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Using Headless Mode in the Java SE Platform--转

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By Artem Ananiev and Alla Redko, June 2006

This article explains how to use the headless mode capabilities of the Java Platform, Standard Edition (Java SE, formerly referred to as J2SE).

Headless mode is a system configuration in which the display device, keyboard, or mouse is lacking. Sounds unexpected, but actually you can perform different operations in this mode, even with graphic data.

Where it is applicable? Let's say that your application repeatedly generates a certain image, for example, a graphical authorization code that must be changed every time a user logs in to the system. When creating an image, your application needs neither the display nor the keyboard. Let's assume now that you have a mainframe or dedicated server on your project that has no display device, keyboard, or mouse. The ideal decision is to use this environment's substantial computing power for the visual as well as the nonvisual features. An image that was generated in the headless mode system then can be passed to the headful system for further rendering.

Many methods in the java.awt.Toolkit and java.awt.GraphicsEnvironment classes, with the exception of fonts, imaging, and printing, require the availability of a display device, keyboard, and mouse. But some classes, such as Canvas or Panel, can be executed in headless mode. Headless mode support has been available since the J2SE 1.4 platform.

Note: This article will point the reader to documentation for version 6 of the Java SE platform. Any API additions or other enhancements to the Java SE platform specification are subject to review and approval by the JSR 270 Expert Group.

Toolkit

The java.awt.Toolkit class is an abstract superclass of all actual implementations of the Abstract Window Toolkit (AWT). Subclasses of Toolkit are used to bind the various AWT components to particular native toolkit implementations.

Many components are affected if a display device, keyboard, or mouse is not supported. An appropriate class constructor throws a HeadlessException :

Such heavyweight components require a peer at the operating-system level, which cannot be guaranteed on headless machines.

Methods related to Canvas, Panel, and Image components do not need to throw aHeadlessException because these components can be given empty peers and treated as lightweight components.

The Headless toolkit also binds the Java technology components to the native resources, but it does so when resources don't include a display device or input devices.

Graphics Environment

The java.awt.GraphicsEnvironment class is an abstract class that describes the collection ofGraphicsDevice objects and Font objects available to a Java technology application on a particular platform. The resources in this GraphicsEnvironment might be local or on a remote machine. GraphicsDevice objects can be monitors, printers, or image buffers and are the destination of Graphics2D drawing methods. Each GraphicsDevice has manyGraphicsConfiguration objects associated with it. These objects specify the different configurations in which the GraphicsDevice can be used.

Table 1 shows the GraphicsEnvironment methods that check for headless mode support.

Table 1. The Headless Mode Methods

Method	Description

`public static boolean` `isHeadless()`	Tests whether the environment is headless, and therefore does not support a display device, keyboard, or mouse. If this method returns`true`, a `HeadlessException` is thrown from areas of the `Toolkit` and`GraphicsEnvironment` classes that depend on a display device, keyboard, or mouse.
`public boolean isHeadlessInstance()`	Returns whether this `GraphicsEnvironment` can support a display device, keyboard, or mouse. If this method returns `true`, a`HeadlessException` is thrown from areas of the`GraphicsEnvironment` that depend on a display device, keyboard, or mouse.

Note: The isHeadless() method checks the specific system property, java.awt.headless, instead of the system's hardware configuration.

A HeadlessException is thrown when code that depends on a display device, keyboard, or mouse is called in an environment that does not support any of these. The exception is derived from an UnsupportedOperationException, which is itself derived from a RuntimeException.

Setting Up Headless Mode

To use headless mode operations, you must first understand how to check and set up the system properties related to this mode. In addition, you must understand how to create a default toolkit to use the headless implementation of the Toolkit class.

System Properties Setup

To set up headless mode, set the appropriate system property by using the setProperty()method. This method enables you to set the desired value for the system property that is indicated by the specific key.

System.setProperty("java.awt.headless", "true");

In this code, java.awt.headless is a system property, and true is a value that is assigned to it.

