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Java读取大文件

2024-08-05 22:35:45   216人阅读

 1 import java.io.BufferedReader; 2 import java.io.File; 3 import java.io.FileReader; 4 import java.io.RandomAccessFile; 5 import java.nio.ByteBuffer; 6 import java.nio.MappedByteBuffer; 7 import java.nio.channels.FileChannel; 8  9 public class ReadBig {10 public static String fff = "C:\\mq\\read\\from.xml";11 12 public static void main1(String[] args) throws Exception {13 14   final int BUFFER_SIZE = 0x300000;// 缓冲区大小为3M15 16   File f = new File(fff);17 18   /**19    * 20    * map(FileChannel.MapMode mode,long position, long size)21    * 22    * 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23    * READ_ONLY、READ_WRITE 或 PRIVATE 之一24    * 25    * position - 文件中的位置，映射区域从此位置开始；必须为非负数26    * 27    * size - 要映射的区域大小；必须为非负数且不大于 Integer.MAX_VALUE28    * 29    * 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,30    * f.length()*7/8,f.length()/8)31    * 32    * 想读取文件所有内容，需要这样写map(FileChannel.MapMode.READ_ONLY, 0,f.length())33    * 34    */35 36   MappedByteBuffer inputBuffer = new RandomAccessFile(f, "r")37     .getChannel().map(FileChannel.MapMode.READ_ONLY,38       f.length() / 2, f.length() / 2);39 40   byte[] dst = new byte[BUFFER_SIZE];// 每次读出3M的内容41 42   long start = System.currentTimeMillis();43 44   for (int offset = 0; offset < inputBuffer.capacity(); offset += BUFFER_SIZE) {45 46    if (inputBuffer.capacity() - offset >= BUFFER_SIZE) {47 48     for (int i = 0; i < BUFFER_SIZE; i++)49 50      dst[i] = inputBuffer.get(offset + i);51 52    } else {53 54     for (int i = 0; i < inputBuffer.capacity() - offset; i++)55 56      dst[i] = inputBuffer.get(offset + i);57 58    }59 60    int length = (inputBuffer.capacity() % BUFFER_SIZE == 0) ? BUFFER_SIZE61      : inputBuffer.capacity() % BUFFER_SIZE;62 63    System.out.println(new String(dst, 0, length));// new64    // 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65 66   }67 68   long end = System.currentTimeMillis();69 70   System.out.println("读取文件文件一半内容花费：" + (end - start) + "毫秒");71 72 }

 

 1 public static void main2(String[] args) throws Exception { 2   int bufSize = 1024; 3   byte[] bs = new byte[bufSize]; 4   ByteBuffer byteBuf = ByteBuffer.allocate(1024); 5   FileChannel channel = new RandomAccessFile(fff, "r").getChannel(); 6   while (channel.read(byteBuf) != -1) { 7    int size = byteBuf.position(); 8    byteBuf.rewind(); 9    byteBuf.get(bs); // 把文件当字符串处理，直接打印做为一个例子。10    System.out.print(new String(bs, 0, size));11    byteBuf.clear();12   }13 14 }

 

1 public static void main(String[] args) throws Exception {2   BufferedReader br = new BufferedReader(new FileReader(fff));3   String line = null;4   while ((line = br.readLine()) != null) {5    System.out.println(line);6   }7 }

 

 1 public static void main(String[] args) throws Exception { 2     int bufSize = 1024; 3     byte[] bs = new byte[bufSize]; 4     ByteBuffer byteBuf = ByteBuffer.allocate(1024); 5     FileChannel channel = new RandomAccessFile("d:\\filename","r").getChannel(); 6     while(channel.read(byteBuf) != -1) { 7       int size = byteBuf.position(); 8       byteBuf.rewind(); 9       byteBuf.get(bs);10       // 把文件当字符串处理，直接打印做为一个例子。11       System.out.print(new String(bs, 0, size));12       byteBuf.clear();13     }14   }15 16 }

 

java 读取大容量文件，内存溢出？怎么按几行读取，读取多次

 1 import java.io.BufferedReader; 2 import java.io.FileNotFoundException; 3 import java.io.FileReader; 4 import java.io.IOException; 5 import java.io.RandomAccessFile; 6 import java.util.Scanner; 7  8 public class TestPrint { 9     public static void main(String[] args) throws IOException {10         String path = "你要读的文件的路径";11         RandomAccessFile br=new RandomAccessFile(path,"rw");//这里rw看你了。要是之都就只写r12         String str = null, app = null;13         int i=0;14         while ((str = br.readLine()) != null) {15             i++;16             app=app+str;17             if(i>=100){//假设读取100行18                 i=0;19 //                这里你先对这100行操作，然后继续读20 app=null;21             }22         }23         br.close();24     }25 26 }

 

 

当逐行读写大于2G的文本文件时推荐使用以下代码

 

 1 void largeFileIO(String inputFile, String outputFile) { 2         try { 3             BufferedInputStream bis = new BufferedInputStream(new FileInputStream(new File(inputFile))); 4             BufferedReader in = new BufferedReader(new InputStreamReader(bis, "utf-8"), 10 * 1024 * 1024);//10M缓存 5             FileWriter fw = new FileWriter(outputFile); 6             while (in.ready()) { 7                 String line = in.readLine(); 8                 fw.append(line + " "); 9             }10             in.close();11             fw.flush();12             fw.close();13         } catch (IOException ex) {14             ex.printStackTrace();15         }

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

1 File file = new File(filepath);   2 BufferedInputStream fis = new BufferedInputStream(new FileInputStream(file));    3 BufferedReader reader = new BufferedReader(new InputStreamReader(fis,"utf-8"),5*1024*1024);// 用5M的缓冲读取文本文件  4   5 String line = "";6 while((line = reader.readLine()) != null){7 //TODO: write your business8 }

 

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Java读取大文件


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