100G的文件如何读取续集 - 第307篇

文章目录

一、大文件读取之文件分割法

二、大文件读取之多线程读取

三、悟纤小结

 

 

一、大文件读取之文件分割法

      

我们来看下这种方法的核心思路就是：不是文件太大了嘛？那么是否可以把文件拆分成几个小的文件，然后使用多线程进行读取呐？具体的步骤：

（1）先分割成多个文件。

（2）多个线程操作多个文件，避免两个线程操作同一个文件

（3）按行读文件

 

1.1 文件分割

       在Mac和Linux都有文件分割的命令，可以使用：

split  -b 1024m  test2.txt   /data/tmp/my/test.txt.

说明：

（1）split：分割命令；

（2）-b 1024m：指定每多少字就要切成一个小文件。支持单位:m,k；这里是将6.5G的文件按照1G进行拆分成7个文件左右。

（3）test2.txt：要分割的文件；

（4）test.txt. ： 切割后文件的前置文件名，split会自动在前置文件名后再加上编号;

其它参数：

（1）-l<行数> : 指定每多少行就要切成一个小文件。

（2）　-C<字节>：与-b参数类似，但切割时尽量维持每行的完整性。

分割成功之后文件是这样子的：

1.2 多线程读取分割文件

       我们使用多线程读取分割的文件，然后开启线程对每个文件进行处理：

	public void readFileBySplitFile(String pathname) {
		//pathname这里是路径，非具体的文件名，比如：/data/tmp/my
		File file = new File(pathname);
		File[] files = file.listFiles();
		List<MyThread> threads = new ArrayList<>();
		for(File f:files) {
			MyThread thread = new MyThread(f.getPath());
			threads.add(thread);
			thread.start();
		}
		for(MyThread t:threads) {
			try {
				t.join();
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
		}
	}
	
	private class MyThread extends Thread{
		private String pathname;
		
		public MyThread(String pathname) {
			this.pathname = pathname;
		}
		
		@Override
		public void run() {
			readFileFileChannel(pathname);
		}
		
	}
	

 

说明：

（1）获取到指定目录下的所有分割的文件信息；

（2）遍历文件路径，将路径使用线程进行处理，这里线程的run使用readFileChannel进行读取每个文件的信息。

（3）join方法：就是让所有线程等待，然后回到主线程，不懂的可以参之前的一篇文章：《悟纤和师傅去女儿国「线程并行变为串行，Thread你好牛」》

测试：6.5G 耗时：4秒

       这个多线程的方式，那么理论上是文件越大，优势会越明显。对于线程开启的个数，这里使用的是文件的个数，在实际中，能这么使用嘛？答案肯定是不行的。相信大家应该知道怎么进行改良下，这里不展开讲解。

 

 

二、大文件读取之多线程读取同一个文件

2.1 多线程1.0版本

我们在看一下这种方式就是使用多线程读取同一个文件，这种方式的思路，就是讲文件进行划分，从不同的位置进行读取，那么满足这种要求的就是RandomAccessFile，因为此类中有一个方法seek，可以指定开始的位置。

	public void readFileByMutiThread(String pathname, int threadCount) {
		BufferedRandomAccessFile randomAccessFile = null;
		try {
			randomAccessFile = new BufferedRandomAccessFile(pathname, "r");
 
			// 获取文件的长度，进行分割
			long fileTotalLength = randomAccessFile.length();
			// 分割的每个大小.
			long gap = fileTotalLength / threadCount;
 
			// 记录每个的开始位置和结束位置.
			long[] beginIndexs = new long[threadCount];
			long[] endIndexs = new long[threadCount];
			// 记录下一次的位置.
			long nextStartIndex = 0;
 
