序列化时未保存对象引用

Question

我参考了B类中的A类，两者都是可序列化的。我在B类的两个不同对象中传递A的相同参考，如下所示：

class A implements Serializable {
    private Integer a;

    A(Integer a) {
        System.out.println("Constructor with argument Called");
        this.a = a;
    }

    A() {
        System.out.println("Constructor Called");
    }

    public Integer getA() {
        return a;
    }

    public void setA(Integer a) {
        this.a = a;
    }
}

class B implements Serializable {
    A a = null;

    public A getA() {
        return a;
    }

    public void setA(A a) {
        this.a = a;
    }


}

public class SerializeDemo {
    public static void main(String[] args) throws Exception {
        A a1 = new A(10);
        B b1 = new B();
        B b2 = new B();
        b1.setA(a1);
        b2.setA(a1);

        FileOutputStream fos = new FileOutputStream("abc.ser");
        ObjectOutputStream os = new ObjectOutputStream(fos);

        os.writeObject(b1);
        b2.getA().setA(20); // here i have set the value to 20 but in output                                                 its coming 10 only
        os.writeObject(b2);

        os.close();

        System.out.println("Serialization Done");
        FileInputStream fis = new FileInputStream("abc.ser");
        ObjectInputStream os1 = new ObjectInputStream(fis);
        B b3 = (B) os1.readObject();
        B b4 = (B) os1.readObject();
        fis.close();

        System.out.println(b3.toString());
        System.out.println(b4.toString());
        System.out.println(b3.getA());
        System.out.println(b4.getA());
        System.out.println(b3.getA().getA());
        System.out.println(b4.getA().getA());
    }
}

输出如下：

Constructor with argument Called
Serialization Done
org.jnbis.B@18f55759
org.jnbis.B@1339a0dc
org.jnbis.A@6096b38e
org.jnbis.A@6096b38e
10
10

由于A也是可序列化的，所以我期待b4的输出为20，但它的即将到来。为什么？