实现 Externalizable 时是否需要显式默认构造函数？

Question

我知道如果 class A 实现 Externalizable 它应该有无参数构造函数，但是如果 class 没有任何构造函数（比如我的 A class) java 为它提供空的无参数构造函数。那为什么我在那里有错误？如果我明确地将无参数构造函数 (public A() {}) 添加到 A class 一切都会好的。
错误：

Exception in thread "main" java.io.InvalidClassException: A; no valid constructor at java.base/java.io.ObjectStreamClass$ExceptionInfo.newInvalidClassException(ObjectStreamClass.java:159) at java.base/java.io.ObjectStreamClass.checkDeserialize(ObjectStreamClass.java:864) at java.base/java.io.ObjectInputStream.readOrdinaryObject(ObjectInputStream.java:2061) at java.base/java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1594) at java.base/java.io.ObjectInputStream.readObject(ObjectInputStream.java:430) at Test.main(Test.java:19)

import java.io.*;

public class Test implements Serializable
{
    public static void main(String[] args) throws IOException, ClassNotFoundException
    {

        // initiaizing
        A a1 = new A();

        // serializing
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        ObjectOutputStream oos = new ObjectOutputStream(baos);
        oos.writeObject(a1);
        oos.close();

        // deserializing
        ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(baos.toByteArray()));
        A a2 = (A) ois.readObject();
    }
}

class A implements Externalizable
{
    @Override
    public void writeExternal(ObjectOutput objectOutput) throws IOException {}

    @Override
    public void readExternal(ObjectInput objectInput) throws IOException, ClassNotFoundException {}
}

Answer 1

Externalizable 需要 public 无参数构造函数，但前提是无参数构造函数具有默认访问修饰符（也称为包私有）试试这个：public class A() {}

实现 Externalizable 时是否需要显式默认构造函数？

Do I need explicit default constructor when implementing Externalizable?

java

externalizable