			// 找到每一段的开始和结束的位置.
			for (int n = 0; n < threadCount; n++) {
				beginIndexs[n] = nextStartIndex;
				// 如果是最后一个的话，剩下的部分，就全部给最后一个线程进行处理了.
				if (n + 1 == threadCount) {
					endIndexs[n] = fileTotalLength;
					break;
				}
				/*
				 * 不是最后一个的话，需要获取endIndexs的位置.
				 */
				// (1)上一个nextStartIndex的位置+gap就是下一个位置.
				nextStartIndex += gap;
 
				// (2)nextStartIndex可能不是刚好这一行的结尾部分，需要处理下.
				// 先将文件移动到这个nextStartIndex的位置，然后往后进行寻找位置.
				randomAccessFile.seek(nextStartIndex);
 
				// 主要是计算回车换行的位置.
				long gapToEof = 0;
				boolean eol = false;
				while (!eol) {
					switch (randomAccessFile.read()) {
					case -1:
						eol = true;
						break;
					case '\n':
						eol = true;
						break;
					case '\r':
						eol = true;
						break;
					default:
						gapToEof++;
						break;
					}
				}
 
				// while循环，那个位置刚好是对应的那一行的最后一个字符的结束,++就是换行符号的位置.
				gapToEof++;
 
				nextStartIndex += gapToEof;
				endIndexs[n] = nextStartIndex;
			}
 
			// 开启线程
			List<MyThread2> threads = new ArrayList<>();
			for (int i = 0; i < threadCount; i++) {
				MyThread2 thread = new MyThread2(pathname, beginIndexs[i], endIndexs[i]);
				threads.add(thread);
				thread.start();
			}
 
			// 等待汇总数据
			for (MyThread2 t : threads) {
				try {
					t.join();
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
			}
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
 
	}

说明：此方法的作用就是对我们的文件根据线程的个数进行位置的分割，每个位置负责一部分的数据处理。

 

       我们看下具体线程的处理：

	private class MyThread2 extends Thread{
		private long begin;
		private long end;
		private String pathname;
		public MyThread2(String pathname,long begin,long end) {
			this.pathname = pathname;
			this.begin = begin;
			this.end = end;
		}
		
		@Override
		public void run() {
			//System.out.println("TestReadFile.MyThread2.run()-"+begin+"--"+end);
			RandomAccessFile randomAccessFile = null;
			try {
				randomAccessFile = new RandomAccessFile(pathname, "r");
				//指定其实读取的位置.
				randomAccessFile.seek(begin);
				
				StringBuffer buffer = new StringBuffer();
				String str;
				while ((str = randomAccessFile.readLine()) != null) {
				    //System.out.println(str+"--"+Thread.currentThread().getName());
					//处理字符串,并不会将字符串保存真正保存到内存中
					 // 这里简单模拟下处理操作.
					 buffer.append(str.substring(0,1));
					 
					 //+1 就是要加上回车换行符号
					 begin += (str.length()+1);
					 if(begin>=end) {
						 break;
					 }
				}
				System.out.println("buffer.length:"+buffer.length()+"--"+Thread.currentThread().getName());
			} catch (IOException e) {
				e.printStackTrace();
			}finally {
				//TODO close处理.
			}
			
		}
		
	}

说明：此线程的主要工作就是根据文件的位置点beginPosition和endPosition读取此区域的数据。

       运行看下效果，6.5G的，居然要运行很久，不知道什么时候要结束，实在等待不了，就结束运行了。

       为啥会这么慢呐？不是感觉这种处理方式很棒的嘛？为什么要伤害我弱小的心灵。

       我们分析下：之前的方法readFileByRandomAccessFile，我们在测试的时候，结果也是很慢，所以可以得到并不是因为我们使用的线程的原因导致了很慢了，那么这个是什么原因导致的呐？

       我们找到RandomAccessFile  的readLin()方法：

  public final String readLine() throws IOException {
        StringBuffer input = new StringBuffer();
        int c = -1;
        boolean eol = false;
 
        while (!eol) {
            switch (c = read()) {
            case -1:
            case '\n':
                eol = true;
                break;
            case '\r':
                eol = true;
                long cur = getFilePointer();
                if ((read()) != '\n') {
                    seek(cur);
                }
                break;
            default:
                input.append((char)c);
                break;
            }
        }
 
        if ((c == -1) && (input.length() == 0)) {
            return null;
        }
        return input.toString();
    }

