原文地址：

CentOS 7 下Tomcat启动超慢的原因及解决方案 - 简书
https://www.jianshu.com/p/de2df2d92d92

现象

CentOS 7系统中安装好openjdk和Tomcat后，启动过程很慢，长达数分钟，日志如下：

17:27:53.596 INFO [main] org.apache.catalina.core.StandardEngine.startInternal Starting Servlet Engine: Apache Tomcat/8.5.38
17:27:53.644 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/docs]
17:32:31.001 WARNING [localhost-startStop-1] org.apache.catalina.util.SessionIdGeneratorBase.createSecureRandom Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [276,660] milliseconds.
17:32:31.022 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/docs] has finished in [277,378] ms
17:32:31.022 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/manager]
17:32:31.101 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/manager] has finished in [79] ms
17:32:31.101 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/examples]
17:32:31.509 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/examples] has finished in [408] ms
17:32:31.510 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/host-manager]
17:32:31.559 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/host-manager] has finished in [49] ms
17:32:31.559 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/ROOT]
17:32:31.576 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/ROOT] has finished in [17] ms
17:32:31.605 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["http-nio-8080"]
17:32:31.660 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["ajp-nio-8009"]
17:32:31.662 INFO [main] org.apache.catalina.startup.Catalina.start Server startup in 278084 ms

tomcat启动耗时278084ms折合278秒，对于刚刚安装的干净tomcat，这肯定是不对劲的。
其中有一条日志引起了笔者的注意：

17:32:31.001 WARNING [localhost-startStop-1] org.apache.catalina.util.SessionIdGeneratorBase.createSecureRandom Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [276,660] milliseconds.

显然tomcat执行到这里时出问题了，google了一下，经过一番搜索明白了其中的缘由。

原因

在tomcat官方wiki文档的HowToFasterStartUp章节中，Entropy Source部分有一段这样的说明：

<DATE> org.apache.catalina.util.SessionIdGenerator createSecureRandom
INFO: Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [5172] milliseconds.

-Djava.security.egd = file:/dev/./urandom

从这里我们得知Tocmat的Session ID是通过SHA1PRNG算法计算得到的，计算Session ID的时候必须有一个密钥，为了提高安全性Tomcat在启动的时候会通过随机生成一个密钥，它强依赖于获取熵池中的随机数来进行创建。
那么什么是/dev/random？什么是熵池？

/dev/random
从维基百科得知，在UNIX操作系统（包括类UNIX系统）中，/dev/random是一个特殊的设备文件，可以用作随机数生成器或伪随机数生成器。
Linux内核中的是第一个以背景噪声产生真正的随机数产生的实现，它允许程序访问来自设备驱动程序或其它来源的背景噪声。
Linux上有两个通用的随机设备：/dev/random和/dev/urandom。其中/dev/random的随机性最好，因为它是一个阻塞的设备。而/dev/random的一个副本是/dev/urandom（“unblocked”，非阻塞的随机数生成器），它会重复使用熵池中的数据以产生伪随机数据。这表示对/dev/urandom的读取操作不会产生阻塞，但其输出的熵可能小于/dev/random的。所以它可以作为生成较低强度密码的伪随机数生成器，不建议用于生成高强度长期密码。

熵池
熵池本质上是若干字节，/proc/sys/kernel/random/entropy_avail中存储了熵池现在的大小，/proc/sys/kernel/random/poolsize是熵池的最大容量，单位都是bit。如果entropy_avail的值小于要产生的随机数bit数，那么/dev/random就会堵塞。

为什么熵池不够用？

熵池实际上是从各种noice source中获取数据，noice source可能是键盘事件、鼠标事件、设备时钟中等。linux内核从2.4升级到2.6时，处于安全性的考虑，废弃了一些source。source减少了，熵池补给的速度当然也变慢，进而不够用。
其实，通过消耗熵池，可以构造DDOS攻击。原理很简单，熵池空了，依赖随机数的业务（SSL，加密等）就不能正常进行。

通过以上信息，笔者得知该问题是由于熵池不足导致的。怎么解决？

解决方案

方法一、降低熵生成的随机性要求

使用非阻塞性的生成器/dev/urandom代替/dev/random。

1、可在JVM环境中配置
通过配置发生器指定熵收集守护进程
修改$JAVA_PATH/jre/lib/security/java.security中参数securerandom.source为：

securerandom.source=file:/dev/./urandom

2、也可在Tomcat环境中配置
通过配置JRE使用非阻塞的Entropy Source获取熵
在$TOMCAT_HOME/bin/catalina.sh中加入：

if [[ "$JAVA_OPTS" != *-Djava.security.egd=* ]]; thenJAVA_OPTS="$JAVA_OPTS -Djava.security.egd=file:/dev/./urandom"
fi

