ObjectInputStream 在使用 ByteArrayInputStream 构造后没有可用字节

Question

我正在构建一个处理二进制 De/Serialization 的 class。方法 open() 接收一个 InputStream 和一个 OutputStream。这些是由另一个接收路径作为参数的 open() 方法创建的。 InputStream 实际上是 ByteArrayInputStream。我已经做了一些测试来证明 InputStream 正在到达带有内容的 open() 方法 - 它确实是。但是当我尝试使用它设置 ObjectInputStream 时，它不起作用。没有抛出异常，但是当我尝试从中读取字节时，它总是给我 -1.

BinaryStrategy class

public class BinaryStrategy implements SerializableStrategy{
  public BinaryStrategy(){
    try{
        open("products.ser");
    }catch(IOException ioe){

    }
  } 
  @Override
  public void open(InputStream input, OutputStream output) throws IOException  {
    try{
        this.ois = new ObjectInputStream(input);
    }catch(Exception ioe){
        System.out.println(ioe);
    }
    this.oos = new ObjectOutputStream(output);
  }
  @Override
  public void writeObject(fpt.com.Product obj) throws IOException {
    oos.writeObject(obj);
    oos.flush();
  }
  @Override
  public Product readObject() throws IOException {
    Product read = new Product();
    try{
        read.readExternal(ois);
    }catch(IOException | ClassNotFoundException exc){
        System.out.println(exc);
    }
    return read;
  }
}

interface SerializableStrategy (just the default method)

    default void open(Path path) throws IOException {
    if (path != null) {
        ByteArrayInputStream in = null;
        if (Files.exists(path)) {
            byte[] data = Files.readAllBytes(path);
            in = new ByteArrayInputStream(data);
        }
        OutputStream out = Files.newOutputStream(path);
        open(in, out);
    }

Product class

public class Product implements java.io.Externalizable {
    @Override
public void writeExternal(ObjectOutput out) throws IOException {
    out.writeLong(getId());
    out.writeObject(getName());
    out.writeObject(getPrice());
    out.writeObject(getQuantity());
}

@Override
public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
    this.setId((Long)in.readLong());
    this.setName((String) in.readObject());
    this.setPrice((Double) in.readObject());
    this.setQuantity((Integer) in.readObject());
}

我必须对其进行个性化设置，因为属性是 SimplePropertys

在 public void open(InputStream input, OutputStream output) 我尝试做一些如下测试：

    public void open(InputStream input, OutputStream output) throws IOException {
    try{
        System.out.println(input.available() + " " + input.read() + " " + input.read());
        //is gives me: 181 172 237
        //181 is the exact size of the file I have, so i think that the Output is ok
        //172 237 - just some chars that are in the file
        //I know that for now on it is going to give me an excepetion because
        // of the position of the index that is reading. I did it just to test
        this.ois = new ObjectInputStream(input);
    }catch(Exception ioe){
        System.out.println(ioe);
    }
    this.oos = new ObjectOutputStream(output);
}

然后是其他测试：

public void open(InputStream input, OutputStream output) throws IOException {
    try{
        this.ois = new ObjectInputStream(input);
        System.out.println(ois.available() + " " + ois.read());
        //here is where I am receiving -1 and 0 available bytes!
        //so something is going wrong right here.
        //i tried to just go on and try to read the object,
        //but I got a EOFException, in other words, -1.
    }catch(Exception ioe){
        System.out.println(ioe);
    }
    this.oos = new ObjectOutputStream(output);
}

Answer 1

请检查 path 代表的文件是否写入了 java 对象。
来自 ObjectInputStream API 文档 https://docs.oracle.com/javase/7/docs/api/java/io/ObjectInputStream.html

An ObjectInputStream deserializes primitive data and objects previously written using an ObjectOutputStream.

ObjectInputStream is used to recover those objects previously serialized.

如果您正在执行 this.ois.readObject()，并且您正在执行 -1，则文件中可能不包含对象。

更新：readObject returns 一个对象而不是 int。如果您在 ois 中使用 read 方法，并且得到 -1，则文件为空。

此外，您的文件有可能包含 -1 作为其内容 ;)

Answer 2

问题是我读 ObjectInputStream 的方式不对。就像：

read.readExternal(ois);

但正确的方法是：

read = (Product)ois.readObject();

并且由于我这样做时遇到的异常，我认为问题出在使用 ByteArrayInputStream 时 ObjectInputStream 的构造上。多么大的错误！ :D

感谢所有试图提供帮助的人。

Answer 3

ObjectInputStream，内部使用一个BlockDataInputStream执行它的读操作。当您调用 read 时，这会读取一个数据块，而不仅仅是我们预期的一个字节。只有当它落在 "block"

时它才读取一个字节

输出也不是我所期望的。但是，如果您查看 ObjectInputStream.read() 的代码，它是有道理的。

因此，在您的情况下，仅使用 readObject 来恢复对象的状态是有意义的。

又是你的代码...

class SimpleJava {

    public static void open(InputStream input, OutputStream output) throws IOException {

        try {
            ObjectInputStream ois = new ObjectInputStream(input);
            System.out.println(ois.available());// 0
            System.out.println(ois.available() + " " + ois.read() + " " + ois.read());// 0 -1 -1
            // Reads the object even if the available returned 0 
            // and ois.read() returned -1
            System.out.println("object:" + ois.readObject());// object:abcd
        }
        catch (Exception ioe) {
            ioe.printStackTrace();
        }
    }

    static void open(Path path) throws IOException {

        if (path != null) {
            ByteArrayInputStream in = null;
            if (Files.exists(path)) {
                byte[] data = Files.readAllBytes(path);
                in = new ByteArrayInputStream(data);
            }
            OutputStream out = Files.newOutputStream(path);
            open(in, out);
        }
    }

    public static void main(String[] args) throws Exception {

        ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(new File("/home/pradhan/temp.object")));
        oos.writeObject("abcd");//writes a string object for us to read later
        oos.close();
        //
        open(FileSystems.getDefault().getPath("/home/user/temp.object"));
    }
}

这是输出...

0
0 -1 -1
object:abcd

ObjectInputStream 在使用 ByteArrayInputStream 构造后没有可用字节

ObjectInputStream doesn't have available bytes after being constructed with a ByteArrayInputStream

java

serialization

inputstream

objectinputstream

bytearrayinputstream