Jsp tag instance

 

1. problem: the People object in the request has a property named men, and men is a Collection with many man. Now, all man names in the collection are displayed.

Obviously, this is a nested Tag problem. Three tags interact with each other: the outermost Tag finds the People object, the Tag in the middle gets the Collection, and the sub-Tag is responsible for printing.

For example:

<Diego: withObject value =$>

<Diego: withCollection property = men>

<Diego: elementout property = name/>

</Diego: withCollection>

</Diego: withObject>

The idea is as follows:

1) Write WithObjectTag to obtain the object from the El expression.

2) Compile WithCollectionTag to retrieve Collection from the object and traverse Collection. Each time a Collection is traversed, the body is executed.

3) Write ElementoutTag to print the name of each men object in the Collection.

2. the complete procedure is as follows:

In the preceding diegoyun. vo package, write the People class.

Package diegoyun. vo;

Import java. util. Collection;

Public class People

{

Private Collection men = null;

Public Collection getMen ()

{

Return men;

}

Public void setMen (Collection men)

{

This. men = men;

}

}

Write withObject. This is the outermost Tag of the People object obtained from the request.

Package diegoyun;

Import javax. servlet. jsp. JspException;

Import javax. servlet. jsp. tagext. BodyTagSupport;

Import org. apache. taglibs. standard. lang. support. ExpressionEvaluatorManager;

Public class WithObjectTag extends BodyTagSupport

{

Private Object value = null;

Public Object getValue ()

{

Return value;

}

Public void setValue (Object value) throws JspException

{

This. value = ExpressionEvaluatorManager. evaluate (value, value. toString (), Object. class, this, pageContext );

}

Public int doStartTag ()

{

Return EVAL_BODY_INCLUDE;

}

Public int doEndTag () throws JspException

{

Return EVAL_PAGE;

}

}

Write WithCollectionTag, which obtains Collection and traverses and executes sub-tags.

Package diegoyun;

Import java. util. Collection;

Import java. util. Iterator;

Import javax. servlet. jsp. JspException;

Import javax. servlet. jsp. tagext. BodyTagSupport;

Import org. apache. commons. beanutils. PropertyUtils;

Public class WithCollectionTag extends BodyTagSupport {

Private Object element = null;

Private Collection list = null;

Private Iterator iterator = null;

Public Object getElement (){

Return element;

}

Public void setProperty (String property) throws JspException {

// Get the parent Tag object and get the Collection

WithObjectTag parent = (WithObjectTag) getParent ();

If (parent = null)

Throw new JspException (parent tag is null );

Try {

Object propertyValue = PropertyUtils. getProperty (parent. getValue (), property );

This. list = (Collection) propertyValue;

If (list = null)

Throw new JspException (Collection is null );

} Catch (Exception e ){

Throw new JspException (e );

}

}

Public int doStartTag () throws JspException {

// Set the first element and then execute the sub-Tag

Iterator = list. iterator ();

If (iterator. hasNext ())

Element = iterator. next ();

Return EVAL_BODY_INCLUDE;

}

Public int doAfterBody (){

If (iterator. hasNext ()){

// If a child element exists, set the child element and execute the child Tag again.

// Loop

// Otherwise, the sub-Tag will not be executed

Element = iterator. next ();

Return EVAL_BODY_AGAIN;

}

Else

Return EVAL_PAGE;

}

}

Write ElementOutputTag

Package diegoyun;

Import java. io. IOException;

Import javax. servlet. jsp. JspException;

Import javax. servlet. jsp. tagext. TagSupport;

Import org. apache. commons. beanutils. PropertyUtils;

Public class ElementOutputTag extends TagSupport

{

Private Object propertyValue = null;

Public void setProperty (String property) throws JspException

{

WithCollectionTag parent = (WithCollectionTag) getParent ();

If (parent = null)

Throw new JspException (parent tag is null );

Try

{

// Determine whether the attribute name exists in the upper-layer tag. If yes, obtain the attribute value. Otherwise, an error is returned.

PropertyValue = PropertyUtils. getProperty (parent. getElement (), property );

}

Catch (Exception e)

{

Throw new JspException (e );

}

}

Public int doEndTag () throws JspException

 

Catch (IOException e)

{

Throw new JspException (e );

}

Return EVAL_PAGE;

}

}

Write tld

<! -- WithObjectTag -->

<Tag>

<Name> withObject </name>

<Tag-class> diegoyun. WithObjectTag </tag-class>

<Body-content> JSP </body-content>

<Attribute>

<Name> value </name>

<Required> false </required>

<Rtexprvalue> true </rtexprvalue>

</Attribute>

</Tag>

<! -- WithCollectionTag -->

<Tag>

<Name> withCollection </name>

<Tag-class> diegoyun. WithCollectionTag </tag-class>

<Body-content> JSP </body-content>

<Attribute>

<Name> property </name>

<Required> false </required>

<Rtexprvalue> true </rtexprvalue>

</Attribute>

</Tag>

<! -- ElementOutputTag -->

<Tag>

<Name> elementout </name>

<Tag-class> diegoyun. ElementOutputTag </tag-class>

<Body-content> empty </body-content>

<Attribute>

<Name> property </name>

<Required> false </required>

<Rtexprvalue> true </rtexprvalue>

</Attribute>

</Tag>

Compile jsp

<% @ Page language = java %>

<% @ Page import = diegoyun. vo. * %>

<% @ Page import = java. util. * %>

<% @ Taglib uri =/WEB-INF/tlds/diego. tld prefix = diego %>

<Html>

<Body bgcolor = # FFFFFF>

<%

Collection c = new ArrayList ();

Man man1 = new Man ();

Man1.setName (diego );

C. add (man1 );

Man man2 = new Man ();

Man2.setName (Zidane );

C. add (man2 );

Man man3 = new Man ();

Man3.setName (Rui );

C. add (man3 );

People p = new People ();

P. setMen (c );

Request. setAttribute (people, p );

%>

Test loop tag:

<Br>

<Diego: withObject value =$>

<Diego: withCollection property = men>

<Diego: elementout property = name/>

<Br>

</Diego: withCollection>

</Diego: withObject>

</Body>

</Html>

Run, you can see:

Test loop tag:

Diego

Zidane

Rui