Bukkit

What version of the product are you using? v0.3.0 of PlayerMarkers (using Java version 1.7.0_91)

Do you have an error log of what happened?

Here's the error:

[22:39:26] [Server thread/ERROR]: Could not load 'plugins/playermarkers.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: com/github/_1am3r/PlayerMarkers/PlayerMarkers : Unsupported major.minor version 52.0
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:135) ~[2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:329) ~[2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:251) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.craftbukkit.v1_8_R3.CraftServer.loadPlugins(CraftServer.java:292) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.craftbukkit.v1_8_R3.CraftServer.reload(CraftServer.java:739) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.Bukkit.reload(Bukkit.java:535) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.command.defaults.ReloadCommand.execute(ReloadCommand.java:25) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:141) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.craftbukkit.v1_8_R3.CraftServer.dispatchCommand(CraftServer.java:641) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.PlayerConnection.handleCommand(PlayerConnection.java:1162) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.PlayerConnection.a(PlayerConnection.java:997) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:45) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.PacketPlayInChat.a(PacketPlayInChat.java:1) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.PlayerConnectionUtils$1.run(SourceFile:13) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [?:1.7.0_91]
        at java.util.concurrent.FutureTask.run(FutureTask.java:262) [?:1.7.0_91]
        at net.minecraft.server.v1_8_R3.SystemUtils.a(SourceFile:44) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.MinecraftServer.B(MinecraftServer.java:715) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.DedicatedServer.B(DedicatedServer.java:374) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.MinecraftServer.A(MinecraftServer.java:654) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at net.minecraft.server.v1_8_R3.MinecraftServer.run(MinecraftServer.java:557) [2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at java.lang.Thread.run(Thread.java:745) [?:1.7.0_91]
Caused by: java.lang.UnsupportedClassVersionError: com/github/_1am3r/PlayerMarkers/PlayerMarkers : Unsupported major.minor version 52.0
        at java.lang.ClassLoader.defineClass1(Native Method) ~[?:1.7.0_91]
        at java.lang.ClassLoader.defineClass(ClassLoader.java:800) ~[?:1.7.0_91]
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142) ~[?:1.7.0_91]
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:449) ~[?:1.7.0_91]
        at java.net.URLClassLoader.access$100(URLClassLoader.java:71) ~[?:1.7.0_91]
        at java.net.URLClassLoader$1.run(URLClassLoader.java:361) ~[?:1.7.0_91]
        at java.net.URLClassLoader$1.run(URLClassLoader.java:355) ~[?:1.7.0_91]
        at java.security.AccessController.doPrivileged(Native Method) ~[?:1.7.0_91]
        at java.net.URLClassLoader.findClass(URLClassLoader.java:354) ~[?:1.7.0_91]
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:101) ~[2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:86) ~[2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at java.lang.ClassLoader.loadClass(ClassLoader.java:425) ~[?:1.7.0_91]
        at java.lang.ClassLoader.loadClass(ClassLoader.java:358) ~[?:1.7.0_91]
        at java.lang.Class.forName0(Native Method) ~[?:1.7.0_91]
        at java.lang.Class.forName(Class.java:278) ~[?:1.7.0_91]
        at org.bukkit.plugin.java.PluginClassLoader.<init>(PluginClassLoader.java:64) ~[2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:131) ~[2015-12-18-20-39-00-spigot.jar:git-Spigot-db6de12-d3e0b6f]
        ... 21 more

Basically the plugin won't launch on start of the server complaining of version issues with Java. Is this compiled for a newer version of Java?