Java 8 Streams: streaming files and MOVING them after read

问题

I want to stream the lines contained in files but MOVING each file to another folder once it has been processed.

The current process is like this:

Explanation:

1. I create a Stream of Files
2. I create a BufferedReader for each one of them
3. I flatMap to the lines Stream of the BufferedReader
4. I print each line.

Code (omited Exceptions for simplicity):

(1)    Stream.generate(localFileProvider::getNextFile)
(2)       .map(file -> new BufferedReader(new InputStreamReader(new FileInputStream(file))))
(3)       .flatMap(BufferedReader::lines)
(4)       .map(System.out::println)
          .MOVE_EACH_FILE_FROM_INPUT_FOLDER_TO_SOME_OTHER_FOLDER;

Would it be possible to move each file once it has been completely read and continue processing the other files in the stream?

Thank you!

回答1:

You can chain a close action to a stream, which will be executed automatically in case of flatMap:

Stream.generate(localFileProvider::getNextFile).takeWhile(Objects::nonNull)

    .flatMap(file -> {
        try {
            Path p = file.toPath();
            return Files.lines(p, Charset.defaultCharset()).onClose(() -> {
                try { // move path/x/y/z to path/x/y/z.moved
                    Files.move(p, p.resolveSibling(p.getFileName()+".moved"));
                } catch(IOException ex) { throw new UncheckedIOException(ex); }
            });
        } catch(IOException ex) { throw new UncheckedIOException(ex); }
    })

    .forEach(System.out::println);

It’s important that the documentation of onClose states:

Close handlers are run when the close() method is called on the stream, and are executed in the order they were added.

So the moving close handler is executed after the already existing close handler that will close the file handle used for reading the lines.

I used Charset.defaultCharset() to mimic the behavior of the nested constructors new InputStreamReader(new FileInputStream(file))) of your question’s code, but generally, you should use a fixed charset, like the Files.lines’s default UTF-8 whenever possible.

回答2:

Actually it will be very easy if you can divide the logic into different method

 public Path readFile(File eachFile) {
   BufferedReader reader = new BufferedReader(new InputStreamReader(new FileInputStream(file)));

  //try-with-resources
 try (Stream<String> lines = reader.lines()) {
lines.forEach(System.out::println);

 } 
catch (IOException e) {
   e.printStackTrace();
 }
   return eachFile.toPath();
  }

And then call this method for each file

(1)    Stream.generate(localFileProvider::getNextFile)
(2)          .map(this::readFile) //process each file
(3)          .forEach(path->Files.move(path,Paths.get("new path"))); //then move each file

回答3:

I would just create two methods:

public void processFile(File f);
public void moveFile(File f, File dstFolder);

then in lambda:

Stream.generate(localFileProvider::getNextFile).forEach(file->
   {
     processFile(file);
     moveFile(file, dstFolder)
   }
}

回答4:

Can do something like this:

    files
        .map( file -> {
            getBufferedReader( file ).lines()
                .forEach( System.out::println );
            return file;
        } )
        .forEach( this::moveFile );

Update for checked exceptions and Reader.close:

Admittedly, this doesn't run close() in a finally block, so that is a downside. The point of this update is mainly to illustrate a way of dealing with checked exceptions in Java 8 streams.

Let's say you have the following utility code available:

private interface ThrowingFunction<I, O>
{
    O apply( I input ) throws Exception;
}

private <I, O> Function<I, O> unchecked( ThrowingFunction<I, O> checked )
{
    return i -> {
        try {
            return checked.apply( i );
        }
        catch ( Exception e ) {
            throw new RuntimeException();
        }
    };
}

private interface ThrowingConsumer<T>
{
    void consume( T input ) throws Exception;
}

private <T> Consumer<T> unchecked( ThrowingConsumer<T> checked )
{
    return t -> {
        try {
            checked.consume( t );
        }
        catch ( Exception e ) {
            throw new RuntimeException();
        }
    };
}

private BufferedReader getBufferedReader( File file ) throws FileNotFoundException
{
    return new BufferedReader( new InputStreamReader( new FileInputStream( file )));
}

Writing the actual code then becomes:

    files
        .map( file -> {
            Stream.of( file )
                .map( unchecked( this::getBufferedReader ))
                .map( reader -> {
                    reader.lines().forEach( System.out::println );
                    return reader;
                } )
                .forEach( unchecked( Reader::close ));
            return file;
        } )
        .forEach( this::moveFile );

来源：https://stackoverflow.com/questions/61914029/java-8-streams-streaming-files-and-moving-them-after-read