Java IO读写大文件的几种方式及测试

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。