1. java 读取大文件的困难
java 读取文件的一般操作是将文件数据全部读取到内存中,然后再对数据进行操作。例如
Path path = Paths.get("file path");
byte[] data = Files.readAllBytes(path);
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这对于小文件是没有问题的,但是对于稍大一些的文件就会抛出异常
Exception in thread "main" java.lang.OutOfMemoryError: Required array size too large
at java.nio.file.Files.readAllBytes(Files.java:3156)
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从错误定位看出,Files.readAllBytes
方法最大支持 Integer.MAX_VALUE - 8
大小的文件,也即最大2GB的文件。一旦超过了这个限度,java 原生的方法就不能直接使用了。
2. 分次读取大文件
既然不能直接全部读取大文件到内存中,那么就应该把文件分成多个子区域分多次读取。这就会有多种方法可以使用。
(1) 文件字节流
对文件建立 java.io.BufferedInputStream
,每次调用 read()
方法时会接连取出文件中长度为 arraySize
的数据到 array
中。这种方法可行但是效率不高。
import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.IOException;
/**
* Created by zfh on 16-4-19.
*/
public class StreamFileReader {
private BufferedInputStream fileIn;
private long fileLength;
private int arraySize;
private byte[] array;
public StreamFileReader(String fileName, int arraySize) throws IOException {
this.fileIn = new BufferedInputStream(new FileInputStream(fileName), arraySize);
this.fileLength = fileIn.available();
this.arraySize = arraySize;
}
public int read() throws IOException {
byte[] tmpArray = new byte[arraySize];
int bytes = fileIn.read(tmpArray);// 暂存到字节数组中
if (bytes != -1) {
array = new byte[bytes];// 字节数组长度为已读取长度
System.arraycopy(tmpArray, 0, array, 0, bytes);// 复制已读取数据
return bytes;
}
return -1;
}
public void close() throws IOException {
fileIn.close();
array = null;
}
public byte[] getArray() {
return array;
}
public long getFileLength() {
return fileLength;
}
public static void main(String[] args) throws IOException {
StreamFileReader reader = new StreamFileReader("/home/zfh/movie.mkv", 65536);
long start = System.nanoTime();
while (reader.read() != -1) ;
long end = System.nanoTime();
reader.close();
System.out.println("StreamFileReader: " + (end - start));
}
}
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(2) 文件通道
对文件建立 java.nio.channels.FileChannel
,每次调用 read()
方法时会先将文件数据读取到分配的长度为 arraySize
的 java.nio.ByteBuffer
中,再从中将已经读取到的文件数据转化到 array
中。这种利用了NIO中的通道的方法,比传统的字节流读取文件是要快一些。
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
/**
* Created by zfh on 16-4-18.
*/
public class ChannelFileReader {
private FileInputStream fileIn;
private ByteBuffer byteBuf;
private long fileLength;
private int arraySize;
private byte[] array;
public ChannelFileReader(String fileName, int arraySize) throws IOException {
this.fileIn = new FileInputStream(fileName);
this.fileLength = fileIn.getChannel().size();
this.arraySize = arraySize;
this.byteBuf = ByteBuffer.allocate(arraySize);
}
public int read() throws IOException {
FileChannel fileChannel = fileIn.getChannel();
int bytes = fileChannel.read(byteBuf);// 读取到ByteBuffer中
if (bytes != -1) {
array = new byte[bytes];// 字节数组长度为已读取长度
byteBuf.flip();
byteBuf.get(array);// 从ByteBuffer中得到字节数组
byteBuf.clear();
return bytes;
}
return -1;
}
public void close() throws IOException {
fileIn.close();
array = null;
}
public byte[] getArray() {
return array;
}
public long getFileLength() {
return fileLength;
}
public static void main(String[] args) throws IOException {
ChannelFileReader reader = new ChannelFileReader("/home/zfh/movie.mkv", 65536);
long start = System.nanoTime();
while (reader.read() != -1) ;
long end = System.nanoTime();
reader.close();
System.out.println("ChannelFileReader: " + (end - start));
}
}
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(3) 内存文件映射
这种方法就是把文件的内容被映像到计算机虚拟内存的一块区域,从而可以直接操作内存当中的数据而无需每次都通过 I/O 去物理硬盘读取文件。这是由当前 java 态进入到操作系统内核态,由操作系统读取文件,再返回数据到当前 java 态的过程。这样就能大幅提高我们操作大文件的速度。
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
/**
* Created by zfh on 16-4-19.
