JavaSercer Pages // Set the first loop value,
JavaSercer Pages // Set the first loop value, if any if (enum != null && enum.hasMoreElements( )) { Object currValue = enum.nextElement( ); pageContext.setAttribute(loopId, currValue); return EVAL_BODY_TAG; } else { return SKIP_BODY; } } After verifying that there really is an object with the specified name, a test is done to see if a property name is specified. If it is, the getProperty( ) method is called to retrieve the property value from the specified object so it can be used for the iteration. If a property name is not specified, the object itself is used. All the supported data structure types can be turned into an Enumeration. That’s done by calling the getEnumeration( ) method. The getProperty( ) method and the getEnumeration( ) method are not shown here, because this code is just plain Java code that has nothing to do with implementing iteration in a tag handler. You can look at the source code to see how they work. When the Enumeration has been created, the doStartTag( ) method initializes the loopId variable and places it in the JSP page scope. As you learned in the previous section, the code generated for the page uses the information gained from the LoopTagExtraInfo class to declare a Java variable and assign it the value it finds in one of the JSP scopes, right after the doStartTag( ) call. When the body has been evaluated, the doAfterBody( ) method is called: public int doAfterBody( ) throws JspException { if (enum.hasMoreElements( )) { Object currValue = enum.nextElement( ); pageContext.setAttribute(loopId, currValue); return EVAL_BODY_TAG; } else { return SKIP_BODY; } } The Enumeration is tested to see if it contains any more values. If it does, the loopId page scope variable is reassigned to the new value, and EVAL_BODY_TAG is returned to evaluate the body again. When the end of the Enumeration is reached, SKIP_BODY is returned to break the iteration. When the doAfterBody( ) method returns SKIP_BODY, the doEndTag( ) method is called: public int doEndTag( ) throws JspException { // Test if bodyContent is set, since it will be null if the // body was never evaluated (doStartTag returned SKIP_BODY) if (getBodyContent( ) != null) { try { getPreviousOut().print(getBodyContent( ).getString( )); } catch (IOException e) {} } return EVAL_PAGE; } For every iteration, the content of the evaluated body is buffered in the BodyContent instance assigned to the tag handler. In the doEndTag( ), the content is simply moved to the parent’s BodyContent instance or the main JspWriter instance for the page. An alternative to accumulating the content until the doEndTag( ) method is called is to write it to the parent’s output stream already in the doAfterBody( ) method, using the same code as shown here. page 228
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