java qr code generator tutorial Displaying Charts from a Portlet in Java

Generate QR Code in Java Displaying Charts from a Portlet

Displaying Charts from a Portlet
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JFreeChart can create PNG or JPEG images out of the charts. Web and portal developers have two choices for displaying charts. The first approach is to create an image file on the file system and then serve the file through the web server to the end user. This works well when an image is created on a schedule. An example is a thread that creates a PNG file named last_day_results.PNG on the file system every 24 hours.
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TIP The PNG format is preferable for charts, because it is a lossless format, and the image quality will be much superior to the JPEG file format. Unless you have to support a web browser that does not support PNG images, avoid the JPEG file format for your charts. They will look terrible.
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Charting with JFreeChart
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Usually, developers will choose to use a servlet to create an image dynamically instead. The source of an HTML image tag would be a URL that points to a servlet with several parameters for customization. This approach works in either web applications or portal applications in both cases, a servlet provides an image. We do not use portlets for this, even though the portlet API supports writing binary output to a render response. Binary output works only for portlets that are contained in an HTML <IFRAME> frame within the portal, and this will be a rare deployment scenario. The <IFRAME> frame will reset to its starting page whenever the portal page refreshes, which is usually not how the user expects the portal page to behave.
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JFreeChart and Servlets
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Servlet support comes with JFreeChart. There are three classes in the org.jfree.chart.servlet package that servlet or portlet application developers can use. The ServletUtilities class provides several utility methods for servlet developers who are serving charts over the Web. The DisplayChart servlet reads chart images out of the Java temporary directory and serves them as binary image files. It is ready for inclusion in a portlet or web application. The ChartDeleter class is a helper class for the servlet or portlet that is managing the content that includes the charts. When a chart is created in the temporary directory, the servlet or portlet registers the chart s filename with ChartDeleter so it is deleted when the user s session expires.
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The ServletUtilities class is useful for developers who need to serve JFreeChart charts from servlets. If the security provided with the DisplayChart servlet is not sufficient, you can develop your own image-serving servlet class. ServletUtilities also provides methods for portlet or servlet developers to save their charts in the temporary directory. The saveChartAsPNG() and saveChartAsJPEG() methods create an image file in the Java temporary directory with the given height and width. They also register the chart for deletion with the user s ChartDeleter. The methods retrieve the ChartDeleter from the user s session. If the ChartDeleter does not exist, it will be created. The ChartDeleter is stored in the session with the key JFreeChart_Deleter .
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public static String saveChartAsPNG(JFreeChart chart, int width, int height, HttpSession session) throws IOException public static String saveChartAsJPEG(JFreeChart chart, int width, int height, HttpSession session) throws IOException
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These methods return the filename on the file system. The DisplayChart servlet uses the filename to serve the images out of the temporary directory. When we create the files, the filename will start with the prefix jfreechart- , followed by a unique number, and then end with the suffix .png for instance, jfreechart-45606.png. There are also two methods for setting and retrieving the temporary file prefix used by the saveChartAsPNG() and saveChartAsJPEG() methods. The default prefix is jfreechart- .
public static String getTempFilePrefix() public static void setTempFilePrefix(String prefix)
Three sendTempFile() methods stream a file in the temporary directory to a user s web browser. They set the HTTP Content-Length and Last-Modified headers on the HTTP response. If the MIME type of the file is either image/jpeg or image/png, they also set the HTTP Content-Type header.
public static void sendTempFile(String filename, HttpServletResponse response) throws IOException public static void sendTempFile(File file, HttpServletResponse response) throws IOException public static void sendTempFile(File file, HttpServletResponse response, String mimeType) throws IOException
You can use any MIME type passed into the third sendTempFile() method listed previously. It is convenient to use the DisplayChart servlet class that comes with JFreeChart to serve your images if possible.
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