JSP Java Server Pages Reference http www apl jhu edu hall java ServletTutorial Servlet Tutorial JSP html Jan 13 2019 A Hello World servlet from the Tomcat installation documentation public class HelloServlet extends HttpServlet public void doGet HttpServletRequest request HttpServletResponse response throws ServletException IOException response setContentType text html PrintWriter out response getWriter String docType DOCTYPE HTML PUBLIC W3C DTD HTML 4 0 Transitional EN n out println docType HTML n HEAD TITLE Hello TITLE HEAD n BODY BGCOLOR FDF5E6 n H1 Hello World H1 n BODY HTML This is mostly Java with a little HTML mixed in 2 Servlets The purpose of a servlet is to create a Web page in response to a client request Servlets are written in Java with a little HTML mixed in JSP Java Server Pages is an alternate way of creating servlets JSP is written as ordinary HTML with a little Java mixed in The Java is enclosed in special tags such as The HTML is known as the template text JSP files must have the extension jsp The HTML is enclosed in out println statements JSP is translated into a Java servlet which is then compiled Servlets are run in the usual way The browser or other client sees only the resultant HTML as usual Tomcat knows how to handle servlets and JSP pages 3 How JSP works When Tomcat needs to use a JSP page it Each call to the JSP servlet is executed in a new Thread Translates the JSP into a Java servlet Compiles the servlet Creates one instance of the JSP servlet Executes the servlet as normal Java code Hence when you are writing JSP you are writing higher level Java code Since there is only one JSP object you have to use synchronization if you use any instance variables of the servlet You have two basic choices when using JSP Let the JSP do all the work of a servlet Write a servlet that does the work and passes the results to JSP to create the resultant HTML page This works because a servlet can call another servlet Bottom line JSP is just a convenient way of writing Java code 4 JSP scripting elements There is more than one type of JSP tag depending on what you want done with the Java expression code The expression is evaluated and the result is inserted into the HTML page The code is inserted into the servlet s service method If code contains declarations they become local variables of the service method This construction is called a scriptlet declarations The declarations are inserted into the servlet class not into a method Hence declarations made here become instance variables 5 Example JSP HTML BODY Hello The time is now new java util Date BODY HTML Notes The tag is used because we are computing a value and inserting it into the HTML The fully qualified name java util Date is used instead of the short name Date because we haven t yet talked about how to do import declarations 6 Variables You can declare your own variables as usual JSP provides several predefined variables request The HttpServletRequest parameter response The HttpServletResponse parameter session The HttpSession associated with the request or null if there is none out A JspWriter like a PrintWriter used to send output to the client Example Your hostname request getRemoteHost 7 Scriptlets Scriptlets are enclosed in tags Scriptlets are executable code and do not directly affect the HTML Scriptlets may write into the HTML with out print value and out println value Example String queryData request getQueryString out println Attached GET data queryData Scriptlets are inserted into the servlet exactly as written and are not compiled until the entire servlet is compiled Example if Math random 0 5 Have a B nice B day else Have a B lousy B day 8 The case against scriptlets One of the principle motivations for JSP is to allow Web designers who are not Java programmers to get some of the features of Java into their pages Hence in some cases it is desirable to put as little actual Java into your JSP as possible Where this is a goal a better approach is to provide the necessary Java functionality via methods in a class that is loaded along with the servlet 9 Declarations Use for declarations to be added to your servlet class not to any particular method Caution Servlets are multithreaded so nonlocal variables must be handled with extreme care If declared with variables are local and OK If declared with variables may need to be synchronized Data can also safely be put in the request or session objects Example private int accessCount 0 Accesses to page since server reboot accessCount You can use to declare methods as easily as to declare variables 10 Directives Directives affect the servlet class itself A directive has the form directive attribute value or directive attribute1 value1 attribute2 value2 attributeN valueN The most useful directive is page which lets you import packages Example page import java util 11 The include directive The include directive inserts another file into the file being parsed Syntax include file URL The included file is treated as just more JSP hence it can include static HTML scripting elements actions and directives The URL is treated as relative to the JSP page If the URL begins with a slash it is treated as relative to the home directory of the Web server The include directive is especially useful for inserting things like navigation bars 12 Actions Actions are XML syntax tags used to control the servlet engine jsp include page URL flush true Inserts the indicated relative URL at execution time not at compile time like the include directive does This is great for rapidly changing data jsp forward page URL jsp forward page JavaExpression Jump to the static URL or the dynamically computed JavaExpression resulting in a URL 13 JSP in XML JSP can be embedded in XML as well as in HTML Due to XML s syntax rules the tags must be different but they do the same things HTML expression XML jsp expression expression jsp expression HTML code XML jsp scriptlet code jsp scriptlet HTML declarations XML jsp declaration declarations jsp declaration HTML include file URL XML jsp directive include file URL 14 Comments You can put two kinds of comments in JSP HTML comment This is an ordinary HTML comment and forms part of the page that you send to the user Hence the user can see it by doing View source JSP scriptlets in HTML comments will be executed JSP comment This kind of comment will be stripped out when the JSP is compiled It s intended for page developers the user will never see it 15 The End
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