I updated both my JRE and JDK to java 8 update 20 this morning, first time updating to java 8. I made sure eclipse switched from using JRE7 to JRE8 and I changed my system variables etc. When trying to launch any of the mods I'm making for testing I get this error.

 

[11:55:50] [main/INFO] [GradleStart]: userProperties: {}

[11:55:50] [main/INFO] [GradleStart]: assetsDir: C:\Users\Mitch\.gradle\caches\minecraft\assets

[11:55:50] [main/INFO] [GradleStart]: assetIndex: 1.7.10

[11:55:50] [main/INFO] [GradleStart]: accessToken: test

[11:55:50] [main/INFO] [GradleStart]: version: 1.6

[11:55:50] [main/INFO] [GradleStart]: tweakClass: cpw.mods.fml.common.launcher.FMLTweaker

[11:55:50] [main/INFO] [GradleStart]: username: {HIDDEN}

[11:55:50] [main/INFO] [GradleStart]: Extra: []

[11:55:50] [main/INFO] [GradleStart]: Password found, attempting login

[11:55:50] [main/INFO]: Logging in with username & password

[11:55:51] [main/INFO] [GradleStart]: Login Succesful!

[11:55:51] [main/INFO] [GradleStart]: Running with arguments: [--userProperties, {}, --assetsDir, C:\Users\Mitch\.gradle\caches\minecraft\assets, --assetIndex, 1.7.10, --accessToken, {REDACTED}, --version, 1.6, --uuid, 91659ea234d4484eaa84ef43b9e19bdb, --tweakClass, cpw.mods.fml.common.launcher.FMLTweaker, --username, 61352151511]

[11:55:51] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLTweaker

[11:55:51] [main/INFO] [LaunchWrapper]: Using primary tweak class name cpw.mods.fml.common.launcher.FMLTweaker

[11:55:51] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLTweaker

[11:55:51] [main/INFO] [FML]: Forge Mod Loader version 7.10.25.1199 for Minecraft 1.7.10 loading

[11:55:51] [main/INFO] [FML]: Java is Java HotSpot 64-Bit Server VM, version 1.8.0_20-ea, running on Windows 7:amd64:6.1, installed at C:\Program Files\Java\jre1.8.0_20

[11:55:51] [main/INFO] [FML]: Managed to load a deobfuscated Minecraft name- we are in a deobfuscated environment. Skipping runtime deobfuscation

[11:55:51] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker

[11:55:51] [main/INFO] [LaunchWrapper]: Loading tweak class name cpw.mods.fml.common.launcher.FMLDeobfTweaker

[11:55:51] [main/INFO] [LaunchWrapper]: Calling tweak class cpw.mods.fml.common.launcher.FMLInjectionAndSortingTweaker

[11:55:51] [main/ERROR] [LaunchWrapper]: Unable to launch

java.util.ConcurrentModificationException

at java.util.ArrayList$Itr.checkForComodification(Unknown Source) ~[?:1.8.0_20-ea]

at java.util.ArrayList$Itr.remove(Unknown Source) ~[?:1.8.0_20-ea]

at net.minecraft.launchwrapper.Launch.launch(Launch.java:117) [launchwrapper-1.9.jar:?]

at net.minecraft.launchwrapper.Launch.main(Launch.java:28) [launchwrapper-1.9.jar:?]

at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[?:1.8.0_20-ea]

at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_20-ea]

at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[?:1.8.0_20-ea]

at java.lang.reflect.Method.invoke(Unknown Source) ~[?:1.8.0_20-ea]

at GradleStart.bounce(GradleStart.java:107) [start/:?]

at GradleStart.startClient(GradleStart.java:100) [start/:?]

at GradleStart.main(GradleStart.java:65) [start/:?]

 

Any help would be appreciated

I made sure eclipse switched from using JRE7 to JRE8 and I changed my system variables etc.

 

Switch them back to 1.7. Just because you installed 1.8 doesn't mean 1.7 is gone. (unless you uninstalled it... in which case: why?)

As far as I've read Minecraft is built on 1.6, and works on 1.7, but 1.8 messes it up a bit. So until Mojang updates you shouldn't update.

 

Edit: Shouldn't update JDKs, I mean. All JREs should be backwards compatible.

I did uninstall it, reason being is because you don't need multiple versions of java installed at once. I updated because I like being as up to date with stuff on my computer as possible and launching from the minecraft launcher itself works just fine and I can play minecraft but when I try launching from eclipse with the GradleStart thing it doesn't work so I think it's a forge issue

I meant JDK not JRE. Go ahead and uninstall the prior JRE since all newer versions are backwards compatible.

But there is no reason to uninstall (or even update) the JDK version because it's best to develop in the same version as the official.

 

Unless you're developing another java app using 1.8, you don't need JDK 1.8 and won't benefit from using it unsupported in your Minecraft mod. If it ain't broke, don't "fix" it.

I made sure eclipse switched from using JRE7 to JRE8 and I changed my system variables etc.

 

Switch them back to 1.7. Just because you installed 1.8 doesn't mean 1.7 is gone. (unless you uninstalled it... in which case: why?)

As far as I've read Minecraft is built on 1.6, and works on 1.7, but 1.8 messes it up a bit. So until Mojang updates you shouldn't update.

 

Edit: Shouldn't update JDKs, I mean. All JREs should be backwards compatible.

Well, how far is a JRE backwards compatible (Java 9 will only be backward compatible with Java 8 when Java 9 is released in a few years...)

 

I did uninstall it, reason being is because you don't need multiple versions of java installed at once. I updated because I like being as up to date with stuff on my computer as possible and launching from the minecraft launcher itself works just fine and I can play minecraft but when I try launching from eclipse with the GradleStart thing it doesn't work so I think it's a forge issue

Well, as an developer there is a reason: I have installed Java 7 and Java 8, because for this few applications that are using Java 7 I compile them with Java 7, because Java 8 didn't support a 100% bug free Java 7 compiler (well, eclipse says that I need to install Java 7, too, if I want to compile a program 100% correct that is using Java 7...)

 

Well, to the error, it could be a problem with the Early Access version. Try it with Java 8 Update 11. (Use stable builds, not ea (Early Access) versions, because this versions can produce problems...)

Well, how far is a JRE backwards compatible (Java 9 will only be backward compatible with Java 8 when Java 9 is released in a few years...)

Java 9 is getting split into 2 code lines near the end of development, so JRE 1.9 is going to be limited to 1.8 as the new code is being designed.

But 1.8 should work back to at least 1.5. (The last major change. More than old enough for MOST java programs out there.) Java 9 is 2 years away though, so...

 

Well, as an developer there is a reason: I have installed Java 7 and Java 8, because for this few applications that are using Java 7 I compile them with Java 7, because Java 8 didn't support a 100% bug free Java 7 compiler (well, eclipse says that I need to install Java 7, too, if I want to compile a program 100% correct that is using Java 7...)

Exactly. Even if you're coding another app in 1.8 there is no need to uninstall 1.6 (or 1.7) since that is the native JDK to Minecraft and thus the best to code in.