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(){               } }