Thursday, September 25, 2014

[YARN] : NullPointerException in MRClientService.getHttpPort()

Have you moved your Hadoop jobs over to Yarn?  Are you seeing the following NullPointerException coming out of your job?
Caused by: java.lang.NullPointerException
 ... 14 more
This appears to be a poorly handled case in Hadoop, where the webApp fails to load.  You need to look at the full log file.  You will likely see the following in the log preceeding the NPE:
ERROR ( - Webapps failed to start. Ignoring for now:
That line is produced by the following code in YARN:"Instantiated MRClientService at " + this.bindAddress);
    try {
      webApp = WebApps.$for("mapreduce", AppContext.class, appContext, "ws").with(conf).
          start(new AMWebApp());
    } catch (Exception e) {
      LOG.error("Webapps failed to start. Ignoring for now:", e);
This means your webApp failed to load.  In my case, this was caused by dependency conflicts on my classpath.  (servlet & jasper compiler)  I've also seen it cause by a dependency mssing, such as:
java.lang.ClassNotFoundException: Class org.apache.hadoop.yarn.server.webproxy.amfilter.AmFilterInitializer
In my situation, the solution was to cleanup my classpath, but the webApp might fail for lots of reasons, which will result in an NPE later in the log file when MRClientService attempts to getHttpPort without a webApp, which is shown in the following code:
public int getHttpPort() {
    return webApp.port();
Anyway, that is probably more detail than you wanted. Happy yarning.

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