       此方法的原理就是：使用while循环，不停的读取字符，如果遇到\n或者\r的话，那么readLine就结束，并且返回此行的数据，那么核心的方法就是read()：

public int read() throws IOException {
        return read0();
    }
 
private native int read0() throws IOException;

       直接调用的是本地方法了。那么这个方法是做了什么呢？我们可以通过注释分析下：

* Reads a byte of data from this file. The byte is returned as an
* integer in the range 0 to 255 ({@code 0x00-0x0ff}). This
* method blocks if no input is yet available.

       通过这里我们可以知道：read()方法会从该文件读取一个字节的数据。 字节返回为介于0到255之间的整数（{@code 0x00-0x0ff}）。 这个如果尚无输入可用，该方法将阻塞。

       到这里，不知道你是否知道这个为啥会这么慢了。一个字节一个字节每次读取，那么肯定是比较慢的嘛。

 

2.2 多线程2.0版本

       那么怎么办呢？有一个类BufferedRandomAccessFile，当然这个类并不属于jdk中的类，需要自己去找下源代码：

package com.kfit.bloomfilter;
  /**
   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  
  import java.io.File;
  import java.io.FileNotFoundException;
  import java.io.IOException;
  import java.io.RandomAccessFile;
  import java.util.Arrays;
  
  /**
   * A <code>BufferedRandomAccessFile</code> is like a
   * <code>RandomAccessFile</code>, but it uses a private buffer so that most
   * operations do not require a disk access.
   * <P>
   * 
   * Note: The operations on this class are unmonitored. Also, the correct
   * functioning of the <code>RandomAccessFile</code> methods that are not
   * overridden here relies on the implementation of those methods in the
   * superclass.
   */
  
  public final class BufferedRandomAccessFile extends RandomAccessFile
  {
      static final int LogBuffSz_ = 16; // 64K buffer
      public static final int BuffSz_ = (1 << LogBuffSz_);
      static final long BuffMask_ = ~(((long) BuffSz_) - 1L);
  
      private String path_;
      
      /*
       * This implementation is based on the buffer implementation in Modula-3's
       * "Rd", "Wr", "RdClass", and "WrClass" interfaces.
       */
      private boolean dirty_; // true iff unflushed bytes exist
      private boolean syncNeeded_; // dirty_ can be cleared by e.g. seek, so track sync separately
      private long curr_; // current position in file
      private long lo_, hi_; // bounds on characters in "buff"
      private byte[] buff_; // local buffer
      private long maxHi_; // this.lo + this.buff.length
      private boolean hitEOF_; // buffer contains last file block?
      private long diskPos_; // disk position
  
