Invalidate Stream without Closing

问题

This is a followup to anonymous file streams reusing descriptors

As per my previous question, I can't depend on code like this (happens to work in JDK8, for now):

RandomAccessFile  r = new RandomAccessFile(...);

FileInputStream f_1 = new FileInputStream(r.getFD());
                      // some io, not shown
                f_1 = null;

                f_2 = new FileInputStream(r.getFD());
                      // some io, not shown
                f_2 = null;

                f_3 = new FileInputStream(r.getFD());
                    // some io, not shown
                f_3 = null;

However, to prevent accidental errors and as a form of self-documentation, I would like to invalidate each file stream after I'm done using it - without closing the underlying file descriptor.

Each FileInputStream is meant to be independent, with positioning controlled by the RandomAccessFile. I share the same FileDescriptor to prevent any race conditions arising from opening the same path multiple times. When I'm done with one FileInputStream, I want to invalidate it so as to make it impossible to accidentally read from it while using the second FileInputStream (which would cause the second FileInputStream to skip data).

How can I do this?

notes:

• the libraries I use require compatibiity with java.io.*
• if you suggest a library (I prefer builtin java semantics if at all possible), it must be commonly available (packaged) for linux (the main target) and usable on windows (experimental target)
• but, windows support isn't a absolutely required

Edit: in response to a comment, here is my workflow:

RandomAccessFile r = new RandomAccessFile(String path, "r");

int header_read;
int header_remaining = 4; // header length, initially

byte[]     ba = new byte[header_remaining];
ByteBuffer bb = new ByteBuffer.allocate(header_remaining);

while ((header_read = r.read(ba, 0, header_remaining) > 0) {
    header_remaining -= header_read;
    bb.put(ba, 0, header_read);
}

byte[] header = bb.array();

// process header, not shown
// the RandomAccessFile above reads only a small amount, so buffering isn't required

r.seek(0);

FileInputStream f_1 = new FileInputStream(r.getFD());

Library1Result result1 = library1.Main.entry_point(f_1)

// process result1, not shown
// Library1 reads the InputStream in large chunks, so buffering isn't required
// invalidate f_1 (this question)

r.seek(0)

int read;
while ((read = r.read(byte[4096] buffer)) > 0 && library1.continue()) {
    library2.process(buffer, read);
}

// the RandomAccessFile above is read in large chunks, so buffering isn't required
// in a previous edit the RandomAccessFile was used to create a FileInputStream. Obviously that's not required, so ignore

r.seek(0)

Reader r_1 = new BufferedReader(new InputStreamReader(new FileInputStream(r.getFD())));

Library3Result result3 = library3.Main.entry_point(r_2)

// process result3, not shown
// I'm not sure how Library3 uses the reader, so I'm providing buffering
// invalidate r_1 (this question) - bonus: frees the buffer

r.seek(0);

FileInputStream f_2 = new FileInputStream(r.getFD());

Library1Result result1 = library1.Main.entry_point(f_2)

// process result1 (reassigned), not shown
// Yes, I actually have to call 'library1.Main.entry_point' *again* - same comments apply as from before
// invalidate f_2 (this question)
//
// I've been told to be careful when opening multiple streams from the same
// descriptor if one is buffered. This is very vague. I assume because I only
// ever use any stream once and exclusively, this code is safe.
//

回答1:

A pure Java solution might be to create a forwarding decorator that checks on each method call whether the stream is validated or not. For InputStream this decorator may look like this:

public final class CheckedInputStream extends InputStream {
  final InputStream delegate;
  boolean validated;

  public CheckedInputStream(InputStream stream) throws FileNotFoundException {
    delegate = stream;
    validated = true;
  }

  public void invalidate() {
    validated = false;
  }

  void checkValidated() {
    if (!validated) {
      throw new IllegalStateException("Stream is invalidated.");
    }
  }

  @Override
  public int read() throws IOException {
    checkValidated();
    return delegate.read();
  }

  @Override
  public int read(byte b[]) throws IOException {
    checkValidated();
    return read(b, 0, b.length);
  }

  @Override
  public int read(byte b[], int off, int len) throws IOException {
    checkValidated();
    return delegate.read(b, off, len);
  }

  @Override
  public long skip(long n) throws IOException {
    checkValidated();
    return delegate.skip(n);
  }

  @Override
  public int available() throws IOException {
    checkValidated();
    return delegate.available();
  }

  @Override
  public void close() throws IOException {
    checkValidated();
    delegate.close();
  }

  @Override
  public synchronized void mark(int readlimit) {
    checkValidated();
    delegate.mark(readlimit);
  }

  @Override
  public synchronized void reset() throws IOException {
    checkValidated();
    delegate.reset();
  }

  @Override
  public boolean markSupported() {
    checkValidated();
    return delegate.markSupported();
  }
}

You can use it like:

CheckedInputStream f_1 = new CheckedInputStream(new FileInputStream(r.getFD()));
                      // some io, not shown
                   f_1.invalidate();

                   f_1.read(); // throws IllegalStateException

回答2:

Under unix you could generally avoid such problems by dup'ing a file descriptor.

Since java does not not offer such a feature one option would be a native library which exposes that. jnr-posix does that for example. On the other hand jnr depends on a lot more jdk implementation properties than your original question.

来源：https://stackoverflow.com/questions/35137475/invalidate-stream-without-closing