java plt_Java读取大文件

Dear 丶 2022-11-03 11:23 156阅读 0赞

importjava.io.BufferedReader;importjava.io.File;importjava.io.FileReader;importjava.io.RandomAccessFile;importjava.nio.ByteBuffer;importjava.nio.MappedByteBuffer;importjava.nio.channels.FileChannel;public classReadBig \{public static String fff = "D:\\\\report\\\\inbound\\\\Citi\_OrderLines.csv";public static void main1(String\[\] args) throwsException \{final int BUFFER\_SIZE = 0x300000;//缓冲区大小为3M

File f= newFile(fff);/\*\*\* map(FileChannel.MapMode mode,long position, long size) mode -

\* 根据是按只读、读取/写入或专用(写入时拷贝)来映射文件，分别为 FileChannel.MapMode 类中所定义的

\* READ\_ONLY、READ\_WRITE 或 PRIVATE 之一 position - 文件中的位置，映射区域从此位置开始；必须为非负数

\* size - 要映射的区域大小；必须为非负数且不大于 Integer.MAX\_VALUE

\* 所以若想读取文件后半部分内容，如例子所写；若想读取文本后1

\* /8内容，需要这样写map(FileChannel.MapMode.READ\_ONLY,

\* f.length()\*7/8,f.length()/8)

\* 想读取文件所有内容，需要这样写map(FileChannel.MapMode.READ\_ONLY, 0,f.length())\*/MappedByteBuffer inputBuffer= new RandomAccessFile(f, "r").getChannel().map(FileChannel.MapMode.READ\_ONLY,

f.length()/ 2, f.length() / 2);byte\[\] dst = new byte\[BUFFER\_SIZE\];//每次读出3M的内容

long start =System.currentTimeMillis();for (int offset = 0; offset < inputBuffer.capacity(); offset +=BUFFER\_SIZE) \{if (inputBuffer.capacity() - offset >=BUFFER\_SIZE) \{for (int i = 0; i < BUFFER\_SIZE; i++)

dst\[i\]= inputBuffer.get(offset +i);

\}else\{for (int i = 0; i < inputBuffer.capacity() - offset; i++)

dst\[i\]= inputBuffer.get(offset +i);

\}int length = (inputBuffer.capacity() % BUFFER\_SIZE == 0) ?BUFFER\_SIZE : inputBuffer.capacity()%BUFFER\_SIZE;

System.out.println(new String(dst, 0, length));//new//String(dst,0,length)这样可以取出缓存保存的字符串，可以对其进行操作

\}long end =System.currentTimeMillis();

System.out.println("读取文件文件一半内容花费：" + (end - start) + "毫秒");