simpledatabase.java

nanjing university cs 的java课件。 对新手很有用。付课件中源码。

import java.io.*;
import java.nio.channels.*;
import java.util.*;
public class SimpleDatabase{
	static public final int NUMSLOTS = 64;
	static public final int SLOTSIZE = 1024;
	private RandomAccessFile raf;
	private FileChannel fc;
	
	public SimpleDatabase( String filename ) throws IOException {
		File file = new File( filename );
		boolean exists = file.exists();
		raf = new RandomAccessFile( file, "rw" );
		fc = raf.getChannel();
		if (!exists) {
			byte b[] = new byte[SLOTSIZE];
			for (int i=0; i<NUMSLOTS; ++i)
				put( i, b );
		}
	}
	
	private FileLock getLock( int slot, boolean shared ) throws IOException {
		long position = slot*SLOTSIZE;
		long size = SLOTSIZE;
		FileLock lock = fc.lock( position, size, shared );
		return lock;
	}
	
	public void put( int slot, byte data[] ) throws IOException {
		FileLock fl = null;
		try {
			if (data.length != SLOTSIZE)
				throw new IllegalArgumentException( "Data wrong size: "+ data.length );
			fl = getLock( slot, false );
			synchronized( raf ) {
				raf.seek( slot*SLOTSIZE );
				raf.write( data, 0, SLOTSIZE/2 );
				Thread.yield();
				raf.seek( slot*SLOTSIZE+(SLOTSIZE/2) );
				raf.write( data, SLOTSIZE/2, SLOTSIZE/2 );
				raf.getFD().sync();
				Thread.yield();
			}
		}
		finally {
			fl.release();
		}
	}
	
	public void get( int slot, byte data[] ) throws IOException {
		FileLock fl = null;
		try {
			if (data.length != SLOTSIZE)
				throw new IllegalArgumentException( "Data wrong size: "+ data.length );
			fl = getLock( slot, true );
			synchronized( raf ) {
				raf.seek( slot*SLOTSIZE );
				raf.read( data );
			}
		} finally {
			fl.release();
		}
	}
	
	public void close() throws IOException {
		raf.close();
	}
	
	static public class SimpleDatabaseTester{
		private Random rand = new Random();
		private final int NUMSLOTS = SimpleDatabase.NUMSLOTS;
		private final int SLOTSIZE = SimpleDatabase.SLOTSIZE;
		private SimpleDatabase sd;
		
		public SimpleDatabaseTester( SimpleDatabase sd ) {
			this.sd = sd;
		}
		
		public void test() throws IOException {
			byte buffer[] = new byte[SLOTSIZE];
			int numOps = 0;
			
			while (true) {
				if (rand.nextInt( 100 )<50) {
					int slot = rand.nextInt( NUMSLOTS );
					generateConstantBuffer( buffer );
					sd.put( slot, buffer );
				}
				else {
					int slot = rand.nextInt( NUMSLOTS );
					sd.get( slot, buffer );
					confirmConstantBuffer( slot, buffer );
				}
				if (((++numOps)%50)==0) {
					System.out.println( numOps+" operations" );
				}
			}
		}
		
		private void generateConstantBuffer( byte buffer[] ) {
			byte b = (byte)rand.nextInt( 256 );
			for (int i=0; i<buffer.length; ++i)
				buffer[i] = b;
		}
		
		private void confirmConstantBuffer( int slot, byte buffer[] ) {
			int b = buffer[0];
			for (int i=1; i<buffer.length; ++i) {
				if (b != buffer[i]) {
					throw new RuntimeException( "Corrupted slot "+slot );
				}
			}
		}
	}

	static public void main( String args[] ) throws IOException {
		if (args.length != 1) {
			System.err.println( "Usage: java SimpleDatabase <filename>" );
			System.exit( 1 );
		}
		String filename = args[0];
		SimpleDatabase sd = new SimpleDatabase( filename );
		SimpleDatabaseTester sdt = new SimpleDatabaseTester( sd );
		sdt.test();
	}
}