new一个ObjectInputStream为什么会出现java.io.EOFException

朴灿烈づ我的快乐病毒、 2024-03-16 10:40 84阅读 0赞

## 一、举例代码 ##

package com.softeem.wolf.homework06;
    
    import java.io.*;
    
    /**
     * Created by 苍狼
     * Time on 2023-05-24
     */
    public class App {
        public static void main(String[] args) throws IOException {
            ObjectInputStream ois = null;
            ObjectOutputStream oos = null;
            ois = new ObjectInputStream(new FileInputStream("src\\com\\softeem\\wolf\\homework06\\student.txt"));
            oos = new ObjectOutputStream(new FileOutputStream("src\\com\\softeem\\wolf\\homework06\\student.txt"));
            
            Student student1 = new Student(1,"张三",'男');
            Student student2 = new Student(2, "李四",'女');
    
            try {
                oos.writeObject(student1);
                oos.writeObject(student2);
                oos.writeObject(null);
                oos.close();
            } catch (IOException ioException) {
                ioException.printStackTrace();
            }
            try {
                Object object;
                while((object=ois.readObject())!=null){
                    System.out.println(object);
                }
                ois.close();
            } catch (IOException ioException) {
                ioException.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
            try {
                ois.close();
            } catch (IOException ioException) {
                ioException.printStackTrace();
            }
        }
    }

在src\\\\com\\\\softeem\\\\wolf\\\\homework06\\\\路径下新建一个student.txt文件，运行程序。

![8ba0298c480d4b9498ed530afb23731f.png][]

发现会出现EOFException异常。这是为什么呢？

## 二、分析原因 ##

我们用debugs的形式一步一步分析原因。

异常信息显示ois = new ObjectInputStream(new FileInputStream("src\\\\com\\\\softeem\\\\wolf\\\\homework06\\\\student.txt"));出现问题，

我们点进去看一ObjectInputStream的构造器

public ObjectInputStream(InputStream in) throws IOException {
            verifySubclass();
            bin = new BlockDataInputStream(in);
            handles = new HandleTable(10);
            vlist = new ValidationList();
            enableOverride = false;
            readStreamHeader();
            bin.setBlockDataMode(true);
        }

发现这里会调用一个readStreamHeader()方法，获取文件中数据流的头，然后我们在点进去看看这个readStreamHeader()方法。

protected void readStreamHeader()
            throws IOException, StreamCorruptedException
        {
            short s0 = bin.readShort();
            short s1 = bin.readShort();
            if (s0 != STREAM_MAGIC || s1 != STREAM_VERSION) {
                throw new StreamCorruptedException(
                    String.format("invalid stream header: %04X%04X", s0, s1));
            }
        }

这个方法调用了bin.readShort（），看看readShort()方法的源码。

public short readShort() throws IOException {
                if (!blkmode) {
                    pos = 0;
                    in.readFully(buf, 0, 2);
                } else if (end - pos < 2) {
                    return din.readShort();
                }
                short v = Bits.getShort(buf, pos);
                pos += 2;
                return v;
            }

发现它会调用in.readFully(buf, 0，2)这个方法，接着去这个方法的源码下看看。

void readFully(byte[] b, int off, int len) throws IOException {
                int n = 0;
                while (n < len) {
                    int count = read(b, off + n, len - n);
                    if (count < 0) {
                        throw new EOFException();
                    }
                    n += count;
                }
            }

接着在这个方法中调用read(b, off+n, len-n), 我们来看看这个源码。

public int read(byte[] b, int off, int len) throws IOException {
                if (len == 0) {
                    return 0;
                } else if (peekb < 0) {
                    return in.read(b, off, len);
                } else {
                    b[off++] = (byte) peekb;
                    len--;
                    peekb = -1;
                    int n = in.read(b, off, len);
                    return (n >= 0) ? (n + 1) : 1;
                }
            }

它会运行到renturn in.read(b, off, len); 我们接着点进去看这个源码。

public int read(byte b[], int off, int len) throws IOException {
            if (b == null) {
                throw new NullPointerException();
            } else if (off < 0 || len < 0 || len > b.length - off) {
                throw new IndexOutOfBoundsException();
            } else if (len == 0) {
                return 0;
            }
    
            int c = read();
            if (c == -1) {
                return -1;
            }
            b[off] = (byte)c;
    
            int i = 1;
            try {
                for (; i < len ; i++) {
                    c = read();
                    if (c == -1) {
                        break;
                    }
                    b[off + i] = (byte)c;
                }
            } catch (IOException ee) {
            }
            return i;
        }

这个方法最终返回的结果是-1；然后一直把结果往上传，传到readFully(byte\[\] b, int off, int len)方法进行判断，如果count<0,则抛出 EOFException异常。这就是new ObjectInputStream（）对象的整个过程。

这个ObjectInputStream构造器中的readStreamHeader()方法就是获取文件头的流，但是由于我们这个student.txt文件是空的，所以获取不到文件头流，导致count为-1，从而出现EOFException异常。

## 三、解决办法 ##

解决办法也很简单。只需要将new ObjectOutputStream（）和new ObjectInputStream()顺序调换就行。

package com.softeem.wolf.homework06;
    
    import java.io.*;
    
    /**
     * Created by 苍狼
     * Time on 2023-05-24
     */
    public class App {
        public static void main(String[] args) throws IOException {
            ObjectInputStream ois = null;
            ObjectOutputStream oos = null;
            oos = new ObjectOutputStream(new FileOutputStream("src\\com\\softeem\\wolf\\homework06\\student.txt"));
            ois = new ObjectInputStream(new FileInputStream("src\\com\\softeem\\wolf\\homework06\\student.txt"));
            Student student1 = new Student(1,"张三",'男');
            Student student2 = new Student(2, "李四",'女');
    
            try {
                oos.writeObject(student1);
                oos.writeObject(student2);
                oos.writeObject(null);
                oos.close();
            } catch (IOException ioException) {
                ioException.printStackTrace();
            }
            try {
                Object object;
                while((object=ois.readObject())!=null){
                    System.out.println(object);
                }
                ois.close();
            } catch (IOException ioException) {
                ioException.printStackTrace();
            } catch (ClassNotFoundException e) {
                e.printStackTrace();
            }
            try {
                ois.close();
            } catch (IOException ioException) {
                ioException.printStackTrace();
            }
        }
    }

运行结果

![30f06941f61345988007b99edf2158df.png][]

因为oos = new ObjectOutputStream(new FileOutputStream("src\\\\com\\\\softeem\\\\wolf\\\\homework06\\\\student.txt"))在运行的时候，构造器中

writeStreamHeader()方法会将文件头写进去，这就可以避免ObjectInputStream()构造器在运行readStreamHeader()方法时检测不到文件头。

下面是ObjectOutputStream中构造器的源码。

public ObjectOutputStream(OutputStream out) throws IOException {
            verifySubclass();
            bout = new BlockDataOutputStream(out);
            handles = new HandleTable(10, (float) 3.00);
            subs = new ReplaceTable(10, (float) 3.00);
            enableOverride = false;
            writeStreamHeader();
            bout.setBlockDataMode(true);
            if (extendedDebugInfo) {
                debugInfoStack = new DebugTraceInfoStack();
            } else {
                debugInfoStack = null;
            }
        }

[8ba0298c480d4b9498ed530afb23731f.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/16/6c9bca41fcb149afb28ebef09ab8cd11.png
[30f06941f61345988007b99edf2158df.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/16/1110ab160e494f62bd6207bfdbbe6ed3.png