I need to pass an InputStream to an object which reads data which I previously stored into a File. I'm assessing a more efficient approach than storing eveything into a File and then passing the FileInputStream. I'd like to do it on the fly.
May someone appoint me the correct approach to do that?
The idea would be passing a Custom InputStream which innerly calls every line I was going to store in the file. I guess I need buffering. I discard storing everything in a String and then build an InputStream on it, as we are in the same situation, waiting to output all the lines before rereading them again.
There already is a stream for this. It's the PipedInputStream. You'll need to have one thread write to the PipedOutputStream, and pass the PipedInputStream to the object that will be reading in another thread.
Related
I have a lot of files in a directory but I only want to read the ones with a certain extension (say .txt). I want these files added to the same BufferedInputStream so that I can read them in one go. When I call read() at the end of a file, the next one should begin.
It really feels like there should be an obvious answer to this but I had no luck finding it.
You might want to take a look at SequenceInputStream:
A SequenceInputStream represents the logical concatenation of other
input streams. It starts out with an ordered collection of input
streams and reads from the first one until end of file is reached,
whereupon it reads from the second one, and so on, until end of file
is reached on the last of the contained input streams.
To me the "obvious answer" is:
Just iterate through all the files in the directory using a proper filter. For each file create a FileInputStream, read it and close it.
I don't think there is an obvious answer to this question.
Probably you need to create a Wrapper InputStream with a list of files you want to read from. Internally you will open/close streams as needed, namely when a file is completely read.
It is not obvious but should not be difficult. This way you can work 'only' with one InputStream for all files.
I'm working with a library that I have to provide an InputStream and a PrintStream. It uses the InputStream to gather data for processing and the PrintStream to provide results. I'm stuck using this library and its API cannot be altered.
There are two issues with this that I think have related solutions.
First, the data that needs to be read via the InputStream is not available upfront. Instead, the data is dynamically created by a different part of the application and given to my code as a String via method call. My code's job is to somehow allow the library to read this data through the InputStream provided as I get it.
Second, I need to somehow get the result that is written to the PrintStream and send it to another part of the application as a String. This needs to happen as immediately after the data is put in to the PrintStream as possible.
What it looks like I need are two stream objects that behave more or less like buffers. I need an InputStream that I can shove data in to whenever I have it and a PrintStream that I can grab it's contents whenever it has some. This seems a little awkward to me, but I'm not sure how else to do it.
I'm wondering if anything already exists that allows this kind of behavior or if there is a different (better) solution that will work in the situation I've described. The only thing I can come up with is to try to implement streams with this behavior, but that can become complicated fast (especially since the InputStream needs to block until data is available).
Any ideas?
Edit: To be clear, I'm not writing the library. I'm writing code that is supposed to provide the library with an InputStream to read data from and a PrintStream to write data to.
Looks like both streams need to be constantly reading/writing so you'll need two threads independent of each other. The pattern resembles JMS a little bit, in which case you're feeding information to a "queue" or "topic", and wait for it to be processed then put on a "output" queue/topic. This may introduce additional moving parts, but you could write a simple client to place info onto a JMS queue, then have a listener to just grab messages, and feed it to the input stream constantly. Then another piece of code to read from output stream, and do what you need with it.
Hope this helps.
I am sending a plain text file to the user through a servlet.
I am using flatworm framework to build the flat file. I receive the file in the browser but is empty. So i want start the debugging analysing the outputstream before being sent.
How i can read the response before i send it in the servlet? I think is the same thing that asking how can i transform an OutputStream to an InputStream.
I already saw solutions that always involve ByteArrayOutputStream , and as you know when i call in the servlet response.getOutputStream() it returns me an OutputStream and not a ByteArrayOutputStream.
There seems to be some confusion somewhere, though I'm not sure exactly where.
What can you do with an OutputStream? Why, you can write to it, and that's about it. That means that if you're given (or look up) an output stream, it's up to you to supply the data - which means you already have it.
Perhaps on the other hand, you're not directly calling write on the OutputStream yourself, but passing this stream into the flatworm library (which will in turn write output to it). In that case, there's your debugging "hook" right there - flatworm will write out the file to any output stream you send it. So in this case, instead of passing in the servlet's stream, you pass in a stream that you've created yourself.
That might be a ByteArrayOutputStream, which (after the flatworm method has returned) you can inspect to get the bytes written. At this point you could manually write them through to the response's output stream. Or maybe you need to do something slightly trickier and create your own stream wrapper which writes straight through to the underlying response stream but logs on the way - and pass this into flatworm.
The bottom line however is that if you're interacting with an output stream, then "your" code already has the data somewhere locally and it's just a matter of capturing/accessing that.
I want to write multiple objects to a file, but the problem is that I dont have all the objects to write at once. I have to write one object and then close the file, and then maybe after sometime I want to add another object to the same file.
I am currently doing it as
FileOutputStream("filename", true)
so that it will append the object to the end of file and not overwrite it. But I get this error :
java.io.StreamCorruptedException: invalid type code: AC
any ideas how can I solve this issue ?
Thanks,
One option is to segment the file into individual messages. When you want to write a message, first serialize it to a ByteArrayOutputStream. Then open the file for appending with DataOutputStream - write the length with writeInt, then write the data.
When you're reading from the stream, you'd open it with DataInputStream, then repeatedly call readInt to find the length of the next message, then readFully to read the message itself. Put the message into ByteArrayInputStream and then deserialize from that.
Alternatively, use a nicer serialization format than the built-in Java serialization - I'm a fan of Protocol Buffers but there are lots of alternatives available. The built-in serialization is too brittle for my liking.
You can't append different ObjectOutputStreams to the same file. You would have to use a different form of serialization, or read the file in and write out all the objects plus the new objects to a new file.
You need to serialize/deserialize the List<T>. Take a look at this stackoverflow thread.
I want to write a program in Java with support for unix pipeline. The problem is that my input files are images and I need in some way to separate them from one another.
I thought that there is no problem because I can read InputStream using ImageIO.read() without reseting position. But it isn't that simple. ImageIO.read() closes the stream every time an image is read. So I can't read more than one file from stdin. Do you have some solution for this?
The API for read() mentions, "This method does not close the provided InputStream after the read operation has completed; it is the responsibility of the caller to close the stream, if desired." You might also check the result for null and verify that a suitable ImageReader is available.