I would like to update specific part of a text file using Java. I would like to be able to scan through the file and select specific lines to be updated, a bit like in a database, for instance given the file:
ID Value
1 100
2 500
4 20
I would like to insert 3 and update 4, e.g.
ID Value
1 100
2 500
3 80
4 1000
Is there a way to achieve this (seemingly) easy task? I know you can append to a file, but I am more interested in a random access
I know you can append to a file, but I am more interested in a random access
You're trying to insert and delete bytes in the middle of a file. You can't do that. File systems simply don't (in general) support that. You can overwrite specific bytes, but you can't insert or delete them.
You could update specific records with random access if your records were fixed-length (in bytes) but it looks like that's not the case.
You could either load the whole file into memory, or read from the original file, writing to a new file with either the old data or the new data as appropriate on a per line basis.
You can do so using Random Access files in java where you can place your current write and read position using available methods. you can explore more here
Load the file into memory, change your value, and then re-write the file
if there's a way to insert into a file without loading it, I haven't heard of it. You have to move the other data out of the way first.
unless you're dealing with huge files, frequently, performance isn't too much of a concern
As said by the previous answers, it's not possible to do that symply using streams. You could try to use properties, that are key, value pairs that can be saved and modified in a text file.
For example you can add to a file a new property with the command
setProperty(String key, String value)
This method adds a new property or, if already existing, modifies the value of the property with the choosen key.
Obviously, new properties are added at the end of the file but the lack of ordering is not a problem for performances because the access to the file is made with the getProperty method that calls the Hashtable method put.
See this tutorial for some examples:
http://docs.oracle.com/javase/tutorial/essential/environment/properties.html
Related
I want to save severals files using Stream to serialize objects, with one serializable object per file.
The reason why I want have one file per object is that I have a list of these objects, I am on Android and at the start of the applicaiton I want load all saved objects, this is the easy part.
But during the execution I want add new elements in this list, and delete elements. And I want update the folder with my files in the same time.
So I supposed I have to create one file per object. But may be there is another solution ?
The main problem is : how name each file ? To avoid overwrite a file, ...
I first thinked that I will have to name my files depending the name of a String given by the user, but I will have to check if the name file is valid,...
So may be the solution is to just name my files with an integer, and at the first load, I just continue the counter from the higher file ?
I suppose my problem is frequent, so what are your solutions to write during the execution a dynamic list of objects ?
Thanks you
I would like to update a .csv file containing 1 million records. I want to update in the same file without creating another file. I am using Opencsv. I don't find any way to update the .csv file. If anyone can help me out that would be great.
For clarification let's say these small .csv file:
Initial csv file:
a,b,c,d
1,2,3,4
5,6,7,8
12,13,14,15
Desired csv file:
a,b,c,d,e,f
1,2,3,4, ,17
5,6,7,8,16,
12,13,14,15,
Basically - you cannot do that.
As csv does not use fixed-length records, most changes will require moving data up or down in the file. That cannot be done without completely rewriting the file.
1,2,3,4
5,6,7,8
...
changing the 8 to a 10 would require every byte from that location onwards in the whole file to be moved up one position.
To achieve the effect of editing the file it is usual to copy the file to a new name making the change you wish to make during the copy. You would then rename the two files so that the new one replaces the old one.
Normally file system doesnt allow to update the file. You need to rewrite the file. Because even though you have all in one file . it will be written in multiple segments. So if you are going to change the file segments will be damaged. So you need to rewrite every thing.
i have a binary file with following format :
[N bytes identifier & record length] [n1 bytes data]
[N bytes identifier & record length] [n2 bytes data]
[N bytes identifier & record length] [n3 bytes data]
as you see i have records with different lengths. in each record i have N bytes fixed which contains and id and the length of data in record.
this file is very big and can contains 3 millions records.
I want to open this file by an application and let user to browse and edit the records.
( Insert / Update / Delete records)
my initial plan is to create and index file from original file and for each record, keep next and previous record address to navigate forward and backward easily. (some sort of linked list but in file not in memory)
is there library (java library) to help me to implement this requirement ?
any recommendation or experience that you think is useful?
----------------- EDIT ----------------------------------------------
Thanks for guides and suggestions,
some more info:
the original file and its format is out of my control (it's a third party file) and i can't change the file format. but i have to read it, let user to navigate over records and edit some of them (insert new record/ update an existing record/ delete a record) and at the end save it back to original file format.
do u still recommend DataBase instead of a normal index file ?
----------------- SECOND EDIT ----------------------------------------------
record size in update mode is fixed. it means updated (edited) record has same length as original record's, unless user delete the record and create another record with different format.
Many Thanks
Seriously, you should NOT be using a binary file for this. You should use a database.
The problems with trying to implement this as a regular file stem from the fact that operating systems do not allow you to insert extra bytes into the middle of an existing file. So if you need to insert a record (anywhere but the end), update a record (with a different size) or remove a record, you would need to:
rewrite other records (after the insertion/update/deletion point) to make or reclaim space, or
implement some kind of free space management within the file.
