Why is a serializable inner class not serializable?

Question

The following code:

public class TestInnerClass {      public static void main(String[] args) throws IOException {         new TestInnerClass().serializeInnerClass();     }      private void serializeInnerClass() throws IOException {         File file = new File("test");         InnerClass inner = new InnerClass();         new ObjectOutputStream(new FileOutputStream(file)).writeObject(inner);     }      private class InnerClass implements Serializable {          private static final long serialVersionUID = 1L;      }  } 

throws the following exception:

Exception in thread "main" java.io.NotSerializableException: TestInnerClass 

I guess the inner class has a TestInnerClass.this field that allows it private access to TestInnerClass's fields and methods. Declaring the inner class static solves it, but what if InnerClass needs this access? Is there a way to serialize a non-static inner class without the enclosing class, e.g. by making the reference to the outer class transient?

edit: for example, access to the outer class could be needed only before serialization. OK, the compiler cannot know that, but I thought that's why the transient keyword exists.

Answer 1

what if InnerClass needs this access?

Then it needs the outer class instance, and it must be serialized along with the inner class.

Is there a way to serialize a non-static inner class without the enclosing class, e.g. by making the reference to the outer class transient?

No. What would happen when you deserialize such a class and then try to call an instance method of the outer class? A NullPointerException?

Answer 2

how about make TestInnerClass serializable?

public class TestInnerClass implements Serializable { }