Basically what's happening is, the Minecraft Launcher, for whatever reason, can't access the main class in the jar. When I click "play", it downloads all the necessary files (jars, etc.), closes the launcher as if it were about to start, and then immediately opens back up with an error message under the Game Output tab.
The error message is this: "OpenJDK 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
Error: Could not find or load main class net.minecraft.client.main.Main"
I have tried deleting the jar file, forcing it to re-download, but it still happens. I have also observed an identical error occurring on a separate computer, running the same operating system, java version, etc.
I don't think the jar is corrupt, because inside the jar, there is, actually, a net.minecraft.client.main.Main class.
UPDATE: I tried again with Minecraft 1.0, and the same thing happened but it said "Error: Could not find or load main class net.minecraft.launchwrapper.Launch". I'm assuming this is because the older versions of Minecraft had a different main class. This shows that the problem is with the launcher and not the actual Minecraft jar.
THIS IS HAPPENING AGAIN AS OF TODAY, 11/19/18.
Honestly, you'd think Mojang would be careful enough to not let a bug like this into production...
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I had this issue AFTER playing the new snapshot 17w43a!
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I just tried with snapshot 17w43a, and it works, but it still doesn't work with any other version.
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I have this same problem using the Linux/other .jar launcher.
Same two errors. Removing the -XX:+CMSIncrementalMode java argument removes the incremental CMS warning. That done, still getting: Error: Could not find or load main class net.minecraft.client.main.Main
Tested with latest jar-based launcher version available on the Minecraft.net website (1.6.82-j)
Tested on two different Windows 10 systems with up-to-date Java version 1.8.0_151 from Oracle.
I also tried deleting the .minecraft folder and the launcher jar and starting from a clean re-download of the jar - same deal.
This worked yesterday (10/24/2017 midnight PST) but no longer worked this morning (10/25/2017 noon PST)
Additionally, it seems to be independent of what minecraft version you select or is even installed in a directory the launcher knows about. I have several old copies with independent minecraft versions each in different %APPDATA%/.minecraft directories (via changing that environment variable) and at least 1.10.2, 1.12.1, and 1.12.2 are affected, which leads me to believe it is strictly a launcher bug.
Also, launching a version of Minecraft Forge has exactly the same problem as launching the Vanilla minecraft version.
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My crash log since OP didn't provide one: https://pastebin.com/raw/HtukkF5x
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I too am having these same issues. It started yesterday. I am yet to find any information on this subject. I will report back here if I manage to find anything.
UPDATE:
This issue has been resolved and they are running out the patch now. The broken version is "1.6.82-j" and they are currently running out version "1.6.84-j."
All we can do now is wait.
bemoty
Thank you for your report!
However, this issue is a Duplicate of MC-121292.
If you have additional information, please add it to that report.
Please search before reporting, as it's likely that one exists already.
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