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Java Forum / General / November 2006How to parse and manipulate a binary stream
Apologies in advance if my question is silly or trivial. I'm trying to write a servlet that reads data from another source in byte[] form and, having parsed this data stream and made a couple of modifications, sends the modified data to an appropriate application whereby it can be rendered in Excel or PDF format. What I need to find out is how I can parse the incoming data, detect certain string patterns in the data, and manipulate that information to generate a new data stream. TIA, tc > Apologies in advance if my question is silly or trivial. I'm trying to > write a servlet that reads data from another source in byte[] form and, [quoted text clipped - 5 lines] > certain string patterns in the data, and manipulate that information to > generate a new data stream. Not a trivial question, exactly, but a bit on the vague side. It's difficult for me to tell what you're having difficultly with. I'm going to assume that you're stuck getting started. First, you need to read binary data from a source. That's exactly the job of an InputStream. It has a read() method that lets to read directly into a byte array. To parse that a portion of a byte[] as a String, you can just use the constructors in the String class. To write the output, you need an OutputStream. You may want to subclass it, and write a write() that asks for its next byte of output from the object responsible for doing the search-and-replace manipulation.  SignatureMark Jeffcoat Austin, TX > Not a trivial question, exactly, but a bit on the > vague side. It's difficult for me to tell what you're > having difficultly with. Sorry, my question *was* very vaguely worded. > I'm going to assume that you're stuck getting started. > [quoted text clipped - 9 lines] > byte of output from the object responsible for doing the > search-and-replace manipulation. My problem is that the conversion of the input stream into character/String data doesn't give me anything meaningful - not enough to parse and manipulate, at any rate. I suppose what I'm wondering is whether there's any reference material that describes how an encoded input stream of data (be it for Excel or PDF) can be "translated" into a String representation in order to do basic String manipulations, and then re-encoded and passed on to the next application. <snip> >> To parse that a portion of a byte[] as a String, you can >> just use the constructors in the String class. [quoted text clipped - 11 lines] > a String representation in order to do basic String manipulations, and > then re-encoded and passed on to the next application. Source data like Excel and GIFs don't have any natural string equivalent and cannot be "parsed" in the sense of parsing strings. PDF is largely text but may have some segments in binary--I don't know offhand how the binary parts work. To "parse" true binary files you have to know the file structure. You can go to http://www.wotsit.org/ to get information on file format. Matt Humphrey matth@ivizNOSPAM.com http://www.iviz.com/ > My problem is that the conversion of the input stream into > character/String data doesn't give me anything meaningful - not enough [quoted text clipped - 3 lines] > a String representation in order to do basic String manipulations, and > then re-encoded and passed on to the next application. Yeah, okay. I gave you a strategy that will work if you've got some Strings in an encoding you already understand surrounded by other miscellaneous bytes that you can ignore; if that's not the case (which it surely can be, if the binary format is trying to be clever with how it stores text), you have a harder problem. The first thing I'd do is run the Unix program "strings" (which you can surely find for Windows, if you have to) on some of the files you're interested in, and see if you're in the happy case. (It's sounds like you've already done something like that, but a quick second opinion won't hurt.) If not, you'll have to handle each format you want to parse separately. I really like the POI library for handling Excel documents in Java. http://jakarta.apache.org/poi/ There is surely something similar for PDF, but I've never had the need of it; your Google will be as good as mine. SignatureMark Jeffcoat Austin, TX > Yeah, okay. I gave you a strategy that will work if you've > got some Strings in an encoding you already understand surrounded > by other miscellaneous bytes that you can ignore; if that's > not the case (which it surely can be, if the binary format > is trying to be clever with how it stores text), you have > a harder problem. I figured out what the problem with the PDF data was. The binary stream that I read in gives me PDF data in compressed form, which I discovered after running a few tests on it. I downloaded a free tool, pdftk, to help me uncompress the source data stream, perform my text manipulations, and then recompress the modified data before passing them on. Thanks for your help, guys! I really appreciate it. - tc Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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