【Hadoop代码笔记】Hadoop作业提交之TaskTracker 启动task

一、概要描述

在上篇博文描述了TaskTracker从Jobtracker如何从JobTracker获取到要执行的Task。在从JobTracker获取到LaunchTaskAction后，执行addToTaskQueue方法来把要执行的Task加入到queue。在本篇博文中，我们来关注下该方法后，TaskTracker怎么来处理这些Task。

实际上，TaskTracker初始化时，会初始化并启动两个TaskLauncher类型的线程，mapLauncher，reduceLauncher。在TaskTracker从JobTracher获取到任务后，对应的会把任务添加到两个TaskLauncher的Queue中，其实是TaskLauncher维护的一个列表List<TaskInProgress> tasksToLaunch。
TaskLauncher线程一直会定时检查TaskTracher上面有slot开业运行新的Task，则启动Task。在这个过程中，先把task运行需要的文件解压到本地，并创建根据Task类型（Map或者Reduce）创建一个TaskRunner线程，在TaskRunner中JvmManager调用JvmManagerForType、JvmRunner来启动一个java进程来执行Map或Reduce任务。

本文只是介绍到启动一个java进程，至于是什么样的java进程，对于maptask和reducetask分别是怎么执行的，在后面的child启动maptask，和child启动reducetask 会比较详细的介绍。

二、 流程描述  

1.tasktracker的offerService方法获取到要执行的task后调用addToTaskQueue方法，其实是调用taskrunner的addToTaskQueue方法
2.TaskLauncher内部维护了一个List<TaskInProgress> tasksToLaunch，只是把task加入到该集合中
3.taskLauncher是一个线程，在其run方法中从tasksToLaunch集合中取出task来执行，调用Tasktracker的startNewTask方法启动task。
4. startNewtask方法中调用localizeJob方法把job相关的配置信息和要运行的jar拷贝到tasktracker本地，然后调用taskInProgress的launchTask方法来启动task。
5.TaskInProgress的launchTask方法先调用localizeTask(task把task相关的配置信息获取到本地。然后创建一个TaskRunner线程来启动task。
6.在TaskRunner的run方法中构建一个java命令的执行的条件，包括引用类，执行目录等，入口类是Child。然后调用JvmManager 的launchJvm方法来调用。
7.JvmManager 进而调用 JvmManagerForType的reapJvm，和spawnNewJvm 方法，发起调用。
8. 在JvmManagerForType的spawnNewJvm 方法中创建了一个JvmRunner线程类执行调用。
9. JvmRunner线程的run反复调用runChild方法来执行 一个命令行的调用。

三、代码详细

1. TaskTracker的 addToTaskQueue方法。

接上文的最后一个方法的在heartbeat中把根据jobtracker的指令把需要launch的task调用addToTaskQueue方法加入task queue。

//根据task的类型不同加入到不同的launcher中。
private void addToTaskQueue(LaunchTaskAction action) {
   if (action.getTask().isMapTask()) {
      mapLauncher.addToTaskQueue(action);
    }else {
      reduceLauncher.addToTaskQueue(action);
    }
  }

2. TaskLauncher 的addToTaskQueue方法，即把要launch的task加入到TaskLauncher内维护的一个列表List<TaskInProgress> tasksToLaunch;中。

public void addToTaskQueue(LaunchTaskAction action) {
      synchronized (tasksToLaunch) {
        TaskInProgress tip = registerTask(action, this);
        tasksToLaunch.add(tip);
        tasksToLaunch.notifyAll();
      }
}

3. TaskLauncher线程的run方法。TaskLauncher是一个线程。一直检查task列表中有数据，取出一个来执行。

public void run() {    
          TaskInProgress tip;
          synchronized (tasksToLaunch) {
            while (tasksToLaunch.isEmpty()) {
              tasksToLaunch.wait();
            }
            //get the TIP
            tip = tasksToLaunch.remove(0);
           //wait for a slot to run
          synchronized (numFreeSlots) {
            while (numFreeSlots.get() == 0) {
              numFreeSlots.wait();
            }
            LOG.info("In TaskLauncher, current free slots : " + numFreeSlots.get()+
                " and trying to launch "+tip.getTask().getTaskID());
            numFreeSlots.set(numFreeSlots.get() - 1);
            assert (numFreeSlots.get() >= 0);
          }
       
