当volatile关键字真的是必要的?

这个代码取自书本 Java线程和并发实用程序 Jeff Friesen:

public class PC {
    public static void main(String[] args) {
        Shared s = new Shared();
        new Producer(s).start();
        new Consumer(s).start();
    }
}

class Shared {
    private char c;
    private volatile boolean writeable = true;

    synchronized void setSharedChar(char c) {
        while (!writeable)
            try {
                wait();
            } catch (InterruptedException ie)  {
            }
        this.c = c;
        writeable = false;
        notify();
    }

    synchronized char getSharedChar() {
        while (writeable)
            try {
                wait();
            } catch (InterruptedException ie)  {
            }
        writeable = true;
        notify();
        return c;
    }
}

class Producer extends Thread {
    private final Shared s;

    Producer(Shared s) {
        this.s = s;
    }

    @Override
    public void run() {
        for (char ch = 'A'; ch <= 'Z'; ch++) {
            synchronized(s) {
                s.setSharedChar(ch);
                System.out.println(ch + " produced by producer.");
            }
        }
    }
}

class Consumer extends Thread {
    private final Shared s;

    Consumer(Shared s) {
        this.s = s;
    }

    @Override
    public void run() {
        char ch;
        do {
            synchronized(s) {
                ch = s.getSharedChar();
                System.out.println(ch + " consumed by consumer.");
            }
        } while (ch != 'Z');
    }
}

我不明白为什么有必要 volatile 这个 writeable 字段,因为它只能由访问 synchronized 方法,其效果与volatile相同(可见性+写入互斥)。以及为什么字段 可写的 需要 不稳定的 而不是字段 c 在这种情况下。

下一个代码也有同样的问题 balance 字段:为什么它需要是可变的,因为它已经被访问了 同步的 withdraw 方法

public class CheckingAccount {
    private volatile int balance;

    public CheckingAccount(int initialBalance) {
        balance = initialBalance;
    }

    public synchronized boolean withdraw(int amount) {
        if (amount <= balance) {
            try {
                Thread.sleep((int) (Math.random() * 200));
            } catch (InterruptedException ie) {
            }
            balance -= amount;
            return true;
        }
        return false;
    }

    public static void main(String[] args) {
        final CheckingAccount ca = new CheckingAccount(100);
        Runnable r = new Runnable() {
            @Override
            public void run() {
                String name = Thread.currentThread().getName();
                for (int i = 0; i < 10; i++)
                    System.out.println (name + " withdraws $10: " +
                ca.withdraw(10));
            }
        };
        Thread thdHusband = new Thread(r);
        thdHusband.setName("Husband");
        Thread thdWife = new Thread(r);
        thdWife.setName("Wife");
        thdHusband.start();
        thdWife.start();
    }
}

有人能解释一下吗?