下面是代码片段，目标是在hadoop/tmp目录中写入一些文件，但得到nullpointerexception（因为hadoop不能创建目录或在/tmp目录中写入），我使用的是单用户，因此没有权限问题，并将chmod 777赋予/tmp目录。

public static class BigBWAMap extends 
        Mapper<Object,Text,IntWritable,Text>
File fout;
FileOutputStream fos;
BufferedWriter bw;
String tmpFileString = "";
String[] values1;
String[] values2;
String tmpDir;
//setup method
@Override
protected void setup(Context context) {
        Configuration conf = context.getConfiguration();
        tmpDir = conf.get("hadoop.tmp.dir","/tmp/");

    if(tmpDir == null || tmpDir.isEmpty()) {
        tmpDir = "/tmp/";
    }
    tmpFileString = tmpDir+"/HadoopTMPFile-"1"-"+213"; // directory where to write:  /tmp/HadoopTMPFile-1-213
    fout = new File(tmpFileString);

    try {
        fos = new FileOutputStream(fout);

    } catch (FileNotFoundException e) {
        e.printStackTrace();

    }
    bw = new BufferedWriter(new OutputStreamWriter(fos));

} 

}

获取bw处的nullpointerexception=new bufferedwriter（new outputstreamwriter（fos））；由于as/tmp/hadooptmpfile-1-213不可写。即使我试着写给/tmp也只能得到相同的npe。