Command line for hadoop streaming - hadoop

Question

I am trying to use hadoop streaming where I have a java class which is used as mapper. To keep the problem simple let us assume the java code is like the following:
import java.io.* ;
class Test {
public static void main(String args[]) {
try {
BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
String input ;
while ((input = br.readLine()) != null) {
System.out.println(input) ;
}
} catch (IOException io) {
io.printStackTrace() ;
}
}
}
I can compile it as "javac Test.java" run it from command line as follows:
[abhattac#eat1-hcl4014 java]$ cat a.dat
abc
[abhattac#eat1-hcl4014 java]$ cat a.dat | java Test
abc
[abhattac#eat1-hcl4014 java]
Let us assume that I have a file in HDFS: a.dat
[abhattac#eat1-hcl4014 java]$ hadoop fs -cat /user/abhattac/a.dat
Abc
[abhattac#eat1-hcl4014 java]$ jar cvf Test.jar Test.class
added manifest
adding: Test.class(in = 769) (out= 485)(deflated 36%)
[abhattac#eat1-hcl4014 java]$
Now I try to use (Test.java) as mapper in hadoop streaming. What do I provide for
[1] -mapper command line option. Should it be like the following?
[2] -file command line option. Do I need to make a jar file out of Test.class? If that is the case do I need to include MANIFEST.MF file to indicate the main class?
I tried all these options but none of them seem to work. Any help will be appreciated.
hadoop jar /export/apps/hadoop/latest/contrib/streaming/hadoop-streaming-1.2.1.45.jar -file Test.jar -mapper 'java Test' -input /user/abhattac/a.dat -output /user/abhattac/output
The command above doesn't work. The error message in task log is:
stderr logs
Exception in thread "main" java.lang.NoClassDefFoundError: Test
Caused by: java.lang.ClassNotFoundException: Test
at java.net.URLClassLoader$1.run(URLClassLoader.java:202)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:190)
at java.lang.ClassLoader.loadClass(ClassLoader.java:306)

Answer 1

Since hadoop streaming is just shoveling work through stdin to a command line executable you can just run "java Test" on your Test.class like you would locally. There's no need to package to a jar.
I ran this successfully myself using your code:
hadoop jar hadoop-streaming.jar -file Test.class -mapper 'java Test' -input /input -output /output
SelimN is right that this is a pretty odd way to go about it though since you could just as well be writing a native java mapper.
Streaming is usually used when you want to use a scripting language such as bash or python instead of using Java.