http://interper56-sohu-com.iteye.com/blog/172303

其實要了解java多線程只需要理解以下幾個事情：
1) java.lang.Thraed類
2) java.lang.Runnable接口
3) synchronized關鍵字
4) wait(),.notify(), notifyAll();

That’s all.

[java.util.Thread類]
Jdk的線程實現類，只要繼承這個類那么我們就可以實現自己的線程。
如果你想要繼承這個類，主要需要重載run方法。

例如：
Java代碼 ?

Pubilc?class?MyThread?{ ??

????Pubilc?MyThread()?{ ??

????????Super(); ??

????} ??

??

????Public?void?run()?{ ??

????????System.out.println(“Hello?world”); ??

????} ??

}??

Pubilc class MyThread {Pubilc MyThread() {Super();}Public void run() {System.out.println(“Hello world”);} }

當你要是用它的時候
Java代碼 ?

Publi?class?ThreadTest?{ ??

????Public?static?void?main(String[]?args)??{ ??

????????MyThread?app?=?new?MyThread(); ??

????????App.start(); ??

????} ??

}??

Publi class ThreadTest {Public static void main(String[] args) {MyThread app = new MyThread();App.start();} }

當你運行這個方法的時候，就會打出Hello world.

[java.lang.Runnable接口]
因為繼承Thread類就無法繼承別的你想要變為線程的類，
所以java.lang.Runnable接口可以實現你的愿望

那么看下面的例子：
食物類：
Java代碼 ?

public?class?Food?{ ??

????private?int?foodNum?=?0; ??

??

????public?Food(int?foodNum)?{ ??

????????this.foodNum?=?foodNum; ??

????} ??

???? ??

????public?void?setFoodNum(int?foodNum)?{ ??

????????this.foodNum?=?foodNum; ??

????} ??

???? ??

????public?int?getFoodNum()?{ ??

????????return?this.foodNum; ??

????} ??

}??

public class Food {private int foodNum = 0;public Food(int foodNum) {this.foodNum = foodNum;}public void setFoodNum(int foodNum) {this.foodNum = foodNum;}public int getFoodNum() {return this.foodNum;} }
消費者類：
Java代碼 ?

public?class?Customer?{ ??

????private?String?name?=?""; ??

??

????public?Customer(String?name)?{ ??

????????this.setName(name); ??

????} ??

??

????protected?void?setName(String?name)?{ ??

????????this.name?=?name; ??

????} ??

??

????protected?String?getName()?{ ??

????????return?this.name; ??

????} ??

}??

public class Customer {private String name = "";public Customer(String name) {this.setName(name);}protected void setName(String name) {this.name = name;}protected String getName() {return this.name;} }
消費者線程類：
Java代碼 ?

public?class?CustomerThread?extends?Customer?implements?Runnable?{ ??

??

????private?Food?food?=?null; ??

??

????public?CustomerThread(Food?food)?{ ??

????????super("Customer"); ??

????????this.food?=?food; ??

????} ??

??

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

????????this.consume(); ??

????} ??

???? ??

????public?void?startConsume()?{ ??

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

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

????} ??

??

????private?void?consume()?{ ??

????????int?num?=?this.food.getFoodNum(); ??

????????num++; ??

????????this.food.setFoodNum(num); ??

????????System.out.println(this.getName()?+?"?"?+?"consume?food"); ??

????} ??

}??

public class CustomerThread extends Customer implements Runnable {private Food food = null;public CustomerThread(Food food) {super("Customer");this.food = food;}public void run() {this.consume();}public void startConsume() {Thread thread = new Thread(this);thread.start();}private void consume() {int num = this.food.getFoodNum();num++;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "consume food");} }
生產者類：
Java代碼 ?

public?class?Productor?{ ??

????private?String?name?=?""; ??

??

????public?Productor(String?name)?{ ??

????????this.name?=?name; ??

????} ??

??

????protected?void?setName(String?name)?{ ??

????????this.name?=?name; ??

????} ??

??

????protected?String?getName()?{ ??

????????return?this.name; ??

????} ??

}??

public class Productor {private String name = "";public Productor(String name) {this.name = name;}protected void setName(String name) {this.name = name;}protected String getName() {return this.name;} }
消費者類：
Java代碼 ?

public?class?ProductorThread?extends?Productor?implements?Runnable?{ ??

??

????private?Food?food?=?null; ??

???? ??

????public?ProductorThread(Food?food)?{ ??

????????super("Productor"); ??

????????this.food?=?food; ??

????} ??

??

???? ??

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

????????this.increase(); ??

????} ??

???? ??

????public?void?startIncrease()?{ ??

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

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

????} ??

??

????private?void?increase()?{ ??

????????int?num?=?this.food.getFoodNum(); ??

????????num--; ??

????????this.food.setFoodNum(num); ??

