java - 多线程Java单例不断重置

Question

我目前正在“Head First Design Patterns”一书第 175 页“The Chocolate Factory”中做一个 Java 练习，为了检验单例线程安全的理论，我实现了自己的多线程驱动程序类。

练习说明:创建一个带有 2 个 boolean 变量的单例巧克力锅炉:empty 和 boiled，默认状态为 empty=true 和 boiled=false。以及写入变量的三个方法:fill()、drain() 和 boil()。

然而，当读取或写入变量“empty”和“boiled”时会出现问题。在线程 #1 填充 ChocolateBoiler 后，它设置 empty=false。然后线程 #2 启动，它说空被设置为默认值 true。这怎么可能？线程 #1 没有更新它吗？或者可能输出不正确，但更改已传播？我已经在所有访问器方法上配置了双重检查锁定，并且变量设置为 static volatile。

我已附加以下代码:

客户端.Java

package creational.singleton.chocolatefactory;

public class Client extends Thread{

    public void run() {
        ChocolateBoiler boiler = ChocolateBoiler.getInstance();
        System.out.println("new Boiler " + Thread.currentThread().getId() + "= isBoiled: " + boiler.isBoiled() + ", isEmpty: " + boiler.isEmpty());
        boiler.fill();
        System.out.println("filled Boiler " + Thread.currentThread().getId() + "= isBoiled: " + boiler.isBoiled() + ", isEmpty: " + boiler.isEmpty());
        boiler.boil();
        System.out.println("boiled Boiler " + Thread.currentThread().getId() + "= isBoiled: " + boiler.isBoiled() + ", isEmpty: " + boiler.isEmpty());
    }

    public static void main(String[] args) {
        Client obj = new Client();
        Thread t1 = new Thread(obj);
        Thread t2 = new Thread(obj);
        Thread t3 = new Thread(obj);

        t1.start();
        t2.start();
        t3.start();
    }
}

和 巧克力锅炉.java

package creational.singleton.chocolatefactory;

public class ChocolateBoiler {
    //volatile guarantees visibility of changes to variables across threads
    //eager initialization for better thread safety
    private volatile static ChocolateBoiler uniqueInstance = new ChocolateBoiler();
    private volatile static boolean empty = true;
    private volatile static boolean boiled = false;

    private ChocolateBoiler() {}

    public static ChocolateBoiler getInstance() {
        return uniqueInstance;
    }

    public void fill() {
        if (isEmpty()) {
            synchronized(uniqueInstance){
                if (isEmpty()) {
                    ChocolateBoiler.empty = false;
                    ChocolateBoiler.boiled = false;
                }
            }
        }
    }

    public void drain(){
        if (!isEmpty() && isBoiled()) {
            synchronized(uniqueInstance){
                if (!isEmpty() && isBoiled()) {
                    ChocolateBoiler.empty = true;
                }
            }
        }
    }

    public void boil(){
        if (!isEmpty() && !isBoiled()) {
            synchronized(uniqueInstance){
                if (!isEmpty() && !isBoiled()) {
                    ChocolateBoiler.boiled = true;
                }
            }
        }
    }

    public boolean isEmpty() {
        synchronized(uniqueInstance){
            return ChocolateBoiler.empty;
        }
    }

    public boolean isBoiled() {
        synchronized(uniqueInstance){
            return ChocolateBoiler.boiled;
        }
    }
}

输出如下:

new Boiler 20= isBoiled: false, isEmpty: true
filled Boiler 20= isBoiled: false, isEmpty: false
new Boiler 19= isBoiled: false, isEmpty: true

注意最后一行:isEmpty: true

它不应该说:isEmpty: false

========================================

= 解决方案

问题出在 Client.java

package creational.singleton.chocolatefactory;

public class Client extends Thread{
    ChocolateBoiler boiler = ChocolateBoiler.getInstance();

    public void run() {
        printState("new");
        boiler.fill();
        printState("filled");
        boiler.boil();
        printState("boiled");
    }

    public synchronized void printState(String state){
        System.out.println(state + " Boiler " + Thread.currentThread().getId() + "= isBoiled: " + boiler.isBoiled() + ", isEmpty: " + boiler.isEmpty());
    }

    public static void main(String[] args) {
        Client obj = new Client();
        Thread t1 = new Thread(obj);
        Thread t2 = new Thread(obj);
        Thread t3 = new Thread(obj);

        t1.start();
        t2.start();
        t3.start();
    }
}

现在输出如下:

new Boiler 19= isBoiled: false, isEmpty: true
filled Boiler 19= isBoiled: false, isEmpty: false
new Boiler 20= isBoiled: true, isEmpty: false
new Boiler 21= isBoiled: true, isEmpty: false
filled Boiler 21= isBoiled: true, isEmpty: false
filled Boiler 20= isBoiled: true, isEmpty: false
boiled Boiler 19= isBoiled: true, isEmpty: false
boiled Boiler 20= isBoiled: true, isEmpty: false
boiled Boiler 21= isBoiled: true, isEmpty: false

Answer 1

虽然各个方法是同步的，但 System.out.println("boiled Boiler"+ Thread.currentThread().getId() + "= isBoiled: "+ boiler.isBoiled() + ", isEmpty: "+锅炉.isEmpty());进行两个单独的方法调用，并且在并发上下文中，无法保证任何事情。

也尝试在同步块(synchronized block)中调用 System.out.println。