completablefuture_example.java

public static CompletableFuture<String> getUrlBytes(String url) {
  CompletableFuture<String> future = new CompletableFuture<>();
  
	final XMLHttpRequest req = new XMLHttpRequest();
	req.open("GET", url, true);
	req.setOnreadystatechange((e) -> {
    if (req.status == 200) {
		  future.complete(req.response);
    } else {
		  future.completeExceptionally(new Exception(req.statusText));
    }
	});
	req.onerror = function() {
    future.completeExceptionally(new Exception("Network Error"));
	};
	req.send();
	
	return future;
}

@JsType( isNative = true, namespace = JsPackage.GLOBAL, name = "XMLHttpRequest" )
public class XMLHttpRequest
{
	@JsMethod
	public native void open( String method, String url, boolean isAsync );

	@JsMethod
	public native void send( Object data );

	@JsProperty
	public native void setOnreadystatechange( ReadyStateHandler handler );

	@JsMethod
	public native void setRequestHeader( String headerName, String value );

	@JsProperty
	public native int getReadyState();

	@JsProperty
	public native int getStatus();

	@JsProperty
	public native String getResponseText();
}

@JsFunction
@FunctionalInterface
public interface ReadyStateHandler
{
	void onStateChanged( Object event );
}