java IO读写大文件的几种方式及测试

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读取文件大小：1.45G

第一种，OldIO:

public static void oldIOReadFile() throws IOException{      BufferedReader br = new BufferedReader(new FileReader("G://lily_947.txt"));      PrintWriter pw = new PrintWriter("G://oldIO.tmp");      char[] c = new char[100*1024*1024];      for(;;){          if(br.read(c)!=-1){              pw.print(c);          }else{              break;          }      }      pw.close();      br.close();  }

耗时70.79s

第二种，newIO:

public static void newIOReadFile() throws IOException{          FileChannel read = new RandomAccessFile("G://lily_947.txt","r").getChannel();          FileChannel writer = new RandomAccessFile("G://newIO.tmp","rw").getChannel();          ByteBuffer bb = ByteBuffer.allocate(200*1024*1024);          while(read.read(bb)!=-1){              bb.flip();              writer.write(bb);              bb.clear();          }          read.close();          writer.close();                }

耗时47.24s

第三种，RandomAccessFile:

public static void randomReadFile() throws IOException{          RandomAccessFile read = new RandomAccessFile("G://lily_947.txt","r");          RandomAccessFile writer = new RandomAccessFile("G://random.tmp","rw");          byte[] b = new byte[200*1024*1024];          while(read.read(b)!=-1){              writer.write(b);          }          writer.close();          read.close();      }

耗时46.65

第四种，MappedByteBuffer：

public static void mappedBuffer() throws IOException{          FileChannel read = new FileInputStream("G://lily_947.txt").getChannel();          FileChannel writer = new RandomAccessFile("G://buffer.tmp","rw").getChannel();          long i = 0;          long size = read.size()/30;          ByteBuffer bb,cc = null;          while(i<read.size()&&(read.size()-i)>size){              bb = read.map(FileChannel.MapMode.READ_ONLY, i, size);              cc = writer.map(FileChannel.MapMode.READ_WRITE, i, size);              cc.put(bb);              i+=size;              bb.clear();              cc.clear();          }          bb = read.map(FileChannel.MapMode.READ_ONLY, i, read.size()-i);          cc.put(bb);          bb.clear();          cc.clear();          read.close();          writer.close();                }

耗时：36

前三种读法对应的资源占用图如下：
相对于最后一种内存直接映射方式前面的测试其实无意义，基本秒杀。。。。。
对于很大的文件直接分块映射时内存会不够，这是因为MappedByteBuffer未被释放造成的，sun未提供直接回收MappedByteBuffer区域的方法，这个时候有两种方法解决，第一种比较愚笨的：

          System.gc();            System.runFinalization();            try {      Thread.sleep(3000);  } catch (InterruptedException e) {            e.printStackTrace();  }

第二种网上找来的，利用反射调用clean方法：

public static void unmap(final MappedByteBuffer buffer) {          if (buffer == null) {              return;          }          AccessController.doPrivileged(new PrivilegedAction<Object>() {              public Object run() {                  try {                      Method getCleanerMethod = buffer.getClass().getMethod("cleaner", new Class[0]);                      if (getCleanerMethod != null) {                          getCleanerMethod.setAccessible(true);                          Object cleaner = getCleanerMethod.invoke(buffer, new Object[0]);                          Method cleanMethod = cleaner.getClass().getMethod("clean", new Class[0]);                          if (cleanMethod != null) {                              cleanMethod.invoke(cleaner, new Object[0]);                          }                      }                  } catch (Exception e) {                      e.printStackTrace();                  }                  return null;              }             });      }

以上两种方法感觉都别扭，还有就是可以自己分割成物理文件再循环调用，这个也不太美观。
速度也会减慢好多。

转载自：http://aronlulu.iteye.com/blog/1018370

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