1. 絮絮叨叨

工作中，Java服务因为fatal error（致命错误，笔者称其为jvm crash），在服务运行日志中出现了致命错误的概要信息：

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x000000010a7d52e8, pid=47989, tid=11011
#
# JRE version: OpenJDK Runtime Environment Temurin-17.0.6+10 (17.0.6+10) (build 17.0.6+10)
# Java VM: OpenJDK 64-Bit Server VM Temurin-17.0.6+10 (17.0.6+10, mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, bsd-amd64)
# Problematic frame:
# V  [libjvm.dylib+0xada2e8]  Unsafe_GetByte(JNIEnv_*, _jobject*, _jobject*, long)+0xd8
#
# No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# An error report file with more information is saved as:
# /Users/xxx/IdeaProjects/study/hs_err_pid47989.log
#
# If you would like to submit a bug report, please visit:
#   https://github.com/adoptium/adoptium-support/issues
#

服务运行在k8s中，由于未提前设置fatal error日志的路径（挂载到宿主机目录），容器重启后该日志会丢失，无法深入排查原因
因此，需要查询jvm的配置，将fatal error日志写入指定目录，保证该日志持久化存储到宿主机磁盘

2. 配置jvm参数，实现日志的持久化存储

2.1 `-XX:ErrorFile`配置fatal error路径

通过查阅资料，了解到可以通过-XX:ErrorFile=filename配置hs_err日志的路径
下面的示例中，将fatal error的日志写入指定目录，文件名的%p会动态替换成改Java程序的PID（进程id）
```
-XX:ErrorFile=/var/log/java/java_error%p.log
```
默认将fatal error日志写入Java程序的working directory，且文件名为hs_err_pid.log；如果空间不足、权限不够等原因，fatal error日志将被写入系统的临时目录
详情见JDK官网的说明：
- JDK 8：A Fatal Error Log
- JDK 17：A Fatal Error Log，Command-Line Options

2.2 笔者的错误配置

考虑到服务每次重启的pid基本一致，如果多次出现fatal error，只使用pid的日志会被覆盖。
笔者结合之前配置heap dump的经验，添加了%t以生成类似2023-08-16_23-33-08的时间戳
```
-XX:ErrorFile=/data_path/var/log/hs_err_pid%p_%t.log
```
当再一次发生fatal error时，发现日志文件名为hs_err_pid6_%t.log，即%t未按照预期进行解析

2.3 `-XX:OnError`配置更新文件名

受问题（How to specify a unique name for the JVM crash log files?）启发，配置-XX:OnError：在日志生成后，执行shell命令为其添加时间戳

-XX:ErrorFile=/data_path/var/log/hs_err.log
-XX:OnError="time=`date +%Y%m%d_%H%M%S` && mv /data_path/var/log/hs_err.log /data_path/var/log/hs_err_\${time}.log"

3. 如何触发fatal error？

不管是验证相关JVM参数的配置，还是学习查看fatal error日志的内容，学会如何在触发fatal error是非常必要的

参考：Write Java code to crash the java virtual machine，通过如下代码可以成功在本地触发fatal error

import sun.misc.Unsafe;
import java.lang.reflect.Field;public class CrashTest {public static void main(String... args) throws Exception {getUnsafe().getByte(0);}private static Unsafe getUnsafe() throws NoSuchFieldException, IllegalAccessException {Field theUnsafe = Unsafe.class.getDeclaredField("theUnsafe");theUnsafe.setAccessible(true);return (Unsafe) theUnsafe.get(null);}
}

4. 待交流的问题

4.1 本地验证OK

按照上面的描述，笔者为CrashTest配置了如下JVM参数

-XX:ErrorFile=/data_path/study/hs_err.log
-XX:OnError="time=`date +%Y%m%d_%H%M%S` && echo $time && mv /data_path/hs_err.log /data_path//hs_err_${time}.log"

程序运行起来后，打印如下信息：

#
# A fatal error has been detected by the Java Runtime Environment:
#
#  SIGSEGV (0xb) at pc=0x000000010a49e2e8, pid=56245, tid=11011
#
# JRE version: OpenJDK Runtime Environment Temurin-17.0.6+10 (17.0.6+10) (build 17.0.6+10)
# Java VM: OpenJDK 64-Bit Server VM Temurin-17.0.6+10 (17.0.6+10, mixed mode, sharing, tiered, compressed oops, compressed class ptrs, g1 gc, bsd-amd64)
# Problematic frame:
# V  [libjvm.dylib+0xada2e8]  Unsafe_GetByte(JNIEnv_*, _jobject*, _jobject*, long)+0xd8
#
# No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
#
# An error report file with more information is saved as:
# /data_path/hs_err.log
#
# If you would like to submit a bug report, please visit:
#   https://github.com/adoptium/adoptium-support/issues
#
#
# -XX:OnError="time=`date +%Y%m%d_%H%M%S` && mv /data_path/hs_err.log /data_path/hs_err_${time}.log"
#   Executing /bin/sh -c "time=`date +%Y%m%d_%H%M%S` && mv /data_path/hs_err.log /data_path/hs_err_${time}.log" ...

最终，fatal error日志的文件名为hs_err_20230827_202458.log，符合预期！

4.2 测试环境验证失败

将此配置移动到线上服务，却发现fatal error日志的文件名为hs_err_.log不符合预期
怀疑：未能正确解析${time}

一个问答： How to add the timestamp of the fatal error occurrence to Java fatal error log filename，遇到了与笔者类似的问题

-XX:ErrorFile={{ .Values.server.data_dir }}/var/log/hs_err.log
-XX:OnError="mv {{ .Values.server.data_dir }}/var/log/hs_err.log {{ .Values.server.data_dir }}/var/log/hs_err_\$(date +%Y%m%d_%H%M%S).log"