CloseableHttpAsyncClient http = HttpAsyncClients.createDefault();
http.start();
http.execute(request, new FutureCallback<HttpResponse>() {
    @Override
    public void completed(HttpResponse response) {
        System.out.println("Hello"); // this works
        // StyledText widget st
        st.setText("Hello"); // never works
        System.out.println("Hello"); // never prints
    }
    
    @Override
    public void failed(Exception e) {
    
    }
    
    @Override
    public void cancelled() {
    
    }
});

HttpResponse response = http.execute(request, null).get();
st.setText("Hello");

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpasyncclient</artifactId>
    <version>4.1.5</version>
</dependency>