容器被无法获取java进程调试

容器被无法获取java进程调试

今天尝试在容器内使用arthas调试，发现无法attch到java进程，jstack命令也无法获取到java进程

问题描述

1. 使用arthas提示错误

   Unable to get pid of LinuxThreads manager thread
   

相关问题连接

1. jmap not happy on alpine · Issue #76 · docker-library/openjdk · GitHub

   结论：

   If you keep bash as PID 1, you'd no longer get signals from docker stop and docker kill (without having to write traps). With docker 1.13 you can use --init to have docker put in tini as PID 1; it'll forward signals and reap zombies.（自行翻译）

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2. (9条消息) docker报错：Unable to get pid of LinuxThreads manager thread及openjdk-alpine镜像无法打印线程堆栈和内存堆栈问题_学亮编程手记-CSDN博客

   结论：

   使用alpine方式打包镜像【FROM openjdk:8-jdk-alpine】，会出现这种情况。通过使用tini插件，可以解决。

解决方法

1. 修改Dockerfile

   添加tini（添加RUN apk add --no-cache tini，并且在ENTRYPOINT添加tini）：

   FROM openjdk:8-jre-alpine
   
   ...
   
   # Install Tini
   RUN apk add --no-cache tini
   
   ENTRYPOINT ["/sbin/tini", "--", "sh", "entrypoint.sh"]
   
   

问题分析

because PID 1 is special and it doesn’t handle any signal unless explicitly declared. So the workaround is to have the java process spawned at non PID 1.

因为启动java的进程ID为1，而PID1是特殊的进程号，不处理任何信号，除非显示声明。所以，我们的解决方案就是：让java进程的ID不为1。而tini就是这个作用：*All Tini does is spawn a single child (Tini is meant to be run in a container), and wait for it to exit. All the while reaping zombies and performing signal forwarding.*

Tini所做的就是生成一个子节点(Tini是在容器中运行的)，然后等待它退出。进行信号转发。

结论

学习了新的技能。