????????System.out.println(this.getName()?+?"?"?+?"increase?food"); ??

????} ??

}??

public class ProductorThread extends Productor implements Runnable {private Food food = null;public ProductorThread(Food food) {super("Productor");this.food = food;}public void run() {this.increase();}public void startIncrease() {Thread thread = new Thread(this);thread.start();}private void increase() {int num = this.food.getFoodNum();num--;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "increase food");} }

測試類：
Java代碼 ?

public?class?MainTest?{ ??

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

????????Food?food?=?new?Food(0); ??

????????ProductorThread?productor?=?new?ProductorThread(food); ??

????????productor.startIncrease(); ??

????????CustomerThread?customer?=?new?CustomerThread(food); ??

????????customer.startConsume(); ??

????} ??

}??

public class MainTest {public static void main(String[] args) {Food food = new Food(0);ProductorThread productor = new ProductorThread(food);productor.startIncrease();CustomerThread customer = new CustomerThread(food);customer.startConsume();} }

上面代碼主要模擬了生產者和消費者生產食物，消費食物的過程，
這里面先讓生產者生產了1個食物，然后讓消費者消費了1個食物。

主要是想說明我們自己實現的Runnable接口的類必須借助Thread類
才可以把它變成一個線程，如果不借助Thread類，即使我們實現了run()方法，
這個類的對象也不會是一個線程。
說白了，就是用Thread(Runnable thread)這個構造方法把我們實現的Runnable
街口的類傳入，然后通過Thread的start方法，來調用我們的run方法，其實
我們實現的Runnable接口的類要想變為線程，是要通過Thread這個載體來實現。

但是上面的實現存在一個問題，我們并不能保證同一時間內只有一個
CustomerThread線程在消費，不能保證在同一時間只有一個ProductorThread
線程在生產。

所以我們引入了synchronized關鍵字

[synchronized關鍵字]
通過在CustomerThraed的consume方法和ProductorThread的increase
方法前面加入synchronized關鍵字就可以解決上面所說的問題。
改動后的方法為：
Java代碼 ?

private?synchronized?void?consume()?{ ??

????int?num?=?this.food.getFoodNum(); ??

????num++; ??

????this.food.setFoodNum(num); ??

????System.out.println(this.getName()?+?"?"?+?"consume?food"); ??

} ??

private?synchronized?void?increase()?{ ??

????int?num?=?this.food.getFoodNum(); ??

????num--; ??

????this.food.setFoodNum(num); ??

????System.out.println(this.getName()?+?"?"?+?"increase?food"); ??

}??

private synchronized void consume() {int num = this.food.getFoodNum();num++;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "consume food");}private synchronized void increase() {int num = this.food.getFoodNum();num--;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "increase food");}
那么synchronized關鍵字到底是干什么用的呢？

就是進入到synchronized關鍵字所包含的代碼塊的線程
都會嘗試獲得對象的鎖，等拿到對象的鎖后就可以進去執行代碼，
如果得不到，就在那里阻塞，等待其它線程釋放鎖，那么怎么用呢？

1)在方法前用synchronized關鍵字，如下：
Java代碼 ?

private?synchronized?void?consume()?{ ??

????int?num?=?this.food.getFoodNum(); ??

????num++; ??

????this.food.setFoodNum(num); ??

????System.out.println(this.getName()?+?"?"?+?"consume?food"); ??

}??

private synchronized void consume() {int num = this.food.getFoodNum();num++;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "consume food");}

當線程進入到這個方法的時候，這個線程就會獲得這個方法所在對象的鎖，
那么其他的進程想要進入這個方法，首先嘗試去獲得這個方法所在對象的鎖，
但是已經被前一個線程霸占了，所只能等待，當前一個線程把這段代碼執行
完畢，那么后來的線程就可以獲得這個對象鎖了，當然它進去后又會把后面的
線程阻塞在外面等待。
這種方法等同于：
Java代碼 ?

private?void?consume()?{ ??

????synchronized(this)?{ ??

????????int?num?=?this.food.getFoodNum(); ??

????????num++; ??

????????this.food.setFoodNum(num); ??

????????System.out.println(this.getName()?+?"?"?+?"consume?food"); ??

????} ??

}??

private void consume() {synchronized(this) {int num = this.food.getFoodNum();num++;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "consume food");}}

但是這種方法我們把整個對象都鎖住了，其他線程想要執行這個類中的其它用
Synchronized方法聲明的方法都不可以了，因為想要進入其它的synchronized
方法也要先獲得這個對象的鎖，所以這種方法比較霸道，我們不建議這么做，
所以出現了第二種方法。
2)聲明一個臨時的對象，讓進入同一個方法的線程去獲得這個臨時對象的鎖，
? 那么獲得這個臨時對象的鎖，并不是整個對象的鎖，所以并不會鎖住整個對象，
? 當然也就避免了上面第一種所遇到的問題：
Java代碼 ?

private?void?consume()?{ ??

