package net.minecraftforge.fml.relauncher;

import java.security.Permission;

/**
 * A custom security manager stopping certain events from happening
 * unexpectedly.
 *
 * @author cpw
 *
 */
public class FMLSecurityManager extends SecurityManager {
    @Override
    public void checkPermission(Permission perm)
    {
        String permName = perm.getName() != null ? perm.getName() : "missing";
        if (permName.startsWith("exitVM"))
        {
            Class<?>[] classContexts = getClassContext();
            String callingClass = classContexts.length > 3 ? classContexts[4].getName() : "none";
            String callingParent = classContexts.length > 4 ? classContexts[5].getName() : "none";
            // FML is allowed to call system exit and the Minecraft applet (from the quit button)
            if (!(callingClass.startsWith("net.minecraftforge.fml.") || "net.minecraft.server.dedicated.ServerHangWatchdog$1".equals(callingClass) || "net.minecraft.server.dedicated.ServerHangWatchdog".equals(callingClass) || ( "net.minecraft.client.Minecraft".equals(callingClass) && "net.minecraft.client.Minecraft".equals(callingParent)) || ("net.minecraft.server.dedicated.DedicatedServer".equals(callingClass) && "net.minecraft.server.MinecraftServer".equals(callingParent))))
            {
                throw new ExitTrappedException();
            }
        }
        else if ("setSecurityManager".equals(permName))
        {
            throw new SecurityException("Cannot replace the FML security manager");
        }
        return;
    }

    public static class ExitTrappedException extends SecurityException {
        private static final long serialVersionUID = 1L;
    }
}

package net.minecraftforge.fml.relauncher;

import java.security.Permission;

/**
 * A custom security manager stopping certain events from happening
 * unexpectedly.
 *
 * @author cpw
 *
 */
public class FMLSecurityManager extends SecurityManager {
    @Override
    public void checkPermission(Permission perm)
    {
        String permName = perm.getName() != null ? perm.getName() : "missing";
        if (permName.startsWith("exitVM"))
        {
            Class<?>[] classContexts = getClassContext();
            String callingClass = classContexts.length > 3 ? classContexts[4].getName() : "none";
            String callingParent = classContexts.length > 4 ? classContexts[5].getName() : "none";
            // FML is allowed to call system exit and the Minecraft applet (from the quit button)
            if (!(callingClass.startsWith("net.minecraftforge.fml.")
                    || "net.minecraft.server.dedicated.ServerHangWatchdog$1".equals(callingClass)
                    || "net.minecraft.server.dedicated.ServerHangWatchdog".equals(callingClass)
                    || ( "net.minecraft.client.Minecraft".equals(callingClass) && "net.minecraft.client.Minecraft".equals(callingParent))
                    || ("net.minecraft.server.dedicated.DedicatedServer".equals(callingClass) && "net.minecraft.server.MinecraftServer".equals(callingParent)))
                    )
            {
                throw new ExitTrappedException();
            }
        }
        else if ("setSecurityManager".equals(permName))
        {
            throw new SecurityException("Cannot replace the FML security manager");
        }
        return;
    }

    public static class ExitTrappedException extends SecurityException {
        private static final long serialVersionUID = 1L;
    }
}

Format a bit better