java synchronized的運用

標籤：synchronized   java   thread   multithread   

這幾天被線程弄暈了

特地惡補了一下java的線程知識

synchronized關鍵字是可以實現一個類對象同一時間只被一個線程調用，其他線程要調用這個對象只能等正在調用的線程結束或停止（一般會用在停止狀態，如果是結束的話直接用join()方法會更方便）才能獲得對象

synchronized使用方法一：鎖定對象方法

動手寫代碼，寫了一個類比買票的情境：

先寫一個Tickets類

class Tickets {public Tickets() {// TODO Auto-generated constructor stub}int total_tickets = 30;void sell_tickets(String thread_name) {synchronized (this) {total_tickets--;System.out.println(thread_name + " sell a ticket, remains "+ total_tickets);}}}

設定一共有30張票，

 sell_tickets(String thread_name)

會將線程名和剩餘張數列印出來，注意sell_tickets()裡面用了synchronized(this)，表示這段代碼同一時間只能被一個線程調用

Thread counter1 = new Thread(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubfor (int i = 0; i < 10; i++) {tickets.sell_tickets("1");try {Thread.sleep(50);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}});Thread counter2 = new Thread(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubfor (int i = 0; i < 10; i++) {tickets.sell_tickets("2");try {Thread.sleep(50);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}});Thread counter3 = new Thread(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubfor (int i = 0; i < 10; i++) {tickets.sell_tickets("3");try {Thread.sleep(50);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}});counter1.start();counter2.start();counter3.start();

建立1個Tickets類對象，3個counter線程，每個線程每隔一段時間就賣1張票，在Thread.sleep的時候就會讓別的等待線程獲得tickets.sell_ticket的使用權

結果如下：

1 sell a ticket, remains 293 sell a ticket, remains 282 sell a ticket, remains 271 sell a ticket, remains 262 sell a ticket, remains 253 sell a ticket, remains 242 sell a ticket, remains 231 sell a ticket, remains 223 sell a ticket, remains 212 sell a ticket, remains 201 sell a ticket, remains 193 sell a ticket, remains 182 sell a ticket, remains 171 sell a ticket, remains 163 sell a ticket, remains 152 sell a ticket, remains 143 sell a ticket, remains 131 sell a ticket, remains 122 sell a ticket, remains 113 sell a ticket, remains 101 sell a ticket, remains 92 sell a ticket, remains 81 sell a ticket, remains 73 sell a ticket, remains 62 sell a ticket, remains 51 sell a ticket, remains 43 sell a ticket, remains 32 sell a ticket, remains 23 sell a ticket, remains 11 sell a ticket, remains 0

可以看到，三個線程調用tickets對象並沒有發生衝突

synchronized使用方法二：線程中鎖定對象

這一次我們並不鎖定Tickets類方法

只做一般的定義

class Tickets {public Tickets() {// TODO Auto-generated constructor stub}int total_tickets = 30;void sell_tickets(String thread_name) {total_tickets--;System.out.println(thread_name + " sell a ticket, remains "+ total_tickets);}}

但我們線上程的runnable中使用synchronized將tickets對象同步

Thread counter1 = new Thread(new Runnable() {@Overridepublic void run() {// TODO Auto-generated method stubfor (int i = 0; i < 10; i++) {synchronized (tickets) {tickets.sell_tickets("1");}try {Thread.sleep(50);} catch (InterruptedException e) {// TODO Auto-generated catch blocke.printStackTrace();}}}});

同樣能實現同步的效果

java synchronized的運用