C/C++程序通过hdfs.h访问HDFS，运行时遇到如下错误，会是什么原因了？（注：hadoop安装在/data/hadoop/hadoop-2.4.0，而/data/hadoop/current是指向它的软链接）：
loadFileSystems error:
(unable to get stack trace for java.lang.NoClassDefFoundError exception: ExceptionUtils::getStackTrace error.)
hdfsBuilderConnect(forceNewInstance=0, nn=172.25.40.171, port=9001, kerbTicketCachePath=(NULL), userName=(NULL)) error:
(unable to get stack trace for java.lang.NoClassDefFoundError exception: ExceptionUtils::getStackTrace error.)
E0507 19:02:57.251287 17859 hdfs_persistence.cpp:31] connect hdfs://172.25.40.171:9001 error: 未知的错误 255
上述信息中的关键项是“NoClassDefFoundError”和“ExceptionUtils”，也就是找不到ExceptionUtils，一般可推断是因为找不到相应的jar文件，Google搜索“ExceptionUtils jar”，发现“ExceptionUtils”应当是在包apache-commons-lang.jar中。
进一步用Google去搜索“apache-commons-lang.jar”，找到下载网址：http://commons.apache.org/proper/commons-lang/download_lang.cgi，上面可以下载commons-lang3-3.3.2-bin.tar.gz，解压后就可以看到commons-lang3-3.3.2.jar。
hadoop的二进制安装包，应当自带了这个文件，通过努力，在hadoop安装目录下的share/hadoop/tools/lib子目录下发现了commons-lang-2.6.jar，应当就是它了。
使用命令“hadoop classpath”可以，可以查看hadoop的classpath：
./hadoop classpath
/data/hadoop/hadoop-2.4.0/etc/hadoop:/data/hadoop/hadoop-2.4.0/share/hadoop/common/lib/*:/data/hadoop/hadoop-2.4.0/share/hadoop/common/*:/data/hadoop/hadoop-2.4.0/share/hadoop/hdfs:/data/hadoop/hadoop-2.4.0/share/hadoop/hdfs/lib/*:/data/hadoop/hadoop-2.4.0/share/hadoop/hdfs/*:/data/hadoop/hadoop-2.4.0/share/hadoop/yarn/lib/*:/data/hadoop/hadoop-2.4.0/share/hadoop/yarn/*:/data/hadoop/hadoop-2.4.0/share/hadoop/mapreduce/lib/*:/data/hadoop/hadoop-2.4.0/share/hadoop/mapreduce/*:/data/hadoop/current/contrib/capacity-scheduler/*.jar
遗憾的是，tools并没有出现，这应当就是问题所在，于是手工将它加进去（hadoop被安装在/data/hadoop/current）：
export CLASSPATH=`/data/hadoop/current/bin/hadoop classpath`:/data/hadoop/current/share/hadoop/tools/lib
重运行，发现还是不行：
loadFileSystems error:
(unable to get stack trace for java.lang.NoClassDefFoundError exception: ExceptionUtils::getStackTrace error.)
hdfsBuilderConnect(forceNewInstance=0, nn=172.25.40.171, port=9001, kerbTicketCachePath=(NULL), userName=(NULL)) error:
(unable to get stack trace for java.lang.NoClassDefFoundError exception: ExceptionUtils::getStackTrace error.)
