I want to come up with a binary format for passing data between application instances in a form of POFs (Plain Old Files ;)).
Prerequisites:
should be cross-platform
information to be persisted includes a single POJO & arbitrary byte[]s (files actually, the POJO stores it's names in a String[])
only sequential access is required
should be a way to check data consistency
should be small and fast
should prevent an average user with archiver + notepad from modifying the data
Currently I'm using DeflaterOutputStream + OutputStreamWriter together with InflaterInputStream + InputStreamReader to save/restore objects serialized with XStream, one object per file. Readers/Writers use UTF8.
Now, need to extend this to support the previously described.
My idea of format:
{serialized to XML object}
{delimiter}
{String file name}{delimiter}{byte[] file data}
{delimiter}
{another String file name}{delimiter}{another byte[] file data}
...
{delimiter}
{delimiter}
{MD5 hash for the entire file}
Does this look sane?
What would you use for a delimiter and how would you determine it?
The right way to calculate MD5 in this case?
What would you suggest to read on the subject?
TIA.
It looks INsane.
why invent a new file format?
why try to prevent only stupid users from changing file?
why use a binary format ( hard to compress ) ?
why use a format that cannot be parsed while being received? (receiver has to receive entire file before being able to act on the file. )
XML is already a serialization format that is compressable. So you are serializing a serialized format.
Would serialization of the model (if you are into MVC) not be another way? I'd prefer to use things in the language (or standard libraries) rather then roll my own if possible. The only issue I can see with that is that the file size may be larger than you want.
1) Does this look sane?
It looks fairly sane. However, if you are going to invent your own format rather than just using Java serialization then you should have a good reason. Do you have any good reasons (they do exist in some cases)? One of the standard reasons for using XStream is to make the result human readable, which a binary format immediately loses. Do you have a good reason for a binary format rather than a human readable one? See this question for why human readable is good (and bad).
Wouldn't it be easier just to put everything in a signed jar. There are already standard Java libraries and tools to do this, and you get compression and verification provided.
2) What would you use for a delimiter and how determine it?
Rather than a delimiter I'd explicitly store the length of each block before the block. It's just as easy, and prevents you having to escape the delimiter if it comes up on its own.
3) The right way to calculate MD5 in this case?
There is example code here which looks sensible.
4) What would you suggest to read on the subject?
On the subject of serialization? I'd read about the Java serialization, JSON, and XStream serialization so I understood the pros and cons of each, especially the benefits of human readable files. I'd also look at a classic file format, for example from Microsoft, to understand possible design decisions from back in the days that every byte mattered, and how these have been extended. For example: The WAV file format.
Let's see this should be pretty straightforward.
Prerequisites:
0. should be cross-platform
1. information to be persisted includes a single POJO & arbitrary byte[]s (files actually, the POJO stores it's names in a String[])
2. only sequential access is required
3. should be a way to check data consistency
4. should be small and fast
5. should prevent an average user with archiver + notepad from modifying the data
Well guess what, you pretty much have it already, it's built-in the platform already:Object Serialization
If you need to reduce the amount of data sent in the wire and provide a custom serialization ( for instance you can sent only 1,2,3 for a given object without using the attribute name or nothing similar, and read them in the same sequence, ) you can use this somehow "Hidden feature"
If you really need it in "text plain" you can also encode it, it takes almost the same amount of bytes.
For instance this bean:
import java.io.*;
public class SimpleBean implements Serializable {
private String website = "http://stackoverflow.com";
public String toString() {
return website;
}
}
Could be represented like this:
rO0ABXNyAApTaW1wbGVCZWFuPB4W2ZRCqRICAAFMAAd3ZWJzaXRldAASTGphdmEvbGFuZy9TdHJpbmc7eHB0ABhodHRwOi8vc3RhY2tvdmVyZmxvdy5jb20=
See this answer
Additionally, if you need a sounded protocol you can also check to Protobuf, Google's internal exchange format.
You could use a zip (rar / 7z / tar.gz / ...) library. Many exists, most are well tested and it'll likely save you some time.
Possibly not as much fun though.
I agree in that it doesn't really sound like you need a new format, or a binary one.
