java/src/main/com/google/appengine/api/datastore/PreGetContext.java

   1 // Copyright 2012 Google. All Rights Reserved.
   2 package com.google.appengine.api.datastore;
   3
   4 import java.util.List;
   5 import java.util.Map;
   6
   7 /**
   8  * Concrete {@link CallbackContext} implementation that is specific to
   9  * intercepted get() operations. Methods annotated with {@link PreGet} that receive
  10  * instances of this class may modify the result of the get() operation by
  11  * calling {@link #setResultForCurrentElement(Entity)}. Keys that receive
  12  * results via this method will not be fetched from the datastore. This is an
  13  * effective way to inject cached results.
  14  *
  15  */
  16 public final class PreGetContext extends BaseCallbackContext<Key> {
  17
  18   /**
  19    * The Map that wil ultimately be populated with the result of the get() RPC.
  20    */
  21   private final Map<Key, Entity> resultMap;
  22
  23   PreGetContext(CurrentTransactionProvider currentTransactionProvider, List<Key> keys,
  24       Map<Key, Entity> resultMap) {
  25     super(currentTransactionProvider, keys);
  26     this.resultMap = resultMap;
  27   }
  28
  29   @Override
  30   String getKind(Key key) {
  31     return key.getKind();
  32   }
  33
  34   /**
  35    * Set the {@link Entity} that will be associated with the {@link Key}
  36    * returned by {@link #getCurrentElement()} in the result of the get()
  37    * operation. This will prevent the get() operation from fetching the
  38    * Entity from the datastore. This is an effective way to inject cached
  39    * results.
  40    *
  41    * @param entity The entity to provide as the result for the current element.
  42    *
  43    * @throws IllegalArgumentException If the key of the provided entity is not
  44    *     equal to the key returned by {@link #getCurrentElement()}.
  45    */
  46   public void setResultForCurrentElement(Entity entity) {
  47     if (entity == null) {
  48       throw new NullPointerException("entity cannot be null");
  49     }
  50     Key curKey = getCurrentElement();
  51     if (!curKey.equals(entity.getKey())) {
  52       throw new IllegalArgumentException("key of provided entity must be equal to current element");
  53     }
  54     resultMap.put(getCurrentElement(), entity);
  55   }
  56 }