序列化期间没有有效的构造函数

Question

在将扩展加载到程序期间,我遇到了一个主要问题.因为没有有效的构造函数,我得到一个异常.

问题在于:

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

我得到类似的东西:

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

该类Dydaktyk应具有可访问(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() { 
  ...
}

来自文档

在反序列化期间,将使用类的public或protected no-arg构造函数初始化非可序列化类的字段.必须可以对可序列化的子类访问no-arg构造函数.可序列化子类的字段将从流中恢复.