【Java反序列化】为什么报错

如题，想从文件读取一个包含自定义类型的ArrayList，然后存到list中，但是不知道为什么报错，求看看

import java.io.File;
import java.io.FileInputStream;
import java.io.ObjectInputStream;
import java.util.ArrayList;

public class Test {
    static ArrayList list = new ArrayList();

    public static void main(String[] args) throws Exception {
        readObject();
    }
    public static void readObject() throws Exception {
        ObjectInputStream in = null;
        FileInputStream fin = null;
        File file = new File("src/test.txt");
        if(file.exists()) {
            fin = new FileInputStream(file);
            in = new ObjectInputStream(fin);
            list = (ArrayList) in.readObject();
        }else {
            file.createNewFile();
        }
        fin.close();
        in.close();
    }
}

简化后：

import java.io.FileInputStream;
import java.io.ObjectInputStream;
import java.util.ArrayList;

public class Test {
    static ArrayList l = new ArrayList();

    public static void main(String[] args) throws Exception {
        readObject();
    }
    public static void readObject() throws Exception {
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("src/test.txt"));
        l = (ArrayList) ois.readObject();
        ois.close();
    }
}

报什么错又不说；怎么writeObject的也不说