*/
public class MappedFileReader {
private FileInputStream fileIn;
private MappedByteBuffer mappedBuf;
private long fileLength;
private int arraySize;
private byte[] array;
public MappedFileReader(String fileName, int arraySize) throws IOException {
this.fileIn = new FileInputStream(fileName);
FileChannel fileChannel = fileIn.getChannel();
this.fileLength = fileChannel.size();
this.mappedBuf = fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, fileLength);
this.arraySize = arraySize;
}
public int read() throws IOException {
int limit = mappedBuf.limit();
int position = mappedBuf.position();
if (position == limit) {
return -1;
}
if (limit - position > arraySize) {
array = new byte[arraySize];
mappedBuf.get(array);
return arraySize;
} else {// 最后一次读取数据
array = new byte[limit - position];
mappedBuf.get(array);
return limit - position;
}
}
public void close() throws IOException {
fileIn.close();
array = null;
}
public byte[] getArray() {
return array;
}
public long getFileLength() {
return fileLength;
}
public static void main(String[] args) throws IOException {
MappedFileReader reader = new MappedFileReader("/home/zfh/movie.mkv", 65536);
long start = System.nanoTime();
while (reader.read() != -1);
long end = System.nanoTime();
reader.close();
System.out.println("MappedFileReader: " + (end - start));
}
}
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看似问题完美解决了,我们肯定会采用内存文件映射的方法去处理大文件。但是运行结果发现,这个方法仍然不能读取超过2GB的文件,明明 FileChannel.map()
方法传递的文件长度是 long
类型的,怎么和 Integer.MAX_VALUE
有关系?
Exception in thread "main" java.lang.IllegalArgumentException: Size exceeds Integer.MAX_VALUE
at sun.nio.ch.FileChannelImpl.map(FileChannelImpl.java:868)
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再从错误定位可以看到
size - The size of the region to be mapped; must be non-negative and no greater than Integer.MAX_VALUE
这可以归结到一些历史原因,还有 int
类型在 java 中的深入程度,但是本质上由于 java.nio.MappedByteBuffer
是直接继承自 java.nio.ByteBuffer
的,而后者的索引变量是 int
类型的,所以前者也只能最大索引到 Integer.MAX_VALUE
的位置。这样的话我们是不是就没有办法了?当然不是,一个内存文件映射不够用,那么试一试用多个就可以了。
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
/**
* Created by zfh on 16-4-19.
*/
public class MappedBiggerFileReader {
private MappedByteBuffer[] mappedBufArray;
private int count = 0;
private int number;
private FileInputStream fileIn;
private long fileLength;
private int arraySize;
private byte[] array;
public MappedBiggerFileReader(String fileName, int arraySize) throws IOException {
this.fileIn = new FileInputStream(fileName);
FileChannel fileChannel = fileIn.getChannel();
this.fileLength = fileChannel.size();
this.number = (int) Math.ceil((double) fileLength / (double) Integer.MAX_VALUE);
this.mappedBufArray = new MappedByteBuffer[number];// 内存文件映射数组
long preLength = 0;
long regionSize = (long) Integer.MAX_VALUE;// 映射区域的大小
for (int i = 0; i < number; i++) {// 将文件的连续区域映射到内存文件映射数组中
if (fileLength - preLength < (long) Integer.MAX_VALUE) {
regionSize = fileLength - preLength;// 最后一片区域的大小
}
mappedBufArray[i] = fileChannel.map(FileChannel.MapMode.READ_ONLY, preLength, regionSize);
preLength += regionSize;// 下一片区域的开始
}
this.arraySize = arraySize;
}
public int read() throws IOException {
if (count >= number) {
return -1;
}
int limit = mappedBufArray[count].limit();
int position = mappedBufArray[count].position();
if (limit - position > arraySize) {
array = new byte[arraySize];
mappedBufArray[count].get(array);
return arraySize;
} else {// 本内存文件映射最后一次读取数据
array = new byte[limit - position];
mappedBufArray[count].get(array);
if (count < number) {
count++;// 转换到下一个内存文件映射
}
return limit - position;
}
}
public void close() throws IOException {
fileIn.close();
array = null;
}
public byte[] getArray() {
return array;
}
public long getFileLength() {
return fileLength;
}
public static void main(String[] args) throws IOException {
MappedBiggerFileReader reader = new MappedBiggerFileReader("/home/zfh/movie.mkv", 65536);
long start = System.nanoTime();
while (reader.read() != -1) ;
long end = System.nanoTime();
reader.close();
System.out.println("MappedBiggerFileReader: " + (end - start));
}
}
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3. 运行结果比较
用上面三种方法读取1GB文件,运行结果如下
StreamFileReader: 11494900386
ChannelFileReader: 11329346316
MappedFileReader: 11169097480
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读取10GB文件,运行结果如下
StreamFileReader: 194579779394
ChannelFileReader: 190430242497
MappedBiggerFileReader: 186923035795
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