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 JFrame("HeadlessExceptionDemo");
      frame.setSize(800, 600);
      frame.setLocationRelativeTo(null);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setVisible(true);

   }
}

the following code also works:

import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

import javax.swing.*;

@Configuration
public class HeadlessExceptionDemo {

   @Bean
   public JFrame frame() {
      JFrame frame = new JFrame("HeadlessExceptionDemo");
      frame.setSize(800, 600);
      frame.setLocationRelativeTo(null);
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      return frame;
   }

   public static void main(String[] args) {

      AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(HeadlessExceptionDemo.class);
      JFrame frame = ctx.getBean(JFrame.class);
      frame.setVisible(true);

   }
}

while the following code does not:

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.context.annotation.Bean;

import javax.swing.*;

@SpringBootApplication
public class HeadlessExceptionDemo {

    @Bean
    public JFrame frame() {
        JFrame frame = new JFrame("HeadlessExceptionDemo");
        frame.setSize(800, 600);
        frame.setLocationRelativeTo(null);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        return frame;
    }

    public static void main(String[] args) {
        ConfigurableApplicationContext ctx = SpringApplication.run(HeadlessExceptionDemo.class, args);
        JFrame frame = ctx.getBean(JFrame.class);
        frame.setVisible(true);
    }
}

Causing exception below:

Caused by: org.springframework.beans.BeanInstantiationException: Failed to instantiate [javax.swing.JFrame]: Factory method 'frame' threw exception; nested exception is java.awt.HeadlessException
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:189) ~[spring-beans-4.2.5.RELEASE.jar:4.2.5.RELEASE]
    at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:588) ~[spring-beans-4.2.5.RELEASE.jar:4.2.5.RELEASE]
    ... 23 common frames omitted
Caused by: java.awt.HeadlessException: null
    at java.awt.GraphicsEnvironment.checkHeadless(GraphicsEnvironment.java:207) ~[na:1.8.0_45]
    at java.awt.Window.<init>(Window.java:536) ~[na:1.8.0_45]
    at java.awt.Frame.<init>(Frame.java:420) ~[na:1.8.0_45]
    at javax.swing.JFrame.<init>(JFrame.java:233) ~[na:1.8.0_45]
    at com.inthemoon.snippets.springboot.HeadlessExceptionDemo.frame(HeadlessExceptionDemo.java:15) [classes/:na]
    at com.inthemoon.snippets.springboot.HeadlessExceptionDemo$$EnhancerBySpringCGLIB$$3680a05b.CGLIB$frame$0(<generated>) ~[classes/:na]
    at com.inthemoon.snippets.springboot.HeadlessExceptionDemo$$EnhancerBySpringCGLIB$$3680a05b$$FastClassBySpringCGLIB$$b7def9bc.invoke(<generated>) ~[classes/:na]
    at org.springframework.cglib.proxy.MethodProxy.invokeSuper(MethodProxy.java:228) ~[spring-core-4.2.5.RELEASE.jar:4.2.5.RELEASE]
    at org.springframework.context.annotation.ConfigurationClassEnhancer$BeanMethodInterceptor.intercept(ConfigurationClassEnhancer.java:355) ~[spring-context-4.2.5.RELEASE.jar:4.2.5.RELEASE]
    at com.inthemoon.snippets.springboot.HeadlessExceptionDemo$$EnhancerBySpringCGLIB$$3680a05b.frame(<generated>) ~[classes/:na]
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_45]
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_45]
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~[na:1.8.0_45]
    at java.lang.reflect.Method.invoke(Method.java:497) ~[na:1.8.0_45]
    at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:162) ~[spring-beans-4.2.5.RELEASE.jar:4.2.5.RELEASE]
    ... 24 common frames omitted

Why? :)

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;
    }