java.io 中装饰流的限制是什么？ [复制]

Question

我想像这样的装饰器模式：

我知道 java io 使用这种模式。

我觉得包装对象存在一些限制，但我无法理解整个规则。

研究代码：

sn-p 1 - 工作代码：

    OutputStream outputStream = new FileOutputStream("2.txt");

    ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);
    ObjectOutputStream objectOutputStreamWrapper = new ObjectOutputStream(objectOutputStream);     //double wrapping

    objectOutputStreamWrapper.writeObject("this is my string");
    objectOutputStreamWrapper.close();

    ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("2.txt"));
    ObjectInputStream   objectInputStreamWrapper = new ObjectInputStream(objectInputStream);


    System.out.println(objectInputStreamWrapper.readObject());
    objectInputStream.close();

sn-p 2 - 不工作：

    OutputStream outputStream = new FileOutputStream("2.txt");

    ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);
    ObjectOutputStream objectOutputStreamWrapper = new ObjectOutputStream(objectOutputStream);     //double wrapping

    objectOutputStreamWrapper.writeObject("this is my string");
    objectOutputStreamWrapper.close();

    ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("2.txt"));
    //ObjectInputStream   objectInputStreamWrapper = new ObjectInputStream(objectInputStream);


    System.out.println(objectInputStream.readObject());
    objectInputStream.close();

输出：

Exception in thread "main" java.io.OptionalDataException
    at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1361)
    at java.io.ObjectInputStream.readObject(ObjectInputStream.java:370)
    at io.WrappingUnwrappingOrderingTest.main(WrappingUnwrappingOrderingTest.java:28)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

sn-p 3 - 不工作

    OutputStream outputStream = new FileOutputStream("2.txt");

    ObjectOutputStream objectOutputStream = new ObjectOutputStream(outputStream);
  //  ObjectOutputStream objectOutputStreamWrapper = new ObjectOutputStream(objectOutputStream);     //double wrapping

    objectOutputStream.writeObject("this is my string");
    objectOutputStream.close();

    ObjectInputStream objectInputStream = new ObjectInputStream(new FileInputStream("2.txt"));
    ObjectInputStream   objectInputStreamWrapper = new ObjectInputStream(objectInputStream);


    System.out.println(objectInputStreamWrapper.readObject());
    objectInputStream.close();

输出：

Exception in thread "main" java.io.EOFException
    at java.io.ObjectInputStream$PeekInputStream.readFully(ObjectInputStream.java:2325)
    at java.io.ObjectInputStream$BlockDataInputStream.readShort(ObjectInputStream.java:2794)
    at java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:801)
    at java.io.ObjectInputStream.<init>(ObjectInputStream.java:299)
    at io.WrappingUnwrappingOrderingTest.main(WrappingUnwrappingOrderingTest.java:25)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

看起来在文件系统上的文件中存储了有关流装饰顺序的信息。这对我来说很奇怪。请澄清这个误解。

我是否总是应该对输入和输出使用完全相同的装饰器顺序？

Answer 1

这里的问题是constructing an ObjectOutputStream 将一些字节写入底层流：

创建一个写入指定 OutputStream 的 ObjectOutputStream。此构造函数将序列化流标头写入底层流

同样，创建 ObjectInputStream 会读取序列化标头。

因此，如果您打开两个 ObjectOutputStream 并仅使用一个读取，反之亦然，您将不会读取所写的那么多序列化标头，从而导致它无法工作，如堆栈跟踪所示：

在 java.io.ObjectInputStream.readStreamHeader(ObjectInputStream.java:801)

这是 ObjectOutputStream 所独有的。例如，将缓冲流包装成缓冲流就不会出现这个问题。