synchronized关键字解决多线程同步存取问题

Lock 和 Condition接口提供了高度的锁定控制，大多数情况下并不需要这样的控制； java中的每一个对象都有一个内部锁；如果一个方法用synchronized关键字进行声明，那么对象的内部锁将保护整个方法

public class UnsynchBankTest {

public static final int NACCOUNTS = 100;
public static final double INITIAL_BALANCE = 1000;
public static final double MAX_AMOUNT = 1000;
public static final int DELAY = 10;

public static void main(String[] args) {
    Bank2 bank = new Bank2(NACCOUNTS, INITIAL_BALANCE);//创建一个银行对象；这个银行有一百个用户;一百个用户使用这个银行
    for (int i = 0; i < NACCOUNTS; i++) {
        int fromAccount = i;
        Runnable r = () -> {
            try
            {
                while (true) {

                    int toAccount = (int) (bank.size() * Math.random());//拿出一个随机账户
                    double amount = MAX_AMOUNT * Math.random();//设定随机一笔钱（支出/得到）
                    bank.transfer(fromAccount, toAccount, amount);//转账操作
                    Thread.sleep((int) (DELAY * Math.random()));//随机的休眠时间
                }
            }
            catch (InterruptedException e) {
                Thread.currentThread().interrupt();//加上这句为好
            }
        };
        Thread t = new Thread(r);//创建一个线程；该线程处于创建状态
        t.start();//使线程处于可运行的状态（可能运行，也可能处于阻塞状态）
    }
}

}

import java.util.Arrays;

public class Bank2 {

private final double[] accounts;

public Bank2(int n, double initialBalance) {
    accounts = new double[n];
    Arrays.fill(accounts, initialBalance);
}

public synchronized void transfer(int from, int to, double amount) throws InterruptedException {

    while (accounts[from] < amount)
        wait();//使线程处于等待集中
    System.out.print(Thread.currentThread());
    accounts[from] -= amount;
    System.out.printf("%10.2f from %d to %d", amount, from, to);
    accounts[to] += amount;
    System.out.printf("Total Balance: %10.2f%n", getTotalBalance());
    notifyAll();//唤醒所有等待的线程
}

public synchronized double getTotalBalance() {

    double sum = 0;

    for (double a : accounts)
        sum += a;

    return sum;
}

public int size() {
    return accounts.length;
}

}