java - 实例变量赋值不粘

Question

我有以下代码:

public class PatientAgent extends Agent {

  private final String HYPHEN = "-";

  private ArrayList<HashSet<Integer>> prefs;

  private AID provider ;

  private boolean hasAppointment;

  private int appointmentNo;

  @Override
  protected void setup() {

      hasAppointment = false;

      appointmentNo = 0; // A value of zero means agent does not have any 
                         // allocated appointments (yet)

      initPrefs(getArguments());

      System.out.println(prefs.toString());

      // Build the description used as template for the subscription
      DFAgentDescription template = new DFAgentDescription();
      ServiceDescription templateSd = new ServiceDescription();
      templateSd.setType("allocate-appointments");
      template.addServices(templateSd);


      SearchConstraints sc = new SearchConstraints();
      // We want to receive 10 results at most
      sc.setMaxResults(new Long(10));

      addBehaviour(new SubscriptionInitiator(this, DFService.createSubscriptionMessage(this, getDefaultDF(), template, sc)) {
          protected void handleInform(ACLMessage inform) {
              System.out.println("Agent "+getLocalName()+": Notification received from DF");
              try {
                  DFAgentDescription[] results = DFService.decodeNotification(inform.getContent());

                  if (results.length > 0) {

                      // Assume there is only one hospital agent
                      assert(results.length == 1);
                      DFAgentDescription dfd = results[0];


                      Iterator it = dfd.getAllServices();
                      while (it.hasNext()) {
                          ServiceDescription sd = (ServiceDescription) it.next();
                          if (sd.getType().equals("allocate-appointments")) {
                              provider = dfd.getName(); 
                              System.out.println("Allocate-appointments service found:");
                              System.out.println("- Service \""+sd.getName()+"\" provided by agent "+provider.getName());

                          }
                      }



                  }

              }
              catch (FIPAException fe) {
                  fe.printStackTrace();
              }
          }
      } );


public AID getProvider() { return provider; }

我清楚地用该行初始化提供程序

   provider = dfd.getName();

但是行

       public AID getProvider() { return provider; }

返回 null，我不明白为什么。有人知道发生了什么事吗？

Answer 1

假设您正在执行赋值语句，请考虑以下事项:

• getProvider() 是否在 handleInform 之前被调用？

• handleInform 和 getProvider() 是否在单独的线程上调用，从而造成竞争或内存可见性问题？

• 在 handleInform 中赋值和使用 getProvider() 检索之间的某个时间，provider 变量是否被覆盖？

我想不出其他的了。您的变量范围看起来不错。您应该向 getProvider() 方法添加一条日志语句，以查看何时相对于 handleInform 调用它，并记录程序中可能覆盖变量的任何其他位置值。