为什么可序列化的内部类不可序列化？

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, eg 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, eg 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

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.

ie

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.

ie

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

Answer 3

如何使TestInnerClass可序列化？

public class TestInnerClass implements Serializable { }