Advanced Java Server Pages An more detailed look at JSPs

Custom methods in JSPs Just as Servlets, JSPs have methods for initialization end destruction JspInit() corresponds to init() in Servlets JspDestroy corresponds to destroy() in Servlets Both JspInit() and JspDestroy() can implemented in your JSPs, i.e you can override the base class Think before you use them  If you need the init and destroy, is it really a JSP you should be writing? Probably not!

Servlets and JSPs Servlets should be used as front ends for JSPs  The Servlet should handle the control logic  The Servlet can initialize Beans that the JSP can use  The Beans can talk to databases, EJBs and so on  This is basic MVC But how is this done?

Before the explanation Assume there is a Bean, DataBean that fetches records from a database  The records are fetched to the bean with the fetch() method  The records are returned from the bean with the getRecords() method

The Servlet part Create the Bean instance Call your methods on the Bean Add the bean to the request or the session Get a RequestDispatcher Use the forward() method in the RequestDispatcher

The Servlet part, using the request public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { DataBean db = new DataBean(); db.fetch(); request.setAttribute("dbBean", db); RequestDispatcher rd = request.getRequestDispatcher("jsppage.jsp"); rd.forward(request, response); }

The Servlet part, using the session public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { HttpSession ses = request.getSession(); DataBean db = new DataBean(); db.fetch(); ses.setAttribute("dbBean", db); RequestDispatcher rd = request.getRequestDispatcher("jsppage.jsp"); rd.forward(request, response); }

The JSP part The JSP uses the to define the bean, but it should not result in a new instance. The instance that was added in the Servlet is used  The same scope in as in the Servlet (request and session in our examples)  The same id in as the name in the Servlet The JSP uses to call methods in the Bean

The JSP part, with request Error, the bean should have been created in the servlet!

The JSP part, with session Error, the bean should have been created in the servlet!

Custom Tags We don’t want Java code in our JSPs The built in tags is quite limited in functionality What we need is a way to extend the built in tags with our own  Custom Tags

Custom Tags Custom Tags separates the logic from the presentation even more that In our BookShop for example, the JSP with a book list and the shopping cart can consist of two lines of code 

Two types of Custom Tags Simple Tag (Tag)  A tag with or without arguments  Doesn’t use the Tag body  Implements javax.servlet.jsp.tagext.Tag Body Tag  With or without arguments  Evaluate the body of the tag This is the body  Implements javax.servlet.jsp.tagext.BodyTag

Simple Tag, an example import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; public class HelloWorldTag implements Tag { private PageContext pageContext; private Tag parent; /** * Constructor */ public HelloWorldTag() { super(); } /** * Method called at start of Tag either a EVAL_BODY_INCLUDE or a SKIP_BODY */ public int doStartTag() throws javax.servlet.jsp.JspTagException { return SKIP_BODY; }

Simple Tag, an example /** * Method Called at end of Tag either EVAL_PAGE or SKIP_PAGE */ public int doEndTag() throws javax.servlet.jsp.JspTagException { try { pageContext.getOut().write("Hello World!"); } catch(java.io.IOException e) { throw new JspTagException("IO Error: " + e.getMessage()); } return EVAL_PAGE; } /** * Method called to releases all resources */ public void release() {} /** Method used by the JSP container to set the current PageContext pageContext, the current PageContext */ public void setPageContext(final javax.servlet.jsp.PageContext pageContext) { this.pageContext=pageContext; }

Simple Tag, an example /** Method used by the JSP container to set the parent of the Tag parent, the parent Tag */ public void setParent(final javax.servlet.jsp.tagext.Tag parent) { this.parent=parent; } /** Method for retrieving the parent the parent */ public javax.servlet.jsp.tagext.Tag getParent() { return parent; }

Simple Tag All methods in the interface must be implemented!  All work is done in the doStartTag() and doEndTag() The doStartTag() is called at the start of the tag The doEndTag() is called at the start of the tag Instead of implementing Tag, extend TagSupport  A helper class included in the package that has implementation for all required methods. Just implement the one that you will use!

Simple Tag, with TagSupport (complete) import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; public class HelloWorldTag extends TagSupport { private PageContext pageContext; private Tag parent; /** * Method Called at end of Tag either EVAL_PAGE or SKIP_PAGE */ public int doEndTag() throws javax.servlet.jsp.JspTagException { try { pageContext.getOut().write("Hello World!"); } catch(java.io.IOException e) { throw new JspTagException("IO Error: " + e.getMessage()); } return EVAL_PAGE; }

Simple Tags In other words, use TagSupport! The HelloTag implements our simple tag Custom Tags are defined in a Tag Library Descriptor (tld)

Tag Library Descriptor mt /mytag My first Tag library helloWorld tags.HelloWorldTag empty A Hello world Tag

Web.xml The tld is referenced in web.xml  Web.xml binds a tag library to a web application mytags /WEB-INF/taglib.tld

Using your tag The uri specified in web.xml is used in the JSP  At the start of the page  Prints “Hello World”

Parameterized tags Both Tag and BodyTag can take parameters  Just as in JavaBeans, use a set-method and a member variable  private String name=“”;  public void setName(_name){ name=_name;}

Parameterized tags import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; public class HelloNameTag extends TagSupport{ private String name=“”; public void setName(String _name){name=_name;} public int doEndTag() throws javax.servlet.jsp.JspTagException{ try{ pageContext.getOut().write("Hello “ + name); } catch(java.io.IOException e){  throw new JspTagException("IO Error: " + e.getMessage()); } return EVAL_PAGE; }

TLD for HelloName hello HelloNameTag empty A Hello Tag name false false

Using the parameterized tag

BodyTag Just as with Tag, there are a lot of methods to write if implementing BodyTag There is a helper class call BodyTagSupport just as with TagSupport BodyTag is often used for looping over some result

Body Tag package se.upright.education.uu.pvk.tags; import javax.servlet.jsp.*; import javax.servlet.jsp.tagext.*; public class LoopTag extends BodyTagSupport { private int iterations = 5; /** * Method used by the JSP container to set the parameter Name. */ public void setIterations(int _iterations) { this.iterations=_iterations; } /** * Method called by the Container to set the BodyContent */ public void setBodyContent(BodyContent bodyContent) { this.bodyContent=bodyContent; }

Body Tag public int doAfterBody() throws JspTagException { if(iterations>1) { //decrease the number of iterations left to do iterations--; return EVAL_BODY_TAG; } else { return SKIP_BODY; }

Body Tag /** * Method Called at end of tag either EVAL_PAGE or SKIP_PAGE */ public int doEndTag() throws JspTagException { try { if(bodyContent != null) // Check if we even entered the body bodyContent.writeOut(bodyContent.getEnclosingWriter()); } catch(java.io.IOException e) { throw new JspTagException("IO Error: " + e.getMessage()); } return EVAL_PAGE; } }

Body Tag tld loop se.upright.education.uu.pvk.tags.LoopTag JSP iterations true

Using Body Tag Looping

Next Time Java Standard Tag Library  A Tag Library with common functionallity XML XSLT