objc[970]: Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/bin/java and /Library/Java/JavaVirtualMachines/jdk1.8.0_91.jdk/Contents/Home/jre/lib/libinstrument.dylib. One of the two will be used. Which one is undefined.

在Mac上裝了Intellij Idea學習Kotlin，隨手用IJ帶的Kotlin Java工程模板創建了一個測試工程，控制臺報了這樣一個Error，意思是這個JavaLaunchHelper類被實現了兩次。無奈之下搜索了萬能的Stack Overflow，找到了解決方案。

鏈接如下：http://stackoverflow.com/questions/43003012/objc3648-class-javalaunchhelper-is-implemented-in-both

引入最高票的回答如下：

You can find all the details here:
IDEA-170117 "objc: Class JavaLaunchHelper is implemented in both ..." warning in Run consoles
It's the old bug in Java on Mac that got triggered by the Java Agent being used by the IDE when starting the app. This message is harmless and is safe to ignore. Oracle developer's comment:
The message is benign, there is no negative impact from this problem since both copies of that class are identical (compiled from the exact same source). It is purely a cosmetic issue.
The problem is fixed in Java 9 and in Java 8 update 152.
If it annoys you or affects your apps in any way (it shouldn't), the workaround for IntelliJ IDEA is to disable idea_rt launcher agent by adding idea.no.launcher=true into idea.properties (Help | Edit Custom Properties...). The workaround will take effect on the next restart of the IDE.
I don't recommend disabling IntelliJ IDEA launcher agent, though. It's used for such features as graceful shutdown (Exit button), thread dumps, workarounds a problem with too long command line exceeding OS limits, etc. Losing these features just for the sake of hiding the harmless message is probably not worth it, but it's up to you.

這位外國碼友清楚地解釋了這個Error的原因，大概意思是說這是Mac上面Java的一個老Bug了，會在那些使用了Java Agent的IDE上運行應用時觸發，但這個Error對程序是無影響的，可以無視。在Java 9和Java 1.8.152版本里已經修復了。

解決方案:
　　點擊IJ最上面菜單的Help-Edit Custom Properties，沒有這個properties文件的話，IJ會提示創建，然后在里面加上
idea.no.launcher=true
重啟IDEA