When I was testing a Bukkit plugin on a local server it worked perfectly, but later on I uploaded it to an online minecraft server and this was the output it gave me:
16.05 20:30:55 [Multicraft] Skipped 12 lines due to rate limit (30/s)
16.05 20:30:54 [Server] INFO at java.net.URLClassLoader.findClass(URLClassLoader.java:354) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.security.AccessController.doPrivileged(Native Method) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.net.URLClassLoader$1.run(URLClassLoader.java:355) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.net.URLClassLoader$1.run(URLClassLoader.java:361) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.net.URLClassLoader.access$100(URLClassLoader.java:71) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.net.URLClassLoader.defineClass(URLClassLoader.java:449) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.lang.ClassLoader.defineClass(ClassLoader.java:800) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.7.0_80]
16.05 20:30:54 [Server] INFO Caused by: java.lang.UnsupportedClassVersionError: nl/calbas/transferEnchantments/TransferEnchantments : Unsupported major.minor version 52.0
16.05 20:30:54 [Server] INFO at java.lang.Thread.run(Thread.java:745) [?:1.7.0_80]
16.05 20:30:54 [Server] INFO at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java:506) [craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at net.minecraft.server.v1_9_R1.DedicatedServer.init(DedicatedServer.java:182) [craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at net.minecraft.server.v1_9_R1.DedicatedPlayerList.<init>(SourceFile:14) [craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at net.minecraft.server.v1_9_R1.PlayerList.<init>(PlayerList.java:69) [craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at org.bukkit.craftbukkit.v1_9_R1.CraftServer.<init>(CraftServer.java:255) [craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at org.bukkit.craftbukkit.v1_9_R1.CraftServer.loadPlugins(CraftServer.java:293) [craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:251) [craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:328) ~[craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:133) ~[craftbukkit-1.9.2.jar:git-Bukkit-4cb3258]
16.05 20:30:54 [Server] INFO org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: nl/calbas/transferEnchantments/TransferEnchantments : Unsupported major.minor version 52.0
16.05 20:30:54 [Server] INFO [20:30:54 ERROR]: Could not load 'plugins/TransferEnchantments.jar' in folder 'plugins'
16.05 20:30:54 [Server] INFO [20:30:54 INFO]: This server is running CraftBukkit version git-Bukkit-4cb3258 (MC: 1.9.2) (Implementing API version 1.9.2-R0.1-SNAPSHOT)
Does someone know what the problem might be? I could upload the code if it is helpful, but the code worked fine on my own computer.
http://wiki.bukkit.org/Setting_Up_Your_Workspace#Which_version_of_the_JDK.3F tells you that most bukkit servers run Java 7, this means that the server you uploaded to probably does too, if you're not using JDK 7, the server will most likely not be able to load your plugin, try to download JDK 7 and compile your plugin with that instead.