前幾天看了一下Spring的部分源碼，發現回調機制被大量使用，覺得有必要把Java回調機制的理解歸納總結一下，以方便在研究類似于Spring源碼這樣的代碼時能更加得心應手。?

注：本文不想扯很多拗口的話來充場面，我的目的是希望以最簡明扼要的語言將Java回調的大概機制說清楚。好了，言歸正傳。?

一句話，回調是一種雙向調用模式，什么意思呢，就是說，被調用方在被調用時也會調用對方，這就叫回調。“If you call me, i will call back”。?
不理解？沒關系，先看看這個可以說比較經典的使用回調的方式：?

• class A實現接口InA ——背景1
• class A中包含一個class B的引用b ——背景2
• class B有一個參數為InA的方法test(InA a) ——背景3
• A的對象a調用B的方法傳入自己，test(a) ——這一步相當于you call me
• 然后b就可以在test方法中調用InA的方法 ——這一步相當于i call you back

是不是清晰一點了？下面再來看一個完全符合這個方式模板的例子?
（PS：這個例子來源于網絡，由于這個例子表現的功能極度拉風，令我感覺想想出一個超越它的例子確實比較困難，所以直接搬過來）?

Java代碼??

//相當于接口InA??

public?interface?BoomWTC{??

??//獲得拉登的決定??

??public?benLaDengDecide();??

??

??//?執行轟炸世貿??

??public?void?boom();??

}??

??

//相當于class?A??

public?class?At$911?implements?BoomWTC{//相當于【背景1】??

??private?boolean?decide;??

??private?TerroristAttack?ta;//相當于【背景2】??

??

??public?At$911(){??

????Date?now=new?Date();??

????SimpleDateFormat?myFmt1=new?SimpleDateFormat("yy/MM/dd?HH:mm");??

????this.dicede=?myFmt.format(dt).equals("01/09/11?09:44");??

????this.ta=new?TerroristAttack();??

??}??

??

??//獲得拉登的決定??

??public?boolean?benLaDengDecide(){??

????return?decide;??

??}??

??

??//?執行轟炸世貿??

??public?void?boom(){??

????ta.attack(new?At$911);//class?A調用class?B的方法傳入自己的對象，相當于【you?call?me】??

??}??

}??

??

//相當于class?B??

public?class?TerroristAttack{??

??public?TerroristAttack(){??

??}??

??

??public?attack(BoomWTC?bmw){——這相當于【背景3】??

????if(bmw.benLaDengDecide()){//class?B在方法中回調class?A的方法，相當于【i?call?you?back】??

?????//let's?go.........??

????}??

??}??

}??

現在應該對回調有一點概念了吧。?
可是問題來了，對于上面這個例子來說，看不出用回調有什么好處，直接在調用方法不就可以了，為什么要使用回調呢？?
事實上，很多需要進行回調的操作是比較費時的，被調用者進行費時操作，然后操作完之后將結果回調給調用者。看這樣一個例子：?

Java代碼??

//模擬Spring中HibernateTemplate回調機制的代碼??

????interface?CallBack{?????

????????public?void?doCRUD();?????

????}????

????????

????public?class?HibernateTemplate?{?????

????????????

????????public?void?execute(CallBack?action){????

????????????getConnection();????

????????????action.doCRUD();????

????????????releaseConnection();????

????????}????

?????????

????????public?void?add(){????

?????????????execute(new?CallBack(){????

????????????????public?void?doCRUD(){????

????????????????????System.out.println("執行add操作...");????

????????????????}????

?????????????});????

????????}?????

????????

????????public?void?getConnection(){????

????????????System.out.println("獲得連接...");????

????????}????

????????????

????????public?void?releaseConnection(){????

????????????System.out.println("釋放連接...");????

????????}????

????????????

????}????

可能上面這個例子你不能一眼看出個所以然來，因為其實這里A是作為一個內部匿名類存在的。好，不要急，讓我們把這個例子來重構一下：?

Java代碼??

interface?CallBack{???//相當于接口InA??

????public?void?doCRUD();?????

}????

??

public?class?A?implements?CallBack{//【背景1】??

????private?B?b;//【背景2】??

????public?void?doCRUD(){????

??????????System.out.println("執行add操作...");????

?????}????

??

?????public?void?add(){????

?????????????b.execute(new?A());//【you?call?me】????

????????}????

}??

??

public?class?B{??

?????public?void?execute(CallBack?action){??//【背景3】??

????????????getConnection();????

????????????action.doCRUD();??//【i?call?you?back】??

????????????releaseConnection();????

????????}????

??

??????public?void?getConnection(){????

????????????System.out.println("獲得連接...");????

????????}????

????????????

????????public?void?releaseConnection(){????

????????????System.out.println("釋放連接...");????

????????}????

}??

好了，現在就明白多了吧，完全可以轉化為上面所說的回調使用方式的模板。?
現在在來看看為什么要使用回調，取得連接getConnection();是費時操作，A希望由B來進行這個費時的操作，執行完了之后通知A即可（即所謂的i call you back）。這就是這里使用回調的原因。?

在網上看到了一個比喻，覺得很形象，這里借用一下：?
你有一個復雜的問題解決不了，打電話給你的同學，你的同學說可以解決這個問題，但是需要一些時間，那么你不可能一直拿著電話在那里等，你會把你的電話號碼告訴他，讓他解決之后打電話通知你。回調就是體現在你的同學又反過來撥打你的號碼。?
結合到前面所分析的，你打電話給你同學就是【you call me】，你同學解決完之后打電話給你就是【i call you back】。?