          //got a free slot. launch the task
          startNewTask(tip);
              return; // ALL DONE
             }
    }
  }

4. TaskTracker的startNewTask 启动一个新task。该方法的主要代码就一句。

 localizeJob(tip);

5. TaskTracker的localizeJob方法。 初始化job的目录 

private void localizeJob(TaskInProgress tip) throws IOException {
        Path localJarFile = null;
        Task t = tip.getTask();
        JobID jobId = t.getJobID();
        Path jobFile = new Path(t.getJobFile());
        FileStatus status = null;
        long jobFileSize = -1;
        status = systemFS.getFileStatus(jobFile);
        jobFileSize = status.getLen();

        Path localJobFile = lDirAlloc.getLocalPathForWrite(
                getLocalJobDir(jobId.toString())
                + Path.SEPARATOR + "job.xml",
                jobFileSize, fConf);
        RunningJob rjob = addTaskToJob(jobId, tip);
        synchronized (rjob) {
            if (!rjob.localized) {

                FileSystem localFs = FileSystem.getLocal(fConf);

                systemFS.copyToLocalFile(jobFile, localJobFile);
                JobConf localJobConf = new JobConf(localJobFile);
                Path workDir = lDirAlloc.getLocalPathForWrite(
                        (getLocalJobDir(jobId.toString())
                                + Path.SEPARATOR + "work"), fConf);

                System.setProperty("job.local.dir", workDir.toString());
                localJobConf.set("job.local.dir", workDir.toString());


                //把job的jar文件拷贝到本地文件系统并且解压。
                String jarFile = localJobConf.getJar();

                Path jarFilePath = new Path(jarFile);
                status = systemFS.getFileStatus(jarFilePath);
                jarFileSize = status.getLen();

                //保证释放的目录容量有5倍的jar文件大小
                localJarFile = new Path(lDirAlloc.getLocalPathForWrite(
                        getLocalJobDir(jobId.toString())
                        + Path.SEPARATOR + "jars",
                        5 * jarFileSize, fConf), "job.jar");

                //把jar文件拷贝到本地
                systemFS.copyToLocalFile(jarFilePath, localJarFile);
                localJobConf.setJar(localJarFile.toString());
                OutputStream out = localFs.create(localJobFile);
                localJobConf.writeXml(out);

                // also unjar the job.jar files 
                RunJar.unJar(new File(localJarFile.toString()),
                        new File(localJarFile.getParent().toString()));
            }
            rjob.keepJobFiles = ((localJobConf.getKeepTaskFilesPattern() != null) ||
                    localJobConf.getKeepFailedTaskFiles());
            rjob.localized = true;
            rjob.jobConf = localJobConf;
        }
    }
    launchTaskForJob(tip, new JobConf(rjob.jobConf)); 
}

6. TaskTracker的addTaskToJob方法。只是把job和task的关系加入到runningJobs中。

private RunningJob addTaskToJob(JobID jobId, 
                                  TaskInProgress tip) {
    synchronized (runningJobs) {
      RunningJob rJob = null;
      if (!runningJobs.containsKey(jobId)) {
        rJob = new RunningJob(jobId);
        rJob.localized = false;
        rJob.tasks = new HashSet<TaskInProgress>();
        runningJobs.put(jobId, rJob);
      } else {
        rJob = runningJobs.get(jobId);
      }
      synchronized (rJob) {
        rJob.tasks.add(tip);
      }
      runningJobs.notify(); //notify the fetcher thread
      return rJob;
    }
  }

7. TaskTracker的launchTaskForJob方法。调用TaskInprogress的launchTask方法。

private void launchTaskForJob(TaskInProgress tip, JobConf jobConf) {
    synchronized (tip) {
      tip.setJobConf(jobConf);
      tip.launchTask();
    }
  }

