java - 如何使用 Guava 在 GWT 中缓存服务器结果？

Question

在我的 GWT 应用程序中，我经常多次引用相同的服务器结果。我也不知道先执行哪个代码。因此，我想使用异步(客户端)结果的缓存。

我想使用现有的缓存库；我正在考虑 guava-gwt。

我找到了这个 Guava 同步 缓存示例(在 guava's documentation 中):

LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
       .build(
           new CacheLoader<Key, Graph>() {
             public Graph load(Key key) throws AnyException {
               return createExpensiveGraph(key);
             }
           });

这就是我尝试异步使用 Guava 缓存的方式(我不知道如何进行这项工作):

LoadingCache<Key, Graph> graphs = CacheBuilder.newBuilder()
       .build(
           new CacheLoader<Key, Graph>() {
             public Graph load(Key key) throws AnyException {

               // I want to do something asynchronous here, I cannot use Thread.sleep in the browser/JavaScript environment.
               service.createExpensiveGraph(key, new AsyncCallback<Graph>() {

                 public void onFailure(Throwable caught) {
                   // how to tell the cache about the failure???
                 }

                 public void onSuccess(Graph result) {
                   // how to fill the cache with that result???
                 }
               });

               return // I cannot provide any result yet. What can I return???
             }
           });

GWT 缺少默认 JRE 中的许多类(尤其是关于线程和并发性)。

如何使用 guava-gwt 缓存异步结果？

Answer 1

据我了解，您想要实现的不仅是异步缓存，而且是惰性缓存，创建一个 GWT 不是最佳位置，因为在使用客户端异步执行实现 GWT 应用程序时存在大问题，因为 GWT 缺少 Future 和/或 Rx 组件的客户端实现(仍然有一些用于 GWT 的 RxJava 实现)。所以在通常的 java 中，你想要创建的东西可以通过以下方式实现:

LoadingCache<String, Future<String>> graphs = CacheBuilder.newBuilder().build(new CacheLoader<String, Future<String>>() {
    public Future<String> load(String key) {
        ExecutorService service = Executors.newSingleThreadExecutor();
        return  service.submit(()->service.createExpensiveGraph(key));
    }
});
Future<String> value = graphs.get("Some Key");
if(value.isDone()){
    // This will block the execution until data is loaded 
    String success = value.get();           
}

但是由于 GWT 没有Future的实现，你需要像这样创建一个

public class FutureResult<T> implements AsyncCallback<T> {
    private enum State {
        SUCCEEDED, FAILED, INCOMPLETE;
    }

    private State state = State.INCOMPLETE;
    private LinkedHashSet<AsyncCallback<T>> listeners = new LinkedHashSet<AsyncCallback<T>>();
    private T value;
    private Throwable error;

    public T get() {
        switch (state) {
        case INCOMPLETE:
            // Do not block browser so just throw ex
            throw new IllegalStateException("The server response did not yet recieved.");
        case FAILED: {
            throw new IllegalStateException(error);
        }
        case SUCCEEDED:
            return value;
        }
        throw new IllegalStateException("Something very unclear");
    }

    public void addCallback(AsyncCallback<T> callback) {
       if (callback == null) return;
       listeners.add(callback);
    }

    public boolean isDone() {
        return state == State.SUCCEEDED;
    }

    public void onFailure(Throwable caught) {
        state = State.FAILED;
        error = caught;
        for (AsyncCallback<T> callback : listeners) {
            callback.onFailure(caught);
        }
    }

    public void onSuccess(T result) {
        this.value = result;
        state = State.SUCCEEDED;
        for (AsyncCallback<T> callback : listeners) {
            callback.onSuccess(value);
        }
    }

}

你的实现将变成:

    LoadingCache<String, FutureResult<String>> graphs = CacheBuilder.newBuilder().build(new CacheLoader<String, FutureResult<String>>() {
        public FutureResult<String> load(String key) {
            FutureResult<String> result = new FutureResult<String>();
            return service.createExpensiveGraph(key, result);
        }
    });

    FutureResult<String> value = graphs.get("Some Key");

    // add a custom handler
    value.addCallback(new AsyncCallback<String>() {
        public void onSuccess(String result) {
            // do something
        }
        public void onFailure(Throwable caught) {
            // do something             
        }
    });
    // or see if it is already loaded / do not wait 
    if (value.isDone()) {
        String success = value.get();
    }

当使用 FutureResult 时，您不仅会缓存执行，还会获得某种惰性，因此您可以在数据加载到缓存中时显示一些 loading screen。