Java多線程程序：多線程對象共享

Question

我正在嘗試用 Java 編寫一個使用多線程的程序。 該程序是關於使用借記卡的夫妻之間共享的銀行帳戶。

我使用 2 個線程編寫了 Java 程序，但我收到了java.lang.NullPointerException

我正在努力積累我在 Java 多線程方面的知識。 幫助我識別此代碼塊中的錯誤。

這是程序：

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;


// A bank account has a balance that can be changed by deposits and withdrawals.

public class BankAccount {

    public double total = 0, amount = 0;

    public void withdraw(double amount) {
        total -= amount;
        System.out.println("Amount Withdrawn is " + amount);
    }

    public void deposit(double amount) {
        total += amount;
        System.out.println("Amount Deposited is " + amount);
    }

    public double getAccount() {
        System.out.println("Total Amount is " + total);
        return total;

    }
}

class WithdrawalRunnable {

    private static final Lock lock = new ReentrantLock();
    final Condition myCond = lock.newCondition();
    BankAccount myAccount;

    public void amountWithdrawn(double money_Withdrawn) throws InterruptedException {
        lock.lock();
        try {
            while (money_Withdrawn > myAccount.total) {
                myCond.await();  
                //when the condition is satisfied then :
                myAccount.withdraw(money_Withdrawn);
                myAccount.getAccount();
            }
        } finally {
            lock.unlock();
        }
    }
}

class DepositRunnable {

    private static final Lock lock = new ReentrantLock();
    final Condition myCond = lock.newCondition();
    BankAccount myAccount;

    public void amountDeposited(double money_deposited) throws InterruptedException {
        lock.lock();
        try {
            myAccount.deposit(money_deposited);
            myAccount.getAccount();
            myCond.signalAll(); 
        } finally {
            lock.unlock();
        }
    }
}

class Husband implements Runnable {

    DepositRunnable myDeposit;
    WithdrawalRunnable myWithdrawal;
    double amount_deposit;
    double amount_withdraw;

    public Husband(double amount_deposit, double amount_withdraw) {
        this.amount_deposit = amount_deposit;
        this.amount_withdraw = amount_withdraw;
    }

    public void run() {
        try {
            myDeposit.amountDeposited(amount_deposit);
            myWithdrawal.amountWithdrawn(amount_withdraw);
        } catch (InterruptedException ex) {
            Logger.getLogger(Wife.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

class Wife implements Runnable {

    DepositRunnable myDeposit;
    WithdrawalRunnable myWithdrawal;
    double amount_deposit;
    double amount_withdraw;

    public Wife(double amount_deposit, double amount_withdraw) {
        this.amount_deposit = amount_deposit;
        this.amount_withdraw = amount_withdraw;
    }

    public void run() {
        try {
            myDeposit.amountDeposited(amount_deposit);
            myWithdrawal.amountWithdrawn(amount_withdraw);
        } catch (InterruptedException ex) {
            Logger.getLogger(Wife.class.getName()).log(Level.SEVERE, null, ex);
        }

    }
}

class RunningThreadTest {

    public static void main(String[] args) throws InterruptedException {

        Husband husb = new Husband(100.0, 0.0);
        Wife wif = new Wife(400.0, 0.0);
        Thread thread1 = new Thread(husb);
        Thread thread2 = new Thread(wif);
        thread1.start();
        thread2.start();
    }
}

Answer 1

這是多線程的一個很好的學習案例，我稍微更改了您的代碼。 請將 BankAccountManager 類與您的兩個類進行比較，我希望這會有所幫助。

import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.logging.Level;
import java.util.logging.Logger;

// A bank account has a balance that can be changed by deposits and withdrawals.

class BankAccount {

    private double total = 0;

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

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

    public double getAccount() {
        return total;

    }
}

class BankAccountManager {

    private static final Lock lock = new ReentrantLock();
    private Condition myCond = lock.newCondition();
    BankAccount myAccount;

    public BankAccountManager(BankAccount myAccount) {
        this.myAccount = myAccount;
    }

    public void amountWithdrawn(double money_Withdrawn)
            throws InterruptedException {
        lock.lock();
        try {
            if (money_Withdrawn > myAccount.getAccount()) {
                myCond.await();
            }
            // when the condition is satisfied then :
            System.out.println("Amount Withdrawn is " + money_Withdrawn);
            myAccount.withdraw(money_Withdrawn);
            System.out.println("Total Amount is " + myAccount.getAccount());
        } finally {
            lock.unlock();
        }
    }

    public void amountDeposited(double money_deposited)
            throws InterruptedException {
        lock.lock();
        try {
            System.out.println("Amount Deposited is " + money_deposited);
            myAccount.deposit(money_deposited);
            System.out.println("Total Amount is " + myAccount.getAccount());
            myCond.signalAll();
        } finally {
            lock.unlock();
        }
    }
}

class Husband implements Runnable {

    private BankAccountManager manager;
    private double amount_deposit;
    private double amount_withdraw;

    public Husband(BankAccountManager manager, double amount_deposit,
            double amount_withdraw) {
        this.manager = manager;
        this.amount_deposit = amount_deposit;
        this.amount_withdraw = amount_withdraw;
    }

    public void run() {
        try {
            manager.amountDeposited(amount_deposit);
            manager.amountWithdrawn(amount_withdraw);
        } catch (InterruptedException ex) {
            Logger.getLogger(Wife.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

class Wife implements Runnable {

    private BankAccountManager manager;
    private double amount_deposit;
    private double amount_withdraw;

    public Wife(BankAccountManager manager, double amount_deposit,
            double amount_withdraw) {
        this.manager = manager;
        this.amount_deposit = amount_deposit;
        this.amount_withdraw = amount_withdraw;
    }

    public void run() {
        try {
            manager.amountDeposited(amount_deposit);
            manager.amountWithdrawn(amount_withdraw);
        } catch (InterruptedException ex) {
            Logger.getLogger(Wife.class.getName()).log(Level.SEVERE, null, ex);
        }

    }
}

public class RunningThreadTest {

    public static void main(String[] args) throws InterruptedException {

        BankAccountManager manager = new BankAccountManager(new BankAccount());

        Husband husb = new Husband(manager, 100.0, 0.0);
        Wife wif = new Wife(manager, 400.0, 1.0);
        Thread thread1 = new Thread(husb);
        Thread thread2 = new Thread(wif);
        thread1.start();
        thread2.start();
    }
}

Answer 2

在DepositRunnable和WithdrawalRunnable ，您從未初始化成員myAccount ，而是嘗試在它們各自的方法中使用它們。 在使用它們之前嘗試初始化這些成員。

BankAccount myAccount = new BankAccount();

可能還有更多的例子，但這些是我首先要看的。

Answer 3

存款和取款 Runnables 中的字段 MyAccount 未初始化

Answer 4

我看到的是屬性

BankAccount myAccount;

出現在類 DepositRunnable 和 WithdrawalRunnable 中的那個沒有被正確初始化，因此你會得到一個空指針異常，例如這里：

myAccount.withdraw(money_Withdrawn);

Answer 5

您的許多類成員變量都沒有被初始化。 您的具體問題是因為Wife.myDeposit從未被初始化，但是一旦您通過了，就會有另一個 NPE 等着您。

Answer 6

是的，謝謝，所有這些類都需要初始化如下：

DepositRunnable myDeposit = new DepositRunnable();
WithdrawalRunnable myWithdrawal = new WithdrawalRunnable();
BankAccount myAccount = new BankAccount();

另一個沒人發現的錯誤是：“total”必須是靜態的。

謝謝大家。