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$1.write(TypeAdapters.java:67)

    at com.google.gson.internal.bind.TypeAdapters$1.write(TypeAdapters.java:61)

    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)

    at com.google.gson.internal.bind.ArrayTypeAdapter.write(ArrayTypeAdapter.java:93)

    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)

    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)

    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)

    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)

    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)

    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)

    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)

    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.write(ReflectiveTypeAdapterFactory.java:89)

    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.write(ReflectiveTypeAdapterFactory.java:195)

    at com.google.gson.internal.bind.ObjectTypeAdapter.write(ObjectTypeAdapter.java:107)

    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.write(TypeAdapterRuntimeTypeWrapper.java:68)

    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:96)

    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.write(CollectionTypeAdapterFactory.java:60)

    at com.google.gson.Gson.toJson(Gson.java:593)

    at com.google.gson.Gson.toJson(Gson.java:572)

    at com.google.gson.Gson.toJson(Gson.java:527)

    at com.google.gson.Gson.toJson(Gson.java:507)

    at com.test.Botest.test2(Botest.java:55)

    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)

    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)

    at java.lang.reflect.Method.invoke(Method.java:597)

    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:45)

    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)

    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:42)

    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)

    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:263)

    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:68)

    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:47)

    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:231)

    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:60)

    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:229)

    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:50)

    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:222)

    at org.junit.runners.ParentRunner.run(ParentRunner.java:300)

    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:50)

    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)

    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)

    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)

    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)

    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)

View Code

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：

 1 Gson gson=new GsonBuilder().registerTypeAdapter(TXq.class, new JsonSerializer<TXq>() {

 2             @Override

 3             public JsonElement serialize(TXq src, Type typeOfSrc,

 4                     JsonSerializationContext context) {

 5                 JsonObject o=new JsonObject();

 6                 o.addProperty("pid",  src.getPid());

 7                 //o.addProperty("name", src.getName());

 8                 return o;

 9             }

10         }).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。