offers a simple and convenient solution. We need two attributes: errorPage and isErrorPage and their synchronization. Where error_url is the url of the error page of that JSP page in the same servlet context. Then It may simply report the exception:<\/p>\n For Example:<\/strong><\/p>\n Explanation:<\/strong> It is a character encoding in which JSP files are encoded. The default value for page encoding is ISO-8859-1. It is a character 8-bit (single-byte) coded graphic character set. Page encoding<\/p>\n 1. Through the value of page encoding in JSP Property group. The pageEncoding and contentType attribute tells about page encoding of the That JSP file in which page directive is physically present. For Example:<\/strong><\/p>\n Explanation:<\/strong> The pageEncoding has been shown in the above example using the default value i.e. ISO-8859-1.<\/p>\n The attribute stands for Expression Language ignored. This attribute specifies the inability of an expression to be evaluated saying that expression is static and will not be evaluated. However, the default value of the attribute is false and expressions are evaluated as specified. For Example:<\/strong><\/p>\n Explanation:<\/strong> It works similarly to the #include of the C preprocessor directory. Its work is to merge the contents of another file into .jsp source input stream at the translation phase.<\/p>\n
\nSyntax:<\/strong><\/p>\n<%@ page attribute=\u201dvalue\u201d attribute=\u201dvalue\u201d\u2026%><\/pre>\n
\n
1. JSP language<\/h3>\n
\nSyntax:\u00a0<\/strong><\/p>\n<%@ page language=\u201cvalue\u201d %> \r\n<\/pre>\n
<% int k = 10; %>\r\nk =\r\n<%= k %>\r\n<\/pre>\n
out.print(\"\\r\\n\");\r\nint k = 10;\r\nout.print(\"\\r\\nk = \");\r\nout.print(k);\r\nout.print(\"\\r\\n\\r\\n\");\r\n<\/pre>\n
<%@ page language=\"javascript\" %>\r\n <% var k = 10; %>\r\n k = <%= k %>\r\n<\/pre>\n
if (scriptEngine == null)\r\n{\r\ntry { scriptEngine = ScriptEngineFactory.getScriptEngine(\"javascript\"); scriptEngine.init(pageContext);\r\n}\r\n catch (Exception e)\r\n{\r\n throw new ServletException (\"Error initializing scripting engine.\", e);\r\n }\r\n if (request.getAttribute(SCRIPT_KEY) != null)\r\n {\r\nscriptEngine.init( pageContext, (String) request.getAttribute(SCRIPT_KEY), (String) request.getAttribute(DECLARATION_KEY));\r\n}\r\nscriptEngine.evaluate(pageContext);\r\n<\/pre>\n<%@ page language = \u201cjava\u201d contentType = \u201ctext\/html\u201d pageEncoding = \u201cISO-8859-1\u201d isErrorPage = \u201cfalse\u201d%>\r\n<\/pre>\n
2. JSP extends<\/h3>\n
<%@ page extends=\"value\" %>\r\n<\/pre>\n
<%@ page language = \u201cjava\u201d contentType = \u201ctext\/html\u201d pageEncoding = \u201cISO-8859-1\u201d %>\r\n<%@ extends = \u201cpack.firstclass\u201d%>\r\n<\/pre>\n
3. JSP import<\/h3>\n
<%@ page import=\u201cvalue\u201d %>\r\n<%@ page import=\"java.io.*,java.sql.*,java.util.*\" %>\r\n<\/pre>\n
<%@ page import=\"java.io.*\" %>\r\n<%@ page import=\"java.sql.*\" %>\r\n<%@ page import=\"java.util.*\" %>\r\n<\/pre>\n
import java.io.*;\r\nimport java.sql.*;\r\nimport java.util.*;\r\n<\/pre>\n
\nb. javax.servlet
\nc. javax.servlet.http
\nd. javax.servlet.jsp<\/p>\n<%@ page language=\u201cjava\u201d contentType=\u201ctext\/html\u201d pageEncoding= \u201cISO-8859-1\u201d isErrorPage = \u201cfalse\u201d import=\u201cjava.util.Calender.Date\u201d %>\r\n<\/pre>\n
\nThe code in the above example imports the instance of Date from the util package.<\/p>\n4. JSP session<\/h3>\n
\nThis value specifies that the page needs an HTTP session. The default value of this attribute is true.<\/p>\n
\nThis value shows that the page doesn\u2019t require an HTTP session. The implicit variable of the session generation can\u2019t be used if the value of the session is false. It causes errors at the time of translation.
