A tag with a body in Java

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Listing 133 A tag with a body
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package comawljspbookch04;
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import javaioIOException; import javaioStringWriter; import javatextSimpleDateFormat; import javautilDate;
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import javaxservlet*; import javaxservlethttp*; import javaxservletjsp*; import javaxservletjsptagext*;
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public class ReverseTag extends BodyTagSupport { private PageContext pageContext; public PageContext getPageContext() {return pageContext;} public void setPageContext(PageContext pageContext) { thispageContext = pageContext; }
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private Tag parent; public Tag getParent() {return parent;} public void setParent(Tag parent) {thisparent = parent;}
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public int doStartTag() throws JspException { return EVAL_BODY_BUFFERED; }
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public int doEndTag() throws JspException { String output = "";
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if(bodyContent != null) { StringWriter sw = new StringWriter(); try { bodyContentwriteOut(sw); output = swtoString(); } catch(javaioIOException e) {} }
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output = doReverse(output);
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try { pageContextgetOut()print(output); } catch(javaioIOException e) {}
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return EVAL_PAGE; }
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public void release() { pageContext = null; parent } = null;
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private String doReverse(String output) { int len char out2[] = outputlength(); = new char[len];
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for(int i=0;i<len;i++) { out2[i] = outputcharAt(len-1-i);
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return new String(out2);
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This listing has many of the same methods as the previous example, which is to be expected, as body tags will need to be initialized with a pageContext and parent, just as any other tag The first difference is that doStartTag() returns EVAL_BODY_BUFFERED, indicating that it will be manipulating the contents of the body This manipulation is accomplished in the doEndTag() method, which first checks whether the body content is available by checking whether the special variable bodyContent is null This variable is defined in the base class If the bodyContent is available, the tag obtains the contents by writing them to a StringWriter, using the writeOut() method This happens after the body has been processed, so if the body has any c:out or other JSP tags, as well as any other JSP tags, they will already have been replaced by the specified values Once the content of the body has been obtained, reversing it is fairly simple and is done by the doReverse() method in this class Sending the contents to the page is then as simple as writing it to out, just as was done for the date in Listing 132 It is now possible to clarify exactly what out is It is an instance of another class, called JspWriter that at the top level will send any data written to it on to the user However, within body tags, the JspWriter will store, or buffer, the data that is written to it, so that the bodyContent object can pass this data to the body tag So in this case, if the awl:reverse tag is being used from within another body tag, out will hold onto the reversed text and pass it along to the other tag This is all managed transparently by the JSP engine, so tag authors almost never need to worry about what exactly out is when they are writing data to it
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134 Using the Expression Language
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The JSP engine does not automatically handle attribute values that use the expression language If a page were to call awl:date with a value of ${param myFormat} as the value of format, the DateFormat object would attempt to use the literal string ${parammyFormat} when formatting the number and would not automatically look up the value of the parameter called myFormat It is the tag's responsibility to interpret any expression language variables that it wishes to make dynamic Fortunately, a few classes make this much easier The primary one is ExpressionEvaluator, which does the evaluation ExpressionEvaluator uses a couple of additional classes: VariableResolver, which is responsible for looking up the values of any variables used within an expression, and FunctionMapper, which can handle more complex kinds of expressions involving calls to functions The functionality is split into these pieces in order to make it easier to customize their behavior It would be possible for a programmer to replace the default VariableResolver with one that obtained values from a database It would even be possible to create an ExpressionEvaluator that handled a different kind of expression language entirely, making it possible to write expressions in other languages, such as Scheme or Perl, if such a thing were ever desired Leaving aside such exotic thoughts, let's look at the basic use of these classes to enable dynamic attributes Listing 134 shows the class that implements the awl:maybeShow tag from 4 Recall that this tag has one attribute, show, which may be yes, no, or reverse Listing 134 A tag that uses the expression language
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package comawljspbookch04;
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import javaioIOException; import javaioStringWriter; import javatextSimpleDateFormat; import javautilDate;
import javaxservlet*; import javaxservlethttp*; import javaxservletjsp*; import javaxservletjsptagext*;
import javaxservletjspel*;
public class MaybeShowTag extends BodyTagSupport { private String show; public String getShow() {return show;} public void setShow(String show) {thisshow = show;}
private PageContext pageContext; public PageContext getPageContext() { return pageContext; } public void setPageContext(PageContext pageContext) { thispageContext = pageContext; }
private Tag parent; public Tag getParent() {return parent;} public void setParent(Tag parent) { thisparent = parent; }
public void release() { pageContext = null; parent } = null;
public int doStartTag() throws JspException { return EVAL_BODY_BUFFERED; }