Java 线程 - 死锁的解决方案？

Question

我整理了一些 Java 代码来演示线程中的死锁。就其本身而言，我通常会得到 2 行输出和一个异常，有时在预期的输出行之前，有时在输出行之后。我得到的异常是 transfer() 方法第一行的 NullPointerException。

我遇到的问题是我想知道如何解决这个死锁问题。我在 StackOverflow 上搜索了这个问题并找到了这个页面:

Avoid Deadlock example

作为解决方案，我尝试了 Will Hartung 和 Dreamcash 发布的内容，但在尝试使用同步或 ReentrantLock 对象时我仍然遇到异常。

代码如下:

账户类别:

public class Account {

    int id;
    double balance;

    public Account(int id, double balance){
        this.id = id;
        this.balance = balance;
    }

    public void withdraw(double amount){
        balance = balance - amount;
    }

    public void deposit(double amount){
        balance = balance + amount;
    }

    public int getID(){
        return id;
    }

    public double getBalance(){
        return balance;
    }

}

Bank 类(单例):

public class Bank{

    static Bank bank;

    Account a1;
    Account a2;

    private Bank(){}

    public static Bank getInstance(){
        if(bank==null){
            bank = new Bank();
            bank.setAccountOne(new Account(1, 100));
            bank.setAccountTwo(new Account(2, 100));
        }

        return bank;
    }


    public void transfer(Account from, Account to, double amount){
        from.withdraw(amount);
        to.deposit(amount);
    }


    public Account getAccountOne(){
        return a1;
    }

    public Account getAccountTwo(){
        return a2;
    }
    public void setAccountOne(Account acc){
        a1 = acc;
    }
    public void setAccountTwo(Account acc){
        a2 = acc;
    }

}

PersonOne 类:

public class PersonOne implements Runnable {

    public void run() {

        Bank bank = Bank.getInstance();     

        Account a1 = bank.getAccountOne();
        Account a2 = bank.getAccountTwo();

        bank.transfer(a2, a1, 10);      

        System.out.println("T1: New balance of A1 is " + a1.getBalance());
        System.out.println("T1: New balance of A2 is " + a2.getBalance());
    }

}

PersonTwo 类:

public class PersonTwo implements Runnable {

    public void run() {
        Bank bank = Bank.getInstance();
        Account a1 = bank.getAccountOne();
        Account a2 = bank.getAccountTwo();

        bank.transfer(a1, a2, 10);

        System.out.println("T2: New balance of A1 is " + a1.getBalance());
        System.out.println("T2: New balance of A2 is " + a2.getBalance());
    }

}

最后，我的主要方法

    public static void main(String[] args){
        PersonOne p1 = new PersonOne();
        PersonTwo p2 = new PersonTwo();

        Thread t1 = new Thread(p1);
        Thread t2 = new Thread(p2);

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

Answer 1

The exception I get is a NullPointerException on the first line of the transfer() method.

The problem I'm having is I'd like to know how to solve this deadlock problem.

您的代码不可能引发任何死锁。它确实引发了写入可见性问题:其中一个线程开始调用惰性 Bank 初始化程序，而另一个线程看不到写入。

要获得死锁，您首先需要任何类型的锁定(synchronized 关键字)。您的特定 NPE 问题将通过将 synchronized 添加到 getInstance 方法来解决，并且不会引入任何死锁。

我的结论是，最好的办法是阅读一些有关 Java 并发性的介绍 Material 。