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Gson error: Attempted to serialize java.lang.Class: org.hibernate.proxy.HibernateProxy

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HibernateProxy异常处理

在使用Hibernate时，那么很可能遇到这样的错误：

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java.lang.UnsupportedOperationException: Attempted to serialize java.lang.Class: org.hibernate.proxy.HibernateProxy. Forgot to register a type adapter?

因为gson在转换时是使用的反射机制，当获取的实体对象还在hibernate代理的时候，例如刚通过Id获取到，这时候获取到的便是代理对象HibernateProxy。

这和直接调用实体对象的get方法不同，获取对象的属性就不能起作用。

解决的方法便是将代理对象实例化，见下面的代码

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    /**
    * This TypeAdapter unproxies Hibernate proxied objects, and serializes them
    * through the registered (or default) TypeAdapter of the base class.
    */

   public class HibernateProxyTypeAdapter extends TypeAdapter<HibernateProxy> {

   public static final TypeAdapterFactory FACTORY = new TypeAdapterFactory() {
       @Override
       @SuppressWarnings("unchecked")
       public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
           return (HibernateProxy.class.isAssignableFrom(type.getRawType()) ? (TypeAdapter<T>) new HibernateProxyTypeAdapter(gson) : null);
       }
   };
   private final Gson context;

   private HibernateProxyTypeAdapter(Gson context) {
       this.context = context;
   }

   @Override
   public HibernateProxy read(JsonReader in) throws IOException {
       throw new UnsupportedOperationException("Not supported");
   }

   @SuppressWarnings({"rawtypes", "unchecked"})
   @Override
   public void write(JsonWriter out, HibernateProxy value) throws IOException {
       if (value == null) {
           out.nullValue();
           return;
       }
       // Retrieve the original (not proxy) class
       Class<?> baseType = Hibernate.getClass(value);
       // Get the TypeAdapter of the original class, to delegate the serialization
       TypeAdapter delegate = context.getAdapter(TypeToken.get(baseType));
       // Get a filled instance of the original class
       Object unproxiedValue = ((HibernateProxy) value).getHibernateLazyInitializer()
               .getImplementation();
       // Serialize the value
       delegate.write(out, unproxiedValue);
   }
}

使用的时候将该TypeAdapter的Factory注册到GsonBuilder,上面的代码变为

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Gson gson = new GsonBuilder().setExcludeStrategy(ts)
.registerTypeAdapterFactory(HibernateProxyTypeAdapter.FACTORY)
.create();

gson.toJson(teacher);

reference: