I'm looking for a theoretical analysis. I mean, how does the buffer system works and what advantage does using flush provide? Kindly illustrate with an example, if possible.
When you are writing to a text file, BufferedWriter does not write it to disk immediately. Instead, it keeps the data in a buffer in memory.
This has the advantage that many small writes will go into the buffer, and then the data will be written to disk in one go, ie. with one big write, instead of many small writes, which would be inefficient.
When the buffer is full, BufferedWriter will write the data out on it's own, ie. it will do the same thing as calling flush() when the buffer is full.
So when should you call flush() manually ?
When you need data to be on disk now. If you have a program which reads data from the file on disk at the same time it is written, you may want to ensure all of the data which is supposed to be on disk is actually there.
If you are writing through a network connection, you may want to call flush() so that the data gets sent through the network immediately.
Usually it is not necessary to call flush(). Just write and call close() when finished, and no need for flush() as close() flushes the buffer for you.
Related
In Java, flush() method is used in streams. But I don't understand what are all the purpose of using this method?
fin.flush();
tell me some suggestions.
From the docs of the flush method:
Flushes the output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.
The buffering is mainly done to improve the I/O performance. More on this can be read from this article: Tuning Java I/O Performance.
When you write data to a stream, it is not written immediately, and it is buffered. So use flush() when you need to be sure that all your data from buffer is written.
We need to be sure that all the writes are completed before we close the stream, and that is why flush() is called in file/buffered writer's close().
But if you have a requirement that all your writes be saved anytime before you close the stream, use flush().
When we give any command, the streams of that command are stored in the memory location called buffer(a temporary memory location) in our computer. When all the temporary memory location is full then we use flush(), which flushes all the streams of data and executes them completely and gives a new space to new streams in buffer temporary location.
-Hope you will understand
If the buffer is full, all strings that is buffered on it, they will be saved onto the disk. Buffers is used for avoiding from Big Deals! and overhead.
In BufferedWriter class that is placed in java libs, there is a one line like:
private static int defaultCharBufferSize = 8192;
If you do want to send data before the buffer is full, you do have control. Just Flush It. Calls to writer.flush() say, "send whatever's in the buffer, now!
reference book: https://www.amazon.com/Head-First-Java-Kathy-Sierra/dp/0596009208
pages:453
In addition to other good answers here, this explanation made it very clear for me:
A buffer is a portion in memory that is used to store a stream of data
(characters). These characters sometimes will only get sent to an
output device (e.g. monitor) when the buffer is full or meets a
certain number of characters. This can cause your system to lag if you
just have a few characters to send to an output device. The flush()
method will immediately flush the contents of the buffer to the output
stream.
Source: https://www.youtube.com/watch?v=MjK3dZTc0Lg
Streams are often accessed by threads that periodically empty their content and, for example, display it on the screen, send it to a socket or write it to a file. This is done for performance reasons. Flushing an output stream means that you want to stop, wait for the content of the stream to be completely transferred to its destination, and then resume execution with the stream empty and the content sent.
For performance issue, first data is to be written into Buffer. When buffer get full then data is written to output (File,console etc.). When buffer is partially filled and you want to send it to output(file,console) then you need to call flush() method manually in order to write partially filled buffer to output(file,console).
I just read that
Some buffered output classes support autoflush, specified by an
optional constructor argument. When autoflush is enabled, certain key
events cause the buffer to be flushed. For example, an autoflush
PrintWriter object flushes the buffer on every invocation of println
or format.
So if I am keeping the reference of any BufferReader for some time being and it gets flushed , then how all the data will be retained back? Is there some call back mechanism that will automatically flush it and again read the content or will I lose the data and again I need to call for it?
So if I am keeping the reference of any BufferReader for some time being and it gets flushed , then how all the data will be retained back?
I think you mean BufferedWriter. (Neither the Reader or InputStream APIs have a flush() method. Flushing doesn't make any sense on a "source".)
The flushed data is written to the stream's "sink"; i.e. the file or socket or whatever. So if you look in the file (or whatever), the data will be there if the stream has been flushed (successfully).
Is there some call back mechanism that will automatically flush it and again read the content
There is no callback mechanism1. (At least, not in any of the buffered stream classes that the standard class library provides: who knows what a custom class might do ...)
Data is flushed automatically when certain things happen. For example, when the application calls println ... for a PrintWriter.
... or will I lose the data and again I need to call for it?
This doesn't make sense, either grammatically or semantically. I don't know what you are trying to ask.
Perhaps you don't understand what flushing does. Flushing simply means pushing the data out of the buffers and out to wherever the stream sends its data. An explicit flush() call or an automatic flush just means "write it NOW".
1 - Incidentally, BufferedWriter doesn't have a finalize() method either. This means that if one of these objects becomes unreachable while it still has output buffered, that output will never be written.
I think you're getting confused between buffered readers and writers. Your statement is talking about buffered writers, so if you're writing out to a stream then you shouldn't really care whether it is physically written or only written to the buffer - it doesn't matter to Java.
I would hope that a buffered reader would never be flushed, but depending on the type of buffer it might be OK. For example, if reading from a file, the buffer could be flushed and the file would just need to be re-read from the file system when you try to read(). However, for other streaming content, you wouldn't want it to be automatically flushed, as you would lose whatever data was in the buffer.
I read that invoking flush method guarantees that the last of the data you thought you had already written actually gets out to the file.I didn't get the meaning of this statement can any one explain clearly what actually flush method invocation will do?
