MappedByteBuffer读写文件

一. MappedByteBuffer

1. java把文件映射到内存中，避免堆内存产生大对象引起full gc。mappedByteBuffer的读写速度都要超过堆内读写文件的速度

   public class AA {
       public static void main(String[] args) throws IOException, InterruptedException {
           map();
           //fis();
       }
   
       private static void fis() throws IOException {
           /*long l = System.currentTimeMillis();
           FileOutputStream fos = new FileOutputStream(new File("d://1234.txt"));
           for(int i=0;i<4000000;i++)
               fos.write('a');
           System.out.println((System.currentTimeMillis()-l));   //13011*/
   
           long l = System.currentTimeMillis();
           FileInputStream fis = new FileInputStream(new File("d://1234.txt"));
           int read;
           for(int i=0;i<4000000;i++) {
               read = fis.read();
           }
           System.out.println((System.currentTimeMillis()-l));  //9774
       }
   
       private static void map() throws IOException {
           /*long l = System.currentTimeMillis();
           FileChannel fileChannel = new RandomAccessFile("d://1234.txt", "rw").getChannel();
           CharBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 4000000*2).asCharBuffer();
           System.out.println("start");
           for(int i=0;i<4000000;i++)
               mappedByteBuffer.put('a');
           System.out.println((System.currentTimeMillis()-l));
           fileChannel.close(); //47*/
   
           long l = System.currentTimeMillis();
           FileChannel fileChannel = new RandomAccessFile("d://1234.txt", "r").getChannel();
           CharBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, fileChannel.size()).asCharBuffer();
           System.out.println("start");
           for(int i=0;i<4000000;i++)
               mappedByteBuffer.get();
           System.out.println((System.currentTimeMillis()-l));
           fileChannel.close();  //40
       }
   }
   
   

2. 关闭MappedByteBuffer

   public final class IOUtil {
       private static final Logger LOGGER = LoggerFactory.getLogger(IOUtil.class);
   
       private IOUtil() {
       }
   
       public static void closeMappedByteBuffer(final MappedByteBuffer byteBuffer) {
           if (byteBuffer == null) {
               return;
           }
   
           if (byteBuffer != null) {
               AccessController.doPrivileged(
                       new PrivilegedAction<Object>() {
                           @Override
                           public Object run() {
                               try {
                                   Method cleanerMethod = byteBuffer.getClass().getMethod("cleaner", new Class[0]);
                                   cleanerMethod.setAccessible(true);
                                   sun.misc.Cleaner cleaner = (sun.misc.Cleaner) cleanerMethod.invoke(byteBuffer,
                                           new Object[0]);
                                   cleaner.clean();
                               } catch (NoSuchMethodException e) {
                                   LOGGER.error("error occurred when clean mapped byte buffer", e);
                               } catch (InvocationTargetException e) {
                                   LOGGER.error("error occurred when clean mapped byte buffer", e);
                               } catch (IllegalAccessException e) {
                                   LOGGER.error("error occurred when clean mapped byte buffer", e);
                               }
                               return null;
                           }
                       }
               );
           }
       }
   }