如何在GWT中将Java回调传递给返回Promise的JSNI方法？

Hey there! Since you're stuck with GWT 2.7 and can't leverage JsInterop, I totally get how tricky it is to handle Promises returned from JSNI methods called from Java. I’ve dealt with this exact scenario before, so let’s break down a practical approach that works:

1. First, Define a Java Callback Interface #

GWT 2.7’s Java code can’t natively understand Promise objects, so we need a way to bridge the Promise’s then/catch callbacks to Java. Start by creating a simple callback interface that your JSNI code can invoke:

public interface PromiseCallback<T> {
  void onSuccess(T result);
  void onFailure(Throwable error);
}

This interface will let us pass success/failure logic from Java into the JSNI layer.

2. Wrap the Promise in JSNI #

Next, write a JSNI method that calls your JavaScript function (the one returning a Promise), then hooks up the Promise’s handlers to trigger your Java callback. Here’s an example:

public native void callJsPromiseMethod(PromiseCallback<String> callback) /*-{
  // Replace this with your actual JS function that returns a Promise
  const jsPromise = $wnd.yourJsFunctionThatReturnsPromise();

  jsPromise.then(function(successResult) {
    // Invoke the Java callback's onSuccess method
    callback.@com.yourpackage.YourClass.PromiseCallback::onSuccess(Ljava/lang/String;)(successResult);
  }).catch(function(error) {
    // Convert JS error to a Java Throwable for error handling
    const javaError = @java.lang.RuntimeException::new(Ljava/lang/String;)(error.message || error.toString());
    callback.@com.yourpackage.YourClass.PromiseCallback::onFailure(Ljava/lang/Throwable;)(javaError);
  });
}-*/;

Make sure to replace com.yourpackage.YourClass with your actual package and class name, and $wnd.yourJsFunctionThatReturnsPromise() with the correct reference to your JavaScript method.

3. Handle Complex Return Types (If Needed) #

If your Promise returns a complex JavaScript object instead of a primitive/string, you’ll need to map it to a Java class. First create a simple Java class to hold the data:

public class JsUserData {
  private String username;
  private int userAge;

  // Empty constructor required for JSNI to instantiate
  public JsUserData() {}

  // Getters and setters
  public String getUsername() { return username; }
  public void setUsername(String username) { this.username = username; }
  public int getUserAge() { return userAge; }
  public void setUserAge(int userAge) { this.userAge = userAge; }
}

Then update your JSNI method to convert the JS object to this Java class:

public native void fetchUserData(PromiseCallback<JsUserData> callback) /*-{
  $wnd.fetchUserPromise().then(function(jsUser) {
    // Create a new instance of your Java data class
    const javaUser = @com.yourpackage.JsUserData::new()();
    // Map JS object properties to Java fields
    javaUser.@com.yourpackage.JsUserData::setUsername(Ljava/lang/String;)(jsUser.username);
    javaUser.@com.yourpackage.JsUserData::setUserAge(I)(jsUser.age);
    // Pass the Java object to the success callback
    callback.@com.yourpackage.PromiseCallback::onSuccess(Ljava/lang/Object;)(javaUser);
  }).catch(function(error) {
    const javaError = @java.lang.RuntimeException::new(Ljava/lang/String;)(error.message);
    callback.@com.yourpackage.PromiseCallback::onFailure(Ljava/lang/Throwable;)(javaError);
  });
}-*/;

4. Use the Callback in Java #

Now you can call your JSNI method from Java and handle the Promise results with your callback:

callJsPromiseMethod(new PromiseCallback<String>() {
  @Override
  public void onSuccess(String result) {
    // Handle successful Promise resolution
    GWT.log("Promise resolved! Result: " + result);
  }

  @Override
  public void onFailure(Throwable error) {
    // Handle Promise rejection
    GWT.log("Promise rejected with error: " + error.getMessage(), error);
  }
});

Key Notes to Remember #

• Reference JS Functions Correctly: Ensure your JavaScript method is accessible via $wnd (the global window object) in JSNI, otherwise GWT won’t find it.
• Async Context: Remember that Promise callbacks run asynchronously in GWT’s event loop—don’t expect immediate results, and avoid blocking the main thread.
• Type Conversion: When passing data between JS and Java, make sure to match types correctly (e.g., JS number maps to Java int/double, JS boolean to Java boolean).
• Error Handling: Always wrap JS errors in a Java Throwable so you can properly log and handle them in your Java code.

内容的提问来源于stack exchange，提问作者DevOne