如何实现可以倒回的OutputStream？

Question

在将一些处理过的内容写入输出流之后,我需要重新访问流的开头并写出一些内容元数据.我写的数据非常大,高达4Gb,可以直接写入文件或内存缓冲区,具体取决于各种环境因素.

如何实现一个OutputStream,允许我在完成内容写入后写出标题？

Answer 1

这是一个随机访问文件输出流.

请注意,如果将其用于大量流式输出,您可以暂时将其包装在BufferedOutputStream中以避免大量小写(在丢弃包装器或直接使用底层流之前,请务必将其刷新).

import java.io.*;

/**
 * A positionable file output stream.
 * <p>
 * Threading Design : [x] Single Threaded  [ ] Threadsafe  [ ] Immutable  [ ] Isolated
 */

public class RandomFileOutputStream
extends OutputStream
{

// *****************************************************************************
// INSTANCE PROPERTIES
// *****************************************************************************

protected RandomAccessFile              randomFile;                             // the random file to write to
protected boolean                       sync;                                   // whether to synchronize every write

// *****************************************************************************
// INSTANCE CONSTRUCTION/INITIALIZATON/FINALIZATION, OPEN/CLOSE
// *****************************************************************************

public RandomFileOutputStream(String fnm) throws IOException {
    this(fnm,false);
    }

public RandomFileOutputStream(String fnm, boolean syn) throws IOException {
    this(new File(fnm),syn);
    }

public RandomFileOutputStream(File fil) throws IOException {
    this(fil,false);
    }

public RandomFileOutputStream(File fil, boolean syn) throws IOException {
    super();

    File                                par;                                    // parent file

    fil=fil.getAbsoluteFile();
    if((par=fil.getParentFile())!=null) { IoUtil.createDir(par); }
    randomFile=new RandomAccessFile(fil,"rw");
    sync=syn;
    }

// *****************************************************************************
// INSTANCE METHODS - OUTPUT STREAM IMPLEMENTATION
// *****************************************************************************

public void write(int val) throws IOException {
    randomFile.write(val);
    if(sync) { randomFile.getFD().sync(); }
    }

public void write(byte[] val) throws IOException {
    randomFile.write(val);
    if(sync) { randomFile.getFD().sync(); }
    }

public void write(byte[] val, int off, int len) throws IOException {
    randomFile.write(val,off,len);
    if(sync) { randomFile.getFD().sync(); }
    }

public void flush() throws IOException {
    if(sync) { randomFile.getFD().sync(); }
    }

public void close() throws IOException {
    randomFile.close();
    }

// *****************************************************************************
// INSTANCE METHODS - RANDOM ACCESS EXTENSIONS
// *****************************************************************************

public long getFilePointer() throws IOException {
    return randomFile.getFilePointer();
    }

public void setFilePointer(long pos) throws IOException {
    randomFile.seek(pos);
    }

public long getFileSize() throws IOException {
    return randomFile.length();
    }

public void setFileSize(long len) throws IOException {
    randomFile.setLength(len);
    }

public FileDescriptor getFD() throws IOException {
    return randomFile.getFD();
    }

} // END PUBLIC CLASS