8. TaskIProgress的 launchTask方法。

    public synchronized void launchTask() throws IOException {
        if (this.taskStatus.getRunState() == TaskStatus.State.UNASSIGNED ||
                this.taskStatus.getRunState() == TaskStatus.State.FAILED_UNCLEAN ||
                this.taskStatus.getRunState() == TaskStatus.State.KILLED_UNCLEAN) {
            localizeTask(task);
            if (this.taskStatus.getRunState() == TaskStatus.State.UNASSIGNED) {
                this.taskStatus.setRunState(TaskStatus.State.RUNNING);
            }
            //创建一个Runner来运行。
            this.runner = task.createRunner(TaskTracker.this, this);
            this.runner.start();
            this.taskStatus.setStartTime(System.currentTimeMillis());
        }

9.TaskinProgress的localizeTask方法。把Task相关的文件拷贝到本地。

private void localizeTask(Task task) throws IOException{

      Path localTaskDir = 
        lDirAlloc.getLocalPathForWrite(
          TaskTracker.getLocalTaskDir(task.getJobID().toString(), 
            task.getTaskID().toString(), task.isTaskCleanupTask()), 
          defaultJobConf );
      
      FileSystem localFs = FileSystem.getLocal(fConf);
      // create symlink for ../work if it already doesnt exist
      String workDir = lDirAlloc.getLocalPathToRead(
                         TaskTracker.getLocalJobDir(task.getJobID().toString())
                         + Path.SEPARATOR  
                         + "work", defaultJobConf).toString();
      String link = localTaskDir.getParent().toString() 
                      + Path.SEPARATOR + "work";
      File flink = new File(link);
      if (!flink.exists())
        FileUtil.symLink(workDir, link);
      
      // 创建task的工作目录
      Path cwd = lDirAlloc.getLocalPathForWrite(
                   getLocalTaskDir(task.getJobID().toString(), 
                      task.getTaskID().toString(), task.isTaskCleanupTask()) 
                   + Path.SEPARATOR + MRConstants.WORKDIR,
                   defaultJobConf);

      Path localTaskFile = new Path(localTaskDir, "job.xml");
      task.setJobFile(localTaskFile.toString());
      localJobConf.set("mapred.local.dir",
                       fConf.get("mapred.local.dir")); 
            
      localJobConf.set("mapred.task.id", task.getTaskID().toString());     
      }
            OutputStream out = localFs.create(localTaskFile);
             localJobConf.writeXml(out);
     
      task.setConf(localJobConf);
    }

10.Task是个抽象类，两个子类分别是MapTask和ReduceTask。先关注Map的TaskRunner。

public TaskRunner createRunner(TaskTracker tracker, 
      TaskTracker.TaskInProgress tip) {
    return new MapTaskRunner(tip, tracker, this.conf);
  }

11. TaskRunner线程的Run方法，有420行代码！主要作用是根据配置信息，构造java命令，启动一个java进程。

拼接一个java指令，启动一个单独的java进程来执行每一个map或者reduce任务。这个java命令的class是Child。即这个java进程最终调用的是Child类的main函数。

public final void run() {
    try {
      
      //before preparing the job localize 
      //all the archives
      TaskAttemptID taskid = t.getTaskID();
      LocalDirAllocator lDirAlloc = new LocalDirAllocator("mapred.local.dir");
      File jobCacheDir = null;
      if (conf.getJar() != null) {
        jobCacheDir = new File(
                          new Path(conf.getJar()).getParent().toString());
      }
      File workDir = new File(lDirAlloc.getLocalPathToRead(
                                TaskTracker.getLocalTaskDir( 
                                  t.getJobID().toString(), 
                                  t.getTaskID().toString(),
                                  t.isTaskCleanupTask())
                                + Path.SEPARATOR + MRConstants.WORKDIR,
                                conf). toString());