All of this is complicated and / or expensive.
Fortunately, there is a class of software that implements this kind of thing. It is called database software. There are a wide range of options, ranging from using a full-scale RDBMS to light-weight solutions like BerkeleyDB files.
In response to your 1st and 2nd edits, a database will still be simpler.
However, here's an alternative that might perform better for this use-case than using a DB... without doing complicated free-space management.
Read the file and build an in-memory index that maps ids to file locations.
Create a second file to hold new and updated records.
Perform the record adds/updates/deletes:
An addition is handled by writing the new record to the end of the second file, and adding an index entry for it.
An update is handled by writing the updated record to the end of the second file, and changing the existing index entry to point to it.
A delete is handled by deleting the index entry for the record's key.
Compact the file as follows:
Create a new file.
Read each record in the old file in order, and check the index for the record's key. If the entry still points to the location of the record, copy the record to the new file. Otherwise skip it.
Repeat the step 4.2 for the second file.
If we completed all of the above successfully, delete the old file and second file.
Note this relies on being able to keep the index in memory. If that is not feasible, then the implementation is going to be more complicated ... and more like a database.
Having a data file and an index file would be the general base idea for such an implementation, but you'd pretty much find yourself dealing with data fragmentation upon repeated data updates/deletion, etc. This kind of project, in itself, should be a separate project and should not be part of your main application. However, essentially, a database is what you need as it is specifically designed for such operations and use cases and will also allow you to search, sort, and extend (alter) your data structure without having to refactor an in-house (custom) solution.
May I suggest you to download Apache Derby and create a local embedded database (derby does it for you want you create a new embedded connection at run-time). It will not only be faster than anything you'll write yourself, but will make your application easier to maintain.
Apache Derby is a single jar file that you can simply include and distribute with your project (check the license if any legal issue may apply in your app). There is no need for a database server or third party software; it's all pure Java.
Bottom line as that it all depends on how large is your application, if you need to share the data across many clients, if speed is a critical aspect of your app, etc.
For a stand-alone, single user project, I recommend Apache Derby. For a n-tier application, you might want to look into MySQL, PostgreSQL or (hrm) even Oracle. Using already made and tested solutions is not only smart, but will cut down your development time (and maintenance efforts).
Cheers.
Generally you are better off letting a library or database do the work for you.
You may not want to have an SQL database and there are plenty of simple databases which don't use SQL. http://nosql-database.org/ lists 122 of them.
At a minimum, if you are going to write this I suggest you read the source for one of these databases to see how they work.
Depending on the size of the records, 3 million isn't that much and I would suggest you keep as much in memory as possible.
The problem you are likely to have is ensuring the data is consistent and recovering the data when a corruption occurs. The second problem is dealing with fragmentation efficiently (some thing the brightest minds working on the GC deal with) The third problem is likely to be maintain the index in a transaction fashion with the source data to ensure there are no inconsistencies.
While this may appear simple at first, there are significant complexities in making sure there data is reliable, maintainable and can be accessed efficiently. This is why most developers use an existing database/datastore library and concentrate on the features which are unqiue to their application.
(Note: My answer is about the problem in general, not considering any Java libraries or - like the other answers also proposed - using a database (library), which might be better than reinventing the wheel)
The idea to create an index is good and will be very helpful performance-wise (although you wrote "index file", I think it should be kept in memory). Generating the index should be quite fast if you read the ID and record length for each entry and then just skip the data with a file seek.
You should also think about the edit functionality. Especially inserting and deleting can be very slow on such a big file if you do it wrong (f.e. deleting and then moving all the following entries to close the gap).
The best option would be to only mark deleted entries as deleted. When inserting, you can overwrite one of those or append to the end of the file.
Insert / Update / Delete records
Inserting (rather than merely appending) and deleting records to a file is expensive because you have to move all the following content of the file to create space for the new record or to remove the space it used. Updating is similarly expensive if the update changes the length of the record (you say they are variable length).
The file format you propose is fundamentally unsuitable for the kinds of operations you want to perform. Others have suggested using a data-base. If you don't want to go that far, adding an index file (as you suggest) is the way to go. I recommend making the index records all the same length.
As others have stated a database would seem a better solution. The following are Java SQL DB's that could be used: H2, Derby or HSQLDB
If you want to use an index file look at Berkley DB or No Sql
If there is some reason for using a file, look at JRecord . It has
Several Classes for reading/writing files with variable length binary records (they where written for Cobol VB files). Any of Mainframe / Fujitsu / Open Cobol VB file structures should do the job.
An Editor for editing JRecord files. The latest version of the Editor can handle large files (it uses Compression / spill file). The editor suffers from having to download the whole file and only one user can edit the file at one time.
The JRecord solution will only work if
There is a limited number (preferably one) users all located in the one location
Fast infostructure
I have a relatively strange question.