You can also use the following command line if you plan to run the same application in both a headless and a traditional environment:

java -Djava.awt.headless=true

Default Toolkit Creation

If a system property named java.awt.headless is set to true, then the headless implementation of Toolkit is used. Use the getDefaultToolkit() method of the Toolkit class to create an instance of headless toolkit:

Toolkit tk = Toolkit.getDefaultToolkit();

Headless Mode Check

To check the availability of headless mode, use the isHeadless() method of theGraphicsEnvironment class:

GraphicsEnvironment ge = 
GraphicsEnvironment.getLocalGraphicsEnvironment(); 
boolean headless_check = ge.isHeadless();

This method checks the java.awt.headless system property. If this property has a true value, then a HeadlessException will be thrown from areas of the Toolkit andGraphicsEnvironment classes that are dependent on a display device, keyboard, or mouse.

Operating in Headless Mode

After setting up headless mode and creating an instance of the headless toolkit, your application can perform the following operations:

Create lightweight components such as Canvas, Panel, and Swing components, except the top levels
Obtain information about available fonts, font metrics, and font settings
Set color for rendering text and graphics
Create and obtain images and prepare images for rendering
Print using java.awt.PrintJob, java.awt.print.*, and javax.print.* classes
Emit an audio beep

Canvas

The following code represents a blank rectangular area of the screen onto which you can draw lines. To create a new Canvas component, use the Canvas class.

        final Canvas c = new Canvas()
        {
            public void paint(Graphics g)
            {
                Rectangle r = getBounds();
                g.drawLine(0, 0, r.width - 1, r.height - 1);
                g.drawLine(0, r.height - 1, r.width - 1, 0);
            }
        };

Fonts

This code shows how to set the font with the Font class to draw a text string. The Graphics object is used to render this string.

      public void paint(Graphics g)
            {
                g.setFont(new Font("Arial", Font.ITALIC, 12));
                g.drawString("Test", 32, 8);
            }

Colors

This code shows how to set the color with the specified red, green, and blue values in order to draw a line. The Graphics object is used to render this line.

  public void paint(Graphics g)
            {
                g.setColor(new Color(255, 127, 0));
                g.drawLine(0, r.height - 1, r.width - 1, 0);
            }

Images

In the following code, the read method of the javax.imageio.ImageIO class decodes thegrapefruit.jpg image file shown in Figure 1 and returns a buffered image.

        Image i = null;
        try
        {
            File f = new File("grapefruit.jpg");
            i = ImageIO.read(f);
        }
        catch (Exception z)
        {
            z.printStackTrace(System.err);
        }

Figure 1. The grapefruit.jpg Image File

Print

This code shows how to print a prepared canvas that enables you to define the printer as the default surface for the paint method.

PrinterJob pj = PrinterJob.getPrinterJob();
       pj.setPrintable(new Printable()
       {
           public int print(Graphics g, PageFormat pf, int pageIndex)
           {
               if (pageIndex > 0)
               {
                   return Printable.NO_SUCH_PAGE;
               }

               ((Graphics2D)g).translate(pf.getImageableX(),
                                         pf.getImageableY());

               // Paint canvas.
               c.paint(g);

               return Printable.PAGE_EXISTS;
           }
       });

Beep

The following code shows how to produce a simple beep sound by using the Toolkit class's beepmethod.

Toolkit tk = Toolkit.getDefaultToolkit();
      tk.beep();

Using Headless Mode: An Example

The following HeadlessBasics example uses all the capabilities described in this article.

To run this example, compile the following code by using the javac compiler. Copy the image filegrapefruit.jpg to the directory that contains the HeadlessBasics class.

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

import javax.imageio.*;

public class HeadlessBasics
{
    public static void main(String[] args)
    {
        // Set system property.
        // Call this BEFORE the toolkit has been initialized, that is,
        // before Toolkit.getDefaultToolkit() has been called.
        System.setProperty("java.awt.headless", "true");

        // This triggers creation of the toolkit.
        // Because java.awt.headless property is set to true, this 
        // will be an instance of headless toolkit.
        Toolkit tk = Toolkit.getDefaultToolkit();
        // Standard beep is available.
        tk.beep();

        // Check whether the application is
        // running in headless mode.
        GraphicsEnvironment ge = 
        GraphicsEnvironment.getLocalGraphicsEnvironment();
        System.out.println("Headless mode: " + ge.isHeadless());

        // No top levels are allowed.
        boolean created = false;
        try
        {
            Frame f = new Frame("Frame");
            created = true;
        }
        catch (Exception z)
        {
            z.printStackTrace(System.err);
            created = false;
        }
        System.err.println("Frame is created: " + created);