怎么樣，現在理解了吧？??

---------------------------------以下為更新----------------------------------?

看了有些朋友的回帖，我又思考了一下，感覺自己之前對回調作用的理解的確存在偏差。?
下面把自己整理之后的想法共享一下，如果有錯誤希望指出！多謝！?

先說上面這段代碼，本來完全可以用模板模式來進行實現：?

Java代碼??

public?abstract?class?B{??

?????public?void?execute(){???

????????????getConnection();????

????????????doCRUD();????

????????????releaseConnection();????

????????}????

??

??????public?abstract?void?doCRUD();??

??

??????public?void?getConnection(){????

????????????System.out.println("獲得連接...");????

????????}????

????????????

????????public?void?releaseConnection(){????

????????????System.out.println("釋放連接...");????

????????}????

}??

??

public?class?A?extends?B{??

????public?void?doCRUD(){????

??????????System.out.println("執行add操作...");????

?????}????

??

?????public?void?add(){????

?????????????doCRUD();??

????????}????

}??

??

public?class?C?extends?B{??

????public?void?doCRUD(){????

??????????System.out.println("執行delete操作...");????

?????}????

??

?????public?void?delete(){????

?????????????doCRUD();??

????????}????

}??

如果改為回調實現是這樣的：?

Java代碼??

interface?CallBack{?????

????public?void?doCRUD();?????

}????

????

public?class?HibernateTemplate?{?????

????public?void?execute(CallBack?action){????

????????getConnection();????

????????action.doCRUD();????

????????releaseConnection();????

????}????

?????

????public?void?add(){????

?????????execute(new?CallBack(){????

????????????public?void?doCRUD(){????

????????????????System.out.println("執行add操作...");????

????????????}????

?????????});????

?????}?????

??

?????public?void?delete(){????

?????????execute(new?CallBack(){????

????????????public?void?doCRUD(){????

????????????????System.out.println("執行delete操作...");????

????????????}????

?????????});????

?????}???

????

????public?void?getConnection(){????

????????System.out.println("獲得連接...");????

????}????

????????

????public?void?releaseConnection(){????

????????System.out.println("釋放連接...");????

????}????

????????

}????

可見摒棄了繼承抽象類方式的回調方式更加簡便靈活。不需要為了實現抽象方法而總是繼承抽象類，而是只需要通過回調來增加一個方法即可，更加的直觀簡潔靈活。這算是回調的好處之一。?

下面再給出一個關于利用回調配合異步調用的很不錯的例子?
回調接口：?

Java代碼??

public?interface?CallBack?{????

????/**??

?????*?執行回調方法??

?????*?@param?objects???將處理后的結果作為參數返回給回調方法??

?????*/????

????public?void?execute(Object...?objects?);????

}????

消息的發送者：?

Java代碼??

/**?

?*?這個類相當于你自己?

?*/??

public?class?Local?implements?CallBack,Runnable{????

?????

????private?Remote?remote;????

????????

????/**??

?????*?發送出去的消息??

?????*/????

????private?String?message;????

????????

????public?Local(Remote?remote,?String?message)?{????

????????super();????

????????this.remote?=?remote;????

????????this.message?=?message;????

????}????

????

????/**??

?????*?發送消息??

?????*/????

????public?void?sendMessage()????

????{????

????????/**當前線程的名稱**/????

????????System.out.println(Thread.currentThread().getName());????

????????/**創建一個新的線程發送消息**/????

????????Thread?thread?=?new?Thread(this);????

????????thread.start();????

????????/**當前線程繼續執行**/????

????????System.out.println("Message?has?been?sent?by?Local~!");????

????}????

????

????/**??

?????*?發送消息后的回調函數??

?????*/????

????public?void?execute(Object...?objects?)?{????

????????/**打印返回的消息**/????

????????System.out.println(objects[0]);????

????????/**打印發送消息的線程名稱**/????

????????System.out.println(Thread.currentThread().getName());????

????????/**中斷發送消息的線程**/????

????????Thread.interrupted();????

????}????

????????

????public?static?void?main(String[]?args)????

????{????

????????Local?local?=?new?Local(new?Remote(),"Hello");????

????????????

????????local.sendMessage();????

????}????

????

????public?void?run()?{????

????????remote.executeMessage(message,?this);??//這相當于給同學打電話，打完電話之后，這個線程就可以去做其他事情了，只不過等到你的同學打回電話給你的時候你要做出響應??

????????????

????}????

}????

消息的接收者：?

Java代碼??

/**?

?*?這個類相當于你的同學?

?*/??

public?class?Remote?{????

????

????/**??

?????*?處理消息??

?????*?@param?msg???接收的消息??

?????*?@param?callBack??回調函數處理類??

?????*/????

????public?void?executeMessage(String?msg,CallBack?callBack)????

????{????

????????/**模擬遠程類正在處理其他事情，可能需要花費許多時間**/????

????????for(int?i=0;i<1000000000;i++)????

????????{????

????????????????

????????}????

????????/**處理完其他事情，現在來處理消息**/????

????????System.out.println(msg);????

????????System.out.println("I?hava?executed?the?message?by?Local");????

????????/**執行回調**/????

????????callBack.execute(new?String[]{"Nice?to?meet?you~!"});??//這相當于同學執行完之后打電話給你??

????}????

????????

}????

由上面這個例子可見，回調可以作為異步調用的基礎來實現異步調用。

轉載于:https://www.cnblogs.com/smilesmile/p/3833385.html

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