E0507 19:52:48.197748 27787 hdfs_persistence.cpp:31] connect hdfs://172.25.40.171:9001 error: 未知的错误 255
好吧，来把硬的，直接指定commons-lang-2.6.jar：
export CLASSPATH=`/data/hadoop/current/bin/hadoop classpath`:/data/hadoop/current/share/hadoop/tools/lib/commons-lang-2.6.jar
重新运行程序，ExceptionUtils错误消失了，但遇到新错误：
loadFileSystems error:
java.lang.NoClassDefFoundError: org/apache/hadoop/fs/FileSystem
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.fs.FileSystem
at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
hdfsBuilderConnect(forceNewInstance=0, nn=172.25.40.171, port=9001, kerbTicketCachePath=(NULL), userName=(NULL)) error:
java.lang.NoClassDefFoundError: org/apache/hadoop/conf/Configuration
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.conf.Configuration
at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
E0507 19:12:56.522274 19834 hdfs_persistence.cpp:31] connect hdfs://172.25.40.171:9001 error: 未知的错误 255
仍然是NoClassDefFoundError错误，原因应当是一样的：classpath中漏了哪个目录，这就要看FileSystem和Configuration在哪个jar中了。
FileSystem是在hadoop-common-2.4.0.jar中，而Configuration在commons-configuration-1.6.jar中，它们两个都已经在classpath上，为何还报错了？
对java不熟，尝试将hadoop-common-2.4.0.jar和commons-configuration-1.6.jar直接加入到classpath：
export CLASSPATH=`/data/hadoop/current/bin/hadoop classpath`:/data/hadoop/current/share/hadoop/tools/lib/commons-lang-2.6.jar:/data/hadoop/current/share/hadoop/common/hadoop-common-2.4.0.jar:/data/hadoop/current/share/hadoop/common/lib/commons-configuration-1.6.jar
发现FileSystem和Configuration错误消失了，说明有效：
loadFileSystems error:
java.lang.NoClassDefFoundError: org/apache/commons/logging/LogFactory
at org.apache.hadoop.fs.FileSystem.(FileSystem.java:95)
Caused by: java.lang.ClassNotFoundException: org.apache.commons.logging.LogFactory
at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 1 more
hdfsBuilderConnect(forceNewInstance=0, nn=172.25.40.171, port=9001, kerbTicketCachePath=(NULL), userName=(NULL)) error:
java.lang.NoClassDefFoundError: org/apache/commons/logging/LogFactory
at org.apache.hadoop.conf.Configuration.(Configuration.java:169)
Caused by: java.lang.ClassNotFoundException: org.apache.commons.logging.LogFactory
at java.net.URLClassLoader$1.run(URLClassLoader.java:372)
at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:360)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 1 more
通过寻找，LogFactory在tools目录下的commons-logging-1.1.3.jar中，把它也加入到classpath中：
export CLASSPATH=`/data/hadoop/current/bin/hadoop classpath`:/data/hadoop/current/share/hadoop/tools/lib/commons-lang-2.6.jar:/data/hadoop/current/share/hadoop/common/hadoop-common-2.4.0.jar:/data/hadoop/current/share/hadoop/common/lib/commons-configuration-1.6.jar:/data/hadoop/current/share/hadoop/common/lib/commons-logging-1.1.3.jar
再次运行，还是报错：
java.util.ServiceConfigurationError: org.apache.hadoop.fs.FileSystem: Provider org.apache.hadoop.fs.viewfs.ViewFileSystem could not be instantiated: java.lang.NoClassDefFoundError: com/google/common/collect/Maps
at java.util.ServiceLoader.fail(ServiceLoader.java:232)
at java.util.ServiceLoader.access$100(ServiceLoader.java:185)
at java.util.ServiceLoader$LazyIterator.nextService(ServiceLoader.java:384)
at java.util.ServiceLoader$LazyIterator.next(ServiceLoader.java:404)
at java.util.ServiceLoader$1.next(ServiceLoader.java:480)
at org.apache.hadoop.fs.FileSystem.loadFileSystems(FileSystem.java:2364)
还是类似的错误，这样下会去搞死人。通过上述的一些操作，估计需要将所有的jar文件一个个的将入到classpath中。由于对java不熟悉，也只有先这样做一做了：
find /data/hadoop/current/ -name *.jar|awk '{ printf("export CLASSPATH=%s:$CLASSPATH\n", $0); }'
终于可以了^_^，好折腾啊。