\nIf the page doesn\u2019t need a http session then Specify it to save the memory and CPU cycles.<\/p>\n<%@ page session= \u201ctrue\/false\u201d %>\r\n<\/pre>\n
<%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d session= \u201cfalse\u201d%>\r\n \r\n<\/pre>\n
\nThe above code suggests that we don\u2019t want to create a session for the particular JSP page.<\/p>\n5. JSP buffer and autoFlush<\/h3>\n
\nOn the other hand, autoFlush can have two values. If it is \u201ctrue\u201d it flushes the output in the buffer if it is full, whereas if the value of autoflush is \u201cfalse\u201d that means it will not work and as soon as the buffer will be filled an exception of buffer overflow occurs.<\/p>\n<%@ page buffer=\u201cvalue\u201d %>\r\n<%@ autoFlush=\u201ctrue\/false\u201d %>\r\n<\/pre>\n
\n\n
\n Buffer<\/b><\/td>\n autoFlush<\/b><\/td>\n Effect<\/b><\/td>\n<\/tr>\n \n None<\/b><\/td>\n True<\/span><\/td>\n The result is directly written to the output stream of the auto generated servlet.<\/span><\/td>\n<\/tr>\n \n None<\/b><\/td>\n False<\/span><\/td>\n This operation is illegal as, if there is no buffer. Nothing will be stored and hence autoflush is of no use.<\/span><\/td>\n<\/tr>\n \n 8<\/b>kb<\/i><\/b><\/td>\n True<\/span><\/td>\n An 8kb of default buffer is initialized. It is flushed automatically once it fills.<\/span><\/td>\n<\/tr>\n \n 8kb<\/b><\/td>\n False<\/span><\/td>\n Again an 8kb size of default buffer is used. As the autoFlush is false, if the buffer fills an exception occurs.<\/span><\/td>\n<\/tr>\n \n size kb<\/b><\/td>\n True<\/span><\/td>\n Only the buffer size changes in this. Users can choose any integer. A size times 1,024-byte buffer is used. As autoflush is true, when this buffer fills, it gets flushed automatically.<\/span><\/td>\n<\/tr>\n \n size kb<\/b><\/td>\n False<\/span><\/td>\n It is similar to the 4th combination. The only difference is this the size of buffer varies according to the user i.e. A size times 1,024-byte buffer is used. When the buffer fills, an exception occurs.<\/span><\/p>\n <%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d buffer=\u201c8kb\u201d autoFlush=\u201ctrue\u201d%>\r\n<\/pre>\n
\nFor the above example, an 8kb buffer will be generated to map the output and it will automatically get flushed once it fills.<\/p>\n6. JSP isThreadSafe<\/h3>\n
\nIf you specify isThreadSafe=”true”, it implies that you take care of any possible thread conflicts. A defined JSP container safely dispatches multiple requests.<\/p>\n<%@ page isThreadSafe=\"true\/false\" %>\r\n<%@ page isThreadSafe=\"true\" %>\r\n<\/pre>\n
public class jrun__Chap10__examples__isThreadSafe__ex12ejsp25\r\nextends allaire.jrun.jsp.HttpJSPServlet\r\nimplements allaire.jrun.jsp.JRunJspPage\r\n \r\n<%@ page isThreadSafe=\"false\" %>\r\n<\/pre>\n
public class jrun__Chap10__examples__isThreadSafe__ex22ejsp25\r\nextends allaire.jrun.jsp.HttpJSPServlet\r\nimplements allaire.jrun.jsp.JRunJspPage, SingleThreadModel \r\n<\/pre>\n
<%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d isThreadsafe= \u201ctrue\u201d%>\r\n \r\n<\/pre>\n
\nThis code suggests that multiple threads will be used and they will be synchronized avoiding any thread conflict.<\/p>\n7. JSP info<\/h3>\n
<%@ page info=\u201cvalue\u201d %>\r\n<%@ page info=\"Shopping Cart Checkout Page\" %>\r\n<\/pre>\n
<%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d info= \u201cJSPDirectives_Kajal\u201d%><\/pre>\n
\nThe value in the info attribute describes that this page is about Directives and thus using getServletInfo() method, servlet retrieves this value.<\/p>\n8. contentType<\/h3>\n
<%@ page contentType=\u201cvalue\u201d %><\/pre>\n
<%@ page contentType=\"type\/subtype; charset=charset\" %><\/pre>\n
<%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d session= \u201cfalse\u201d%>\r\n<\/pre>\n
\ncontentType sets the type of response as text or html. The character set is specified too for the JSP and response page.<\/p>\n9. JSP errorPage and isErrorPage<\/h3>\n
\nWhat a JSP can do is to indicate that whenever a page throws an exception, then an error page can be displayed. The Syntax can be:<\/strong><\/p>\n<%@ page errorPage=\"value\" %><\/pre>\n
\nNow, if the JSP page contains the attribute isErrorPage then it becomes capable of receiving exceptions of the JSP pages that have error pages. That JSP page generated after error needs to have following described in its page directive:<\/p>\n<%@ page isErrorPage=\"true\/false\" %>\r\n<\/pre>\n
<%@ page isErrorPage=\"true\" session=\"false\"%>\r\n<H3>Application Error<\/H3>\r\nThe error message is:\r\n<B><%= exception.getMessage() %><\/B>\r\n<\/pre>\n
<%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d isErrorPage=\u201ctrue\u201d errorPage= \u201cHandleexception.jsp\u201d%>\r\n<\/pre>\n
\nIn this code, if the page contains error, then it should be referred to the Handle Exception page.<\/p>\n10. JSP pageEncoding<\/h3>\n
\n2. Through the value present in page encoding attribute. A translation error is generated if the page encoding of a JSP page is different from its property groups.
\n3. Through the contentType attribute that has charset value.<\/p>\n
\nSyntax:<\/strong><\/p>\n<%@ pageEncoding= \u201cvalue\u201d %><\/pre>\n
<%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d session= \u201cfalse\u201d%><\/pre>\n
11. JSP isELignored<\/h3>\n
\nSyntax:<\/strong><\/p>\n<%@ isELignored = \u201ctrue\/false\u201d%><\/pre>\n
<%@ page language=\u201cjava\u201d contentType = \u201ctext\/html; charset=ISO-8859-1\u201d pageEncoding = \u201cISO-8859-1\u201d isELignored= \u201cfalse\u201d%>\r\n<\/pre>\n
\nFor the above example, all the expression language statements will be calculated as they were specified. They will not be ignored.<\/p>\nJSP Include Directive<\/h2>\n