MappedByteBuffer是java nio引入的文件内存映射方案，读写性能极高。在NIO中主要用到普通的输入流，带缓冲的输入流，RandomAccessFile和MappedByteBuffer。
现在我们来看看这四种流的效率，废话少说直接上代码。
我们采用CRC32来循环冗余校验。CRC32在java.util.zip包中。
1.普通的输入流。
public static long checksumInputStream(Path filename) throws IOException
{
try (InputStream in = Files.newInputStream(filename))
{
CRC32 crc = new CRC32();
int c;
while ((c = in.read()) != -1)
crc.update(c);
return crc.getValue();
}
}
2.带缓冲的输入流
public static long checksumBufferedInputStream(Path filename) throws IOException
{
try (InputStream in = new BufferedInputStream(Files.newInputStream(filename)))
{
CRC32 crc = new CRC32();
int c;
while ((c = in.read()) != -1)
crc.update(c);
return crc.getValue();
}
}
3.RandomAccessFile
public static long checksumRandomAccessFile(Path filename) throws IOException
{
try (RandomAccessFile file = new RandomAccessFile(filename.toFile(), "r"))
{
long length = file.length();
CRC32 crc = new CRC32();
for (long p = 0; p < length; p++)
{
file.seek(p);
int c = file.readByte();
crc.update(c);
}
return crc.getValue();
}
}
4.MappedByteBuffer
public static long checksumMappedFile(Path filename) throws IOException
{
try (FileChannel channel = FileChannel.open(filename))
{
CRC32 crc = new CRC32();
int length = (int) channel.size();
MappedByteBuffer buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, length);
for (int p = 0; p < length; p++)
{
int c = buffer.get(p);
crc.update(c);
}
return crc.getValue();
}
}
每个流对应的方法写好之后我们来开始测试
public static void main(String[] args) throws IOException {
String name = "txt.txt";
Path filename = Paths.get(name);
long start, crcValue, end;
System.out.println("Input Stream:");
start = System.currentTimeMillis();
crcValue = checksumInputStream(filename);
end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start) + " milliseconds");
System.out.println("Buffered Input Stream:");
start = System.currentTimeMillis();
crcValue = checksumBufferedInputStream(filename);
end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start) + " milliseconds");
System.out.println("Random Access File:");
start = System.currentTimeMillis();
crcValue = checksumRandomAccessFile(filename);
end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start) + " milliseconds");
System.out.println("Mapped File:");
start = System.currentTimeMillis();
crcValue = checksumMappedFile(filename);
end = System.currentTimeMillis();
System.out.println(Long.toHexString(crcValue));
System.out.println((end - start) + " milliseconds");
}
为了保证每种流相互之间对结果没有影响，我们可以分别把main方法中其他流的代码注释掉。
最后可以看到MappedByteBuffer效率最高，所消耗的时间最少。