      /*
      * To describe the above fields, we introduce the following abstractions for
      * the file "f":
      *
      * len(f) the length of the file curr(f) the current position in the file
      * c(f) the abstract contents of the file disk(f) the contents of f's
      * backing disk file closed(f) true iff the file is closed
      *
      * "curr(f)" is an index in the closed interval [0, len(f)]. "c(f)" is a
      * character sequence of length "len(f)". "c(f)" and "disk(f)" may differ if
      * "c(f)" contains unflushed writes not reflected in "disk(f)". The flush
      * operation has the effect of making "disk(f)" identical to "c(f)".
      *
      * A file is said to be *valid* if the following conditions hold:
      *
      * V1. The "closed" and "curr" fields are correct:
      *
      * f.closed == closed(f) f.curr == curr(f)
      *
      * V2. The current position is either contained in the buffer, or just past
      * the buffer:
      *
      * f.lo <= f.curr <= f.hi
      *
      * V3. Any (possibly) unflushed characters are stored in "f.buff":
      *
      * (forall i in [f.lo, f.curr): c(f)[i] == f.buff[i - f.lo])
      *
      * V4. For all characters not covered by V3, c(f) and disk(f) agree:
      *
      * (forall i in [f.lo, len(f)): i not in [f.lo, f.curr) => c(f)[i] ==
      * disk(f)[i])
      *
      * V5. "f.dirty" is true iff the buffer contains bytes that should be
      * flushed to the file; by V3 and V4, only part of the buffer can be dirty.
      *
      * f.dirty == (exists i in [f.lo, f.curr): c(f)[i] != f.buff[i - f.lo])
      *
      * V6. this.maxHi == this.lo + this.buff.length
      *
      * Note that "f.buff" can be "null" in a valid file, since the range of
      * characters in V3 is empty when "f.lo == f.curr".
      *
      * A file is said to be *ready* if the buffer contains the current position,
      * i.e., when:
      *
      * R1. !f.closed && f.buff != null && f.lo <= f.curr && f.curr < f.hi
      *
      * When a file is ready, reading or writing a single byte can be performed
      * by reading or writing the in-memory buffer without performing a disk
      * operation.
      */
      
      /**
       * Open a new <code>BufferedRandomAccessFile</code> on <code>file</code>
       * in mode <code>mode</code>, which should be "r" for reading only, or
       * "rw" for reading and writing.
       */
      public BufferedRandomAccessFile(File file, String mode) throws IOException
      {
          this(file, mode, 0);
      }
      
      public BufferedRandomAccessFile(File file, String mode, int size) throws IOException
      {
          super(file, mode);
          path_ = file.getAbsolutePath();
          this.init(size);
      }
      
      /**
       * Open a new <code>BufferedRandomAccessFile</code> on the file named
       * <code>name</code> in mode <code>mode</code>, which should be "r" for
       * reading only, or "rw" for reading and writing.
       */
      public BufferedRandomAccessFile(String name, String mode) throws IOException
      {
          this(name, mode, 0);
      }
      
      public BufferedRandomAccessFile(String name, String mode, int size) throws FileNotFoundException
      {
          super(name, mode);
          path_ = name;
          this.init(size);
      }
      
      private void init(int size)
      {
          this.dirty_ = false;
          this.lo_ = this.curr_ = this.hi_ = 0;
          this.buff_ = (size > BuffSz_) ? new byte[size] : new byte[BuffSz_];
          this.maxHi_ = (long) BuffSz_;
          this.hitEOF_ = false;
          this.diskPos_ = 0L;
      }
  
      public String getPath()
      {
          return path_;
      }
  
      public void sync() throws IOException
      {
          if (syncNeeded_)
          {
              flush();
              getChannel().force(true);
              syncNeeded_ = false;
          }
      }
  
//      public boolean isEOF() throws IOException
//      {
//          assert getFilePointer() <= length();
//          return getFilePointer() == length();
//      }
  
      public void close() throws IOException
      {
          this.flush();
          this.buff_ = null;
          super.close();
      }
      
      /**
       * Flush any bytes in the file's buffer that have not yet been written to
       * disk. If the file was created read-only, this method is a no-op.
       */
      public void flush() throws IOException
      {        
          this.flushBuffer();
      }
      
      /* Flush any dirty bytes in the buffer to disk. */
      private void flushBuffer() throws IOException
      {   
          if (this.dirty_)
          {
              if (this.diskPos_ != this.lo_)
                  super.seek(this.lo_);
              int len = (int) (this.curr_ - this.lo_);
              super.write(this.buff_, 0, len);
              this.diskPos_ = this.curr_;             
              this.dirty_ = false;
          }
      }
      