      URI[] archives = DistributedCache.getCacheArchives(conf);
      URI[] files = DistributedCache.getCacheFiles(conf);
      FileStatus fileStatus;
      FileSystem fileSystem;
      Path localPath;
      String baseDir;

      if ((archives != null) || (files != null)) {
        if (archives != null) {
          String[] archivesTimestamps = 
                               DistributedCache.getArchiveTimestamps(conf);
          Path[] p = new Path[archives.length];
          for (int i = 0; i < archives.length;i++){
            fileSystem = FileSystem.get(archives[i], conf);
            fileStatus = fileSystem.getFileStatus(
                                      new Path(archives[i].getPath()));
            String cacheId = DistributedCache.makeRelative(archives[i],conf);
            String cachePath = TaskTracker.getCacheSubdir() + 
                                 Path.SEPARATOR + cacheId;
            
            localPath = lDirAlloc.getLocalPathForWrite(cachePath,
                                      fileStatus.getLen(), conf);
            baseDir = localPath.toString().replace(cacheId, "");
            p[i] = DistributedCache.getLocalCache(archives[i], conf, 
                                                  new Path(baseDir),
                                                  fileStatus,
                                                  true, Long.parseLong(
                                                        archivesTimestamps[i]),
                                                  new Path(workDir.
                                                        getAbsolutePath()), 
                                                  false);
            
          }
          DistributedCache.setLocalArchives(conf, stringifyPathArray(p));
        }
        if ((files != null)) {
          String[] fileTimestamps = DistributedCache.getFileTimestamps(conf);
          Path[] p = new Path[files.length];
          for (int i = 0; i < files.length;i++){
            fileSystem = FileSystem.get(files[i], conf);
            fileStatus = fileSystem.getFileStatus(
                                      new Path(files[i].getPath()));
            String cacheId = DistributedCache.makeRelative(files[i], conf);
            String cachePath = TaskTracker.getCacheSubdir() +
                                 Path.SEPARATOR + cacheId;
            
            localPath = lDirAlloc.getLocalPathForWrite(cachePath,
                                      fileStatus.getLen(), conf);
            baseDir = localPath.toString().replace(cacheId, "");
            p[i] = DistributedCache.getLocalCache(files[i], conf, 
                                                  new Path(baseDir),
                                                  fileStatus,
                                                  false, Long.parseLong(
                                                           fileTimestamps[i]),
                                                  new Path(workDir.
                                                        getAbsolutePath()), 
                                                  false);
          }
          DistributedCache.setLocalFiles(conf, stringifyPathArray(p));
        }
        Path localTaskFile = new Path(t.getJobFile());
        FileSystem localFs = FileSystem.getLocal(conf);
        localFs.delete(localTaskFile, true);
        OutputStream out = localFs.create(localTaskFile);
        try {
          conf.writeXml(out);
        } finally {
          out.close();
        }
      }
          
      if (!prepare()) {
        return;
      }

      String sep = System.getProperty("path.separator");
      StringBuffer classPath = new StringBuffer();
      // start with same classpath as parent process
      classPath.append(System.getProperty("java.class.path"));
      classPath.append(sep);
      if (!workDir.mkdirs()) {
        if (!workDir.isDirectory()) {
          LOG.fatal("Mkdirs failed to create " + workDir.toString());
        }
      }
      
      String jar = conf.getJar();
      if (jar != null) {       
        // if jar exists, it into workDir
        File[] libs = new File(jobCacheDir, "lib").listFiles();
        if (libs != null) {
          for (int i = 0; i < libs.length; i++) {
            classPath.append(sep);            // add libs from jar to classpath
            classPath.append(libs[i]);
          }
        }
        classPath.append(sep);
        classPath.append(new File(jobCacheDir, "classes"));
        classPath.append(sep);
        classPath.append(jobCacheDir);
       
      }

      // include the user specified classpath
          
      //archive paths
      Path[] archiveClasspaths = DistributedCache.getArchiveClassPaths(conf);
      if (archiveClasspaths != null && archives != null) {
        Path[] localArchives = DistributedCache
          .getLocalCacheArchives(conf);
        if (localArchives != null){
          for (int i=0;i<archives.length;i++){
            for(int j=0;j<archiveClasspaths.length;j++){
              if (archives[i].getPath().equals(
                                               archiveClasspaths[j].toString())){
                classPath.append(sep);
                classPath.append(localArchives[i]
                                 .toString());
              }
            }
          }
        }
      }
      //file paths
      Path[] fileClasspaths = DistributedCache.getFileClassPaths(conf);
      if (fileClasspaths!=null && files != null) {
        Path[] localFiles = DistributedCache
          .getLocalCacheFiles(conf);
        if (localFiles != null) {
          for (int i = 0; i < files.length; i++) {
            for (int j = 0; j < fileClasspaths.length; j++) {
              if (files[i].getPath().equals(
                                            fileClasspaths[j].toString())) {
                classPath.append(sep);
                classPath.append(localFiles[i].toString());
              }
            }
          }
        }
      }

      classPath.append(sep);
      classPath.append(workDir);
      //  Build exec child jmv args.
      Vector<String> vargs = new Vector<String>(8);
      File jvm =                                  // use same jvm as parent
        new File(new File(System.getProperty("java.home"), "bin"), "java");

      vargs.add(jvm.toString());

