public class Data {
    private String filename;    //修改是的名字
    private String content;     // 资料的内容
    private boolean changed;    //修改后的内容还没存储的话，值为true

    public Data(String filename, String content) {
        this.filename = filename;
        this.content = content;
        this.changed = true;
    }

    // 修改资料内容
    public synchronized void change(String newContent) {        
        content = newContent;                                   
        changed = true;                                           
    }                                                           

    // 若有资料修改，就存储到挡安里
    public synchronized void save() throws IOException {      
        if (!changed) {                                           
            System.out.println(Thread.currentThread().getName() + " balks");
            return;                                             
        }                                                       
        doSave();                                             
        changed = false;                                          
    }                                                           

    // 实际资料储存到挡案里用的方法
    private void doSave() throws IOException {
        System.out.println(Thread.currentThread().getName() + " calls doSave, content = " + content);
        Writer writer = new FileWriter(filename);
        writer.write(content);
        writer.close();
    }
}

public class SaverThread extends Thread {
    private Data data;
    public SaverThread(String name, Data data) {
        super(name);
        this.data = data;
    }
    public void run() {
        try {
            while (true) {
                data.save();            // 存储资料
                Thread.sleep(1000);     // 休息约1秒
            }
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class ChangerThread extends Thread {
    private Data data;
    private Random random = new Random();
    public ChangerThread(String name, Data data) {
        super(name);
        this.data = data;
    }
    public void run() {
        try {
            for (int i = 0; true; i++) {
                data.change("No." + i);             //修改资料
                Thread.sleep(random.nextInt(1000)); // 模拟去做别的事情
                data.save();                        // 明确的要求存档
            }
        } catch (IOException e) {
            e.printStackTrace();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Data data = new Data("data.txt", "(empty)");
        new ChangerThread("ChangerThread", data).start();
        new SaverThread("SaverThread", data).start();
    }
}

条件不满足，则跳过。并不wait()