Java AWT: Printing - JDK 5 Documentation v1.1.8, Java 2 SDK 英文文档

Java AWT: Printing

Last Updated: February 3, 1997

Purpose

The goal for the AWT printing API is to give developers an easy mechanism to print the AWT components using native platform facilities. This API is designed for use with the current AWT Graphics model.

The Printing API

The printing API is very simple and consists primarily of one method in class java.awt.Toolkit:

	PrintJob getPrintJob(Frame f, String jobtitle, Properties props)

and a new class responsible for encapsulating all the information associated with a printing request:

	java.awt.PrintJob

The getPrintJob() method in Toolkit will take care of posting any platform specific print dialogs so that when it returns, the PrintJob object has already been configured to match the user's request. The "properties" parameter provides the ability to pass in any platform-dependent default property values for the print job (i.e. "printer name", "last-page-first", etc). This allows app-specific printing configuration properties appropriate for a particular program to be stored and applied in individual circumstances. If the user changes the default print settings in the print dialog, those changes are saved into the Properties object passed in as the parameter (so the program can choose to store those new defaults if desired).

The PrintJob object provides access to the relevant printing properties chosen by the user (page size, etc) as well as the print graphics contexts used to transparently render to the print device.

Print Graphics Context

The print graphics context is provided by a new interface:

	java.awt.PrintGraphics

The PrintJob object provides the method to obtain a handle to an object which is a subclass of Graphics AND implements the PrintGraphics interface:

	Graphics getGraphics()

Since this object is a subclass of the Graphics class, it can simply be passed into existing paint() or print() methods (which use the standard Graphics drawing methods) and the underlying AWT implementation takes care of translating those calls to the appropriate print device (this is straightforward on Mac/Windows which support graphic print devices; on Solaris, a PostScript graphics object is required to generate appropriate PostScript).

It is often the case that from within a paint() or print() method, your program may want to detect whether it is rendering to the screen or to a print device (for example it would not necessarily want to print `highlighted' text which has been selected by the user). Since the graphics object returned from the PrintJob.getGraphics() method implements the PrintGraphics interface, this can easily be detected with the following test:

	public void paint(Graphics g) {

		if (g instanceof PrintGraphics)

			// printing is occurring

...

The primary method in the PrintGraphics interface is a method to obtain a handle to the associated PrintJob object:

	PrintJob getPrintJob()

Pagination

It is the program's responsibility to handle pagination. Each call to getGraphics() will return a print graphics context for a single page; when the program calls dispose() on the print graphics context, that page is flushed to the printer. The program must take care of determining appropriate page content and page breaking, given the specified page dimensions and resolution.

These printing attributes can be obtained by the following methods on java.awt.PrintJob:

	Dimension getPageDimension()

	int getPageResolution()

Printing Hierarchies of Components

If a program needs to print an entire containment hierarchy of AWT components (i.e. a Panel and any nested controls/containers within it), it should call the following java.awt.Component method on the root of the containment hierarchy (passing in the appropriate print graphics context):

     public void printAll(Graphics g)

Calling this method will result in the complete traversal of the containment hierarchy such that the print() method will be called on all descendent components. The default implementation of the print() method simply calls paint(), so if a program has created a component which renders itself in the paint() method, printing should work "automagically". Note that all the base AWT components (Button, Scrollbar, etc.) are capable of printing themselves when their printAll() method is called (even though their rendering does not take place in the paint() method).

Print Dialogs

Although it is important for some programs to either extend or create their own print dialogs, this capability will not be in the first release of the API.

Security

Because it was considered dangerous to allow potentially untrusted applets to initiate print job requests (and potentially quietly print anything devious to the printer that they desire) the initiation of a print job (via Toolkit.getPrintJob) is defined to be an operation bound by the Applet security restrictions implemented in a new SecurityManager method.

It's important to note that this restriction does not limit applets' capability to render using a PrintGraphics object returned from a printjob, it just disallows untrusted applets to initiate the job. A more reasonable policy will be provided when the Java security architecture allows more customization.

Sample Code

A VERY simple example of how a print job can be initiated to print an AWT component:

	
import java.awt.*;
import java.awt.event.*;

public class PrintingTest extends Frame implements ActionListener {

    PrintCanvas canvas;

    public PrintingTest() {
        super("Printing Test"); 
        canvas = new PrintCanvas();
        add("Center", canvas);
        
        Button b = new Button("Print");
        b.setActionCommand("print");
        b.addActionListener(this);
        add("South", b);

        pack();
    }

    public void actionPerformed(ActionEvent e) {
        String cmd = e.getActionCommand();
        if (cmd.equals("print")) {
            PrintJob pjob = getToolkit().getPrintJob(this,
                               "Printing Test", null);

            if (pjob != null) {          
                Graphics pg = pjob.getGraphics();

                if (pg != null) {
                    canvas.printAll(pg);
                    pg.dispose(); // flush page
                }
                pjob.end();

            }
        }
    }

    public static void main(String args[]) {
        PrintingTest test = new PrintingTest();
        test.show();
    }
}

class PrintCanvas extends Canvas {

    public Dimension getPreferredSize() {
        return new Dimension(100, 100);
    }
  
    public void paint(Graphics g) {
        Rectangle r = getBounds();

        g.setColor(Color.yellow);
        g.fillRect(0, 0, r.width, r.height);

        g.setColor(Color.blue);
        g.drawLine(0, 0, r.width, r.height);

        g.setColor(Color.red);
        g.drawLine(0, r.height, r.width, 0);
    }
}

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