java - 序列化和反序列化 transient 对象的棘手情况

Question

这是我的代码:

class Collar {
  int size;
}

class Dog implements Serializable {
  int weight;
  transient Collar c;

  public Dog(int weight, int size) {
    this.weight = weight;
    c = new Collar();
    c.size = size;
  }

  public void writeObject(ObjectOutputStream os) throws Exception {
    os.defaultWriteObject();
    os.writeInt(c.size);
  }

  public void readObject(ObjectInputStream is) throws Exception {
    is.defaultReadObject();
    c = new Collar();
    c.size = is.readInt();
  }
}

public class test {
  public static final String FILE_REVISION = "$Revision$";

  public static void main(String ar[]) throws Exception {
    Dog dOut = new Dog(20, 2);

    System.out.println("DOut Weight: " + dOut.weight + " Size: " + dOut.c.size);

    FileOutputStream fo = new FileOutputStream("Dog.ser");
    ObjectOutputStream os = new ObjectOutputStream(fo);
    os.writeObject(dOut);

    FileInputStream fi = new FileInputStream("Dog.ser");
    ObjectInputStream is = new ObjectInputStream(fi);
    Dog dIn = (Dog) is.readObject();

    System.out.println("DIn Weight: " + dIn.weight + " Size: " + dIn.c.size);
  }
}

这是输出:

DOut Weight: 20 Size: 2
Exception in thread "main" java.lang.NullPointerException
    at test.main(test.java:55)

第 55 行是包含 System.out.println() 的最后一行代码

程序能够取回可序列化的对象，这里就是 Dog。但是，当我使用自定义 readObject 方法回读时，它无法创建新的“遏制”对象 Collar。我哪里出错了？

如您所见，“dIn.c.size”语句中的输出为 NullPointerException。尽管我在自定义 readObject 方法中将 c 设置为新对象，但它并没有真正工作。

Answer 1

因为 Collar c 在 readObject 内是 transient ，因此您需要首先初始化 c

c.size = is.readInt();<-- NullPointerException Here

这里需要先初始化C。

c = new Collar();
c.size = is.readInt();

两个方法签名都是错误的，它们应该如下所示:

 private void writeObject(ObjectOutputStream stream) throws IOException {
    stream.defaultWriteObject();
    stream.writeInt(c.size);
}

private void readObject(ObjectInputStream stream) throws IOException,
        ClassNotFoundException {
    stream.defaultReadObject();
    c = new Collar();
    c.size = stream.readInt();
}

您可以阅读有关序列化的更多详细信息 here