这个系统属性egd表示熵收集守护进程(entropy gathering daemon)。

方法二、使用熵生成器补充熵池（推荐）

1、[硬件随机数生成器]安装并使用rng-tools作为额外的熵随机数生成器（推荐）

# 安装rng-tools
yum install rng-tools -y
# 测试rngd
rngd -f
# 启动rngd
systemctl start rngd
# 设置自启动
systemctl enable rngd
# 查看自启动列表
systemctl list-unit-files

cat /dev/random命令会消耗熵池，rngd守护进程会补充熵池，可使用如下命令来测试随机数生成的情况：

cat /dev/random | rngtest -c 100
# 测试结果
rngtest 6
Copyright (c) 2004 by Henrique de Moraes Holschuh
This is free software; see the source for copying conditions.  There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.rngtest: starting FIPS tests...
rngtest: bits received from input: 2000032
rngtest: FIPS 140-2 successes: 100
rngtest: FIPS 140-2 failures: 0
rngtest: FIPS 140-2(2001-10-10) Monobit: 0
rngtest: FIPS 140-2(2001-10-10) Poker: 0
rngtest: FIPS 140-2(2001-10-10) Runs: 0
rngtest: FIPS 140-2(2001-10-10) Long run: 0
rngtest: FIPS 140-2(2001-10-10) Continuous run: 0
rngtest: input channel speed: (min=13.823; avg=42.721; max=10761.019)Kibits/s
rngtest: FIPS tests speed: (min=93.041; avg=129.531; max=136.239)Mibits/s
rngtest: Program run time: 45733452 microseconds

2、[软件随机数生成器]在rng-tools仍不满足的情况下，可使用haveged作为额外的熵随机数生成器

要检查是否需要 Haveged，可使用下面命令查看当前收集到的熵：

cat /proc/sys/kernel/random/entropy_avail

如果结果比较低 (<1000)，建议安装 haveged，否则加密程序会处于等待状态，直到系统有足够的熵。
 

# 安装haveged
yum install haveged -y
# 启动haveged
systemctl start haveged
# 设置自启动
systemctl enable haveged

安装 haveged 之后，可以再次查看系统熵看下有无提升。

因为方法一存在一定的不安全性，且需要对环境进行配置，为了满足熵的需要，这里笔者选择了第二种方法，使用rng-tools作为额外的熵随机数生成器，同以上操作后顺利解决了问题。

操作后重启tomcat日志如下，启动速度快了两个数量级：

17:58:07.068 INFO [main] org.apache.catalina.core.StandardEngine.startInternal Starting Servlet Engine: Apache Tomcat/8.5.38
17:58:07.088 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/docs]
17:58:07.740 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/docs] has finished in [652] ms
17:58:07.740 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/manager]
17:58:07.815 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/manager] has finished in [75] ms
17:58:07.816 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/examples]
17:58:08.241 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/examples] has finished in [425] ms
17:58:08.241 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/host-manager]
17:58:08.268 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/host-manager] has finished in [27] ms
17:58:08.269 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deploying web application directory [/opt/tomcat_8_5_38/webapps/ROOT]
17:58:08.306 INFO [localhost-startStop-1] org.apache.catalina.startup.HostConfig.deployDirectory Deployment of web application directory [/opt/tomcat_8_5_38/webapps/ROOT] has finished in [37] ms
17:58:08.335 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["http-nio-8080"]
17:58:08.424 INFO [main] org.apache.coyote.AbstractProtocol.start Starting ProtocolHandler ["ajp-nio-8009"]
17:58:08.429 INFO [main] org.apache.catalina.startup.Catalina.start Server startup in 1411 ms

参考文档：
https://stackoverflow.com/questions/40383430/tomcat-takes-too-much-time-to-start-java-securerandom
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=6202721
http://openjdk.java.net/jeps/123
https://zh.wikipedia.org/zh-hans//dev/random
https://wiki.apache.org/tomcat/HowTo/FasterStartUp
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/security_guide/sect-security_guide-encryption-using_the_random_number_generator
https://wiki.archlinux.org/index.php/Rng-tools
https://wiki.archlinux.org/index.php/Haveged
https://www.digitalocean.com/community/tutorials/how-to-setup-additional-entropy-for-cloud-servers-using-haveged

作者：风澈vio
链接：https://www.jianshu.com/p/de2df2d92d92
来源：简书
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