Pentaho does display chart in an web application

I embed the report in my web application. without the chart, the report is displaying properly, but if i put a chart , then error message will occur . I try pie chart and bar chart , got same error. I have include all needed jar file into my application, such as libbase.jar, jfreechart.jar,pentaho-reporting-engine-classic-core.jar,pentaho-reporting-engine-legacy-charts.jar...
the error message like:
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org.pentaho.reporting.libraries.xmlns.parser.ParseException: Expression 'org.pentaho.plugin.jfreereport.reportcharts.BarChartExpression' is not valid. [Location: Line=22 Column=190]
at org.pentaho.reporting.engine.classic.core.modules.parser.bundle.layout.elements.AttributeExpressionReadHandler.startParsing(AttributeExpressionReadHandler.java:96)
---------------------

I open the layout.xml file, the 22th line is :
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<attribute-expression namespace="http://reporting.pentaho.org/namespaces/engine/attributes/core" name="value" class="org.pentaho.plugin.jfreereport.reportcharts.BarChartExpression">
-------------

further, I open the source code of AttributeExpressionReadHandler.java. the related code is pasted at follow:
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if (expression == null && expressionClassName != null)
{


expression = (Expression) ObjectUtilities.loadAndInstantiate(expressionClassName, getClass(), Expression.class);
if (expression == null)
{
throw new ParseException("Expression '" + expressionClassName +
"' is not valid.", getLocator());
}
}
--------
I suspect that ObjectUtilities.loadAndInstantiate function return a null, but why it reurn null ? the reason I can't find out. Anybody can help me? I am very appreciate for see your response！