Why does Swing think it's headless under Spring Boot, but not under Spring or plain Java?

Question

The following code works:

import javax.swing.*;

public class HeadlessExceptionDemo {

   public static void main(String[] args) {

      JFrame frame = new          


        

Answer 1

Spring Boot sets java.awt.headless to true by default as seen in the source code for SpringApplication.java:

private boolean headless = true;

...

private void configureHeadlessProperty() {
        System.setProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, System.getProperty(
                SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, Boolean.toString(this.headless)));
}

As to why it does this, there is a comment in the source code on the setHeadless method about preventing icons from appearing:

    /**
     * Sets if the application is headless and should not instantiate AWT. Defaults to
     * {@code true} to prevent java icons appearing.
     * @param headless if the application is headless
     */
    public void setHeadless(boolean headless) {
        this.headless = headless;
    }