在agent启动时，会启动Channel,SourceRunner,SinkRunner，比如在org.apache.flume.agent.embedded.EmbeddedAgent类的doStart方法中：

  private void doStart() {boolean error = true;try {channel.start(); //调用Channel.start启动ChannelsinkRunner.start(); //调用SinkRunner.start启动SinkRunnersourceRunner.start(); //调用SourceRunner.start启动SourceRunnersupervisor.supervise(channel,new SupervisorPolicy.AlwaysRestartPolicy(), LifecycleState.START);supervisor.supervise(sinkRunner,new SupervisorPolicy.AlwaysRestartPolicy(), LifecycleState.START);supervisor.supervise(sourceRunner,new SupervisorPolicy.AlwaysRestartPolicy(), LifecycleState.START);error = false;
.....

SourceRunner目前有两大类PollableSourceRunner和EventDrivenSourceRunner，分别对应PollableSource和EventDrivenSource，PollableSource相关类需要外部驱动来确定source中是否有消息可以使用，而EventDrivenSource相关类不需要外部驱动，自己实现了事件驱动机制，目前常见的Source类都属于EventDrivenSource类型。在org.apache.flume.conf.source.SourceType中定义了常见的Source类：

OTHER(null),
SEQ("org.apache.flume.source.SequenceGeneratorSource"),
NETCAT("org.apache.flume.source.NetcatSource"),
EXEC("org.apache.flume.source.ExecSource"),
AVRO("org.apache.flume.source.AvroSource"),
SYSLOGTCP("org.apache.flume.source.SyslogTcpSource"),
MULTIPORT_SYSLOGTCP("org.apache.flume.source.MultiportSyslogTCPSource"),
SYSLOGUDP("org.apache.flume.source.SyslogUDPSource"),
SPOOLDIR("org.apache.flume.source.SpoolDirectorySource"),
HTTP("org.apache.flume.source.http.HTTPSource");

可以由org.apache.flume.source.DefaultSourceFactory获取其对应的实例.
SourceRunner则负责启动Source,比如在org.apache.flume.source.EventDrivenSourceRunner的start方法：

  public void start() {Source source = getSource(); //通过getSource获取Source对象ChannelProcessor cp = source.getChannelProcessor(); //获取ChannelProcessor 对象cp.initialize(); //调用ChannelProcessor.initialize方法source.start(); //调用Source.start方法lifecycleState = LifecycleState. START;}

这里以execsource为例，配置source的类型为EXEC时，使用org.apache.flume.source.ExecSource.
主要原理是通过启动操作系统中的进程把每一行文本信息转换为Event，exec source不提供事务的特性（不能保证数据被成功发送到channel中），execsource关心的是标准输出，标准错误输出会被忽略（虽然可以打印到log中，但不会作为Event发送），可以设置restart的特性让source在退出时自动重启
主要方法分析：
1.configure方法用来设置相关参数
参数项：

command  执行的命令
restart 在命令退出后是否自动重启，默认是false
restartThrottle 重启命令的等待时间，默认是10s
logStderr 是否记录错误日志（只会放到日志中，不会做为Event）,默认是false
batchSize 一次向channel发送的event的数量，批量发送的功能，默认是20
charset 字符编码设置，默认是UTF-8

2.start方法，SourceRunner的start方法会调用这个start方法：

  public void start() {logger.info( "Exec source starting with command:{}" , command );executor = Executors. newSingleThreadExecutor(); // 初始化一个单线程的线程池,private ExecutorService executor;counterGroup = new CounterGroup(); //初始化性能计数器runner = new ExecRunnable( command, getChannelProcessor(), counterGroup,restart, restartThrottle, logStderr , bufferCount , charset ); //实例化一个ExecRunnable线程// FIXME : Use a callback-like executor / future to signal us upon failure.runnerFuture = executor.submit( runner);  //private Future<?> runnerFuture;super.start(); //设置状态为start,lifecycleState = LifecycleState.START;logger.debug( "Exec source started");}

3.核心是一个实现Runnable接口的线程类ExecRunnable，用于启动读取数据的进程（在一个do...while循环中）

public void run() {do {String exitCode = "unknown";BufferedReader reader = null;try {String[] commandArgs = command.split("\\s+"); //由命令生成字符串数组process = new ProcessBuilder(commandArgs).start(); //生成一个Process对象，启动命令进程reader = new BufferedReader(new InputStreamReader(process.getInputStream(), charset)); //读取标准输出至缓冲区对象// StderrLogger dies as soon as the input stream is invalidStderrReader stderrReader = new StderrReader(new BufferedReader(new InputStreamReader(process.getErrorStream(), charset)), logStderr); //StderrReader线程用于记录标准错误日志stderrReader.setName("StderrReader-[" + command + "]");stderrReader.setDaemon(true);stderrReader.start();/*while((line = input.readLine()) != null) {if(logStderr) {logger.info("StderrLogger[{}] = '{}'", ++i, line); //日志只会写到log中，不会转换为Event}}*/String line = null;List<Event> eventList = new ArrayList<Event>();while ((line = reader.readLine()) != null) { //循环读取缓冲区中的内容counterGroup.incrementAndGet("exec.lines.read");eventList.add(EventBuilder.withBody(line.getBytes(charset)));  //使用EventBuilder.withBody转换成Event并放入eventList中if(eventList.size() >= bufferCount) { //达到batchSize的设置时调用ChannelProcessor.processEventBatch处理EventchannelProcessor.processEventBatch(eventList);eventList.clear();}}if(!eventList.isEmpty()) {channelProcessor.processEventBatch(eventList);}}
....} while(restart); //如果设置了restart，会在process退出后重新运行do代码块，否则退出
}