基本打印程序

This section explains how to create a basic printing program that displays a print dialog and prints the text "Hello World" to the selected printer.

Printing task usually consists of two parts:

• Job control — Creating a print job, associating it with a printer, specifying the number of copies, and user print dialog interaction.
• Page Imaging — Drawing content to a page, and managing content that spans pages (pagination).

First create the printer job. The class representing a printer job and most other related classes is located in the java.awt.print package.

import java.awt.print.*;

PrinterJob job = PrinterJob.getPrinterJob();

Next provide code that renders the content to the page by implementing the Printable interface.

class HelloWorldPrinter
              implements Printable { ... }
...
job.setPrintable(new HelloWorldPrinter());

An application typically displays a print dialog so that the user can adjust various options such as number of copies, page orientation, or the destination printer.

boolean doPrint = job.printDialog();

This dialog appears until the user either approves or cancels printing. The doPrint variable will be true if the user gave a command to go ahead and print. If the doPrint variable is false, the user cancelled the print job. Since displaying the dialog at all is optional, the returned value is purely informational.

If the doPrint variable is true, then the application will request that the job be printed by calling the PrinterJob.print method.

if (doPrint) {
    try {
        job.print();
    } catch (PrinterException e) {
        // The job did not successfully
        // complete
    }
}

The PrinterException will be thrown if there is problem sending the job to the printer. However, since the PrinterJob.print method returns as soon as the job is sent to the printer, the user application cannot detect paper jams or paper out problems. This job control boilerplate is sufficient for basic printing uses.

The Printable interface has only one method:

public int print(Graphics graphics,
           PageFormat pf, int page)
           throws PrinterException;

The PageFormat class describes the page orientation (portrait or landscape) and its size and imageable area in units of 1/72nd of an inch. Imageable area accounts for the margin limits of most printers (hardware margin). The imageable area is the space inside these margins, and in practice if is often further limited to leave space for headers or footers.

A page parameter is the zero-based page number that will be rendered.

The following code represents the full Printable implementation:

import java.awt.print.*;
import java.awt.*;

public class HelloWorldPrinter
    implements Printable {

  public int print(Graphics g, PageFormat pf, int page)
      throws PrinterException {

    // We have only one page, and 'page'
    // is zero-based
    if (page > 0) {
         return NO_SUCH_PAGE;
    }

    // User (0,0) is typically outside the
    // imageable area, so we must translate
    // by the X and Y values in the PageFormat
    // to avoid clipping.
    Graphics2D g2d = (Graphics2D)g;
    g2d.translate(pf.getImageableX(), pf.getImageableY());

    // Now we perform our rendering
    g.drawString("Hello world!", 100, 100);

    // tell the caller that this page is part
    // of the printed document
    return PAGE_EXISTS;
  }
}

The complete code for this example is in HelloWorldPrinter.java.

Sending a Graphics instance to the printer is essentially the same as rendering it to the screen. In both cases you need to perform the following steps:

• To draw a test string is as easy as the other operations that were described for drawing to a Graphics2D.
• Printer graphics have a higher resolution, which should be transparent to most code.
• The Printable.print() method is called by the printing system, just as the Component.paint() method is called to paint a Component on the display. The printing system will call the Printable.print() method for page 0, 1,.. etc until the print() method returns NO_SUCH_PAGE.
• The print() method may be called with the same page index multiple times until the document is completed. This feature is applied when the user specifies attributes such as multiple copies with collate option.
• The PageFormat's imageable area determines the clip area. Imageable area is also important in calculating pagination, or how to span content across printed pages, since page breaks are determined by how much can fit on each page.

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  注意: A call to the print() method may be skipped for certain page indices if the user has specified a different page range that does not involve a particular page index.

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