java/src/main/com/google/appengine/spi/FactoryProvider.java

   1 // Copyright 2012 Google Inc. All rights reserved.
   2
   3 package com.google.appengine.spi;
   4
   5 import com.google.common.base.Objects;
   6 import com.google.common.primitives.Ints;
   7
   8 /**
   9  * A base class for service factory creation that can be registered with the ProviderRegistry.
  10  *
  11  * @param <I> is the interface the provided factory must implement.
  12  */
  13 public abstract class FactoryProvider<I> implements Comparable<FactoryProvider<?>> {
  14   private final Class<I> baseInterface;
  15
  16   protected FactoryProvider(Class<I> baseInterface) {
  17     this.baseInterface = baseInterface;
  18   }
  19
  20   protected Class<I> getBaseInterface() {
  21     return baseInterface;
  22   }
  23
  24   /**
  25    * Return an instance of the factory
  26    */
  27   protected abstract I getFactoryInstance();
  28
  29   private int getPrecedence() {
  30     ServiceProvider annotation = getClass().getAnnotation(ServiceProvider.class);
  31     return (annotation != null) ? annotation.precedence() : ServiceProvider.DEFAULT_PRECEDENCE;
  32   }
  33
  34   /**
  35    * This ensures that a list of these will be sorted so that higher precedence entries come later
  36    * in the list.
  37    */
  38   @Override
  39   public int compareTo(FactoryProvider<?> o) {
  40     int result =
  41         getBaseInterface().getCanonicalName().compareTo(o.getBaseInterface().getCanonicalName());
  42     return result != 0 ? result : Ints.compare(getPrecedence(), o.getPrecedence());
  43   }
  44
  45   @Override
  46   public int hashCode() {
  47     return Objects.hashCode(getBaseInterface().getCanonicalName(), getPrecedence());
  48   }
  49
  50   /**
  51    * Included to support sorting by precedence (@see #compareTo(FactoryProvider))
  52    */
  53   @SuppressWarnings("unchecked")
  54   @Override
  55   public boolean equals(Object o) {
  56     if (o == null || !(o instanceof FactoryProvider)) {
  57       return false;
  58     }
  59     return (compareTo((FactoryProvider<I>) o) == 0);
  60   }
  61
  62 }