Hadoop压缩机制的了解

通过一定的算法对数据进行特殊编码，使得数据占用的存储空间比较小，这个过程我们称之为压缩，反之为解压缩
不管哪种压缩工具都需要权衡时间和空间
在大数据领域内还要考虑压缩文件的可分割性
Hadoop支持的压缩工具有：DEFLATE、gzip、bzip以及Snappy
压缩与解压：CompressTest.java

public class CompressTest {
    public static void main(String[] args) throws IOException, ClassNotFoundException {
        //compress("block.txt", "org.apache.hadoop.io.compress.GzipCodec");//解压时注释掉

        //压缩方式：
        //gzip => org.apache.hadoop.io.compress.GzipCodec
        //bzip => org.apache.hadoop.io.compress.BZipCodec
        //snappy => org.apache.hadoop.io.compress.SnappyCodec
        //DEFLATE => org.apache.hadoop.io.compress.DefaultCodec

        decompress(new File("block.txt.gz"));//压缩时注释掉
    }

    private static File compress(String fileName, String compressClassName) throws ClassNotFoundException, IOException {
        Class<?> codecClass = Class.forName(compressClassName);
        Configuration configuration = new Configuration();
        CompressionCodec codec = (CompressionCodec) ReflectionUtils.newInstance(codecClass, configuration);

        File fileOut = new File(fileName + codec.getDefaultExtension());
        fileOut.delete();

        OutputStream out = new FileOutputStream(fileOut);

        CompressionOutputStream cout = codec.createOutputStream(out);

        File fileIn = new File(fileName);
        InputStream in = new FileInputStream(fileIn);
        IOUtils.copyBytes(in, cout, 4096, false);

        in.close();
        cout.close();

        return fileOut;
    }

    private static void decompress(File file) throws IOException {
        Configuration configuration = new Configuration();
        CompressionCodecFactory factory = new CompressionCodecFactory(configuration);

        CompressionCodec codec = factory.getCodec(new Path(file.getName()));

        if (codec == null) {
            System.out.println("Can not find codec for file " + file);
            return;
        }

        File fileOut = new File(file.getName() + "-.txt");
        InputStream in = codec.createInputStream(new FileInputStream(file));

        OutputStream outputStream = new FileOutputStream(fileOut);
        IOUtils.copyBytes(in, outputStream, 4096, false);

        in.close();
        outputStream.close();
    }
}

在WordCount.java主函数中增加压缩设置：

FileOutputFormat.setCompressOutput(job,true);
FileOutputFormat.setOutputCompressorClass(job, SnappyCodec.class);