Java-无法将对象作为文件写入目录

Question

I have a weird problem with my program. I wrote a small program to write an object to a file in a specific directory.

Player-Class:

package readwrite;

import java.io.Serializable;

public class Player implements Serializable {

    private String name;
    private int level;

    public Player(String name, int level) {
        this.name = name;
        this.level = level;
    }

    public String getName() {
        return name;
    }

    public int getLevel() {
        return level;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setLevel(int level) {
        this.level = level;
    }

    public String toString() {
        return "Name: " + name + ", Level: " + level;
    }

    public void printInfo() {
        System.out.println(this.toString());
    }

}

PlayerReaderWriter-Class:

package readwrite;

import java.io.*;

public class PlayerReaderWriter {

    public static Player readPlayer(String path) {
        Player p = null;

        try {
            FileInputStream fileIn = new FileInputStream(path);
            ObjectInputStream in = new ObjectInputStream(fileIn);
            p = (Player) in.readObject();
            in.close();
            fileIn.close();
        } catch(IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }

        return p;
    }

    public static void writePlayer(String path, Player p) {
        try {
            File file = new File(path);
            if (file.exists()) {
                file.delete();
                file.createNewFile();
            } else {
                file.mkdirs();
                file.createNewFile();
            }

            FileOutputStream fileOut = new FileOutputStream(path);
            ObjectOutputStream out = new ObjectOutputStream(fileOut);
            out.writeObject(p);
            out.close();
            fileOut.close();
        } catch(IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        Player p = new Player("Foo", 64);
        p.printInfo();
        writePlayer("players/Player.ser", p);

        Player q = readPlayer("players/Player.ser");
        q.printInfo();
    }

}

The problem is: When I run my program for the first time, when the directory "players" doesn't exist, I get the following error:

D:\Projects>java readwrite/PlayerReaderWriter
Name: Foo, Level: 64
java.io.FileNotFoundException: players\Player.ser (Access is denied)
        at java.io.FileOutputStream.open0(Native Method)
        at java.io.FileOutputStream.open(FileOutputStream.java:270)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:213)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:101)
        at readwrite.PlayerReaderWriter.writePlayer(PlayerReaderWriter.java:34)
        at readwrite.PlayerReaderWriter.main(PlayerReaderWriter.java:47)
java.io.FileNotFoundException: players\Player.ser (Access is deniedt)
        at java.io.FileInputStream.open0(Native Method)
        at java.io.FileInputStream.open(FileInputStream.java:195)
        at java.io.FileInputStream.<init>(FileInputStream.java:138)
        at java.io.FileInputStream.<init>(FileInputStream.java:93)
        at readwrite.PlayerReaderWriter.readPlayer(PlayerReaderWriter.java:11)
        at readwrite.PlayerReaderWriter.main(PlayerReaderWriter.java:49)
Exception in thread "main" java.lang.NullPointerException
        at readwrite.PlayerReaderWriter.main(PlayerReaderWriter.java:50)

Now the directory "players" has been created by the program and for some reason it also created a subdirectory named "Player.ser".

When I run it a second time, it works and creates a "Player.ser" file within the "players" directory.

When I run it with just a empty directory named "players" it also won't work.

Regards :)

Answer 1

Look at this code:

if (file.exists()) {
    file.delete();
    file.createNewFile();
} else {
    file.mkdirs();
    file.createNewFile();
}

You're saying that if players/Player.ser doesn't exist, you should create it as a directory - and then also as a file. That's broken.

You don't need to delete the file at all if it already exists - you can just use a FileOutputStream and let it overwrite the file, truncating it to start with, as is the default behaviour.

You just need to check whether the directory exists first, eg

File file = new File(path);
File parent = file.getParentFile();
// TODO: Handle parent existing, but not being a directory,
// or file existing, but being a directory...
if (!parent.exists()) {
    parent.mkdirs();
}

try (OutputStream output = new FileOutputStream(file)) {
    // ...
}

Note the try-with-resources statement so that your output stream is always closed even if an exception is thrown.

Answer 2

Have a look at the JavaDoc on File#mkdirs() :

Creates the directory named by this abstract pathname ...

That says that the file is assumed to represent a directory, hence Player.ser is created as a directory. Use file.getParentFile().mkdirs() instead (keep in mind that depending on the path there might be no parent).

Since your code is creating a directory any write operation to that File object will fail since you can't write data directly into a directory.