hibernate结合使用gson转换json中一点看法

转发请注明出处：http://www.cnblogs.com/shizhongtao/p/3680216.html

在前后台的交互中，经常需要把bean对象与xml或者json，这里就把自己以前遇到的问题做一下说明。

• 如果bean中没有配置one-to-many或者many-to-many等映射关系，转化时候一般没有问题
• 在配置了单项映射关系中，如果把(fetch = FetchType.LAZY)的话。当应用gson对其进行转换时候，会出现这个问题：

java.lang.UnsupportedOperationException: Attempted to serialize java.lang.Class: org.hibernate.proxy.HibernateProxy. Forgot to register a type adapter?
    at com.google.gson.internal.bind.TypeAdapters$.write(TypeAdapters.java:)
    at com.google.gson.internal.bind.TypeAdapters$.write(TypeAdapters.java:)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:)
    at com.google.gson.internal.bind.ArrayTypeAdapter.write(ArrayTypeAdapter.java:)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$.write(ReflectiveTypeAdapterFactory.java:)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$.write(ReflectiveTypeAdapterFactory.java:)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$.write(ReflectiveTypeAdapterFactory.java:)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:)
    at com.google.gson.internal.bind.ObjectTypeAdapter.write(ObjectTypeAdapter.java:)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:)
    at com.google.gson.Gson.toJson(Gson.java:)
    at com.google.gson.Gson.toJson(Gson.java:)
    at com.google.gson.Gson.toJson(Gson.java:)
    at com.google.gson.Gson.toJson(Gson.java:)
    at com.test.Botest.test2(Botest.java:)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:)
    at java.lang.reflect.Method.invoke(Method.java:)
    at org.junit.runners.model.FrameworkMethod$.runReflectiveCall(FrameworkMethod.java:)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:)
    at org.junit.runners.ParentRunner$.run(ParentRunner.java:)
    at org.junit.runners.ParentRunner$.schedule(ParentRunner.java:)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:)
    at org.junit.runners.ParentRunner.access$(ParentRunner.java:)
    at org.junit.runners.ParentRunner$.evaluate(ParentRunner.java:)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:)

原因应该是在gson进行转换json字符串时候，得不到映射实体（除了id以外）属性。如果想解决这个问题，最简单的方法就是把FetchType改成EAGER。或者你可以为gson注册一个转换器。如下：

 Gson gson=new GsonBuilder().registerTypeAdapter(TXq.class, new JsonSerializer<TXq>() {
             @Override
             public JsonElement serialize(TXq src, Type typeOfSrc,
                     JsonSerializationContext context) {
                 JsonObject o=new JsonObject();
                 o.addProperty("pid",  src.getPid());
                 //o.addProperty("name", src.getName());
                 return o;
             }
         }).create();

上面的类的转换器中注释的那句会引发hibernate查询数据库。

• 如果你配置了映射的双向关联。那么使用json时候就会报错。因为他们互相引用，gson转换时候就会进入一个无穷的死循环中。这时候你可以利用注册转换器的方法（如上），或者把这个属性忽略掉。

小节：应该利用xmlstream等转化工具时候都会遇到这个问题。