Why is a serializable inner class not serializable?

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 fi开发者_如何学运维eld 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.

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?

how about make TestInnerClass serializable?

public class TestInnerClass implements Serializable { }

InnerClass cannot be serialised because to instantiate it (as required during deserialization) you need a reference to an instance of the outer class

Instances of inner classes cannot exist without an instance of the outer class.

i.e

OuterClass o = new OuterClass();
OuterClass.InnerClass o_i = o.new InnerClass();

If you use static inner class, it will be possible to serialise the static inner class. The instances of static inner classes can be created standalone.

i.e

OuterClass o = new OuterClass();
OuterClass.InnerClass i = new OuterClass.InnerClass();