Java線程(八)：鎖對象Lock-同步問題更完美的處理方式 · Java線程

# Java線程(八)：鎖對象Lock-同步問題更完美的處理方式 Lock是[java.util.concurrent.locks](http://blog.csdn.net/ghsau/article/details/7461369)包下的接口，Lock?實現提供了比使用synchronized?方法和語句可獲得的更廣泛的鎖定操作，它能以更優雅的方式處理線程同步問題，我們拿[Java線程(二)](http://blog.csdn.net/ghsau/article/details/7424694)中的一個例子簡單的實現一下和sychronized一樣的效果，代碼如下： ~~~ public?class?LockTest?{?? ????public?static?void?main(String[]?args)?{?? ????????final?Outputter1?output?=?new?Outputter1();?? ????????new?Thread()?{?? ????????????public?void?run()?{?? ????????????????output.output("zhangsan");?? ????????????};?? ????????}.start();???????? ????????new?Thread()?{?? ????????????public?void?run()?{?? ????????????????output.output("lisi");?? ????????????};?? ????????}.start();?? ????}?? }?? class?Outputter1?{?? ????private?Lock?lock?=?new?ReentrantLock();//?鎖對象?? ????public?void?output(String?name)?{?? ????????//?TODO?線程輸出方法?? ????????lock.lock();//?得到鎖?? ????????try?{?? ????????????for(int?i?=?0;?i? ????????????????System.out.print(name.charAt(i));?? ????????????}?? ????????}?finally?{?? ????????????lock.unlock();//?釋放鎖?? ????????}?? ????}?? }?? ~~~ 這樣就實現了和sychronized一樣的同步效果，需要注意的是，用sychronized修飾的方法或者語句塊在代碼執行完之后鎖自動釋放，而用Lock需要我們手動釋放鎖，所以為了保證鎖最終被釋放(發生異常情況)，要把互斥區放在try內，釋放鎖放在finally內。如果說這就是Lock，那么它不能成為同步問題更完美的處理方式，下面要介紹的是讀寫鎖(ReadWriteLock)，我們會有一種需求，在對數據進行讀寫的時候，為了保證數據的一致性和完整性，需要讀和寫是互斥的，寫和寫是互斥的，但是讀和讀是不需要互斥的，這樣讀和讀不互斥性能更高些，來看一下不考慮互斥情況的代碼原型： ~~~ public?class?ReadWriteLockTest?{?? ????public?static?void?main(String[]?args)?{?? ????????final?Data?data?=?new?Data();?? ????????for?(int?i?=?0;?i?3;?i++)?{?? ????????????new?Thread(new?Runnable()?{?? ????????????????public?void?run()?{?? ????????????????????for?(int?j?=?0;?j?5;?j++)?{?? ????????????????????????data.set(new?Random().nextInt(30));?? ????????????????????}?? ????????????????}?? ????????????}).start();?? ????????}????????? ????????for?(int?i?=?0;?i?3;?i++)?{?? ????????????new?Thread(new?Runnable()?{?? ????????????????public?void?run()?{?? ????????????????????for?(int?j?=?0;?j?5;?j++)?{?? ????????????????????????data.get();?? ????????????????????}?? ????????????????}?? ????????????}).start();?? ????????}?? ????}?? }?? class?Data?{?????? ????private?int?data;//?共享數據?????? ????public?void?set(int?data)?{?? ????????System.out.println(Thread.currentThread().getName()?+?"準備寫入數據");?? ????????try?{?? ????????????Thread.sleep(20);?? ????????}?catch?(InterruptedException?e)?{?? ????????????e.printStackTrace();?? ????????}?? ????????this.data?=?data;?? ????????System.out.println(Thread.currentThread().getName()?+?"寫入"?+?this.data);?? ????}????? ????public?void?get()?{?? ????????System.out.println(Thread.currentThread().getName()?+?"準備讀取數據");?? ????????try?{?? ????????????Thread.sleep(20);?? ????????}?catch?(InterruptedException?e)?{?? ????????????e.printStackTrace();?? ????????}?? ????????System.out.println(Thread.currentThread().getName()?+?"讀取"?+?this.data);?? ????}?? }?? ~~~ 部分輸出結果： ~~~ Thread-1準備寫入數據?? Thread-3準備讀取數據?? Thread-2準備寫入數據?? Thread-0準備寫入數據?? Thread-4準備讀取數據?? Thread-5準備讀取數據?? Thread-2寫入12?? Thread-4讀取12?? Thread-5讀取5?? Thread-1寫入12?? ~~~ 我們要實現寫入和寫入互斥，讀取和寫入互斥，讀取和讀取互斥，在set和get方法加入sychronized修飾符： ~~~ public?synchronized?void?set(int?data)?{...}?????? public?synchronized?void?get()?{...}?? ~~~ 部分輸出結果： ~~~ Thread-0準備寫入數據?? Thread-0寫入9?? Thread-5準備讀取數據?? Thread-5讀取9?? Thread-5準備讀取數據?? Thread-5讀取9?? Thread-5準備讀取數據?? Thread-5讀取9?? Thread-5準備讀取數據?? Thread-5讀取9?? ~~~ 我們發現，雖然寫入和寫入互斥了，讀取和寫入也互斥了，但是讀取和讀取之間也互斥了，不能并發執行，效率較低，用讀寫鎖實現代碼如下： ~~~ class?Data?{?????? ????private?int?data;//?共享數據?? ????private?ReadWriteLock?rwl?=?new?ReentrantReadWriteLock();????? ????public?void?set(int?data)?{?? ????????rwl.writeLock().lock();//?取到寫鎖?? ????????try?{?? ????????????System.out.println(Thread.currentThread().getName()?+?"準備寫入數據");?? ????????????try?{?? ????????????????Thread.sleep(20);?? ????????????}?catch?(InterruptedException?e)?{?? ????????????????e.printStackTrace();?? ????????????}?? ????????????this.data?=?data;?? ????????????System.out.println(Thread.currentThread().getName()?+?"寫入"?+?this.data);?? ????????}?finally?{?? ????????????rwl.writeLock().unlock();//?釋放寫鎖?? ????????}?? ????}????? ????public?void?get()?{?? ????????rwl.readLock().lock();//?取到讀鎖?? ????????try?{?? ????????????System.out.println(Thread.currentThread().getName()?+?"準備讀取數據");?? ????????????try?{?? ????????????????Thread.sleep(20);?? ????????????}?catch?(InterruptedException?e)?{?? ????????????????e.printStackTrace();?? ????????????}?? ????????????System.out.println(Thread.currentThread().getName()?+?"讀取"?+?this.data);?? ????????}?finally?{?? ????????????rwl.readLock().unlock();//?釋放讀鎖?? ????????}?? ????}?? }?? ~~~ 部分輸出結果： ~~~ Thread-4準備讀取數據?? Thread-3準備讀取數據?? Thread-5準備讀取數據?? Thread-5讀取18?? Thread-4讀取18?? Thread-3讀取18?? Thread-2準備寫入數據?? Thread-2寫入6?? Thread-2準備寫入數據?? Thread-2寫入10?? Thread-1準備寫入數據?? Thread-1寫入22?? Thread-5準備讀取數據?? ~~~ 從結果可以看出實現了我們的需求，這只是鎖的基本用法，鎖的機制還需要繼續深入學習。