无法将SnappyCodec与hadoop jar一起使用：NullPointerException

Question

我试图在简单的Java程序中使用Hadoop的压缩库。 但是我无法使用Snappy编解码器：执行会在SnappyCodec.createCompressor方法中产生NullPointerException 。

请注意 ，我没有得到由于未设置LD_LIBRARY_PATH和JAVA_LIBRARY_PATH环境变量而导致的典型java.lang.UnsatisfiedLinkError 。 Snappy已随CDH正确安装，通过运行hadoop checknative报告为可用，并且当我在hadoop checknative文件上执行hdfs dfs -text hadoop checknative ，Snappy的解压缩有效。

$ hadoop jar SnappyTool-0.0.1-SNAPSHOT.jar com.mycorp.SnappyCompressor
Exception in thread "main" java.lang.NullPointerException
        at org.apache.hadoop.io.compress.SnappyCodec.createCompressor(SnappyCodec.java:145)
        at org.apache.hadoop.io.compress.CodecPool.getCompressor(CodecPool.java:152)
        at org.apache.hadoop.io.compress.CodecPool.getCompressor(CodecPool.java:165)
        at com.mycorp.SnappyCompressor.main(SnappyCompressor.java:19)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:606)
        at org.apache.hadoop.util.RunJar.run(RunJar.java:221)
        at org.apache.hadoop.util.RunJar.main(RunJar.java:136)
$
$ hadoop checknative | grep snappy 2>/dev/null
$ snappy:  true /opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop/lib/native/libsnappy.so.1
$ ls /opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop/lib/native/
libhadoop.a       libhadoop.so.1.0.0  libnativetask.a         libsnappy.so
libhadooppipes.a  libhadooputils.a    libnativetask.so        libsnappy.so.1
libhadoop.so      libhdfs.a           libnativetask.so.1.0.0
libsnappy.so.1.1.4
$ export LD_LIBRARY_PATH=/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop/lib/native/
$ java -Djava.library.path=/opt/cloudera/parcels/CDH-5.5.1-1.cdh5.5.1.p0.11/lib/hadoop/lib/native/ -cp `hadoop classpath`:SnappyTool-0.0.1-SNAPSHOT.jar com.mycorp.SnappyCompressor
Exception in thread "main" java.lang.NullPointerException
        at org.apache.hadoop.io.compress.SnappyCodec.createCompressor(SnappyCodec.java:145)
        at org.apache.hadoop.io.compress.CodecPool.getCompressor(CodecPool.java:152)
        at org.apache.hadoop.io.compress.CodecPool.getCompressor(CodecPool.java:165)
        at com.mycorp.SnappyCompressor.main(SnappyCompressor.java:19)

Java代码如下所示，最后一行是罪魁祸首：

        SnappyCodec.checkNativeCodeLoaded();
        CompressionCodec codec = new SnappyCodec();
        Compressor comp = CodecPool.getCompressor(codec);

我错过了什么？

Answer 1

好的，事实证明，正如该答案所指出的那样，问题是CompressionCodec需要正确的配置。

获取已配置的Snappy压缩器的简单方法如下：

Configuration conf = new Configuration();
CompressionCodecFactory ccf = new CompressionCodecFactory(conf);
CompressionCodec codec = ccf.getCodecByClassName(SnappyCodec.class.getName());
Compressor comp = codec.createCompressor();

可以使用原始问题中使用的命令行来运行生成的jar。