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A JSP directive affects the overall structure of the servlet class. It usually has the following form −

<% directive attribute = "value" %>

JSP comment marks text or statements that the JSP container should ignore. A JSP comment is useful when you want to hide or "comment out" part of your JSP page.

Following is the syntax of JSP comments −

<%-- This is JSP comment --%>

A JSP expression element contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file.

The expression element can contain any expression that is valid according to the Java Language Specification but you cannot use a semicolon to end an expression.

Its syntax is −

<%= expression %>

A declaration declares one or more variables or methods that you can use in Java code later in the JSP file. You must declare the variable or method before you use it in the JSP file.

<%! declaration; [ declaration; ]+ ... %>

A scriptlet can contain any number of JAVA language statements, variable or method declarations, or expressions that are valid in the page scripting language.

Following is the syntax of Scriptlet −

<% code fragment %>

A JSP Lifecycle consists of following steps −

• Compilation − When a browser asks for a JSP, the JSP engine first checks to see whether it needs to compile the page. If the page has never been compiled, or if the JSP has been modified since it was last compiled, the JSP engine compiles the page.

  The compilation process involves three steps −

  • Parsing the JSP.

  • Turning the JSP into a servlet.

  • Compiling the servlet.

• Initialization − When a container loads a JSP it invokes the jspInit() method before servicing any requests

• Execution − Whenever a browser requests a JSP and the page has been loaded and initialized, the JSP engine invokes the _jspService() method in the JSP.The _jspService() method of a JSP is invoked once per a request and is responsible for generating the response for that request and this method is also responsible for generating responses to all seven of the HTTP methods ie. GET, POST, DELETE etc.

• Cleanup − The destruction phase of the JSP life cycle represents when a JSP is being removed from use by a container.The jspDestroy() method is the JSP equivalent of the destroy method for servlets.

Regular HTML, of course, cannot contain dynamic information.

JavaScript can generate HTML dynamically on the client but can hardly interact with the web server to perform complex tasks like database access and image processing etc.

SSI is really only intended for simple inclusions, not for "real" programs that use form data, make database connections, and the like.

It is more convenient to write (and to modify!) regular HTML than to have plenty of println statements that generate the HTML. Other advantages are −

• Embedding of Java code in HTML pages.

• Platform independence.

• Creation of database-driven Web applications.

• Server-side programming capabilities.

The advantages of JSP are twofold.

First, the dynamic part is written in Java, not Visual Basic or other MS specific language, so it is more powerful and easier to use.

Second, it is portable to other operating systems and non-Microsoft Web servers.

JSP offer several advantages as listed below −

• Performance is significantly better because JSP allows embedding Dynamic Elements in HTML Pages itself.

• JSP are always compiled before it's processed by the server unlike CGI/Perl which requires the server to load an interpreter and the target script each time the page is requested.

• JavaServer Pages are built on top of the Java Servlets API, so like Servlets, JSP also has access to all the powerful Enterprise Java APIs, including JDBC, JNDI, EJB, JAXP etc.

• JSP pages can be used in combination with servlets that handle the business logic, the model supported by Java servlet template engines.