10.5.4 Integrating FieldGetter with IterationTagSupport
Having established the nature of the FieldGetter, how do we integrate it into the
iteration process? The answer is in the updated implementation of IterationTagSupport
wherein FieldGetter was integrated. An updated listing of Iteration-TagSupport
is in listing 10.16 (for clarity, unmodified code was omitted and
whenever new and old code are mixed, the new code is in bold).
Listing 10.16 An Updated IterationTagSupport with FieldGetter Integration
package book.iteration;
import java.io.Reader;
import java.io.IOException;
import book.util.LocalStrings;
import book.util.FieldGetter;
import book.util.ExBodyTagSupport;
import javax.servlet.jsp.JspWriter;
import javax.servlet.jsp.JspException;
public abstract class IterationTagSupport
extends ExBodyTagSupport {
static LocalStrings ls =
LocalStrings.getLocalStrings(IterationTagSupport.class);
protected IterationSupport elementsList = null;
protected Object current; 
protected FieldGetter fGetter = null;
// Some unmodified code was removed
public int doAfterBody()
throws JspException
{
try {
if( null == fGetter) { 
getBodyContent().writeOut(getPreviousOut());
} else {
populateFields();
}
getBodyContent().clear();
} catch(java.io.IOException ioe) {
// User probably disconnected ...
// Log and throw a JspTagException
}
if( elementsList.hasNext()) {
exportVariables();
return EVAL_BODY_TAG;
}
return SKIP_BODY;
}
protected void populateFields()
throws JspException
{
String field = null;
try {
Reader r = getBodyContent().getReader();
JspWriter w = getPreviousOut();
fGetter. setObject(current); 
int ch = r. read(); 
while(-1 != ch) {
if( '< ' == ch) {
ch = r.read();
if( '$ ' == ch) {
/* found a field reference */
field = readFieldName( r); 
w. print( fGetter.getField(field));
ch = r.read();
} else {
w.write( '< ');
}
} else {
w.write( ch);
ch = r.read();
}
}
} catch(IllegalAccessException e) {
// Throw a JspTagException
} catch( IOException ioe) {
// Throw a JspTagException
}
}
protected String readFieldName(Reader r)
throws JspException, IOException
{
StringBuffer sb = new StringBuffer();
int ch = r.read();
while(-1 != ch) { 
if( '$ ' == ch) {
ch = r.read();
if( '> ' == ch) {
/* found a field ending mark */
return sb.toString().trim();
} else {
sb.append((char)ch);
}
} else {
sb.append((char)ch);
ch = r.read();
}
}
// Throw a JspTagException (parse error, directive
// was not terminated)
}
// Some unmodified code was removed
protected void exportVariables()
throws JspException
{
current = elementsList.getNext(); 
pageContext.setAttribute(id, current);
}
// Some unmodified code was removed
protected void clearServiceState()
{
elementsList = null;
current = null;
fGetter = null;
}
}
Two new instance variables for the field substitution The majority of new code
that was added has to do with parsing the body and propagating the current iterator
value to the field substitution code. Propagating the value of the current iterator is
needed because doAfterBody() does not know the value. Implementing the propagation
involves adding an instance variable to carry the iterator value as well as initialize
this value whenever a new iterator value is exported.
If a field getter is available, field substitution is on Now that
the iterator value is available for all methods, we can use doAfterBody()
to process the body. Body processing
is turned on whenever a value is set to the class FieldGetter member, fGetter,
which informs IterationTagSupport that field substitution is required
and populateFields() is being called.
Sets the current iterator into the field getter to make it possible to get field values
from the iterator Searches for a directive starting prefix (<$)
Reads the
field name and prints its value using the getter
Looks for the directive-terminating
sequence ($>) populateFields() and readFieldName() are those that actually
implement the field substitution. populateFields() parses through the body looking
for the substitution directive-starting prefix. Whenever populateFields() finds this
directive it will ask readFieldName() to read the rest of the directive (including its suffix)
and return the name of the field referenced therein. Once populateFields()
holds the referenced field name, it uses the FieldGetter to obtain the field's value,
print it, and continue parsing the body (looking for other directives).
Stores the current iterator for later use in doEndBody().
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