      /*
       * Read at most "this.buff.length" bytes into "this.buff", returning the
       * number of bytes read. If the return result is less than
       * "this.buff.length", then EOF was read.
       */
      private int fillBuffer() throws IOException
      {
          int cnt = 0;
          int rem = this.buff_.length;
          while (rem > 0)
          {
              int n = super.read(this.buff_, cnt, rem);
              if (n < 0)
                  break;
              cnt += n;
              rem -= n;
          }
          if ( (cnt < 0) && (this.hitEOF_ = (cnt < this.buff_.length)) )
          {
              // make sure buffer that wasn't read is initialized with -1
              Arrays.fill(this.buff_, cnt, this.buff_.length, (byte) 0xff);
          }
          this.diskPos_ += cnt;
          return cnt;
      }
      
      /*
       * This method positions <code>this.curr</code> at position <code>pos</code>.
       * If <code>pos</code> does not fall in the current buffer, it flushes the
       * current buffer and loads the correct one.<p>
       * 
       * On exit from this routine <code>this.curr == this.hi</code> iff <code>pos</code>
       * is at or past the end-of-file, which can only happen if the file was
       * opened in read-only mode.
       */
      public void seek(long pos) throws IOException
      {
          if (pos >= this.hi_ || pos < this.lo_)
          {
              // seeking outside of current buffer -- flush and read             
              this.flushBuffer();
              this.lo_ = pos & BuffMask_; // start at BuffSz boundary
              this.maxHi_ = this.lo_ + (long) this.buff_.length;
              if (this.diskPos_ != this.lo_)
              {
                  super.seek(this.lo_);
                  this.diskPos_ = this.lo_;
              }
              int n = this.fillBuffer();
              this.hi_ = this.lo_ + (long) n;
          }
          else
          {
              // seeking inside current buffer -- no read required
              if (pos < this.curr_)
              {
                  // if seeking backwards, we must flush to maintain V4
                  this.flushBuffer();
              }
          }
          this.curr_ = pos;
      }
      
      public long getFilePointer()
      {
          return this.curr_;
      }
  
      public long length() throws IOException
      {
          // max accounts for the case where we have written past the old file length, but not yet flushed our buffer
          return Math.max(this.curr_, super.length());
      }
  
      public int read() throws IOException
      {
          if (this.curr_ >= this.hi_)
          {
              // test for EOF
              // if (this.hi < this.maxHi) return -1;
              if (this.hitEOF_)
                  return -1;
              
              // slow path -- read another buffer
              this.seek(this.curr_);
              if (this.curr_ == this.hi_)
                  return -1;
          }
          byte res = this.buff_[(int) (this.curr_ - this.lo_)];
          this.curr_++;
          return ((int) res) & 0xFF; // convert byte -> int
      }
      
      public int read(byte[] b) throws IOException
      {
          return this.read(b, 0, b.length);
      }
      
      public int read(byte[] b, int off, int len) throws IOException
      {
          if (this.curr_ >= this.hi_)
          {
              // test for EOF
              // if (this.hi < this.maxHi) return -1;
              if (this.hitEOF_)
                  return -1;
              
              // slow path -- read another buffer
              this.seek(this.curr_);
              if (this.curr_ == this.hi_)
                  return -1;
          }
          len = Math.min(len, (int) (this.hi_ - this.curr_));
          int buffOff = (int) (this.curr_ - this.lo_);
          System.arraycopy(this.buff_, buffOff, b, off, len);
          this.curr_ += len;
          return len;
      }
      
      public void write(int b) throws IOException
      {
          if (this.curr_ >= this.hi_)
          {
              if (this.hitEOF_ && this.hi_ < this.maxHi_)
              {
                  // at EOF -- bump "hi"
                  this.hi_++;
              }
              else
              {
                  // slow path -- write current buffer; read next one
                  this.seek(this.curr_);
                  if (this.curr_ == this.hi_)
                  {
                      // appending to EOF -- bump "hi"
                      this.hi_++;
                  }
              }
          }
          this.buff_[(int) (this.curr_ - this.lo_)] = (byte) b;
          this.curr_++;
          this.dirty_ = true;
          syncNeeded_ = true;
      }
      
      public void write(byte[] b) throws IOException
      {
          this.write(b, 0, b.length);
      }
      
      public void write(byte[] b, int off, int len) throws IOException
      {        
          while (len > 0)
          {              
              int n = this.writeAtMost(b, off, len);
              off += n;
              len -= n;
              this.dirty_ = true;
              syncNeeded_ = true;
          }
      }
      
