为什么除非我使用 .join()，否则同步方法不会被锁定？

Question

在 Java 中，标记一个方法 synchronized 应该禁用 竞争条件 ，这将导致两个线程调用正在访问和修改一个方法的方法同一对象中的字段。

但出于某种原因，synchronized 在以下示例中没有按预期工作，除非我在主程序的两个线程上调用 .join()。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);
        }
    }
}

Answer 1

正如@flakes 所阐明的那样，代码需要主线程join() 两个线程以保证两个线程都终止，即在打印出结束之前完成对余额的修改balance.

更简洁的实现方式是使用 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balance之前完成。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);
        }
    }
}

为什么除非我使用 .join()，否则同步方法不会被锁定？

Why is the synchronized method not getting locked unless I use .join()?

java

concurrency

synchronized