java读取大文件 超大文件的几种方法

时间:2021-10-04 03:46:23
java 读取一个巨大的文本文件既能保证内存不溢出又能保证性能  
import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
public class ReadBig {
public static String fff = "C:\\mq\\read\\from.xml";
public static void main1(String[] args) throws Exception {
  final int BUFFER_SIZE = 0x300000;// 缓冲区大小为3M
  File f = new File(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");
}
public static void main2(String[] args) throws Exception {
  int bufSize = 1024;
  byte[] bs = new byte[bufSize];
  ByteBuffer byteBuf = ByteBuffer.allocate(1024);
  FileChannel channel = new RandomAccessFile(fff, "r").getChannel();
  while (channel.read(byteBuf) != -1) {
   int size = byteBuf.position();
   byteBuf.rewind();
   byteBuf.get(bs); // 把文件当字符串处理,直接打印做为一个例子。
   System.out.print(new String(bs, 0, size));
   byteBuf.clear();
  }
}
public static void main(String[] args) throws Exception {
  BufferedReader br = new BufferedReader(new FileReader(fff));
  String line = null;
  while ((line = br.readLine()) != null) {
   System.out.println(line);
  }
}
public static void main(String[] args) throws Exception {
    int bufSize = 1024;
    byte[] bs = new byte[bufSize];
    ByteBuffer byteBuf = ByteBuffer.allocate(1024);
    FileChannel channel = new RandomAccessFile("d:\\filename","r").getChannel();
    while(channel.read(byteBuf) != -1) {
      int size = byteBuf.position();
      byteBuf.rewind();
      byteBuf.get(bs);
      // 把文件当字符串处理,直接打印做为一个例子。
      System.out.print(new String(bs, 0, size));
      byteBuf.clear();
    }
  }
}
java 读取大容量文件,内存溢出?怎么按几行读取,读取多次
 import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.Scanner;
public class TestPrint {
public static void main(String[] args) throws IOException {
String path = "你要读的文件的路径";
RandomAccessFile br=new RandomAccessFile(path,"rw");//这里rw看你了。要是之都就只写r
String str = null, app = null;
int i=0;
while ((str = br.readLine()) != null) {
i++;
app=app+str;
if(i>=100){//假设读取100行
i=0;
// 这里你先对这100行操作,然后继续读
app=null;
}
}
br.close();
}
}
当逐行读写大于2G的文本文件时推荐使用以下代码
void largeFileIO(String inputFile, String outputFile) {
        try {
            BufferedInputStream bis = new BufferedInputStream(new FileInputStream(new File(inputFile)));
            BufferedReader in = new BufferedReader(new InputStreamReader(bis, "utf-8"), 10 * 1024 * 1024);//10M缓存
            FileWriter fw = new FileWriter(outputFile);
            while (in.ready()) {
                String line = in.readLine();
                fw.append(line + " ");
            }
            in.close();
            fw.flush();
            fw.close();
        } catch (IOException ex) {
            ex.printStackTrace();
        }
jdk本身就支持超大文件的读写。
网上的文章基本分为两大类,一类是使用BufferedReader类读写超大文件;另一类是使用RandomAccessFile类读取,经过比较,最后使用了前一种方式进行超大文件的读取,下面是相关代码,其实很简单
-------------------------------------------------------------------
File file = new File(filepath);   
BufferedInputStream fis = new BufferedInputStream(new FileInputStream(file));    
BufferedReader reader = new BufferedReader(new InputStreamReader(fis,"utf-8"),5*1024*1024);// 用5M的缓冲读取文本文件  
String line = "";
while((line = reader.readLine()) != null){
//TODO: write your business
}
---------------------------------------------------------------------
注意代码,在实例化BufferedReader时,增加一个分配缓存的参数即可