How do I create a listener when a new file editor is opened

see com.intellij.openapi.fileEditor.FileEditorManagerListener which can be registered via com.intellij.openapi.fileEditor.FileEditorManager#addFileEditorManagerListener()

Thank you for your answer. It works

FileEditorManager.addFileEditorManagerListener() is deprecated. You can use MessageBus.

MessageBus messageBus = project.getMessageBus();
messageBus.connect().subscribe(FileEditorManagerListener.FILE_EDITOR_MANAGER, new FileEditorManagerAdapter() {
    @Override
    public void fileOpened(@NotNull FileEditorManager source, @NotNull VirtualFile file) {
        super.fileOpened(source, file);
    }

    @Override
    public void fileClosed(@NotNull FileEditorManager source, @NotNull VirtualFile file) {
        super.fileClosed(source, file);
    }

    @Override
    public void selectionChanged(@NotNull FileEditorManagerEvent event) {
        super.selectionChanged(event);
    }
});