1 package simpledb.file;
2
3 import static simpledb.file.Page.BLOCK_SIZE;
4 import java.io.*;
5 import java.nio.ByteBuffer;
6 import java.nio.channels.FileChannel;
7 import java.util.*;
8
9 /**
10 * The SimpleDB file manager.
11 * The database system stores its data as files within a specified directory.
12 * The file manager provides methods for reading the contents of
13 * a file block to a Java byte buffer,
14 * writing the contents of a byte buffer to a file block,
15 * and appending the contents of a byte buffer to the end of a file.
16 * These methods are called exclusively by the class {@link simpledb.file.Page Page},
17 * and are thus package-private.
18 * The class also contains two public methods:
19 * Method {@link #isNew() isNew} is called during system initialization by {@link simpledb.server.SimpleDB#init}.
20 * Method {@link #size(String) size} is called by the log manager and transaction manager to
21 * determine the end of the file.
22 * @author Edward Sciore
23 */
24 public class FileMgr {
25 private File dbDirectory;
26 private boolean isNew;
27 private Map<String,FileChannel> openFiles = new HashMap<String,FileChannel>();
28
29 /**
30 * Creates a file manager for the specified database.
31 * The database will be stored in a folder of that name
32 * in the user's home directory.
33 * If the folder does not exist, then a folder containing
34 * an empty database is created automatically.
35 * Files for all temporary tables (i.e. tables beginning with "temp") are deleted.
36 * @param dbname the name of the directory that holds the database
37 */
38 public FileMgr(String dbname) {
39 String homedir = System.getProperty("user.home");
40 dbDirectory = new File(homedir, dbname);
41 isNew = !dbDirectory.exists();
42
43 // create the directory if the database is new
44 if (isNew && !dbDirectory.mkdir())
45 throw new RuntimeException("cannot create " + dbname);
46
47 // remove any leftover temporary tables
48 for (String filename : dbDirectory.list())
49 if (filename.startsWith("temp"))
50 new File(dbDirectory, filename).delete();
51 }
52
53 /**
54 * Reads the contents of a disk block into a bytebuffer.
55 * @param blk a reference to a disk block
56 * @param bb the bytebuffer
57 */
58 synchronized void read(Block blk, ByteBuffer bb) {
59 try {
60 bb.clear();
61 FileChannel fc = getFile(blk.fileName());
62 fc.read(bb, blk.number() * BLOCK_SIZE);
63 }
64 catch (IOException e) {
65 throw new RuntimeException("cannot read block " + blk);
66 }
67 }
68
69 /**
70 * Writes the contents of a bytebuffer into a disk block.
71 * @param blk a reference to a disk block
72 * @param bb the bytebuffer
73 */
74 synchronized void write(Block blk, ByteBuffer bb) {
75 try {
76 bb.rewind();
77 FileChannel fc = getFile(blk.fileName());
78 fc.write(bb, blk.number() * BLOCK_SIZE);
79 }
80 catch (IOException e) {
81 throw new RuntimeException("cannot write block" + blk);
82 }
83 }
84
85 /**
86 * Appends the contents of a bytebuffer to the end
87 * of the specified file.
88 * @param filename the name of the file
89 * @param bb the bytebuffer
90 * @return a reference to the newly-created block.
91 */
92 synchronized Block append(String filename, ByteBuffer bb) {
93 int newblknum = size(filename);
94 Block blk = new Block(filename, newblknum);
95 write(blk, bb);
96 return blk;
97 }
98
99 /**
100 * Returns the number of blocks in the specified file.
101 * @param filename the name of the file
102 * @return the number of blocks in the file
103 */
104 public synchronized int size(String filename) {
105 try {
106 FileChannel fc = getFile(filename);
107 return (int)(fc.size() / BLOCK_SIZE);
108 }
109 catch (IOException e) {
110 throw new RuntimeException("cannot access " + filename);
111 }
112 }
113
114 /**
115 * Returns a boolean indicating whether the file manager
116 * had to create a new database directory.
117 * @return true if the database is new
118 */
119 public boolean isNew() {
120 return isNew;
121 }
122
123 /**
124 * Returns the file channel for the specified filename.
125 * The file channel is stored in a map keyed on the filename.
126 * If the file is not open, then it is opened and the file channel
127 * is added to the map.
128 * @param filename the specified filename
129 * @return the file channel associated with the open file.
130 * @throws IOException
131 */
132 private FileChannel getFile(String filename) throws IOException {
133 FileChannel fc = openFiles.get(filename);
134 if (fc == null) {
135 File dbTable = new File(dbDirectory, filename);
136 RandomAccessFile f = new RandomAccessFile(dbTable, "rws");
137 fc = f.getChannel();
138 openFiles.put(filename, fc);
139 }
140 return fc;
141 }
142 }