Attach a new security manager in order to listen to File and Socket operations on the main thread.

BlockingSecurityManager.java

package com.github.games647.lagmonitor;

import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Sets;

import java.io.FilePermission;
import java.net.SocketPermission;
import java.security.Permission;
import java.util.Arrays;
import java.util.Map.Entry;
import java.util.Set;
import java.util.logging.Level;

import org.bukkit.Bukkit;
import org.bukkit.plugin.Plugin;

public class BlockingSecurityManager extends SecurityManager {

    private final LagMonitor plugin;
    private final SecurityManager delegate;

    private final Set<PluginViolation> violations = Sets.newConcurrentHashSet();
    private final Set<String> violatedPlugins = Sets.newConcurrentHashSet();
    private final Set<String> fileWhitelist = ImmutableSet.of(".jar", "session.lock");

    public BlockingSecurityManager(LagMonitor plugin, SecurityManager delegate) {
        this.plugin = plugin;

        this.delegate = delegate;
    }

    public BlockingSecurityManager(LagMonitor plugin) {
        this(plugin, null);
    }

    public SecurityManager getOldSecurityManager() {
        return delegate;
    }

    @Override
    public void checkPermission(Permission perm, Object context) {
        if (delegate != null) {
            delegate.checkPermission(perm, context);
        }

        checkMainThreadOperation(perm);
    }

    @Override
    public void checkPermission(Permission perm) {
        if (delegate != null) {
            delegate.checkPermission(perm);
        }

        checkMainThreadOperation(perm);
    }

    private void checkMainThreadOperation(Permission perm) {
        if (Bukkit.isPrimaryThread() && isBlockingAction(perm)) {
            //warn the admin
        }
    }

    private boolean isBlockingAction(Permission permission) {
        String actions = permission.getActions();

        if (permission instanceof FilePermission) {
            //which could executed by the main thread, doesn't it`?
            return (actions.contains("read") || actions.contains("write"))
                    && fileWhitelist.stream().noneMatch(ignored -> permission.getName().contains(ignored));
            //read write
        } else if (permission instanceof SocketPermission) {
            //on new socket connections
        }

        return false;
    }
}