Custom implementation of InputStream

To send data to a file on my FTP server, I need to create a custom InputStream implementation that reads database data row by row, converts it to CSV and publishes it via its read() methods: from the database, 开发者_运维问答I get a List<Application> object with the data. For each Application object, I want to create a line in the CSV file.

My idea is to load all the data in the constructor and then override the read method. Do I need to override all InputStream's methods? I tried googling for some examples but didn't succeed - could you eventually give me a link to one?

You only nead to implement the read() method without parameters. All other methods are implemented as calls to that method. For performance reasons (and even ease of implementation) it might be easier to implement the three-argument read() method instead and re-implement the no-args read() method in terms of that method.

Some very important points which I met when implementing my InputStream.

1. Override available(). As the Javadoc says:

   The available method for class InputStream always returns 0. This method should be overridden by subclasses.

   not overriding this method will causes that any tempt to test whether this stream is readable return false. For example, if you feed your inputStream to a inputStreamReader, this reader will always return false when you invoke reader.ready().

2. return -1 in the read(). The doc didn't emphasize it:

   If no byte is available because the end of the stream has been reached, the value -1 is returned. This method blocks until input data is available, the end of the stream is detected, or an exception is thrown.

   if you choose to block read() when no data is available, you have to remember to return -1 at some situations. Not doing this may causes that another read(byte b[], int off, int len) blocks for the following code in the source:

   for (; i < len ; i++) {// default len is a relative large number (8192 - readPosition)
       c = read();
       if (c == -1) {
           break;
       }
       b[off + i] = (byte)c;
   }
   

   And this causes some(if not all) high level read block, like a reader's readLine(), read() etc.

For possibly large data you can use com.google.common.io.FileBackedOutputStream from guava.

Javadoc: An OutputStream that starts buffering to a byte array, but switches to file buffering once the data reaches a configurable size.

Using out.getSupplier().getInput() you get your InputStream.

There's absolutely no need to create a custom InputStream. Use ByteArrayInputStream, something like this:

public static InputStream createStream(){
    final String csv = createCsvFromDataBaseValues();
    return new ByteArrayInputStream(csv.getBytes());
}

Especially given this quote:

My idea is to load all the data in the constructor and then override the read method.

If you do it like this, you gain absolutely nothing by implementing a custom InputStream. It's pretty much equivalent to the approach I outlined above.

Why do you need a custon inputstream? why not just write the csv data as you generate it to the outputstream being written to the ftp server?

If the data is not too large, you could:

• Read it all
• Convert to CSV (text)
• Get the text bytes (via String.getBytes(encoding))
• But the byte array in a ByteArrayInputStream