? ? ? ? Lock是java.util.concurrent.locks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)中的一個例子簡單的實現一下和sychronized一樣的效果，代碼如下：

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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?<?name.length();?i++)?{??

????????????????System.out.print(name.charAt(i));??

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

????????}?finally?{??

????????????lock.unlock();//?釋放鎖 ??

????????}??

????}??

}??

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 < name.length(); 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，而是用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：

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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);??

????}??

}??

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：

?

?

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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??

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：

?

?

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public?synchronized?void?set(int?data)?{...}??????

public?synchronized?void?get()?{...}??

public synchronized void set(int data) {...} public synchronized void get() {...}

? ? ? ? 部分輸出結果：
?

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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??

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：

?

?

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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 ??

????????}??

????}??

}??

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：

?

[java] view plaincopyprint?

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??

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。

轉載于:https://blog.51cto.com/519124/1114616

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