目前公司大部分服务器操作系统还是centos6.5，tomcat用的是7，平时基本上没什么问题，启动也比较快，但是，最近有部分项目服务器更新至centos7.3 ，有些机器启动tomcat的时候巨慢无比，甚至达到10分钟左右，无法忍受，于是想办法解决这个问题，网上帖子也比较多，基本上能解决问题，还是简单记录一下吧：
tomcat wiki上有这么一段话：
Entropy Source Tomcat 7+ heavily relies on SecureRandom class to provide random values for its session ids and in other places. Depending on your JRE it can cause delays during startup if entropy source that is used to initialize SecureRandom is short of entropy. You will see warning in the logs when this happens, e.g.: <DATE> org.apache.catalina.util.SessionIdGenerator createSecureRandom
INFO: Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [5172] milliseconds.
There is a way to configure JRE to use a non-blocking entropy source by setting the following system property: -Djava.security.egd=file:/dev/./urandom Note the "/./" characters in the value. They are needed to work around known Oracle JRE bug #6202721. See also JDK Enhancement Proposal 123. It is known that implementation of SecureRandom was improved in Java 8 onwards. Also note that replacing the blocking entropy source (/dev/random) with a non-blocking one actually reduces security because you are getting less-random data. If you have a problem generating entropy on your server (which is common), consider looking into entropy-generating hardware products such as "EntropyKey".
从这段话中，可以找到解决方案，在启动的时候添加一个参数： -Djava.security.egd=file:/dev/./urandom
另外一种方式，网上帖子也有说，那就是修改jre/lib/security/java.security文件:
#securerandom.source=file:/dev/urandom
securerandom.source=file:/dev/./urandom
有人还推荐了另外一种方式，前提是需要服务器cpu支持DRNG特性：
1、查看是否支持：
cat /proc/cpuinfo | grep rdrand
2、如果支持，安装rngd服务(熵服务)：
yum install rngd-tools
3、启动服务
systemctl start rngd
如果你的CPU不支持DRNG特性或者使用虚拟机，可以使用/dev/unrandom来模拟。
cp /usr/lib/systemd/system/rngd.service /etc/systemd/system
编辑/etc/systemd/system/rngd.service service，ExecStart=/sbin/rngd -f -r /dev/urandom
systemctl daemon-reload重新载入服务
systemctl restart rngd重启服务
观察/proc/sys/kernel/random/entropy_avail：
watch -n 1 cat /proc/sys/kernel/random/entropy_avail
新打开一个shell，用dd命令测试随机数。dd if=/dev/random of=random.dat count=40960
参考：
https://wiki.apache.org/tomcat/HowTo/FasterStartUp
http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6202721
http://udn.yyuap.com/thread-129351-1-1.html