https://www.graalvm.org/ 
public interface org.graalvm.tools.insight.test.InsightAPI$OnCloseHandler extends org.graalvm.tools.insight.test.InsightAPI$Handler
  minor version: 0
  major version: 59
  flags: flags: (0x0601) ACC_PUBLIC, ACC_INTERFACE, ACC_ABSTRACT
  this_class: org.graalvm.tools.insight.test.InsightAPI$OnCloseHandler
  super_class: java.lang.Object
{
  public abstract void closed();
    descriptor: ()V
    flags: (0x0401) ACC_PUBLIC, ACC_ABSTRACT
}
SourceFile: "InsightAPI.java"
NestHost: org.graalvm.tools.insight.test.InsightAPI
InnerClasses:
  public abstract Handler = org.graalvm.tools.insight.test.InsightAPI$Handler of org.graalvm.tools.insight.test.InsightAPI
  public abstract OnCloseHandler = org.graalvm.tools.insight.test.InsightAPI$OnCloseHandler of org.graalvm.tools.insight.test.InsightAPI
    RuntimeVisibleAnnotations: 
      java.lang.FunctionalInterface()