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java - 除非我使用 .join()，否则为什么同步方法不会被锁定？

问题描述

在Java中，标记一个方法synchronized应该禁用由两个线程调用该方法导致的竞争条件，该方法正在访问和修改同一对象中的字段。

但由于某种原因，synchronized除非我.join()在主程序中调用两个线程，否则在以下示例中无法按预期工作。这是为什么？

package example;

public class Account {
    private double balance;

    public Account(double balance) {
        super();
        this.balance = balance;
    }

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

    public double getBalance() {
        return balance;
    }
}

package example;

public class AccountTester extends Thread {
    private Account account;
    private double amount;

    public AccountTester(Account account, double amount) {
        this.account = account;
        this.amount = amount;
    }

    public static void main(String[] args) {
        Account account = new Account(0);
        AccountTester tester1 = new AccountTester(account, 1.0);
        AccountTester tester2 = new AccountTester(account, 2.0);
        tester1.start();
        tester2.start();
        // Why do I need the main thread to join threads
        //      tester1 and tester2 for synchronized to work?
        try {
            tester1.join();
            tester2.join();
        } catch (InterruptedException e) {
            System.err.println(e);
        }
        System.out.println("End balance: " + account.getBalance());
    }

    @Override
    public void run() {
        for (int i = 0; i < 1000; i++) {
            account.deposit(amount);
        }
    }
}

标签： javaconcurrencysynchronized

解决方案

正如@flakes 所阐明的那样，代码需要两个线程的主线程join()来保证两个线程都被终止，即在打印出 end 之前完成对余额的修改balance。

实现这一点的一种更简洁的方法是使用java.util.concurrent.ExecutorService接口。这里它的shutdown()方法保证线程池中的两个线程都在打印结束之前完成balance。这是我的实现：

package example;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class AccountTester implements Runnable {
    private Account account;
    private double amount;

    public AccountTester(Account account, double amount) {
        this.account = account;
        this.amount = amount;
    }

    public static void main(String[] args) {
        Account account = new Account(0);
        ExecutorService executorService = Executors.newFixedThreadPool(2);
        executorService.execute(new AccountTester(account, 1.0));
        executorService.execute(new AccountTester(account, 2.0));
        executorService.shutdown();
        System.out.println("End balance: " + account.getBalance());
    }

    @Override
    public void run() {
        for (int i = 0; i < 1000; i++) {
            account.deposit(amount);
        }
    }
}

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