基于MappedByteBuffer & FileChannel 的大文件读取工具

╰半夏微凉° 2022-01-11 21:33 160阅读 0赞

import java.io.FileInputStream;
    import java.io.IOException;
    import java.nio.MappedByteBuffer;
    import java.nio.channels.FileChannel;
    
    /**
     * 大文件读取工具
     **/
    public class BigFileReader {
    
    	private MappedByteBuffer[] mappedByteBuffers;
    	private FileInputStream inputStream;
    	private FileChannel fileChannel;
    
    	private int bufferCountIndex = 0;
    	private int bufferCount;
    
    	private int byteBufferSize;
    	private byte[] byteBuffer;
    
    	public BigFileReader(String fileName, int byteBufferSize) throws IOException {
    		this.inputStream = new FileInputStream(fileName);
    		this.fileChannel = inputStream.getChannel();
    		long fileSize = fileChannel.size();
    		this.bufferCount = (int) Math.ceil((double) fileSize / (double) Integer.MAX_VALUE);
    		this.mappedByteBuffers = new MappedByteBuffer[bufferCount];
    		this.byteBufferSize = byteBufferSize;
    
    		long preLength = 0;
    		long regionSize = Integer.MAX_VALUE;
    		for (int i = 0; i < bufferCount; i++) {
    			if (fileSize - preLength < Integer.MAX_VALUE) {
    				regionSize = fileSize - preLength;
    			}
    			mappedByteBuffers[i] = fileChannel.map(FileChannel.MapMode.READ_ONLY, preLength, regionSize);
    			preLength += regionSize;
    		}
    	}
    
    	public synchronized int read() {
    		if (bufferCountIndex >= bufferCount) {
    			return -1;
    		}
    
    		int limit = mappedByteBuffers[bufferCountIndex].limit();
    		int position = mappedByteBuffers[bufferCountIndex].position();
    
    		int realSize = byteBufferSize;
    		if (limit - position < byteBufferSize) {
    			realSize = limit - position;
    		}
    		byteBuffer = new byte[realSize];
    		mappedByteBuffers[bufferCountIndex].get(byteBuffer);
    
    		if (realSize < byteBufferSize && bufferCountIndex < bufferCount) {
    			bufferCountIndex++;
    		}
    		return realSize;
    	}
    
    	public void close() throws IOException {
    		fileChannel.close();
    		inputStream.close();
    		for (MappedByteBuffer byteBuffer: mappedByteBuffers) {
    			byteBuffer.clear();
    		}
    		byteBuffer = null;
    	}
    
    	public synchronized byte[] getCurrentBytes() {
    		return byteBuffer;
    	}
    
    	public static void main(String[] args) throws Exception {
    		BigFileReader reader = new BigFileReader("文件路径", 1024);
    		while (reader.read() != -1) {
    			byte[] bytes = reader.getCurrentBytes();
    			System.out.println(new String(bytes));
    		}
    		reader.close();
    	}
    }