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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"><!--NewPage--><HTML><HEAD><!-- Generated by javadoc (build 1.5.0_07) on Sun May 06 17:59:51 GMT 2007 --><TITLE>DefaultBlockFileSystem (Heritrix 1.12.1)</TITLE><META NAME="keywords" CONTENT="org.archive.util.ms.DefaultBlockFileSystem class"><LINK REL ="stylesheet" TYPE="text/css" HREF="../../../../stylesheet.css" TITLE="Style"><SCRIPT type="text/javascript">function windowTitle(){ parent.document.title="DefaultBlockFileSystem (Heritrix 1.12.1)";}</SCRIPT><NOSCRIPT></NOSCRIPT></HEAD><BODY BGCOLOR="white" onload="windowTitle();"><!-- ========= START OF TOP NAVBAR ======= --><A NAME="navbar_top"><!-- --></A><A HREF="#skip-navbar_top" title="Skip navigation links"></A><TABLE BORDER="0" WIDTH="100%" CELLPADDING="1" CELLSPACING="0" SUMMARY=""><TR><TD COLSPAN=2 BGCOLOR="#EEEEFF" CLASS="NavBarCell1"><A NAME="navbar_top_firstrow"><!-- --></A><TABLE BORDER="0" CELLPADDING="0" CELLSPACING="3" SUMMARY=""> <TR ALIGN="center" VALIGN="top"> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../../../overview-summary.html"><FONT CLASS="NavBarFont1"><B>Overview</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="package-summary.html"><FONT CLASS="NavBarFont1"><B>Package</B></FONT></A> </TD> <TD BGCOLOR="#FFFFFF" CLASS="NavBarCell1Rev"> <FONT CLASS="NavBarFont1Rev"><B>Class</B></FONT> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="class-use/DefaultBlockFileSystem.html"><FONT CLASS="NavBarFont1"><B>Use</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="package-tree.html"><FONT CLASS="NavBarFont1"><B>Tree</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../../../deprecated-list.html"><FONT CLASS="NavBarFont1"><B>Deprecated</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../../../index-all.html"><FONT CLASS="NavBarFont1"><B>Index</B></FONT></A> </TD> <TD BGCOLOR="#EEEEFF" CLASS="NavBarCell1"> <A HREF="../../../../help-doc.html"><FONT CLASS="NavBarFont1"><B>Help</B></FONT></A> </TD> </TR></TABLE></TD><TD ALIGN="right" VALIGN="top" ROWSPAN=3><EM></EM></TD></TR><TR><TD BGCOLOR="white" CLASS="NavBarCell2"><FONT SIZE="-2"> <A HREF="../../../../org/archive/util/ms/Cp1252.html" title="class in org.archive.util.ms"><B>PREV CLASS</B></A> <A HREF="../../../../org/archive/util/ms/DefaultEntry.html" title="class in org.archive.util.ms"><B>NEXT CLASS</B></A></FONT></TD><TD BGCOLOR="white" CLASS="NavBarCell2"><FONT SIZE="-2"> <A HREF="../../../../index.html?org/archive/util/ms/DefaultBlockFileSystem.html" target="_top"><B>FRAMES</B></A> <A HREF="DefaultBlockFileSystem.html" target="_top"><B>NO FRAMES</B></A> <SCRIPT type="text/javascript"> <!-- if(window==top) { document.writeln('<A HREF="../../../../allclasses-noframe.html"><B>All Classes</B></A>'); } //--></SCRIPT><NOSCRIPT> <A HREF="../../../../allclasses-noframe.html"><B>All Classes</B></A></NOSCRIPT></FONT></TD></TR><TR><TD VALIGN="top" CLASS="NavBarCell3"><FONT SIZE="-2"> SUMMARY: NESTED | FIELD | <A HREF="#constructor_summary">CONSTR</A> | <A HREF="#method_summary">METHOD</A></FONT></TD><TD VALIGN="top" CLASS="NavBarCell3"><FONT SIZE="-2">DETAIL: FIELD | <A HREF="#constructor_detail">CONSTR</A> | <A HREF="#method_detail">METHOD</A></FONT></TD></TR></TABLE><A NAME="skip-navbar_top"></A><!-- ========= END OF TOP NAVBAR ========= --><HR><!-- ======== START OF CLASS DATA ======== --><H2><FONT SIZE="-1">org.archive.util.ms</FONT><BR>Class DefaultBlockFileSystem</H2><PRE>java.lang.Object <IMG SRC="../../../../resources/inherit.gif" ALT="extended by "><B>org.archive.util.ms.DefaultBlockFileSystem</B></PRE><DL><DT><B>All Implemented Interfaces:</B> <DD><A HREF="../../../../org/archive/util/ms/BlockFileSystem.html" title="interface in org.archive.util.ms">BlockFileSystem</A></DD></DL><HR><DL><DT><PRE>public class <B>DefaultBlockFileSystem</B><DT>extends java.lang.Object<DT>implements <A HREF="../../../../org/archive/util/ms/BlockFileSystem.html" title="interface in org.archive.util.ms">BlockFileSystem</A></DL></PRE><P>Default implementation of the Block File System. <p>The overall structure of a BlockFileSystem file (such as a .doc file) is as follows. The file is divided into blocks, which are of uniform length (512 bytes). The first block (at file pointer 0) is called the header block. It's used to look up other blocks in the file. <p>Subfiles contained within the .doc file are organized using a Block Allocation Table, or BAT. The BAT is basically a linked list; given a block number, the BAT will tell you the next block number. Note that the header block has no number; block #0 is the first block after the header. Thus, to convert a block number to a file pointer: <code>int filePointer = (blockNumber + 1) * BLOCK_SIZE</code>. <p>The BAT itself is discontinuous, however. To find the blocks