blocks|key|3951048|text|该值将异步返回给回调函数，但您可以强制程序在需要该值时等待返回。一种方法是返回一个值持有者类，它会在您尝试获取值时阻塞，直到它被设置，然后回调设置该值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3951049|例如，下面的类使用Phaser来控制此等待行为：|3951050|import+java.util.concurrent.ExecutionException;
import+java.util.concurrent.Phaser;

public+class+FutureValue<T>+{

++++private+T+value+=+null;
++++private+Exception+ex+=+null;
++++private+Phaser+phaser+=+new+Phaser(1);

++++/**
+++++*+Get+blocks+until+a+value+or+an+exception+is+set,+after+that+it+always
+++++*+returns+immediately
+++++*
+++++*+@return+the+value
+++++*+@throws+ExecutionException
+++++*/
++++public+T+get()+throws+ExecutionException+{
++++++++phaser.register();
++++++++phaser.arriveAndAwaitAdvance();
++++++++if+(ex+!=+null)+{
++++++++++++throw+new+ExecutionException(ex);
++++++++}
++++++++return+value;
++++}

++++/**
+++++*+Sets+the+value+and+unblocks+awaiting+threads
+++++*
+++++*+@param+val
+++++*/
++++public+void+set(T+val)+{
++++++++this.value+=+val;
++++++++phaser.forceTermination();
++++}

++++/**
+++++*+Sets+an+exception+and+unblocks+awaiting+threads
+++++*/
++++public+void+setException(InterruptedException+ex)+{
++++++++this.ex+=+ex;
++++++++phaser.forceTermination();
++++}
}|code-block|syntax|javascript|3951051|您可以更改您的方法以返回一个FutureValue：|3951052|++++public+FutureValue<String>+getUsernamebyUPN(String+UPN)+{

+++++++final+FutureValue<String>+future+=+new+FutureValue<String>();
+++++++client.users(UPN).buildRequest().get(new+ICallback<User>()+{
++++++++++++@Override
++++++++++++public+void+success(User+user)+{
++++++++++++++++future.set(user.displayName);
++++++++++++}

++++++++++++@Override
++++++++++++public+void+failure(ClientException+ex)+{
++++++++++++++++future.setException(ex);
++++++++++++}
++++++++});
++++++++return+future;
++++}|3951053|另一种选择是更改您的逻辑，使其不获取值，但观察事件，并由回调发送事件。|3951054|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|S|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|T|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|U|8|@]|9|@]|A|$G|H]]|$1|M|3|N|5|6|7|V|8|@]|9|@]|A|$]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

The value will be returned asynchronously to the callback function, but you can force your program to wait the return when it needs the value. One way to do it is to return a value holder class that blocks when you try to get the value until it is set, and the callback sets the value.

For example, the following class users a Phaser to control this waiting behavior:

<pre class="lang-java prettyprint-override"><code>import java.util.concurrent.ExecutionException;
import java.util.concurrent.Phaser;

public class FutureValue&lt;T&gt; {

 private T value = null;
 private Exception ex = null;
 private Phaser phaser = new Phaser(1);

 /**
 * Get blocks until a value or an exception is set, after that it always
 * returns immediately
 *
 * @return the value
 * @throws ExecutionException
 */
 public T get() throws ExecutionException {
 phaser.register();
 phaser.arriveAndAwaitAdvance();
 if (ex != null) {
 throw new ExecutionException(ex);
 }
 return value;
 }

 /**
 * Sets the value and unblocks awaiting threads
 *
 * @param val
 */
 public void set(T val) {
 this.value = val;
 phaser.forceTermination();
 }

 /**
 * Sets an exception and unblocks awaiting threads
 */
 public void setException(InterruptedException ex) {
 this.ex = ex;
 phaser.forceTermination();
 }
}
</code></pre>

And you can change your method to return a FutureValue:

<pre class="lang-java prettyprint-override"><code> public FutureValue&lt;String&gt; getUsernamebyUPN(String UPN) {

 final FutureValue&lt;String&gt; future = new FutureValue&lt;String&gt;();
 client.users(UPN).buildRequest().get(new ICallback&lt;User&gt;() {
 @Override
 public void success(User user) {
 future.set(user.displayName);
 }

 @Override
 public void failure(ClientException ex) {
 future.setException(ex);
 }
 });
 return future;
 }
</code></pre>

Another option is to change your logic to not get the values, but observe events, with the callbacks sending the events.

Sorry for that weird thread title but I didn't know how to describe my problem better than this.

I'm currently working on a Java project which uses the Microsoft Graph API (it's my first time that I'm using Microsoft Graph API).

I think my problem is a general problem with Java. I have a class which provides a Graph API client and a second class with all the operations I want to execute with the Graph API. The class with the operation methods should return the results to the client class.

Some code.
My Client class (relevant part where I call an operation):

<pre><code>client = GraphServiceClientManager.getInstance().getGraphServiceClient();
Operations task = new Operations(client);
System.out.println(task.getUsernamebyUPN("USER UPN"));
</code></pre>

My class with the operations:

<pre><code>public String username = "0";

 public String getUsernamebyUPN(String UPN) {
 client.users(UPN).buildRequest().get(new ICallback&lt;User&gt;() {
 @Override
 public void success(User user) {
 username = user.displayName;

 }

 @Override
 public void failure(ClientException ex) {
 ex.printStackTrace();
 }
 });
 return username;
 }
</code></pre>

So the problem is when I run the code it will return the username with value 0 instead of the username I want. The username variable will be changed after the return.

If I add a System.out.prntln(username) under the "username = user.displayName;" line in the success method my output would be:

<pre><code>0
The username I want
</code></pre>

How can I solve this problem? Do you need more code?

Thank you very much for your help. :)

Method returns wrong value because return is too early

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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很抱歉这个奇怪的帖子标题，但我不知道如何比这个更好地描述我的问题。我目前正在开发一个使用Microsoft Graph API的Java项目(这是我第一次使用Microsoft Graph API)。我认为我的问题是Java的一个普遍问题。我有一个类，它提供了一个Graph API客户端，第二个类包含我想要使用Grap...

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