覆盖通过FileDescriptor FD获得的FileOutputStream中的属性文件 [英] Overwrite a properties file from the FileOutputStream obtained through FileDescriptor FD

问题描述

特殊的问题是 当我将properties.store代码与从文件路径获得的FileOutputStream一起使用时，以上方法都可以正常工作，但是当我从从FileDescriptor获得的FileOutputStream执行此操作时，将属性文件附加到该文件中，则不会覆盖

Peculiar problem is When I am using the properties.store code with the FileOutputStream obtained from file path, the above method works all fine but when I do that from FileOutputStream obtained from FileDescriptor the properties file append to it, doesn't overwrite.

由于我正在使用FileLock，并且现在无法再次通过文件获取FileOutputStream，所以现在的约束是使用后一种方法.

Now my constraint is to use the later approach since I am using FileLock and cant get the FileOutputStream through file again.

1. 有可能吗?使用后面的方法做些事情并覆盖和
2. 如果没有，我有什么选择?

这两段代码是

第一种方法

OutputStream out = null;
    try {
        if (portFile != null && portFile.exists()) {
            out = new FileOutputStream(portFile);
        } else {
            try {
                portFile.createNewFile();
            } catch (IOException e) {
                LOGGER.error("error in creating properties file ", e);
            }
            out = new FileOutputStream(portFile);
        }
    } catch (FileNotFoundException e) {
        LOGGER.error("Not able to get outputstream on properties file ", e);
    }

    try {
        properties.store(out, CJDPluginConstants.PROP_NAME_PORT_MIN);
    } catch (IOException e) {
        LOGGER.error("Not able to save properties file ", e);
    }

第二种方法

// so we move to raf now
    OutputStream out = null;
    if (portFileRandomAccess != null && channel != null) {
        //ByteBuffer buffer = ByteBuffer.allocate(1024);
        try {
            if (buffer != null) {
                //if (buffer != null) {
                buffer.flip();
                LOGGER.info("buffer what we get we are writing ? " + buffer.asCharBuffer());
                out = new ByteBufferBackedOutputStream(buffer);
                FileOutputStream fos = new FileOutputStream(portFileRandomAccess.getFD());
                //fos.flush();
                properties.store(fos, CJDPluginConstants.PROP_NAME_PORT_MIN);
                buffer.clear();
                buffer = null;
                //}
            }
        } catch (IOException e) {
            LOGGER.error("Not able to save properties file ", e);
        }
    }

    //release lock, close channel
    if (lock != null) {
        try {
            lock.release();
        } catch (IOException e) {
            LOGGER.error("Error releasing lock", e);
        }
    }

    if (channel != null) {
        try {
            channel.close();
        } catch (IOException e) {
            LOGGER.error("Error closing channel", e);
        }
    }

推荐答案

如果您有FileOutputStream，则无论创建方式如何，都很容易将关联文件的长度截断为零:

If you have a FileOutputStream, regardless of how you created it, it’s easy to truncate the associated file to a zero length:

fileOutputStream.getChannel().truncate(0);

然后文件为空，然后向其中写入新内容.

Then the file is empty and you write new contents to it.

还可以进行真正的覆盖，即将旧内容保留在您未写入的区域:

It’s also possible to do a real overwrite, i.e. keeping old contents at regions you don’t write to:

fileOutputStream.getChannel().position(0);

然后，下一次写入将移至指定位置，覆盖与实际写入一样多的字节，但保留所有其他字节.

Then the next writes go to the specified position overwriting as much bytes as you really write, but retaining all other bytes.

这篇关于覆盖通过FileDescriptor FD获得的FileOutputStream中的属性文件的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！