java - 跨对象实例同步

Question

我有一个转子对象，它有一个 goalSpeed 和一个 currentSpeed。每个人都尝试改变其当前速度以匹配设置的目标速度。我有 4 个这样的转子，运行 4 个独立的线程。 Controller 定期为每个目标分配一个新的 goalSpeed。

当我尝试在每个转子中更改其当前速度时，我永远不能超过所有转子当前速度的总和以超过 X 值。 sum(currentSpeed(Rotor1) + ... + currentSpeed(Rotor2)) !> X.

这是我的问题:当我检查是否可以增加转子的当前速度时，我对速度总和条件做了一个 if 语句。然而，在检查之后，由于每个转子都是一个单独的线程，因此另一个转子可能会更改其值。因此我在另一个线程中的检查不再有效。如何确保当我使用一个转子的 setNewSpeed() 方法时，其他转子不会改变其当前速度？

class Rotor implements Runnable {
    private int id;
    private int goalSpeed;
    private int currentSpeed;
    private Controller controller;
    private int Y;
    private int failedAttempts;
    private int successAttempts;
    private int maxSpeed;

    public int getSuccessAttempts() {
        return successAttempts;
    }

    public void setSuccessAttempts(int successAttempts) {
        this.successAttempts = successAttempts;
    }

    public int getMaxSpeed() {
        return maxSpeed;
    }

    public void setMaxSpeed(int maxSpeed) {
        this.maxSpeed = maxSpeed;
    }

    public int getFailedAttempts() {
        return failedAttempts;
    }

    public Rotor(Controller c, int Y, int id){
        this.controller = c;
        this.Y = Y;
        this.id = id;
        this.currentSpeed = 0;
        this.failedAttempts = 0;
        this.goalSpeed = 0;
        this.maxSpeed = 0;
        this.successAttempts = 0;
    }

    synchronized public void setGoalSpeed(int s){
        this.goalSpeed = s;
    }

    public int getCurrentSpeed(){
        return currentSpeed;
    }

    synchronized private void setNewSpeed(){
        int currentDrain = 0;
        for(Rotor r : controller.getRotors()){
            currentDrain = currentDrain + r.getCurrentSpeed();
        }
        if((currentDrain + (goalSpeed - currentSpeed)) > 20){
            //we cannot increase by total amount because drain too high
            System.out.println("failed");
            this.failedAttempts++;
            currentSpeed = currentSpeed + (20 - currentDrain);
            System.out.println("currentSpeed:" + currentSpeed);
        } else {
            System.out.println("success");
            successAttempts++;
            currentSpeed = goalSpeed;
        }
//      System.out.println("goalSpeed:" + goalSpeed);
//      System.out.println("currentDrain:" + currentDrain);

}

    public void run() {
        try {
            while(true){
                setNewSpeed();
                if(currentSpeed > maxSpeed){
                    maxSpeed = currentSpeed;
                }

                Thread.sleep(Y);
            }   
        } catch (InterruptedException e) {
            System.out.println("Rotor " + id + ": checks=" + (int)(successAttempts + failedAttempts) + ", success rate=" + successAttempts + ", failedAttempts=" + failedAttempts + ", max=" + maxSpeed);
        }
    }
}

Answer 1

在所有转子之间共享的锁上进行同步。现在它们每个人都在自己的锁(即 this)上进行同步，因此即使该方法是同步的，它也可以同时在不同的对象上调用。