Spring Boot启动第一次访问慢Creation of SecureRandom Instance is Slow in Spring Boot

Spring Boot启动第一次访问慢Creation of SecureRandom Instance is Slow in Spring Boot

Linux服务器上启动了一个Spring Boot项目后，第一次请求用了很长的时间，查看日志后发现一个关键内容：Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [155,058] milliseconds.

2020-11-18 14:49:57.096  INFO 29140 --- [nio-8181-exec-1] o.s.web.servlet.DispatcherServlet        : Completed initialization in 18 ms
2020-11-18 14:52:12.835  WARN 29140 --- [nio-8181-exec-1] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [135,666] milliseconds.
2020-11-18 14:52:12.843  WARN 29140 --- [nio-8181-exec-4] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [112,500] milliseconds.
2020-11-18 14:52:12.844  WARN 29140 --- [nio-8181-exec-6] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [21,618] milliseconds.
2020-11-18 14:52:12.844  WARN 29140 --- [nio-8181-exec-3] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [45,144] milliseconds.
2020-11-18 14:52:12.844  WARN 29140 --- [nio-8181-exec-2] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [47,941] milliseconds.
2020-11-18 14:52:12.844  WARN 29140 --- [nio-8181-exec-7] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [49,850] milliseconds.
2020-11-18 14:52:12.844  WARN 29140 --- [nio-8181-exec-8] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [82,910] milliseconds.
2020-11-18 14:52:12.844  WARN 29140 --- [nio-8181-exec-9] o.a.c.util.SessionIdGeneratorBase        : Creation of SecureRandom instance for session ID generation using [SHA1PRNG] took [109,737] milliseconds.

原因分析

从日志的内容我们基本可以知道，是服务接收到第一个请求时，需要为当前会话创建一个Session ID。而创建这个Session ID需要用到 java.security.SecureRandom 这个类来生成一个随机数。就是创建这个类的实例消耗了很长的时间。在类UNIX的系统上，SecureRandom 会默认使用 /dev/random 文件来获取随机数，通过该文件获取随机数是可能会阻塞的。具体内容可参考维基百科：https://en.wikipedia.org/wiki//dev/random。

除了 /dev/random 文件，其实还可以通过另一个文件 /dev/urandom 来获取随机数，两者最主要的一个区别就是通过 /dev/urandom 文件来获取随机数是不会阻塞的。

解决办法

查明原因后，我们就可以通过以下办法来解决这个问题，我们只需要在启动Spring Boot项目时通过命令行指定获取随机数的文件为 /dev/urandom 即可：

java -jar -Djava.security.egd=file:/dev/./urandom project.jar