在hadoop中很多地方都用到了servlet，并且使用jetty作为servlet的容器来提供http的服务，其主要是通过org.apache.hadoop.http.HttpServer类实现的，HttpServer类是对Jetty的简单封装，通过调用HttpServer类的addServlet方法增加可以实现增加servlet到jetty的功能：
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public
void
addServlet(String name, String pathSpec,
Class<?
extends
HttpServlet> clazz) {
//名称，url访问路径，处理类
addInternalServlet(name, pathSpec, clazz,
false
);
addFilterPathMapping(pathSpec, webAppContext);
}
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默认在HttpServer的构造函数中，会调用addDefaultServlets添加需要增加的servlets:
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public
HttpServer(String name, String bindAddress,
int
port,
boolean
findPort, Configuration conf, AccessControlList adminsAcl,
Connector connector, String[] pathSpecs)
throws
IOException {
....
webAppContext =
new
WebAppContext();
webAppContext.setDisplayName(name);
webAppContext.setContextPath(
"/"
);
webAppContext.setWar(appDir +
"/"
+ name);
....
addDefaultServlets();
....
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启动addDefaultServlets定义了默认加载的servlet:
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protected
void
addDefaultServlets() {
// set up default servlets
addServlet(
"stacks"
,
"/stacks"
, StackServlet.
class
);
addServlet(
"logLevel"
,
"/logLevel"
, LogLevel.Servlet.
class
);
addServlet(
"metrics"
,
"/metrics"
, MetricsServlet.
class
);
addServlet(
"jmx"
,
"/jmx"
, JMXJsonServlet.
class
);
addServlet(
"conf"
,
"/conf"
, ConfServlet.
class
);
}
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hadoop在多个地方都用到了HttpServer这个类：
比如在org.apache.hadoop.hdfs.server.datanode.DataNode类中：
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DataNode的构造函数--->startDataNode-->initDataXceiver+startInfoServer
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其中startInfoServer就是调用HttpServer类启动jetty的：
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private
HttpServer infoServer =
null
;
...
private
void
startInfoServer(Configuration conf)
throws
IOException {
// create a servlet to serve full-file content
InetSocketAddress infoSocAddr = DataNode.getInfoAddr(conf);
String infoHost = infoSocAddr.getHostName();
int
tmpInfoPort = infoSocAddr.getPort();
this
.infoServer = (secureResources ==
null
)
?
new
HttpServer(
"datanode"
, infoHost, tmpInfoPort, tmpInfoPort ==
0
,
conf,
new
AccessControlList(conf.get(DFS_ADMIN,
" "
)))
:
new
HttpServer(
"datanode"
, infoHost, tmpInfoPort, tmpInfoPort ==
0
,
conf,
new
AccessControlList(conf.get(DFS_ADMIN,
" "
)),
secureResources.getListener());
LOG.info(
"Opened info server at "
+ infoHost +
":"
+ tmpInfoPort);
.....
this
.infoServer.addInternalServlet(
null
,
"/streamFile/*"
, StreamFile.
class
);
//添加datanode专属的servlet
this
.infoServer.addInternalServlet(
null
,
"/getFileChecksum/*"
,
FileChecksumServlets.GetServlet.
class
);
this
.infoServer.setAttribute(
"datanode"
,
this
);
this
.infoServer.setAttribute(JspHelper.CURRENT_CONF, conf);
this
.infoServer.addServlet(
null
,
"/blockScannerReport"
,
DataBlockScanner.Servlet.
class
);
if
(WebHdfsFileSystem.isEnabled(conf, LOG)) {
infoServer.addJerseyResourcePackage(DatanodeWebHdfsMethods.
class
.getPackage().getName() +
";"
+ Param.
class
.getPackage().getName(),
WebHdfsFileSystem.PATH_PREFIX +
"/*"
);
}
this
.infoServer.start();
}
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小结如下：
1)HttpServer是对Jetty的简单封装
2)hadoop各个组件都会用到HttpServer,datanode/namenode,resourcemanager等
其主要功能有：Hadoop的内部状态显示，运行和管理
3)HttpServer的addDefaultServlets方法定义了通用的几个servlet（比如更改日志级别的servlet），在每个类中又会定义属于自己的servlet
本文转自菜菜光 51CTO博客，原文链接：http://blog.51cto.com/caiguangguang/1592799，如需转载请自行联系原作者
