Unsupported class version error

Hello, I was working on a plugin and used my 1.2.5 plugin code as a basic, seems that or something updated with 1.3 that causes this error or I used the wrong libs?

Code:

18:31:16 [SEVERE] Could not load 'plugins/greylist.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: be/zipron/main/Main : Unsupported major.minor version 51.0

This is the full log:

Code:

(zipronServer) bukkit ~/server ./bukkit_start.sh
195 recipes
27 achievements
18:31:15 [INFO] Starting minecraft server version 1.3.1
18:31:15 [INFO] Loading properties
18:31:15 [INFO] Default game type: SURVIVAL
18:31:15 [INFO] Generating keypair
18:31:16 [INFO] Starting Minecraft server on *:25565
18:31:16 [INFO] This server is running CraftBukkit version git-Bukkit-1.3.1-R2.0-b2340jnks (MC: 1.3.1) (Implementing API version 1.3.1-R2.0)
18:31:16 [SEVERE] Could not load 'plugins/greylist.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.UnsupportedClassVersionError: be/zipron/main/Main : Unsupported major.minor version 51.0
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:155)
        at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:305)
        at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:230)
        at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:222)
        at org.bukkit.craftbukkit.CraftServer.<init>(CraftServer.java:198)
        at net.minecraft.server.ServerConfigurationManagerAbstract.<init>(ServerConfigurationManagerAbstract.java:50)
        at net.minecraft.server.ServerConfigurationManager.<init>(SourceFile:11)
        at net.minecraft.server.DedicatedServer.init(DedicatedServer.java:105)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:378)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:539)
Caused by: java.lang.UnsupportedClassVersionError: be/zipron/main/Main : Unsupported major.minor version 51.0
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(ClassLoader.java:634)
        at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
        at java.net.URLClassLoader.defineClass(URLClassLoader.java:277)
        at java.net.URLClassLoader.access$000(URLClassLoader.java:73)
        at java.net.URLClassLoader$1.run(URLClassLoader.java:212)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(URLClassLoader.java:205)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:44)
        at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:321)
        at java.lang.ClassLoader.loadClass(ClassLoader.java:266)
        at java.lang.Class.forName0(Native Method)
        at java.lang.Class.forName(Class.java:264)
        at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:144)
        ... 9 more
18:31:16 [INFO] Preparing level "world"
18:31:16 [INFO] Preparing start region for level 0 (Seed: -984384394565405406)
18:31:17 [INFO] Preparing spawn area: 69%
18:31:18 [INFO] Preparing start region for level 1 (Seed: -4566416240848725813)
18:31:18 [INFO] Preparing spawn area: 56%
18:31:19 [INFO] Preparing start region for level 2 (Seed: -4566416240848725813)
18:31:19 [INFO] Server permissions file permissions.yml is empty, ignoring it
18:31:19 [INFO] Done (3,248s)! For help, type "help" or "?"

This is my main class

Code:

package be.zipron.main;

import be.zipron.db.*;
import be.zipron.listener.*;
import java.util.logging.Logger;
import org.bukkit.configuration.file.FileConfiguration;
import org.bukkit.plugin.PluginDescriptionFile;
import org.bukkit.plugin.PluginManager;
import org.bukkit.plugin.java.JavaPlugin;

public class Main extends JavaPlugin {
    public final Logger log = Logger.getLogger("Minecraft");
    public EventListeners eventListener = null;
    private String website = "";

    public void onDisable() {
        PluginDescriptionFile pdf = getDescription();
        this.log.info(pdf.getName() + " is now disabled");
    }

    public void onEnable() {
        /* LOGGER */
        this.log.info(name() + "enabled");
        /* **************************** */

        /* YML CONFIG */
        FileConfiguration config = getConfig();
        config.addDefault("database.host", "127.0.0.1");
        config.addDefault("database.database", "minecraft");
        config.addDefault("database.user", "root");
        config.addDefault("database.password", "");
        config.addDefault("database.authentication table", "users");
        config.addDefault("database.authentication username field", "user");
        config.addDefault("information.website", "www.website.com");
        config.options().copyDefaults(true);
        saveConfig();
        this.log.info(name() + "config loaded");
        
        website = getConfig().getString("information.website");
        /* **************************** */
        
        /* INITIAL DB CONNECTION + TABLE CREATION */
        dbConnect();
        DB db = dbConnect();
        try {
            db.createTable();
            this.log.info(name() + "connected to database");
        } catch (Exception localException) {
            
        }
        /* **************************** */

        /* EVENTS LISTENERS */
        this.eventListener = new EventListeners(this, db);
        PluginManager pm = getServer().getPluginManager();
        pm.registerEvents(this.eventListener, this);
        this.log.info(name() + "event listeners loaded");
        /* **************************** */

        
        /* COMMAND EXECUTORS 
        CmdExec cmdEx = new CmdExec(this);
        getCommand("inv").setExecutor(cmdEx);
        getCommand("invs").setExecutor(cmdEx);
        this.log.info(name() + "command executors loaded");
        /* **************************** */
    }
    
    /* DATABASE RECONNECTION */
    public DB dbConnect() {
        DB db = null;
        String host = getConfig().getString("database.host");
        String database = getConfig().getString("database.database");
        String user = getConfig().getString("database.user");
        String password = getConfig().getString("database.password");
        String authTable = getConfig().getString("database.authentication table");
        String authUsername = getConfig().getString("database.authentication username field");
        String dbCon = "jdbc:mysql://" + host + "/" + database;
        try {
            db = new DB(dbCon, user, password, authTable, authUsername);
        } catch (Exception e) {
            e.printStackTrace();
            this.log.info(error() + "could not connect to database.");
        }
        return db;
    }
    /* **************************** */

    /* returns plugin name */
    public String name() {
        PluginDescriptionFile pdf = getDescription();
        return "[" + pdf.getName() + "] > ";
    }
    public String error() {
        return "[ERROR]" + name();
    }
    public String getWebsite() {
        return website;
    }
}

 

UnsupportedClassVersionError, very commonly posted ...
http://forums.bukkit.org/threads/unsupportedclassversionerror.44223/