If you truly want a binary format, why not consider one of these first:
Binary XML (fast infoset, Bnux)
Hessian
google packet buffers
But besides that, many textual formats should work just fine (or perhaps better) too; easier to debug, extensive tool support, compresses to about same size as binary (binary compresses poorly, and information theory suggests that for same effective information, same compression rate is achieved -- and this has been true in my testing).
So perhaps also consider:
Json works well; binary support via base64 (with, say, http://jackson.codehaus.org/)
XML not too bad either; efficient streaming parsers, some with base64 support (http://woodstox.codehaus.org/, "typed access API" under 'org.codehaus.stax2.typed.TypedXMLStreamReader').
So it kind of sounds like you just want to build something of your own. Nothing wrong with that, as a hobby, but if so you need to consider it as such.
It likely is not a requirement for the system you are building.
Perhaps you could explain how this is better than using an existing file format such as JAR.
Most standard files formats of this type just use CRC as its faster to calculate. MD5 is more appropriate if you want to prevent deliberate modification.
Bencode could be the way to go.
Here's an excellent implementation by Daniel Spiewak.
Unfortunately, bencode spec doesn't support utf8 which is a showstopper for me.
Might come to this later but currently xml seems like a better choice (with blobs serialized as a Map).
Related
I have a file that contains a serialized HashMap containing an element of type MyObject:
�� sr java.util.HashMap���`� F
loadFactorI thresholdxp?# w t (a54d88e06612d820bc3be72877c74f257b561b19sr com.myproject.MyObject C�m�I�/ I partitionL hashcodet Ljava/lang/String;L idt Ljava/lang/Long;L offsetq ~ L timestampq ~ L topicq ~ xp q ~ ppppx
Now, I also have some other MyObject objects that I would like to add to that map. However, I dont want to first read and deserialize the map back into memory, then update it and then write the whole updated map back to file. How would one update the serialization in the file in a more efficient way?
How would one update the serialization in the file in a more efficient way?
Basically by reverse engineering the binary protocol that Java uses when serializing objects into their binary representation. That would enable you to understand which elements in that binary blob would need to be updated in which way.
Other people have already done that, see here for example.
Anything else is just work. You sitting down and writing code.
Or you write the few lines of code that read in the existing files, and write out a new file with that map plus the other object you need in there.
You see, efficiency depends on the point of view:
do you think the single update of a file with binary serialized objects is so time critical that it needs to be done by manually "patching" that binary file
do you think it is more efficient to spend hours and hours to learn the underlying binary format, to correctly update its content?
The only valid reason (I can think of) why to do that: to learn exactly such things: binary data formats, and how to patch content. But even then there might be "better" assignments that give you more insights (of real value in the real world) than ... spending your time re-implementing Java binary serialization.
From the homepage:
ANTLR (ANother Tool for Language Recognition) is a powerful parser generator for [...] or binary files.
I have read through the docs now for some hours and think that I have some basic understanding of ANTLR, but I have a hard time to find any references to processing binary files. And I'm not the only one as it seems.
I need to create a parser for some binary data and would like to decide if ANTLR is of any help or not.
Binary data structure
That binary data is structured in logical fields like field1, which is followed by field2, which is followed by field3 etc. and all those fields have a special purpose. The length of all those fields may differ AND may not be known at the time the parser is generated, so e.g. I do know that field1 is e.g. 4 bytes always, field2 might simply be 1 byte and field3 might be 1 to 10 bytes and might be followed by additional field3s with n bytes, depending on the actual value of the data. That is the second problem, I know the fields are there and e.g. with field1 I know it's 4 bytes, but I don't know the actual value, but that is what I'm interested in. Same goes for the other fields, I need the values from all of those.
What I need in ANTLR
This sounds like a common structure and use case for some arbitrary binary data to me, but I don't see any special handling of such data in ANTLR. All examples are using some kind of texts and I don't see some value extraction callbacks or such. Additionally, I think I would need some callbacks influencing the parsing process itself, so for e.g. one callback is called on the first byte of field3, I check that, decide that one to N additional bytes need to be consumed and that those are logically part of field3 and tell the parser that, so it's able to proceed "somehow".
In the end, I would get some higher level "field" objects and ANTLR would provide the underlying parse logic with callbacks and listener infrastructure, walking abilities etc.
Did anyone ever do something like that and can provide some hints to examples or the concrete documentation I seem to have missed? Thanks!