that comprise the BAT, you have to look in the header block. The header block contains an array of 109 pointers to the blocks that comprise the BAT. If more than 109 BAT blocks are required (in other words, if the .doc file is larger than ~6 megabytes), then something called the XBAT comes into play. <p>XBAT blocks contain pointers to the 110th BAT block and beyond. The first XBAT block is stored at a file pointer listed in the header. The other XBAT blocks are always stored in order after the first; the XBAT table is continuous. One is inclined to wonder why the BAT itself is not so stored, but oh well. <p>The BAT only tells you the next block for a given block. To find the first block for a subfile, you have to look up that subfile's directory entry. Each directory entry is a 128 byte structure in the file, so four of them fit in a block. The number of the first block of the entry list is stored in the header. To find subsequent entry blocks, the BAT must be used. <p>I'm telling you all this so that you understand the caching that this class provides. <p>First, directory entries are not cached. It's assumed that they will be looked up at the beginning of a lengthy operation, and then forgotten about. This is certainly the case for <CODE>Doc#getText(BlockFileSystem)</CODE>. If you need to remember directory entries, you can manually store the Entry objects in a map or something, as they don't grow stale. <p>This class keeps all 512 bytes of the header block in memory at all times. This prevents a potentially expensive file pointer repositioning every time you're trying to figure out what comes next. <p>BAT and XBAT blocks are stored in a least-recently used cache. The <i>n</i> most recent BAT and XBAT blocks are remembered, where <i>n</i> is set at construction time. The minimum value of <i>n</i> is 1. For small files, this can prevent file pointer repositioning for BAT look ups. <p>The BAT/XBAT cache only takes up memory as needed. If the specified cache size is 100 blocks, but the file only has 4 BAT blocks, then only 2048 bytes will be used by the cache. <p>Note this class only caches BAT and XBAT blocks. It does not cache the blocks that actually make up a subfile's contents. It is assumed that those blocks will only be accessed once per operation (again, this is what {Doc.getText(BlockFileSystem)} typically requires.)<P><P><DL><DT><B>Author:</B></DT> <DD>pjack</DD><DT><B>See Also:</B><DD><CODE>http://jakarta.apache.org/poi/poifs/fileformat.html</CODE></DL><HR><P><!-- =========== FIELD SUMMARY =========== --><A NAME="field_summary"><!-- --></A><TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""><TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"><TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2"><B>Field Summary</B></FONT></TH></TR></TABLE> <A NAME="fields_inherited_from_class_org.archive.util.ms.BlockFileSystem"><!-- --></A><TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""><TR BGCOLOR="#EEEEFF" CLASS="TableSubHeadingColor"><TH ALIGN="left"><B>Fields inherited from interface org.archive.util.ms.<A HREF="../../../../org/archive/util/ms/BlockFileSystem.html" title="interface in org.archive.util.ms">BlockFileSystem</A></B></TH></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><A HREF="../../../../org/archive/util/ms/BlockFileSystem.html#BLOCK_SIZE">BLOCK_SIZE</A></CODE></TD></TR></TABLE> <!-- ======== CONSTRUCTOR SUMMARY ======== --><A NAME="constructor_summary"><!-- --></A><TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""><TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"><TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2"><B>Constructor Summary</B></FONT></TH></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD><CODE><B><A HREF="../../../../org/archive/util/ms/DefaultBlockFileSystem.html#DefaultBlockFileSystem(org.archive.io.SeekInputStream, int)">DefaultBlockFileSystem</A></B>(<A HREF="../../../../org/archive/io/SeekInputStream.html" title="class in org.archive.io">SeekInputStream</A> input, int batCacheSize)</CODE><BR> Constructor.</TD></TR></TABLE> <!-- ========== METHOD SUMMARY =========== --><A NAME="method_summary"><!-- --></A><TABLE BORDER="1" WIDTH="100%" CELLPADDING="3" CELLSPACING="0" SUMMARY=""><TR BGCOLOR="#CCCCFF" CLASS="TableHeadingColor"><TH ALIGN="left" COLSPAN="2"><FONT SIZE="+2"><B>Method Summary</B></FONT></TH></TR><TR BGCOLOR="white" CLASS="TableRowColor"><TD ALIGN="right" VALIGN="top" WIDTH="1%"><FONT SIZE="-1"><CODE>(package private) <A HREF="../../../../org/archive/util/ms/Entry.html" title="interface in org.archive.util.ms">Entry</A></CODE></FONT></TD><TD><CODE><B><A HREF="../../../../org/archive/util/ms/DefaultBlockFileSystem.html#getEntry(int)">getEntry</A></B>(int entryNumber)</CODE><BR>?? 快捷鍵說明
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