elasticsearch之JAVA环境变量报错：could not find java; set JAVA_HOME or ensure java is in PATH

在以RPM包安装elasticsearch过程中出现报错JAVA环境的问题：

● elasticsearch.service - Elasticsearch
   Loaded: loaded (/usr/lib/systemd/system/elasticsearch.service; enabled; vendor preset: disabled)
   Active: failed (Result: exit-code) since Mon 2019-01-14 11:27:16 CST; 5s ago
     Docs: http://www.elastic.co
  Process: 2035 ExecStart=/usr/share/elasticsearch/bin/elasticsearch -p ${PID_DIR}/elasticsearch.pid --quiet (code=exited, status=1/FAILURE)
 Main PID: 2035 (code=exited, status=1/FAILURE)

Jan 14 11:27:16 ip-172-31-30-62.ec2.internal systemd[1]: Started Elasticsearch.
Jan 14 11:27:16 ip-172-31-30-62.ec2.internal systemd[1]: Starting Elasticsearch...
Jan 14 11:27:16 ip-172-31-30-62.ec2.internal elasticsearch[2035]: which: no java in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin)
Jan 14 11:27:16 ip-172-31-30-62.ec2.internal elasticsearch[2035]: could not find java; set JAVA_HOME or ensure java is in PATH
Jan 14 11:27:16 ip-172-31-30-62.ec2.internal systemd[1]: elasticsearch.service: main process exited, code=exited, status=1/FAILURE
Jan 14 11:27:16 ip-172-31-30-62.ec2.internal systemd[1]: Unit elasticsearch.service entered failed state.
Jan 14 11:27:16 ip-172-31-30-62.ec2.internal systemd[1]: elasticsearch.service failed.

也就是说就算是你设置好JAVA全局环境变量后，elasticsearch启动的时候依然是在上面红色字体的PATH变量里面找java变量。这个不清楚究竟是什么原因。我们暂时把它当做是丢环境变量的原因吧。不过我们倒是有其他的解决方法：

在/etc/sysconfig/elasticsearch的这个文件里面设置JAVA_HOME环境变量：

################################
# Elasticsearch
################################

# Elasticsearch home directory
#ES_HOME=/usr/share/elasticsearch

# Elasticsearch Java path
JAVA_HOME=/usr/local/jdk

# Elasticsearch configuration directory
ES_PATH_CONF=/etc/elasticsearch

# Elasticsearch PID directory
#PID_DIR=/var/run/elasticsearch

就是上面背景为青色的字体，设置一下环境变量后就可以正常启动了。

有时候也是编译环境的问题，比如上面这个问题就是，JAVA的jdk是源码安装的，而elasticsearch是yum一键安装的，因此很容易出现问题，所以我建议两者的安装尽量一致，jdk也使用yum安装比较好，这个问题就不会出现了。