Spring ⑥ @Lazy 注解原理详解

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本章的内容是对 bean 创建过程中依赖项注入时 @Lazy 注解源码进行分析。

场景

protected Object buildLazyResolutionProxy(final DependencyDescriptor descriptor, final @Nullable String beanName) {
   BeanFactory beanFactory = getBeanFactory();
   Assert.state(beanFactory instanceof DefaultListableBeanFactory,
         "BeanFactory needs to be a DefaultListableBeanFactory");
   final DefaultListableBeanFactory dlbf = (DefaultListableBeanFactory) beanFactory;

   TargetSource ts = new TargetSource() {
      @Override
      public Class<?> getTargetClass() {
         return descriptor.getDependencyType();
      }
      @Override
      public boolean isStatic() {
         return false;
      }
      @Override
      public Object getTarget() {
         Set<String> autowiredBeanNames = (beanName != null ? new LinkedHashSet<>(1) : null);
         Object target = dlbf.doResolveDependency(descriptor, beanName, autowiredBeanNames, null);
         if (target == null) {
            Class<?> type = getTargetClass();
            if (Map.class == type) {
               return Collections.emptyMap();
            }
            else if (List.class == type) {
               return Collections.emptyList();
            }
            else if (Set.class == type || Collection.class == type) {
               return Collections.emptySet();
            }
            throw new NoSuchBeanDefinitionException(descriptor.getResolvableType(),
                  "Optional dependency not present for lazy injection point");
         }
         if (autowiredBeanNames != null) {
            for (String autowiredBeanName : autowiredBeanNames) {
               if (dlbf.containsBean(autowiredBeanName)) {
                  dlbf.registerDependentBean(autowiredBeanName, beanName);
               }
            }
         }
         return target;
      }
      @Override
      public void releaseTarget(Object target) {
      }
   };

   ProxyFactory pf = new ProxyFactory();
   pf.setTargetSource(ts);
   Class<?> dependencyType = descriptor.getDependencyType();
   if (dependencyType.isInterface()) {
      pf.addInterface(dependencyType);
   }
   return pf.getProxy(dlbf.getBeanClassLoader());
}