序列化期间没有有效的构造函数

Question

在将扩展程序加载到程序中时遇到了一个重大问题。我得到一个异常，因为没有有效的构造函数。

问题出在一行：

 ekstensja =  (ArrayList<Dydaktyk>) ois.readObject();

我得到了类似的东西：

java.io.InvalidClassException: Dydaktyk; no valid constructor
    at java.io.ObjectStreamClass$ExceptionInfo.newInvalidClassException(Unknown Source)
    at java.io.ObjectStreamClass.checkDeserialize(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at java.util.ArrayList.readObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at Dydaktyk.wczytajEkstensje(Dydaktyk.java:81)
    at Dydaktyk.<clinit>(Dydaktyk.java:69)
    at java.io.ObjectStreamClass.hasStaticInitializer(Native Method)
    at java.io.ObjectStreamClass.computeDefaultSUID(Unknown Source)
    at java.io.ObjectStreamClass.access$100(Unknown Source)
    at java.io.ObjectStreamClass$1.run(Unknown Source)
    at java.io.ObjectStreamClass$1.run(Unknown Source)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.io.ObjectStreamClass.getSerialVersionUID(Unknown Source)
    at java.io.ObjectStreamClass.initNonProxy(Unknown Source)
    at java.io.ObjectInputStream.readNonProxyDesc(Unknown Source)
    at java.io.ObjectInputStream.readClassDesc(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at java.util.ArrayList.readObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.defaultReadFields(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at java.util.ArrayList.readObject(Unknown Source)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at java.io.ObjectStreamClass.invokeReadObject(Unknown Source)
    at java.io.ObjectInputStream.readSerialData(Unknown Source)
    at java.io.ObjectInputStream.readOrdinaryObject(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at Przedmiot.wczytajEkstensje(Przedmiot.java:99)
    at Przedmiot.<clinit>(Przedmiot.java:87)
    at GUI.main(GUI.java:100)

---

static {
    wczytajEkstensje(); // load Extension
}

public static void wczytajEkstensje() {// load extension
    FileInputStream fis = null;
    ObjectInputStream ois = null;
    try {
        fis = new FileInputStream("dydaktyk.ser");
        ois = new ObjectInputStream(fis);
        // here is the problem
        ekstensja =  (ArrayList<Dydaktyk>) ois.readObject();
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } finally {
        try {
            if (ois != null) {
                ois.close();
            }
        } catch (IOException e) {
        }
        try {
            if (fis != null) {
                fis.close();
            }
        } catch (IOException e) {
        }
    }
}

public static void zapiszEkstensje() {// save extension
    FileOutputStream fos = null;
    ObjectOutputStream oos = null;
    try {
        fos = new FileOutputStream("dydaktyk.ser");
        oos = new ObjectOutputStream(fos);

        oos.writeObject(ekstensja); // serialization
    } catch (FileNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        try {
            if (oos != null) {
                oos.close();
            }
        } catch (IOException e) {
        }
        try {
            if (fos != null) {
                fos.close();
            }
        } catch (IOException e) {
        }
    }
}

Answer 1

Dydaktyk 类应该有一个可访问（公共或受保护）的无参数构造函数，以便序列化反射机制可以创建该类的实例：

public Dydaktyk() { 
  ...
}

来自docs

在反序列化期间，不可序列化类的字段将使用类的公共或受保护的无参数构造函数进行初始化。可序列化的子类必须可以访问无参数构造函数。可序列化子类的字段将从流中恢复。

Answer 2

如 Oracle 网站中关于 Serialization 接口的说明：

在反序列化期间，不可序列化类的字段将 使用公共或受保护的无参数构造函数初始化 班级。 子类必须可以访问无参数构造函数 可序列化。可序列化子类的字段将被恢复 来自流。

我猜Dydaktyk 是某个类的子类。并且您已经在 superclass 中定义了 parametric constructor 并且没有在其中定义 parameter-less constructor 。并且编译器没有为superclass 插入默认构造函数。所以，你也应该在你的超类中定义一个parameter-less constructor。