=解决方案

Question

我目前正在本书中进行Java练习＆＃34; Head First Design Patterns＆＃34;第175页＆＃34;巧克力工厂＆＃34;，并测试单例是线程安全的理论我已经实现了我自己的多线程驱动程序类。

练习说明：用2个布尔变量创建一个单独的巧克力锅：空和煮沸，默认状态为empty = true，boiled = false。以及写入变量的三种方法：fill（），drain（）和boil（）。

然而，当读取或写入变量时出现问题＆＃34;空＆＃34;和＆＃34;煮沸＆＃34;。线程＃1填充ChocolateBoiler后，它设置为empty = false。然后启动线程＃2，并将空值设置为默认值true。这怎么可能？线程＃1没有更新吗？或者输出可能不正确，但改变了传播？我已在所有访问器方法上配置了双重检查锁定，并且变量设置为静态volatile。

我附上了以下代码：

Client.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 ChocolateBoiler.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

注意最后一行说：isEmpty：true

不应该说：isEmpty：false

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

=解决方案

问题在于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

虽然个别方法是同步的，但系统System.out.println（＆＃34; boiler Boiler＆＃34; + Thread.currentThread（）。getId（）+＆＃34; = isBoiled：＆＃34; + boiler.isBoiled（）+＆＃34;，isEmpty：＆＃34; + boiler.isEmpty（））;进行两个单独的方法调用，并且在并发上下文中，没有任何东西可以保证。

尝试在同步块中调用System.out.println。

多线程Java单例保持重置

=解决方案

1 个答案: