引数のチェックとかやってないけどそのへんは許して

src_Main.java

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main {

    public static void main(String[] args) {
        System.setProperty("debug", "true");

        ExecutorService service = Executors.newFixedThreadPool(4);
        Util util = new Util(2);
        service.submit(() -> {
            util.lockStart(2);
            System.out.println("A");
            util.lockEnd(2);
        });
        service.submit(() -> {
            util.lockStart(1);
            System.out.println("B");
            util.lockEnd(1);
        });

        try {
            //これどうしよう・・・
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        util.unlockOrder();
        service.shutdown();
    }
}

src_Util.java

public class Util {

    private static final boolean enable;

    static {
        enable = Boolean.valueOf(System.getProperty("debug", "false"));
    }

    private volatile Object[] lockObjects;
    public Util(int size) {
        lockObjects = new Object[size];
    }

    public void lockStart(int seq) {
        if (!enable) return;

        lockObjects[seq - 1] = new Object();
        synchronized (lockObjects[seq - 1]) {
            try {
                System.out.println("wait before: " + seq);
                lockObjects[seq - 1].wait();
                System.out.println("wait after: " + seq);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    public void lockEnd(int seq) {
        if (!enable) return;

        if (lockObjects.length != seq) {
            synchronized (lockObjects[seq]) {
                lockObjects[seq].notify();
            }
        }
    }

    public void unlockOrder() {
        if (!enable) return;

        lockEnd(0);
    }
}