      /*
       * Write at most "len" bytes to "b" starting at position "off", and return
       * the number of bytes written.
       */
      private int writeAtMost(byte[] b, int off, int len) throws IOException
      {        
          if (this.curr_ >= this.hi_)
          {
              if (this.hitEOF_ && this.hi_ < this.maxHi_)
              {
                  // at EOF -- bump "hi"
                  this.hi_ = this.maxHi_;
              }
              else
              {                                
                  // slow path -- write current buffer; read next one                
                  this.seek(this.curr_);
                  if (this.curr_ == this.hi_)
                  {
                      // appending to EOF -- bump "hi"
                      this.hi_ = this.maxHi_;
                  }
              }
          }
          len = Math.min(len, (int) (this.hi_ - this.curr_));
          int buffOff = (int) (this.curr_ - this.lo_);
          System.arraycopy(b, off, this.buff_, buffOff, len);
          this.curr_ += len;
          return len;
      }
  }

       然后将我们在上面使用到的类RandomAccessFile  替换成BufferedRandomAccessFile 即可。

       来测试下吧：

如果是前面的方法：

TestReadFile.readFileByBufferedRandomAccessFile(pathname2);

6.5G 耗时：32秒

       相比之前一直不能读取的情况下，已经是好很多了，但是相对于nio的话，还是慢了。

       测试下多线程版本的吧：

6.5G 耗时：2个线程20秒，3个线程16秒，4个线程14秒，5个线程11秒，6个线程8秒，7个线程8秒，8个线程9秒

       我这个Mac电脑是6核处理器，所以在6核的时候，达到了性能的最高点，在开启的更多的时候，线程的上下文切换会浪费这个时间，所以时间就越越来越高。但和上面的版本好像还是不能媲美。

 

2.3 多线程3.0版本

       RandomAccessFile的绝大多数功能，在JDK 1.4以后被nio的”内存映射文件(memory-mapped files)”给取代了MappedByteBuffer，大家可以自行去尝试下，本文就不展开讲解了。

 

三、悟纤小结

师傅：本文有点难，也有点辣眼睛和骚脑，今天就为师给你总结下。

徒儿：师傅，我太难了，我都要听睡着了。

师傅：文件操作本身就会比较复杂，在一个项目中，也不是所有人都会去写IO流的代码。

       来个小结，主要讲了两个知识点。

（1）第一：使用文件分隔的方式读取大文件，配套NIO的技术，速度会有提升。核心的思路就是：使用Mac/Linx下的split命令，将大文件分割成几个小的文件，然后使用多线程分别读取每个小文件。13.56G ：分割为6个文件，耗时8秒；26G,耗时16秒。按照这样的情况，那么读取100G的时间，也就是1分钟左右的事情了，当然实际耗时，还是和你具体的获取数据的处理方法有很大的关系，比如你使用系统的System.out的话，那么这个时间就很长了。

（2）第二：使用多线程读取大文件。核心的思路就是：根据文件的长度将文件分割成n段，然后开启多线程利用类RandomAccessFile的位置定位seek方法，直接从此位置开启读取。13.56G ：6个线程耗时23秒。

       另外实际上NIO的FileChannel单线程下的读取速度也是挺快的：13.56G  ：耗时15秒，之前就提到过了Java天然支持大文件的处理，这就是Java ，不仅Write once ，而且Write happy。

       最后要注意下，ByteBuffer读取到的是很多行的数据，不是一行一行的数据。

我就是我，是颜色不一样的烟火。
我就是我，是与众不同的小苹果。





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