Class JavaLaunchHelper is implemented in both /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/bin/java (0x10d19c4c0) and /Library/Java/JavaVirtualMachines/jdk1.8.0_121.jdk/Contents/Home/jre/lib/libinstrument.dylib (0x10ea194e0). One of the two will be used. Which one is undefined.

昨晚在Mac上裝了Intellij Idea，隨手用IJ帶的Java工程模板建立了一個測試工程，控制檯報了這樣一個Error。

意思是這個JavaLaunchHelper類被實現了兩次。無奈之下搜尋了萬能的Stack Overflow，找到了解決方案。

引用高票回答如下

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.

If it annoys you or affects your apps in any way, 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...).

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上執行應用時觸發，但這個Error對程式是無影響的，可以無視。在Java 9和Java 1.8.152版本里已經修復了。

解決方案:

點選IJ最上面選單的Help-Edit Custom Properties，沒有這個properties檔案的話，IJ會提示建立，然後在裡面加上