????Object?lock?=?new?Object(); ??

????synchronized(lock)?{ ??

????????int?num?=?this.food.getFoodNum(); ??

????????num++; ??

????????this.food.setFoodNum(num); ??

????????System.out.println(this.getName()?+?"?"?+?"consume?food"); ??

????} ??

}??

private void consume() {Object lock = new Object();synchronized(lock) {int num = this.food.getFoodNum();num++;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "consume food");}}
Obj是一個臨時的對象，當多個線程進入到這個方法的時候都會嘗試去獲得這個
Obj對象的鎖，誰先獲得，誰就可以繼續執行，否則阻塞在外面，等待前一個進程
結束執行synchronized內的代碼，出了synchronized包含的代碼塊之后，
會馬上自動釋放對這個obj對象的鎖。

[wait(), notify()和notifyAll()]
其實采用了上面的synchronized關鍵字之后，上面的代碼還是有問題，
什么問題？
我們再來仔細分析一下Synchronzied關鍵字，
Synchronzied關鍵字對線程來說就是這么一回事：
1) 線程進入synchronized代碼塊：嘗試獲得對象鎖
2) 線程出了synchronized代碼塊：釋放對象鎖
說白了，就是誰有鎖，誰就可以繼續干活，沒有就得等。
加鎖就是限制同一時間有多個線程同時去訪問公共的資源。

但是問題也就來了，synchronized可以限制對公共的資源訪問，
但是無法決定線程訪問公共資源的順序，所以引入了wait(),
Notify(), notifyAll 等原語來控制線程訪問的順序。
注意這3個原語是當前對象的方法，不是當前線程的方法。

那么讓我們來看看這三個原語有什么用：
1) wait(): 使當前線程阻塞，釋放它所獲得的對象的鎖
2) notify(): 通知虛擬機當前線程準備要釋放它所獲得的對象的鎖，
?????????????? 當調用了wait()方法或者當這個線程出了synchronized
?????????????? 代碼塊之后，這兩個動作就是釋放了當前線程對對象的鎖的持有，
?????????????? 那么其它的被阻塞的線程又可以執行了。
3) notify(): 跟notify()沒有什么大區別，notify是通知1個被阻塞的線程做準備，
?????????????? notifyAll()是通知所有被阻塞的線程做準備，至于哪個線程可以獲得
?????????????? 這個鎖，那就看JVM的啦！

所以對于上面的生產者與消費者的例子，
正確的流程是，
如果消費者有東西可以消費，那么我們就讓他消費，
如果還需要生產，還可以生產，那么我們就讓生產者生產，
只需要修改CustomerThread類ProductorThread類，
Food類 MainTest類。
那么我們的最終代碼如下：
CustomerThread類：
Java代碼 ?

public?class?CustomerThread?extends?Customer?implements?Runnable?{ ??

??

????private?Food?food?=?null; ??

??

????public?CustomerThread(Food?food)?{ ??

????????super("Customer"); ??

????????this.food?=?food; ??

????} ??

??

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

????????this.consume(); ??

????} ??

???? ??

????public?void?startConsume()?{ ??

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

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

????} ??

??

????private?void?consume()?{ ??

????????synchronized(food)?{ ??

????????????while(true)?{ ??

????????????????if(food.getFoodNum()?<=?0)?{ ??

????????????????????try?{ ??

????????????????????????food.wait(); ??

????????????????????}?catch?(InterruptedException?e)?{ ??

????????????????????????//?Do?nothing ??

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

????????????????}?else?{ ??

????????????????????break; ??

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

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

????????????int?num?=?this.food.getFoodNum(); ??

????????????num--; ??

????????????this.food.setFoodNum(num); ??

????????????System.out.println(this.getName()?+?"?"?+?"consume?food?"?+?num); ??

????????????food.notify(); ??

????????} ??

????} ??

}??

public class CustomerThread extends Customer implements Runnable {private Food food = null;public CustomerThread(Food food) {super("Customer");this.food = food;}public void run() {this.consume();}public void startConsume() {Thread thread = new Thread(this);thread.start();}private void consume() {synchronized(food) {while(true) {if(food.getFoodNum() <= 0) {try {food.wait();} catch (InterruptedException e) {// Do nothing}} else {break;}}int num = this.food.getFoodNum();num--;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "consume food " + num);food.notify();}} }

ProductorThread類：
Java代碼 ?

public?class?ProductorThread?extends?Productor?implements?Runnable?{ ??

??

????private?Food?food?=?null; ??

???? ??

????public?ProductorThread(Food?food)?{ ??

????????super("Productor"); ??

????????this.food?=?food; ??

????} ??