        // No other components except Canvas and Panel are allowed.
        created = false;
        try
        {
            Button b = new Button("Button");
            created = true;
        }
        catch (Exception z)
        {
            z.printStackTrace(System.err);
            created = false;
        }
        System.err.println("Button is created: " + created);
        
        // Canvases can be created.
        final Canvas c = new Canvas()
        {
            public void paint(Graphics g)
            {
                Rectangle r = getBounds();
                g.drawLine(0, 0, r.width - 1, r.height - 1);
                // Colors work too.
                g.setColor(new Color(255, 127, 0));
                g.drawLine(0, r.height - 1, r.width - 1, 0);
                // And fonts
                g.setFont(new Font("Arial", Font.ITALIC, 12));
                g.drawString("Test", 32, 8);
            }
        };
        // And all the operations work correctly.
        c.setBounds(32, 32, 128, 128);

        // Images are available.
        Image i = null;
        try
        {
            File f = new File("grapefruit.jpg");
            i = ImageIO.read(f);
        }
        catch (Exception z)
        {
            z.printStackTrace(System.err);
        }
        final Image im = i;
        
        // Print system is available.
        PrinterJob pj = PrinterJob.getPrinterJob();
        pj.setPrintable(new Printable()
        {
            public int print(Graphics g, PageFormat pf, int pageIndex)
            {
                if (pageIndex > 0)
                {
                    return Printable.NO_SUCH_PAGE;
                }
                ((Graphics2D)g).translate(pf.getImageableX(),
                                          pf.getImageableY());

                // Paint the canvas.
                c.paint(g);

                // Paint the image.
                if (im != null)
                {
                    g.drawImage(im, 32, 32, 64, 64, null);
                }

                return Printable.PAGE_EXISTS;
            }
        });
        try
        {
            pj.print();
        }
        catch (Exception z)
        {
            z.printStackTrace(System.err);
        }
    }
}

Figure 2 shows the printed output from this example.

Figure 2. The HeadlessBasics Printed Output

Also, you can expect to see the following messages:

Headless mode: true
java.awt.HeadlessException
at java.awt.GraphicsEnvironment.checkHeadless(Unknown Source)
at java.awt.Window.<init>(Unknown Source)
at java.awt.Frame.<init>(Unknown Source)
at HeadlessBasics.main(HeadlessBasics.java:24)
Frame is created: false
java.awt.HeadlessException
at java.awt.GraphicsEnvironment.checkHeadless(Unknown Source)
at java.awt.Button.<init>(Unknown Source)
at HeadlessBasics.main(HeadlessBasics.java:39)
Button is created: false

Note: For demonstration purposes, the original sample code causes the application to throw twojava.awt.HeadlessExceptions.

As an alternative, you can forward the standard output messages to a file and print out the file. In this case, use the following command line to run this example:

java HeadlessBasics 2> standard_output.txt

Converting an Existing Application to Headless Mode

How can you convert your existing application to execute it in headless mode? The most effective way to perform this conversion is to analyze your source code in order to determine any functionality that is dependent on headless mode. In other words, to implement the same functionality, you have to find those methods and classes that can throw a HeadlessExceptionand replace them with the methods and classes that are independent of headless mode.

You can use the Java SE 6 API specification to determine whether a specific method or class is supported in headless mode or not. If a specific component is not supported in headless mode, the only exception your application needs to catch is a HeadlessException. It will be thrown first among the other possible exceptions. That is why the code sample in the section "Example: Using Headless Mode" contains no special requirement to catch the other exceptions.

You will certainly find other useful ways to apply the advantages of headless mode. We hope that this article can help you to accomplish this task and open new horizons in the Java SE platform.

For More Information

AWT Enhancements in J2SE 1.4: Headless Support
J2SE 1.4 platform documentation: HeadlessException
The New Modality API in Java SE 6
The java.awt.Toolkit Class
The java.awt.GraphicsEnvironment Class

About the Authors

Artem Ananiev is a Sun Microsystems software engineer in Saint Petersburg, Russia. He has been working in the Abstract Window Toolkit (AWT) project for several years, with his primary functional areas in modality, robot, and multiscreen systems.

Alla Redko is a Sun Microsystems technical writer in Saint Petersburg, Russia. She supports documentation for the AWT project and updates the Java Tutorial. Prior to her assignment for Sun, she worked as a technical writer for eight years.