将类型a的类反序列化为具有相同SerialVersionId、字段和方法的类型B

Question

给我java。lang.ClassCastException:slz。不能向slz施放A。B为什么？这两个类都有相同的字段和方法。为什么在反序列化时无法将A类型的序列化对象转换为B类型？

public class A implements Serializable {

  private  String var1;
  private String var2;
  private static final long serialVersionUID = 4L;

  public String getVar1() {
    return var1;
  }

  public void setVar1(String var1) {
    this.var1 = var1;
  }

  public String getVar2() {
    return var2;
  }

  public void setVar2(String var2) {
    this.var2 = var2;
  }
}


public class B implements Serializable {

  private  String var1;
  private String var2;
  private static final long serialVersionUID = 4L;

  public String getVar1() {
    return var1;
  }

  public void setVar1(String var1) {
    this.var1 = var1;
  }

  public String getVar2() {
    return var2;
  }

  public void setVar2(String var2) {
    this.var2 = var2;
  }

  @Override
  public String toString() {
    return var1 + " " + var2;
  }
}

public class Test {

  public static void main(String[] args)  {
    A a = new A();
    a.setVar1("d");
    a.setVar2("e");

    B b = null;

    try(FileOutputStream fileOut = new FileOutputStream("Test.ser");
        ObjectOutputStream outputStream = new ObjectOutputStream(fileOut);
        FileInputStream inputStream = new FileInputStream("Test.ser");
        ObjectInputStream objectInputStream = new ObjectInputStream(inputStream);){
      outputStream.writeObject(a);
      b = (B) objectInputStream.readObject();

    } catch (FileNotFoundException e) {
      e.printStackTrace();
    } catch (IOException e) {
      e.printStackTrace();
    } catch (ClassNotFoundException e) {
      e.printStackTrace();
    }
  }
}

两个类A和B实现了serializable，它们具有相同的字段和方法以及相同的串行版本id

Answer 1

public class A extends B {}

将你的类A更改为上面的代码，这应该会起作用。发生类强制转换异常是因为类A和B的类型不同。反序列化过程返回类型为A的对象，您试图将其强制转换为B，这就是它失败的原因。

Answer 2

不能将A反序列化为B，因为它们是不同的类。他们实际上是一样的并不重要，他们仍然是不同的阶级。这就像试图将一只狗反序列化为一只猫，它们有相同数量的腿等，但它们是不同的动物。

Answer 3

毕竟他们是不同的阶级。在序列化过程中，进程需要跟踪其类型，因为在反序列化过程中，运行时需要知道实例化哪种类型。当类型不匹配时，将出现类强制转换异常。

但是，您应该研究readResolve（）方法，该方法将在反序列化期间调用，您可以使用它从流中创建不同的类型。如果将以下方法添加到类A中，反序列化将起作用：

public Object readResolve() throws ObjectStreamException{
  return new B();
}

从Serializable接口中的Javadocs：

 * <p>Serializable classes that need to designate an alternative object to be
 * used when writing an object to the stream should implement this
 * special method with the exact signature:
 *
 * <PRE>
 * ANY-ACCESS-MODIFIER Object writeReplace() throws ObjectStreamException;
 * </PRE><p>
 *
 * This writeReplace method is invoked by serialization if the method
 * exists and it would be accessible from a method defined within the
 * class of the object being serialized. Thus, the method can have private,
 * protected and package-private access. Subclass access to this method
 * follows java accessibility rules. <p>
 *
 * Classes that need to designate a replacement when an instance of it
 * is read from the stream should implement this special method with the
 * exact signature.
 *
 * <PRE>
 * ANY-ACCESS-MODIFIER Object readResolve() throws ObjectStreamException;
 * </PRE><p>
 *
 * This readResolve method follows the same invocation rules and
 * accessibility rules as writeReplace.<p>