Balking Pattern不需要就不用做

word自动保存功能，如果文档被修改了，后台线程每隔一段时间会自动执行保存功能，但是如果用户在自动保存之前用Ctrl+S手动保存呢？自动保存还会执行吗？答案是不会，因为这个操作时不需要重复做的。

public class BalkingTest {
	public static void main(String[] args) {
		Data data = new Data("data.txt","(empty)");

		new ChangerThread("ChangerThread", data).start();
		new AutoSaverThread("AutoSaverThread", data).start();
	}
}

class ChangerThread extends Thread{
	private Data data;
	private Random random = new Random();

	public ChangerThread(String name, Data data){
		super(name);

		this.data=data;
	}

	@Override
	public void run() {
		try{
			for(int i=0; true; i++){
				data.change("No." + i);
				Thread.sleep(random.nextInt(1000));
				data.save();
			}
		}
		catch(IOException e){
			e.printStackTrace();
		}catch(InterruptedException e){
			e.printStackTrace();
		}
	}
}

class AutoSaverThread extends Thread{
	private Data data;
	public AutoSaverThread(String name, Data data){
		super(name);

		this.data=data;
	}
	@Override
	public void run() {
		try{
			while(true){
				data.save();
				Thread.sleep(3000);
			}
		}catch(IOException e){
			e.printStackTrace();
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

class Data{
	private final String filename;
	private String content;
	private boolean changed;

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

	public synchronized void change(String newContent){
		this.content = newContent;
		this.changed=true;
	}

	public synchronized void save() throws IOException{
		if(!changed){
			return;
		}

		doSave();
		changed=false;
	}

	private void doSave() throws IOException {
		System.err.println(Thread.currentThread().getName()+" call doSave(), content = "+ content);

		Writer writer = new FileWriter(filename);
		writer.write(content);
		writer.close();
	}
}

　　

这样就不会重复保存了，执行结果如下：

AutoSaverThread call doSave(), content = No.0
ChangerThread call doSave(), content = No.1
ChangerThread call doSave(), content = No.2
ChangerThread call doSave(), content = No.3
ChangerThread call doSave(), content = No.4
AutoSaverThread call doSave(), content = No.5
ChangerThread call doSave(), content = No.6
ChangerThread call doSave(), content = No.7
ChangerThread call doSave(), content = No.8
ChangerThread call doSave(), content = No.9
ChangerThread call doSave(), content = No.10
AutoSaverThread call doSave(), content = No.11
ChangerThread call doSave(), content = No.12
ChangerThread call doSave(), content = No.13
ChangerThread call doSave(), content = No.14
ChangerThread call doSave(), content = No.15
AutoSaverThread call doSave(), content = No.16

　　还有一种情况，就是一个类的一个方法只想被执行一次，我们可以用Balking 模式处理。

class RunOnceTime{
	private boolean initialized = false;
	public synchronized void init(){
		if(initialized == true){
			return;
		}

		doInit();

		initialized = true;
	}

	private void doInit(){

	}
}