I have a file that is 6 gigabytes long. What I need to do, is scan the entire file, line by line, and determine all rows that match an id number of any other row in the file. Essentially, its like analyzing a web log file where there are many session ids that are organized by the time of each click rather than by userID.
I tried to do the simple (dumb) thing, which was to create 2 file readers. One that scans the file line by line getting the userID, and the next to 1. verify that the userID has not been processed already and 2. If it hasn't been processed, read every line that begins with the userID that is contained in the file and store (some value X, related to the rows)
Any advice or tips on how I can make this process work more efficiently?
Import file into SQL database
Use SQL
Performance!
Seriously, that's it. Databases are optimized exactly for this kind of thing. Alternatively, if you have a machine with enough RAM, just put all the data into a HashMap for easy lookup.
Easiest: create a datamodel and import the file in a database and take benefit of JDBC and SQL powers. You can if necessary (when the file format is pretty specific) write a some Java which does import line by line with help of under each BufferedReader#readLine() and PreparedStatement#addBatch().
Hardest: write your Java code so that it doesn't unnecessarily keep large amounts of data in the memory. You're then basically reinventing what the average database already does.
For each row R in the file {
Let N be the number that you need to
extract from R.
Check if there is a file called N. If
not, create it.
Append R to the file called N
}
How much data are you storing about each line, compared with the size of the line? Do you have enough memory to maintain the state for each distinct ID (e.g. number of log lines seen, number of exceptions or whatever)? That's what I'd do if possible.
Otherwise, you'll either need to break the log file into separate chunks (e.g. split it based on the first character of the ID) and then parse each file separately, or perhaps have some way of pretending you have enough memory to maintain the state for each distinct ID: have an in-memory cache which dumps values to disk (or reads them back) only when it has to.
You don't mention whether or not this is a regular, ongoing thing or an occasional check.
Have you considered pre-processing the data? Not practical for dynamic data, but if you can sort it based on the field you're interested in, it makes solving the problem much easier. Extracting only the fields you care about may reduce the data volume to a more manageable size as well.
Alot of the other advice here is good but assumes that you'll be able to load what you need into memory without running out of memory. If you can do that that would be better than the 'worst case' solution I'm mentioning.
If you have large files you may end up needing to sort them first. In the past I've dealt with multiple large files where I needed to match them up based on a key (sometimes matches were in all files, sometimes only in a couple, etc). If this is the case the first thing you need to do is sort your files. Hopefully you're on a box where you can easily do this (for example there are many good Unix scripts for this). After you've sorted each file read each file until you get matching IDs then process.
I'd suggest:
1. Open both files and read the first record
2. See if you have matching IDs and processing accordingly
3. Read the file(s) for the key just processed and do step 2 again until EOF.
For example if you had a key of 1,2,5,8 in FILE1 and 2,3,5,9 in FILE2 you'd:
1. Open and read both files (FILE1 has ID 1, FILE2 had ID2).
2. Process 1.
3. Read FILE1 (FILE1 has ID 2)
4. Process 2.
5. Read FILE1 (ID 5) and FILE2 (ID 3)
6. Process 3.
7. Read FILE 2 (ID 5)
8. Process 5.
9. Read FILE1 (ID 8) and FILE2 (ID 9).
10. Process 8.
11. Read FILE1 (EOF....no more FILE1 processing).
12. Process 9.
13. Read FILE2 (EOF....no more FILE2 processing).
Make sense?
I have some 1000 key-pair values that I will use in my j2me application, reading it from a resource file. However I will be using only a few of those values at any time, say 10, based on the record number being generated inside the application logic. Loading all the values into memory and then looking up is fairly not an efficient option as I will not be using all the records. Is there a better scheme to store the values in the file, some indexing or something so that I can retrieve those key-pair values by skipping the amount of bytes in the file to reach and read the appropriate record? As this is a resource file in the jar there wont be any modifications to it.
If you know the record length when they are created, you could write the records out in binary format to a file. But at the start of each record, you could first write a number indicating its size in bytes and use a RandomAccessFile to access the records by moving the file pointer.
But in terms of speed, loading into memory will be faster than reading from a file, but if memory is at a premium, then a file wouldn't be a bad way to go.
Jeff
Skipping bytes in a compressed resource file inside a jar is not really going to be optimal either and the implementation of InputStream you get as a result of calling Class.getResourceAsInputStream() may be fragmented if you plan on running your application on several devices.
EDIT after additional info in comment:
It could be that the best way to do this is actually to store the (question, answer) data in 1000 different classes.
It's going to feel very weird as a solution but the class loader should only load the 10 classes you actually use, you can generate the 1000 source files with a simple J2SE program and you can load 10 random classes based on an integer inside their name using java.lang.Class.forName().
If the jar file doesn't become too big to use, you're basically relying on the indexing of its zip file format for the class loader performances...