“没有X11 DISPLAY变量” - 这是什么意思？

我正在尝试在我的Linux机器（Slackware）上安装一个Java应用程序。

我收到了以下错误，但我不理解它。

你能建议我如何解决这个问题吗？谢谢。

这是我的输出：（我看到需要设置一些X11 DISPLAY变量，但应该给它什么值以及如何设置？）

~$ java -jar gate-5.0-beta1-build3048-installer.jar
- ERROR -

java.awt.HeadlessException:
No X11 DISPLAY variable was set, but this program performed an operation which requires it.
java.awt.HeadlessException:
No X11 DISPLAY variable was set, but this program performed an operation which requires it.
at java.awt.GraphicsEnvironment.checkHeadless(Graphic sEnvironment.java:159)
at java.awt.Window.<init>(Window.java:407)
at java.awt.Frame.<init>(Frame.java:402)
at net.sourceforge.mlf.metouia.borders.MetouiaDotsBuf fer.<init>(MetouiaDotsBuffer.java:105)
at net.sourceforge.mlf.metouia.borders.MetouiaDots.<i nit>(MetouiaDots.java:66)
at net.sourceforge.mlf.metouia.borders.MetouiaToolBar Border.<init>(MetouiaToolBarBorder.java:49)
at net.sourceforge.mlf.metouia.MetouiaLookAndFeel.ini tComponentDefaults(MetouiaLookAndFeel.java:241)
at javax.swing.plaf.basic.BasicLookAndFeel.getDefault s(BasicLookAndFeel.java:130)
at javax.swing.plaf.metal.MetalLookAndFeel.getDefault s(MetalLookAndFeel.java:1591)
at javax.swing.UIManager.setLookAndFeel(UIManager.jav a:537)
at javax.swing.UIManager.setLookAndFeel(UIManager.jav a:581)
at com.izforge.izpack.installer.GUIInstaller.loadLook AndFeel(GUIInstaller.java:373)
at com.izforge.izpack.installer.GUIInstaller.<init>(G UIInstaller.java:116)
at sun.reflect.NativeConstructorAccessorImpl.newInsta nce0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInsta nce(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newI nstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Construc tor.java:513)
at java.lang.Class.newInstance0(Class.java:355)
at java.lang.Class.newInstance(Class.java:30
at com.izforge.izpack.installer.Installer.main(Instal ler.java:62)