通过 log4j 发送 JNI C stderr/stdout

2022-01-25 00:00:00 logging java java-native-interface

我的 java 应用程序使用 JNI 调用用 C 编写的库.这个本机库将错误记录到 stderr，但我想以某种方式通过我的 log4j 记录器重定向错误流.这可能吗?

My java app uses JNI to invoke a library written in C. This native library logs errors to stderr but I would like to redirect the error stream through my log4j logger somehow. Is this possible?

C 库是外部的 - 我没有源代码，所以无法更改.

The C library is external - I don't have the source, so cannot change it.

谢谢

推荐答案

注意:这个答案我没试过；YMMV.

Note: I have not tried this answer; YMMV.

POSIX 方法 freopen 将更改与流关联的基础文件.正如手册页所述:freopen() 函数的主要用途是更改与标准文本流(stderr、stdin 或 stdout)关联的文件".

The POSIX method freopen will change the underlying file associated with a stream. As the manpage states: "The primary use of the freopen() function is to change the file associated with a standard text stream (stderr, stdin, or stdout)".

因此，您可以创建自己的 JNI 库，将流重定向到文件中.但是，要完成这项工作有几个严重的障碍:

So, you could create your own JNI library that simply redirects the stream into a file. However, there are several serious roadblocks to making this work:

  1. 您需要确保您的 Java程序本身不使用标准流，因为它们会也重定向了.一种解决方法这是为了改变 System.out 等当你的程序启动时，其他的东西.
  2. 有可能第三方库绕过标准文本流，并直接写入底层文件描述符.它是不太可能但可能.我不能记住是否 freopen() 只是更改使用的文件描述符缓冲的流，或者实际上重新打开底层 fd.
  3. 你会还是有连接问题将文件"更改为记录器.
  1. You'll need to ensure that your Java program doesn't itself use standard streams, because they'll be redirected too. A workaround for this is to change System.out et al to something else when your program starts up.
  2. It's possible that the third-party library bypasses the standard text streams, and writes directly to the underlying file descriptor. It's unlikely but possible. I can't remember whether freopen() just changes the file descriptor used by the buffered stream, or actually re-opens the underlying fd.
  3. You'll still have the problem of connecting changes to the "file" to a logger.

最后一点是迄今为止最大的一点:stdio 写入操作系统级别的文件描述符.您必须创建一些 Java 级别的代码来读取该描述符并写入日志.我的第一个想法是您必须使用命名管道，然后启动一个新的 Java 线程来读取该管道.但这不能移植到不同的操作系统，并且您可以在程序的配置中管理管道名称.

That last point is by far the biggest: stdio writes to an OS-level file descriptor. You would have to create some Java-level code to read that descriptor and write to a log. My first thought is that you'd have to use a named pipe, and then spin up a new Java thread to read that pipe. But that won't be portable to different operating systems, and will that you manage the pipe name in your program's configuration.

假设这是一个服务器程序，并且本身不处理标准 IO 流，我认为您最好的解决方案是使用控制台记录器配置 Log4J，并将所有控制台输出重定向到一个文件.

Assuming that this is a server program, and does not itself process the standard IO streams, I think your best solution will be to configure Log4J with a console logger, and simply redirect all console output to a file.

或者与编写库的人交谈，让他们添加可配置的日志记录.

Or talk to the people who wrote the library, to get them to add configurable logging.

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