      // Add child (task) java-vm options.
      //
      // The following symbols if present in mapred.child.java.opts value are
      // replaced:
      // + @taskid@ is interpolated with value of TaskID.
      // Other occurrences of @ will not be altered.
      //
      // Example with multiple arguments and substitutions, showing
      // jvm GC logging, and start of a passwordless JVM JMX agent so can
      // connect with jconsole and the likes to watch child memory, threads
      // and get thread dumps.
      //
      //  <property>
      //    <name>mapred.child.java.opts</name>
      //    <value>-verbose:gc -Xloggc:/tmp/@taskid@.gc 
      //           -Dcom.sun.management.jmxremote.authenticate=false 
      //           -Dcom.sun.management.jmxremote.ssl=false 
      //    </value>
      //  </property>
      //
      String javaOpts = conf.get("mapred.child.java.opts", "-Xmx200m");
      javaOpts = javaOpts.replace("@taskid@", taskid.toString());
      String [] javaOptsSplit = javaOpts.split(" ");
      
      // Add java.library.path; necessary for loading native libraries.
      //
      // 1. To support native-hadoop library i.e. libhadoop.so, we add the 
      //    parent processes' java.library.path to the child. 
      // 2. We also add the 'cwd' of the task to it's java.library.path to help 
      //    users distribute native libraries via the DistributedCache.
      // 3. The user can also specify extra paths to be added to the 
      //    java.library.path via mapred.child.java.opts.
      //
      String libraryPath = System.getProperty("java.library.path");
      if (libraryPath == null) {
        libraryPath = workDir.getAbsolutePath();
      } else {
        libraryPath += sep + workDir;
      }
      boolean hasUserLDPath = false;
      for(int i=0; i<javaOptsSplit.length ;i++) { 
        if(javaOptsSplit[i].startsWith("-Djava.library.path=")) {
          javaOptsSplit[i] += sep + libraryPath;
          hasUserLDPath = true;
          break;
        }
      }
      if(!hasUserLDPath) {
        vargs.add("-Djava.library.path=" + libraryPath);
      }
      for (int i = 0; i < javaOptsSplit.length; i++) {
        vargs.add(javaOptsSplit[i]);
      }

      // add java.io.tmpdir given by mapred.child.tmp
      String tmp = conf.get("mapred.child.tmp", "./tmp");
      Path tmpDir = new Path(tmp);
      
      // if temp directory path is not absolute 
      // prepend it with workDir.
      if (!tmpDir.isAbsolute()) {
        tmpDir = new Path(workDir.toString(), tmp);
      }
      FileSystem localFs = FileSystem.getLocal(conf);
      if (!localFs.mkdirs(tmpDir) && !localFs.getFileStatus(tmpDir).isDir()) {
        throw new IOException("Mkdirs failed to create " + tmpDir.toString());
      }
      vargs.add("-Djava.io.tmpdir=" + tmpDir.toString());

      // Add classpath.
      vargs.add("-classpath");
      vargs.add(classPath.toString());

      // Setup the log4j prop
      long logSize = TaskLog.getTaskLogLength(conf);
      vargs.add("-Dhadoop.log.dir=" + 
          new File(System.getProperty("hadoop.log.dir")
          ).getAbsolutePath());
      vargs.add("-Dhadoop.root.logger=INFO,TLA");
      vargs.add("-Dhadoop.tasklog.taskid=" + taskid);
      vargs.add("-Dhadoop.tasklog.totalLogFileSize=" + logSize);

      if (conf.getProfileEnabled()) {
        if (conf.getProfileTaskRange(t.isMapTask()
                                     ).isIncluded(t.getPartition())) {
          File prof = TaskLog.getTaskLogFile(taskid, TaskLog.LogName.PROFILE);
          vargs.add(String.format(conf.getProfileParams(), prof.toString()));
        }
      }

