一、前面主要是怎么使用datax和datax的插件编写，后面主要说明源码阅读部分，python相关文件
二、datax关键代码（python datax.py test.json）
1、datax.py文件
printCopyright()
parser = getOptionParser(sys.argv[1:])
options, args = parser.parse_args(sys.argv[1:])
if options.reader is not None and options.writer is not None:
generateJobConfigTemplate(options.reader,options.writer)
sys.exit(RET_STATE['OK'])
if len(args) == 0:
parser.print_help()
sys.exit(RET_STATE['FAIL'])
startCommand = buildStartCommand(options, args)
child_process = subprocess.Popen(startCommand, shell=True)
上面代码关键部分是函数buildStartCommand(options, args)拼出来的java命令，然后启动一个新的子进程去执行java命令
def buildStartCommand(options, args):
commandMap = {}
tempJVMCommand = DEFAULT_JVM
if options.jvmParameters:
tempJVMCommand = tempJVMCommand + " " + options.jvmParameters
if options.remoteDebug:
tempJVMCommand = tempJVMCommand + " " + REMOTE_DEBUG_CONFIG
print 'local ip: ', getLocalIp()
if options.loglevel:
tempJVMCommand = tempJVMCommand + " " + ("-Dloglevel=%s" % (options.loglevel))
if options.mode:
commandMap["mode"] = options.mode
# jobResource 可能是 URL，也可能是本地文件路径（相对,绝对）
jobResource = args[0]
if not isUrl(jobResource):
jobResource = os.path.abspath(jobResource)
if jobResource.lower().startswith("file://"):
jobResource = jobResource[len("file://"):]
jobParams = ("-Dlog.file.name=%s") % (jobResource[-20:].replace('/', '_').replace('.', '_'))
if options.params:
jobParams = jobParams + " " + options.params
if options.jobid:
commandMap["jobid"] = options.jobid
commandMap["jvm"] = tempJVMCommand
commandMap["params"] = jobParams
commandMap["job"] = jobResource
return Template(ENGINE_COMMAND).substitute(**commandMap)
java命令是通过模板ENGINE_COMMAND和commandMap拼接而成，其中ENGINE_COMMAND是主要的java脚本命令
ENGINE_COMMAND = "java -server ${jvm} %s -classpath %s ${params} com.alibaba.datax.core.Engine -mode ${mode} -jobid ${jobid} -job ${job}" % (DEFAULT_PROPERTY_CONF, CLASS_PATH)
commandMap中包含jvm参数、任务动态参数和job文件信息（json配置文件），根据这个python文件，用户可以自行修改jvm相关参数，如
DEFAULT_JVM = "-XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=%s/log" % (DATAX_HOME)
2、上述python文件执行完毕之后会启动java命令，主函数是com.alibaba.datax.core.Engine，至此python文件主要流程完毕