EN 13757-3:2012
I don't think it makes understanding my question really easier, but the binary data I'm referring to is defined in the standard EN 13757-3:2012:
Communication systems for and remote reading of meters - Part
3: Dedicated application layer
The standard is not freely available on the net (anymore?), but the following PDF might provide you an overview of how example data looks like in page 4. Especially that bytes of the mentioned fields are not constant, only the overall structure of the datagram is defined.
http://fastforward.ag/downloads/docu/FAST_EnergyCam-Protocol-wirelessMBUS.pdf
The tokens for the grammar would be the fields, implemented by a different amount of bytes, but with a value etc. Regarding the self-description of ANTLR, I would expected such things to work somehow...
Alternative: Kaitai.io
Whoever is in a comparable position like me currently, have a look at Kaitai.io, which reads very promising:
https://stackoverflow.com/a/40527106/2055163
The SolrJ library offers different parsers for Solr's responses.
Namely:
BinaryResponseParser
StreamingBinaryResponseParser
NoOpResponseParser
XMLResponseParser
Sadly the documentation doesn't say much about them, other than:
SolrJ uses a binary format, rather than XML, as its default format.
Users of earlier Solr releases who wish to continue working with XML
must explicitly set the parser to the XMLResponseParser, like so:
server.setParser(new XMLResponseParser());
So it looks like the XMLResponseParser is there mainly for legacy purposes.
What are the differences between the others parsers?
Can I expect performance improvements by using an other parser over the XMLResponseParser?
The Binary Stream Parsers is meant to work directly with the Java Object Format (the binary POJO format) to make the creation of data objects as smooth as possible on the client side.
The XML parser was designed to work with the old response format where there wasn't any real alternatives (as there was no binary response format in Solr). It's a lot more work to consider all the options for an XML format than use the binary format directly.
The StreamingBinaryResponseParser does the same work as the BinaryResponseParser, but has been designed to make streaming documents (i.e. not creating a list of documents and returning that list, but instead return each document by itself without having to hold them all in memory at the same time) possible. See SOLR-2112 for a description of the feature and why it was added.
Lastly, yes, if you're using SolrJ, use the binary response format, unless you have a very good reason for using the XML based one. If you have to ask the question, you're probably better off with the binary format.
I have some Java's serialized objects (arrays of doubles) in MySql database fields that I generated previously. Now I needed to read them from Python and I have just realized that it is probably not possible to do directly.
Then I tried to convert them to strings in java (simply comma delimited), and manually parse them from Python. But, it turned out that parsing from Python works painfully slow this way. Is there any better way for serializing arrays that is compatible between Java and Python?
Edit: Sorry, my parsing code was the problem, of course. I replaced it with this:
stringList = string.split(', ')
svdVector = [float(x) for x in stringList]
..and now it is almost instant for my case of 1000x1000 doubles. Although it still feels wrong to store doubles as strings instead of binary, but since it's easy to code and runs fast enough, it is fine.
Python comes with modules for CSV files, XML, and JSON, so one of those would likely do the trick quite well.
If you really want to try binary serialization, check out the built-in struct and array modules for help with interpreting the data in Python.
At the server end (GAE), I've got a java Hashtable.
At the client end (iPhone), I'm trying to create an NSDictionary.
myHashTable.toString() gets me something that looks darned-close-to-but-not-quite-the-same-as [myDictionary description]. If they were the same, I could write the string to a file and do:
NSDictionary *dict = [NSDictionary dictionaryWithContentsOfFile:tmpFile];
I could write a little parser in obj-C to deal with myHashtable.toString(), but I'm sort-of hoping that there's a shortcut already built into something, somewhere -- I just can't seem to find it.
(So, being a geek, I'll spend far longer searching the web for a shortcut than it would take me to write & debug the parser... ;)
Anyway -- hints?
Thanks!
I would convert the Hashtable into something JSON-like and take it on the iPhone side.
Hashtable.toString() is not ideal, it will have problem with spaces, comma and quotation marks.
For JSON-to-NSDictionary, you can find the json-framework tools under http://www.json.org/
As j-16 SDiZ mentioned, you need to serialize your hashtable. It can be to json, xml or some other format. Once serialized, you need to deserialize them into an NSDictionary. JSON is probably the easiest format to do this with plenty of libraries for both Objective-C and Java. http://json.org has a list of libraries.