      // Add main class and its arguments 
      vargs.add(Child.class.getName());  // main of Child
      // pass umbilical address
      InetSocketAddress address = tracker.getTaskTrackerReportAddress();
      vargs.add(address.getAddress().getHostAddress()); 
      vargs.add(Integer.toString(address.getPort())); 
      vargs.add(taskid.toString());                      // pass task identifier

      String pidFile = lDirAlloc.getLocalPathForWrite(
            (TaskTracker.getPidFile(t.getJobID().toString(), 
             taskid.toString(), t.isTaskCleanupTask())),
            this.conf).toString();
      t.setPidFile(pidFile);
      tracker.addToMemoryManager(t.getTaskID(), conf, pidFile);

      // set memory limit using ulimit if feasible and necessary ...
      String[] ulimitCmd = Shell.getUlimitMemoryCommand(conf);
      List<String> setup = null;
      if (ulimitCmd != null) {
        setup = new ArrayList<String>();
        for (String arg : ulimitCmd) {
          setup.add(arg);
        }
      }

      // Set up the redirection of the task's stdout and stderr streams
      File stdout = TaskLog.getTaskLogFile(taskid, TaskLog.LogName.STDOUT);
      File stderr = TaskLog.getTaskLogFile(taskid, TaskLog.LogName.STDERR);
      stdout.getParentFile().mkdirs();
      tracker.getTaskTrackerInstrumentation().reportTaskLaunch(taskid, stdout, stderr);

      Map<String, String> env = new HashMap<String, String>();
      StringBuffer ldLibraryPath = new StringBuffer();
      ldLibraryPath.append(workDir.toString());
      String oldLdLibraryPath = null;
      oldLdLibraryPath = System.getenv("LD_LIBRARY_PATH");
      if (oldLdLibraryPath != null) {
        ldLibraryPath.append(sep);
        ldLibraryPath.append(oldLdLibraryPath);
      }
      env.put("LD_LIBRARY_PATH", ldLibraryPath.toString());
      jvmManager.launchJvm(this, 
          jvmManager.constructJvmEnv(setup,vargs,stdout,stderr,logSize, 
              workDir, env, pidFile, conf));
      synchronized (lock) {
        while (!done) {
          lock.wait();
        }
      }
      tracker.getTaskTrackerInstrumentation().reportTaskEnd(t.getTaskID());
      if (exitCodeSet) {
        if (!killed && exitCode != 0) {
          if (exitCode == 65) {
            tracker.getTaskTrackerInstrumentation().taskFailedPing(t.getTaskID());
          }
          throw new IOException("Task process exit with nonzero status of " +
              exitCode + ".");
        }
      }
    } catch (FSError e) {
      LOG.fatal("FSError", e);
      try {
        tracker.fsError(t.getTaskID(), e.getMessage());
      } catch (IOException ie) {
        LOG.fatal(t.getTaskID()+" reporting FSError", ie);
      }
    } catch (Throwable throwable) {
      LOG.warn(t.getTaskID()+" Child Error", throwable);
      ByteArrayOutputStream baos = new ByteArrayOutputStream();
      throwable.printStackTrace(new PrintStream(baos));
      try {
        tracker.reportDiagnosticInfo(t.getTaskID(), baos.toString());
      } catch (IOException e) {
        LOG.warn(t.getTaskID()+" Reporting Diagnostics", e);
      }
    } finally {
      try{
        URI[] archives = DistributedCache.getCacheArchives(conf);
        URI[] files = DistributedCache.getCacheFiles(conf);
        if (archives != null){
          for (int i = 0; i < archives.length; i++){
            DistributedCache.releaseCache(archives[i], conf);
          }
        }
        if (files != null){
          for(int i = 0; i < files.length; i++){
            DistributedCache.releaseCache(files[i], conf);
          }
        }
      }catch(IOException ie){
        LOG.warn("Error releasing caches : Cache files might not have been cleaned up");
      }
      tip.reportTaskFinished();
    }
  }

