我们通过在命令行输入start-all.sh启动hadoop服务，该脚本通过SSH运行各个节点上的TaskTracker类的main()来启动TaskTracker，它是作为一个单独的JVM来运行的。TaskTracker类实现了MapReduce模型中的TaskTracker的功能。TaskTracker的main函数如下：
main（）{
TaskTracker tt = new TaskTracker();
tt. run();
.......
}
TaskTracker的run()方法主要调用了offerService()，其主要代码如下：
while (running && !shuttingDown) {//TaskTracker运行期间一直执行，每隔一段时间就想JobTracker发送一次心跳包，并接受JobTracker发送的HeartbeatResponse
try {
//判断是否该发送下一个心跳包
while (remaining > 0) {
...
}
// TaskTracker 刚启动:
// 1. 验证buildVersion
// 2. 为TaskTracker创建一个System Directory
if(justInited) {
String jobTrackerBV = jobClient.getBuildVersion();
...
systemDirectory = new Path(dir);
systemFS = systemDirectory.getFileSystem(fConf);
}
//每个一段时间检查一下TaskTrack的工作目录
now = System.currentTimeMillis();
if (now > (lastCheckDirsTime + diskHealthCheckInterval)) {
localStorage.checkDirs();
lastCheckDirsTime = now;
int numFailures = localStorage.numFailures();
// Re-init the task tracker if there were any new failures
if (numFailures > lastNumFailures) {
lastNumFailures = numFailures;
return State.STALE;
}
}
//发送心跳包，并向JobTracker请求Task
HeartbeatResponse heartbeatResponse = transmitHeartBeat(now);
...
//从心跳包的响应中获取它要执行的任务
TaskTrackerAction[] actions = heartbeatResponse.getActions();
//将相应的Task放入到特定的Queue
if (actions != null){
for(TaskTrackerAction action: actions) {
//将要运行一个Task
if (action instanceof LaunchTaskAction) {
addToTaskQueue((LaunchTaskAction)action);
} else if (action instanceof CommitTaskAction) {//
CommitTaskAction commitAction = (CommitTaskAction)action;
if (!commitResponses.contains(commitAction.getTaskID())) {
LOG.info("Received commit task action for " +
commitAction.getTaskID());
commitResponses.add(commitAction.getTaskID());
}
} else {//待清理的Task
tasksToCleanup.put(action);
}
}
}
}