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POIs there a safe, programmatic way to determine if it's safe to open a Swing window?
 

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    <p>I have a java app that the user might invoke from the command line by running java -jar app.jar or from a desktop environment by double-clicking the .jar file. </p> <p>I would like the app to be able to detect if it can safely launch a Swing GUI or if it has to use a command-line interface. If I just start calling Swing functions without checking for the availability of a window system, Java bails out with an InternalError.</p> <p>For instance, if I unset DISPLAY and run the application, I get:</p> <pre> java.lang.InternalError: Can't connect to X11 window server using '' as the value of the DISPLAY variable. at sun.awt.X11GraphicsEnvironment.initDisplay(Native Method) at sun.awt.X11GraphicsEnvironment.access$200(X11GraphicsEnvironment.java:62) at sun.awt.X11GraphicsEnvironment$1.run(X11GraphicsEnvironment.java:178) at java.security.AccessController.doPrivileged(Native Method) at sun.awt.X11GraphicsEnvironment.(X11GraphicsEnvironment.java:142) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:186) at java.awt.GraphicsEnvironment.getLocalGraphicsEnvironment(GraphicsEnvironment.java:82) at sun.awt.X11.XToolkit.(XToolkit.java:112) at java.lang.Class.forName0(Native Method) at java.lang.Class.forName(Class.java:186) at java.awt.Toolkit$2.run(Toolkit.java:849) at java.security.AccessController.doPrivileged(Native Method) at java.awt.Toolkit.getDefaultToolkit(Toolkit.java:841) [etc] </pre> <p>I could just try...catch the InternalError and run the command-line interface in that case, but I'd heard that one shouldn't catch Errors because they might leave the JVM in an inconsistent state. (Please correct me if I'm wrong.) </p> <p>Is there any way that I can check to see if I can safely open a Swing window without catching an InternalError? (I could just check if the DISPLAY environment variable is not empty, but a non-empty DISPLAY is no guarantee that the X server actually works. Nor would that approach work in non-X11 environments.)</p>
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    1. COWhile you should not catch `Error` in general, in this specific case it would probably be ok to catch the `InternalError` (only on a very limited `catch` block that only calls `getDefaultToolkit()`). But since there's a better solution (`isHeadless()`), it isn't necessary.
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      1. USJoachim Sauer
 

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