12.JvmManager的 launchJvm方法。在TaskRunner的run方法，是构造一个java命令的参数，调用JvmManager的launchJvm方法执行。

public void launchJvm(TaskRunner t, JvmEnv env) {
    if (t.getTask().isMapTask()) {
      mapJvmManager.reapJvm(t, env);
    } else {
      reduceJvmManager.reapJvm(t, env);
    }
  }

13. JvmManagerForType的reapJvm方法

private synchronized void reapJvm( 
        TaskRunner t, JvmEnv env) {
      if (t.getTaskInProgress().wasKilled()) {
       //如果task被杀死则直接返回
        return;
      }
      boolean spawnNewJvm = false;
      JobID jobId = t.getTask().getJobID();
      //检查是否有空闲的槽，如果小于最大jvm数，则重新开启一个jvm，不让你从现有job的空闲jvm中选择一个，或者杀死另外job的空闲jvm
      int numJvmsSpawned = jvmIdToRunner.size();
      JvmRunner runnerToKill = null;
      if (numJvmsSpawned >= maxJvms) {
        //go through the list of JVMs for all jobs.
        Iterator<Map.Entry<JVMId, JvmRunner>> jvmIter = 
          jvmIdToRunner.entrySet().iterator();
        
        while (jvmIter.hasNext()) {
          JvmRunner jvmRunner = jvmIter.next().getValue();
          JobID jId = jvmRunner.jvmId.getJobId();
          //look for a free JVM for this job; if one exists then just break
          if (jId.equals(jobId) && !jvmRunner.isBusy() && !jvmRunner.ranAll()){
            setRunningTaskForJvm(jvmRunner.jvmId, t); //reserve the JVM
            LOG.info("No new JVM spawned for jobId/taskid: " + 
                     jobId+"/"+t.getTask().getTaskID() +
                     ". Attempting to reuse: " + jvmRunner.jvmId);
            return;
          }
      
          if ((jId.equals(jobId) && jvmRunner.ranAll()) ||
              (!jId.equals(jobId) && !jvmRunner.isBusy())) {
            runnerToKill = jvmRunner;
            spawnNewJvm = true;
          }
        }
      } else {
        spawnNewJvm = true;
      }

      if (spawnNewJvm) {
        if (runnerToKill != null) {
          LOG.info("Killing JVM: " + runnerToKill.jvmId);
          runnerToKill.kill();
        }
        spawnNewJvm(jobId, env, t);
        return;
      }
}

14. JvmManagerForType的spawnNewJvm方法。重新启动一个jvm。

 private void spawnNewJvm(JobID jobId, JvmEnv env,  
        TaskRunner t) {
      JvmRunner jvmRunner = new JvmRunner(env,jobId);
      jvmIdToRunner.put(jvmRunner.jvmId, jvmRunner);
      jvmRunner.setDaemon(true);
      jvmRunner.setName("JVM Runner " + jvmRunner.jvmId + " spawned.");
      setRunningTaskForJvm(jvmRunner.jvmId, t);
      LOG.info(jvmRunner.getName());
      jvmRunner.start();
    }

15. JvmRunner线程的run方法。

  public void run() {
        runChild(env);
      }

16. JvmRunner线程的runChild方法。其中掉ShellCommandExecutor的execute方法。ShellCommandExecutor封装了shell执行。即把前面步骤构造的JvmEnv类型的执行信息分装成一个字符串列表，使用该列表构造一个ShellCommandExecutor来执行命令。

public void runChild(JvmEnv env) {
env.vargs.add(Integer.toString(jvmId.getId()));
          List<String> wrappedCommand = 
            TaskLog.captureOutAndError(env.setup, env.vargs, env.stdout, env.stderr,
                env.logSize, env.pidFile);
          shexec = new ShellCommandExecutor(wrappedCommand.toArray(new String[0]), 
              env.workDir, env.env);
          shexec.execute();
}

 完.

为了转载内容的一致性、可追溯性和保证及时更新纠错，转载时请注明来自：http://www.cnblogs.com/douba/p/hadoop_mapreduce_tasktracker_launch_task.html。谢谢！