RandomAccessFile读取大文件时效率很低，现进行改进
2021-04-21 本文已影响0人
atdoking
/**
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.Arrays;
/**
* A <code>BufferedRandomAccessFile</code> is like a
* <code>RandomAccessFile</code>, but it uses a private buffer so that most
* operations do not require a disk access.
* <P>
*
* Note: The operations on this class are unmonitored. Also, the correct
* functioning of the <code>RandomAccessFile</code> methods that are not
* overridden here relies on the implementation of those methods in the
* superclass.
*/
public final 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 <code>BufferedRandomAccessFile</code> on <code>file</code>
* in mode <code>mode</code>, 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 <code>BufferedRandomAccessFile</code> on the file named
* <code>name</code> in mode <code>mode</code>, 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 <code>this.curr</code> at position <code>pos</code>.
* If <code>pos</code> does not fall in the current buffer, it flushes the
* current buffer and loads the correct one.<p>
*
* On exit from this routine <code>this.curr == this.hi</code> iff <code>pos</code>
* 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;
}
}
