java import java.io.File;import java.io.FileNotFoundException;import java.io.IOException;import java.io.RandomAccessFile;import java.util.Arrays;public class BufferedRandomAccessFile extends RandomAccessFile { static final int LogBuffSz_ = 16; // 64K buffer public static final int BuffSz_ = (1 << LogBuffSz_); static final long BuffMask_ = ~(((long) BuffSz_) - 1L); private String path_; /* * This implementation is based on the buffer implementation in Modula-3's * "Rd", "Wr", "RdClass", and "WrClass" interfaces. */ private boolean dirty_; // true iff unflushed bytes exist private boolean syncNeeded_; // dirty_ can be cleared by e.g. seek, so track sync separately private long curr_; // current position in file private long lo_, hi_; // bounds on characters in "buff" private byte[] buff_; // local buffer private long maxHi_; // this.lo + this.buff.length private boolean hitEOF_; // buffer contains last file block? private long diskPos_; // disk position /* * To describe the above fields, we introduce the following abstractions for * the file "f": * * len(f) the length of the file curr(f) the current position in the file * c(f) the abstract contents of the file disk(f) the contents of f's * backing disk file closed(f) true iff the file is closed * * "curr(f)" is an index in the closed interval [0, len(f)]. "c(f)" is a * character sequence of length "len(f)". "c(f)" and "disk(f)" may differ if * "c(f)" contains unflushed writes not reflected in "disk(f)". The flush * operation has the effect of making "disk(f)" identical to "c(f)". * * A file is said to be *valid* if the following conditions hold: * * V1. The "closed" and "curr" fields are correct: * * f.closed == closed(f) f.curr == curr(f) * * V2. The current position is either contained in the buffer, or just past * the buffer: * * f.lo <= f.curr <= f.hi * * V3. Any (possibly) unflushed characters are stored in "f.buff": * * (forall i in [f.lo, f.curr): c(f)[i] == f.buff[i - f.lo]) * * V4. For all characters not covered by V3, c(f) and disk(f) agree: * * (forall i in [f.lo, len(f)): i not in [f.lo, f.curr) => c(f)[i] == * disk(f)[i]) * * V5. "f.dirty" is true iff the buffer contains bytes that should be * flushed to the file; by V3 and V4, only part of the buffer can be dirty. * * f.dirty == (exists i in [f.lo, f.curr): c(f)[i] != f.buff[i - f.lo]) * * V6. this.maxHi == this.lo + this.buff.length * * Note that "f.buff" can be "null" in a valid file, since the range of * characters in V3 is empty when "f.lo == f.curr". * * A file is said to be *ready* if the buffer contains the current position, * i.e., when: * * R1. !f.closed && f.buff != null && f.lo <= f.curr && f.curr < f.hi * * When a file is ready, reading or writing a single byte can be performed * by reading or writing the in-memory buffer without performing a disk * operation. */ /** * Open a new BufferedRandomAccessFile on file * in mode mode, which should be "r" for reading only, or * "rw" for reading and writing. */ public BufferedRandomAccessFile(File file, String mode) throws IOException { this(file, mode, 0); } public BufferedRandomAccessFile(File file, String mode, int size) throws IOException { super(file, mode); path_ = file.getAbsolutePath(); this.init(size); } /** * Open a new BufferedRandomAccessFile on the file named * name in mode mode, which should be "r" for * reading only, or "rw" for reading and writing. */ public BufferedRandomAccessFile(String name, String mode) throws IOException { this(name, mode, 0); } public BufferedRandomAccessFile(String name, String mode, int size) throws FileNotFoundException { super(name, mode); path_ = name; this.init(size); } private void init(int size) { this.dirty_ = false; this.lo_ = this.curr_ = this.hi_ = 0; this.buff_ = (size > BuffSz_) ? new byte[size] : new byte[BuffSz_]; this.maxHi_ = (long) BuffSz_; this.hitEOF_ = false; this.diskPos_ = 0L; } public String getPath() { return path_; } public void sync() throws IOException { if (syncNeeded_) { flush(); getChannel().force(true); syncNeeded_ = false; } }// public boolean isEOF() throws IOException// {// assert getFilePointer() <= length();// return getFilePointer() == length();// } public void close() throws IOException { this.flush(); this.buff_ = null; super.close(); } /** * Flush any bytes in the file's buffer that have not yet been written to * disk. If the file was created read-only, this method is a no-op. */ public void flush() throws IOException { this.flushBuffer(); } /* Flush any dirty bytes in the buffer to disk. */ private void flushBuffer() throws IOException { if (this.dirty_) { if (this.diskPos_ != this.lo_) super.seek(this.lo_); int len = (int) (this.curr_ - this.lo_); super.write(this.buff_, 0, len); this.diskPos_ = this.curr_; this.dirty_ = false; } } /* * Read at most "this.buff.length" bytes into "this.buff", returning the * number of bytes read. If the return result is less than * "this.buff.length", then EOF was read. */ private int fillBuffer() throws IOException { int cnt = 0; int rem = this.buff_.length; while (rem > 0) { int n = super.read(this.buff_, cnt, rem); if (n < 0) break; cnt += n; rem -= n; } if ((cnt < 0) && (this.hitEOF_ = (cnt < this.buff_.length))) { // make sure buffer that wasn't read is initialized with -1 Arrays.fill(this.buff_, cnt, this.buff_.length, (byte) 0xff); } this.diskPos_ += cnt; return cnt; } /* * This method positions this.curr at position pos. * If pos does not fall in the current buffer, it flushes the * current buffer and loads the correct one. * * On exit from this routine this.curr == this.hi iff pos * is at or past the end-of-file, which can only happen if the file was * opened in read-only mode. */ public void seek(long pos) throws IOException { if (pos >= this.hi_ || pos < this.lo_) { // seeking outside of current buffer -- flush and read this.flushBuffer(); this.lo_ = pos & BuffMask_; // start at BuffSz boundary this.maxHi_ = this.lo_ + (long) this.buff_.length; if (this.diskPos_ != this.lo_) { super.seek(this.lo_); this.diskPos_ = this.lo_; } int n = this.fillBuffer(); this.hi_ = this.lo_ + (long) n; } else { // seeking inside current buffer -- no read required if (pos < this.curr_) { // if seeking backwards, we must flush to maintain V4 this.flushBuffer(); } } this.curr_ = pos; } public long getFilePointer() { return this.curr_; } public long length() throws IOException { // max accounts for the case where we have written past the old file length, but not yet flushed our buffer return Math.max(this.curr_, super.length()); } public int read() throws IOException { if (this.curr_ >= this.hi_) { // test for EOF // if (this.hi < this.maxHi) return -1; if (this.hitEOF_) return -1; // slow path -- read another buffer this.seek(this.curr_); if (this.curr_ == this.hi_) return -1; } byte res = this.buff_[(int) (this.curr_ - this.lo_)]; this.curr_++; return ((int) res) & 0xFF; // convert byte -> int } public int read(byte[] b) throws IOException { return this.read(b, 0, b.length); } public int read(byte[] b, int off, int len) throws IOException { if (this.curr_ >= this.hi_) { // test for EOF // if (this.hi < this.maxHi) return -1; if (this.hitEOF_) return -1; // slow path -- read another buffer this.seek(this.curr_); if (this.curr_ == this.hi_) return -1; } len = Math.min(len, (int) (this.hi_ - this.curr_)); int buffOff = (int) (this.curr_ - this.lo_); System.arraycopy(this.buff_, buffOff, b, off, len); this.curr_ += len; return len; } public void write(int b) throws IOException { if (this.curr_ >= this.hi_) { if (this.hitEOF_ && this.hi_ < this.maxHi_) { // at EOF -- bump "hi" this.hi_++; } else { // slow path -- write current buffer; read next one this.seek(this.curr_); if (this.curr_ == this.hi_) { // appending to EOF -- bump "hi" this.hi_++; } } } this.buff_[(int) (this.curr_ - this.lo_)] = (byte) b; this.curr_++; this.dirty_ = true; syncNeeded_ = true; } public void write(byte[] b) throws IOException { this.write(b, 0, b.length); } public void write(byte[] b, int off, int len) throws IOException { while (len > 0) { int n = this.writeAtMost(b, off, len); off += n; len -= n; this.dirty_ = true; syncNeeded_ = true; } } /* * Write at most "len" bytes to "b" starting at position "off", and return * the number of bytes written. */ private int writeAtMost(byte[] b, int off, int len) throws IOException { if (this.curr_ >= this.hi_) { if (this.hitEOF_ && this.hi_ < this.maxHi_) { // at EOF -- bump "hi" this.hi_ = this.maxHi_; } else { // slow path -- write current buffer; read next one this.seek(this.curr_); if (this.curr_ == this.hi_) { // appending to EOF -- bump "hi" this.hi_ = this.maxHi_; } } } len = Math.min(len, (int) (this.hi_ - this.curr_)); int buffOff = (int) (this.curr_ - this.lo_); System.arraycopy(b, off, this.buff_, buffOff, len); this.curr_ += len; return len; }}