??

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

????????this.increase(); ??

????} ??

???? ??

????public?void?startIncrease()?{ ??

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

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

????} ??

??

????private?void?increase()?{ ??

????????synchronized(food)?{ ??

????????????while(true)?{ ??

????????????????if(food.getFoodNum()?==?Food.MAX_NUM)?{ ??

????????????????????try?{ ??

????????????????????????food.wait(); ??

????????????????????}?catch?(InterruptedException?e)?{ ??

????????????????????????//?Do?nothing ??

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

????????????????}?else?{ ??

????????????????????break; ??

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

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

????????????int?num?=?this.food.getFoodNum(); ??

????????????num++; ??

????????????this.food.setFoodNum(num); ??

????????????System.out.println(this.getName()?+?"?"?+?"increase?food?"?+?num); ??

????????????food.notify(); ??

????????} ??

????} ??

}??

public class ProductorThread extends Productor implements Runnable {private Food food = null;public ProductorThread(Food food) {super("Productor");this.food = food;}public void run() {this.increase();}public void startIncrease() {Thread thread = new Thread(this);thread.start();}private void increase() {synchronized(food) {while(true) {if(food.getFoodNum() == Food.MAX_NUM) {try {food.wait();} catch (InterruptedException e) {// Do nothing}} else {break;}}int num = this.food.getFoodNum();num++;this.food.setFoodNum(num);System.out.println(this.getName() + " " + "increase food " + num);food.notify();}} }

Food類：
Java代碼 ?

public?class?Food?{ ??

??

????public?static?final?int?MAX_NUM?=?10; ??

??

????private?int?foodNum?=?0; ??

??

????public?Food(int?foodNum)?{ ??

????????this.foodNum?=?foodNum; ??

????} ??

??

????public?void?setFoodNum(int?foodNum)?{ ??

????????this.foodNum?=?foodNum; ??

????} ??

??

????public?int?getFoodNum()?{ ??

????????return?this.foodNum; ??

????} ??

}??

public class Food {public static final int MAX_NUM = 10;private int foodNum = 0;public Food(int foodNum) {this.foodNum = foodNum;}public void setFoodNum(int foodNum) {this.foodNum = foodNum;}public int getFoodNum() {return this.foodNum;} }

MainTest類：
Java代碼 ?

public?class?MainTest?{ ??

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

????????Food?food?=?new?Food(0); ??

???????? ??

????????CustomerThread?customer?=?new?CustomerThread(food); ??

????????customer.startConsume(); ??

????????CustomerThread?customer1?=?new?CustomerThread(food); ??

????????customer1.startConsume(); ??

????????CustomerThread?customer2?=?new?CustomerThread(food); ??

????????customer2.startConsume(); ??

????????CustomerThread?customer3?=?new?CustomerThread(food); ??

????????customer3.startConsume(); ??

????????CustomerThread?customer4?=?new?CustomerThread(food); ??

????????customer4.startConsume(); ??

????????CustomerThread?customer5?=?new?CustomerThread(food); ??

????????customer5.startConsume(); ??

???????? ??

????????ProductorThread?productor?=?new?ProductorThread(food); ??

????????productor.startIncrease(); ??

????????ProductorThread?productor1?=?new?ProductorThread(food); ??

????????productor1.startIncrease(); ??

????????ProductorThread?productor2?=?new?ProductorThread(food); ??

????????productor2.startIncrease(); ??

????????ProductorThread?productor3?=?new?ProductorThread(food); ??

????????productor3.startIncrease(); ??

????????ProductorThread?productor4?=?new?ProductorThread(food); ??

????????productor4.startIncrease(); ??

????????ProductorThread?productor5?=?new?ProductorThread(food); ??

????????productor5.startIncrease(); ??

??

??

??

????} ??

}??

public class MainTest {public static void main(String[] args) {Food food = new Food(0);CustomerThread customer = new CustomerThread(food);customer.startConsume();CustomerThread customer1 = new CustomerThread(food);customer1.startConsume();CustomerThread customer2 = new CustomerThread(food);customer2.startConsume();CustomerThread customer3 = new CustomerThread(food);customer3.startConsume();CustomerThread customer4 = new CustomerThread(food);customer4.startConsume();CustomerThread customer5 = new CustomerThread(food);customer5.startConsume();ProductorThread productor = new ProductorThread(food);productor.startIncrease();ProductorThread productor1 = new ProductorThread(food);productor1.startIncrease();ProductorThread productor2 = new ProductorThread(food);productor2.startIncrease();ProductorThread productor3 = new ProductorThread(food);productor3.startIncrease();ProductorThread productor4 = new ProductorThread(food);productor4.startIncrease();ProductorThread productor5 = new ProductorThread(food);productor5.startIncrease();} }

終于寫完了，累死我了~~~

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