The writers are usually buffered so it waits for the buffer to be filled before it writes it to the file. Flush tells to write the buffer even though it might not be filled yet. It's usually useful when you finish the writing since the last buffer may not be full but you want to finish the writing.
Many streams have internal buffers which they use to store data before it is passed on. This prevents a file stream from having to continually write each individual byte to disk (which can be quite expensive). The flush command forces a stream to clear its internal buffers so that, in this case, everything is forced to disk.
In Java, flush() method is used in streams. But I don't understand what are all the purpose of using this method?
fin.flush();
tell me some suggestions.
From the docs of the flush method:
Flushes the output stream and forces any buffered output bytes to be written out. The general contract of flush is that calling it is an indication that, if any bytes previously written have been buffered by the implementation of the output stream, such bytes should immediately be written to their intended destination.
The buffering is mainly done to improve the I/O performance. More on this can be read from this article: Tuning Java I/O Performance.
When you write data to a stream, it is not written immediately, and it is buffered. So use flush() when you need to be sure that all your data from buffer is written.
We need to be sure that all the writes are completed before we close the stream, and that is why flush() is called in file/buffered writer's close().
But if you have a requirement that all your writes be saved anytime before you close the stream, use flush().
When we give any command, the streams of that command are stored in the memory location called buffer(a temporary memory location) in our computer. When all the temporary memory location is full then we use flush(), which flushes all the streams of data and executes them completely and gives a new space to new streams in buffer temporary location.
-Hope you will understand
If the buffer is full, all strings that is buffered on it, they will be saved onto the disk. Buffers is used for avoiding from Big Deals! and overhead.
In BufferedWriter class that is placed in java libs, there is a one line like:
private static int defaultCharBufferSize = 8192;
If you do want to send data before the buffer is full, you do have control. Just Flush It. Calls to writer.flush() say, "send whatever's in the buffer, now!
reference book: https://www.amazon.com/Head-First-Java-Kathy-Sierra/dp/0596009208
pages:453
In addition to other good answers here, this explanation made it very clear for me:
A buffer is a portion in memory that is used to store a stream of data
(characters). These characters sometimes will only get sent to an
output device (e.g. monitor) when the buffer is full or meets a
certain number of characters. This can cause your system to lag if you
just have a few characters to send to an output device. The flush()
method will immediately flush the contents of the buffer to the output
stream.
Source: https://www.youtube.com/watch?v=MjK3dZTc0Lg
Streams are often accessed by threads that periodically empty their content and, for example, display it on the screen, send it to a socket or write it to a file. This is done for performance reasons. Flushing an output stream means that you want to stop, wait for the content of the stream to be completely transferred to its destination, and then resume execution with the stream empty and the content sent.
For performance issue, first data is to be written into Buffer. When buffer get full then data is written to output (File,console etc.). When buffer is partially filled and you want to send it to output(file,console) then you need to call flush() method manually in order to write partially filled buffer to output(file,console).
I have a question in my mind that, while writing into the file, before closing is done, should we include flush()??. If so what it will do exactly? dont streams auto flush??
EDIT:
So flush what it actually do?
Writers and streams usually buffer some of your output data in memory and try to write it in bigger blocks at a time. flushing will cause an immediate write to disk from the buffer, so if the program crashes that data won't be lost. Of course there's no guarantee, as the disk may not physically write the data immediately, so it could still be lost. But then it wouldn't be the Java program's fault :)
PrintWriters auto-flush (by default) when you write an end-of-line, and of course streams and buffers flush when you close them. Other than that, there's flushing only when the buffer is full.
I would highly recommend to call flush before close. Basically it writes remaining bufferized data into file.
If you call flush explicitly you may be sure that any IOException coming out of close is really catastrophic and related to releasing system resources.
When you flush yourself, you can handle its IOException in the same way as you handle your data write exceptions.
You don't need to do a flush because close() will do it for you.
From the javadoc:
"Close the stream, flushing it first. Once a stream has been closed, further write() or flush() invocations will cause an IOException to be thrown. Closing a previously-closed stream, however, has no effect."
To answer your question as to what flush actually does, it makes sure that anything you have written to the stream - a file in your case - does actually get written to the file there and then.
Java can perform buffering which means that it will hold onto data written in memory until it has a certain amount, and then write it all to the file in one go which is more efficient. The downside of this is that the file is not necessarily up-to-date at any given time. Flush is a way of saying "make the file up-to-date.
Close calls flush first to ensure that after closing the file has what you would expect to see in it, hence as others have pointed out, no need to flush before closing.
Close automatically flushes. You don't need to call it.
There's no point in calling flush() just before a close(), as others have said. The time to use flush() is if you are keeping the file open but want to ensure that previous writes have been fully completed.
As said, you don't usually need to flush.
It only makes sense if, for some reason, you want another process to see the complete contents of a file you're working with, without closing it. For example, it could be used for a file that is concurrently modified by multiple processes, although with a LOT of care :-)
FileWriter is an evil class as it picks up whatever character set happens to be there, rather than taking an explicit charset. Even if you do want the default, be explicit about it.
The usual solution is OutputStreamWriter and FileOutputStream. It is possible for the decorator to throw an exception. Therefore you need to be able to close the stream even if the writer was never constructed. If you are going to do that, you only need to flush the writer (in the happy case) and always close the stream. (Just to be confusing, some decorators, for instance for handling zips, have resources that do require closing.)
Another usecase for flushing in program is writing progress of longrunning job into file (so it can be stopped and restarted later. You want to be sure that data is safe on the drive.
while (true) {
computeStuff();
progresss += 1;
out.write(String.format("%d", progress));
out.flush();
}
out.close();