在Java項(xiàng)目中實(shí)現(xiàn)多線(xiàn)程并發(fā)編程的方法?很多新手對(duì)此不是很清楚,為了幫助大家解決這個(gè)難題,下面小編將為大家詳細(xì)講解,有這方面需求的人可以來(lái)學(xué)習(xí)下,希望你能有所收獲。
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Java 中的鎖通常分為兩種:
通過(guò)關(guān)鍵字 synchronized 獲取的鎖,我們稱(chēng)為同步鎖,上一篇有介紹到:Java 多線(xiàn)程并發(fā)編程 Synchronized 關(guān)鍵字。
java.util.concurrent(JUC)包里的鎖,如通過(guò)繼承接口 Lock 而實(shí)現(xiàn)的 ReentrantLock(互斥鎖),繼承 ReadWriteLock 實(shí)現(xiàn)的 ReentrantReadWriteLock(讀寫(xiě)鎖)。
本篇主要介紹 ReentrantLock(互斥鎖)。
ReentrantLock(互斥鎖)
ReentrantLock 互斥鎖,在同一時(shí)間只能被一個(gè)線(xiàn)程所占有,在被持有后并未釋放之前,其他線(xiàn)程若想獲得該鎖只能等待或放棄。
ReentrantLock 互斥鎖是可重入鎖,即某一線(xiàn)程可多次獲得該鎖。
公平鎖 and 非公平鎖
public ReentrantLock() { sync = new NonfairSync(); } public ReentrantLock(boolean fair) { sync = fair ? new FairSync() : new NonfairSync(); }
由 ReentrantLock 的構(gòu)造函數(shù)可見(jiàn),在實(shí)例化 ReentrantLock 的時(shí)候我們可以選擇實(shí)例化一個(gè)公平鎖或非公平鎖,而默認(rèn)會(huì)構(gòu)造一個(gè)非公平鎖。
公平鎖與非公平鎖區(qū)別在于競(jìng)爭(zhēng)鎖時(shí)的有序與否。公平鎖可確保有序性(FIFO 隊(duì)列),非公平鎖不能確保有序性(即使也有 FIFO 隊(duì)列)。
然而,公平是要付出代價(jià)的,公平鎖比非公平鎖要耗性能,所以在非必須確保公平的條件下,一般使用非公平鎖可提高吞吐率。所以 ReentrantLock 默認(rèn)的構(gòu)造函數(shù)也是“不公平”的。
一般使用
DEMO1:
public class Test { private static class Counter { private ReentrantLock mReentrantLock = new ReentrantLock(); public void count() { mReentrantLock.lock(); try { for (int i = 0; i < 6; i++) { System.out.println(Thread.currentThread().getName() + ", i = " + i); } } finally { // 必須在 finally 釋放鎖 mReentrantLock.unlock(); } } } private static class MyThread extends Thread { private Counter mCounter; public MyThread(Counter counter) { mCounter = counter; } @Override public void run() { super.run(); mCounter.count(); } } public static void main(String[] var0) { Counter counter = new Counter(); // 注:myThread1 和 myThread2 是調(diào)用同一個(gè)對(duì)象 counter MyThread myThread1 = new MyThread(counter); MyThread myThread2 = new MyThread(counter); myThread1.start(); myThread2.start(); } }
DEMO1 輸出:
Thread-0, i = 0 Thread-0, i = 1 Thread-0, i = 2 Thread-0, i = 3 Thread-0, i = 4 Thread-0, i = 5 Thread-1, i = 0 Thread-1, i = 1 Thread-1, i = 2 Thread-1, i = 3 Thread-1, i = 4 Thread-1, i = 5
DEMO1 僅使用了 ReentrantLock 的 lock 和 unlock 來(lái)提現(xiàn)一般鎖的特性,確保線(xiàn)程的有序執(zhí)行。此種場(chǎng)景 synchronized 也適用。
鎖的作用域
DEMO2:
public class Test { private static class Counter { private ReentrantLock mReentrantLock = new ReentrantLock(); public void count() { for (int i = 0; i < 6; i++) { mReentrantLock.lock(); // 模擬耗時(shí),突出線(xiàn)程是否阻塞 try{ Thread.sleep(100); System.out.println(Thread.currentThread().getName() + ", i = " + i); } catch (InterruptedException e) { e.printStackTrace(); } finally { // 必須在 finally 釋放鎖 mReentrantLock.unlock(); } } } public void doOtherThing(){ for (int i = 0; i < 6; i++) { // 模擬耗時(shí),突出線(xiàn)程是否阻塞 try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " doOtherThing, i = " + i); } } } public static void main(String[] var0) { final Counter counter = new Counter(); new Thread(new Runnable() { @Override public void run() { counter.count(); } }).start(); new Thread(new Runnable() { @Override public void run() { counter.doOtherThing(); } }).start(); } }
DEMO2 輸出:
Thread-0, i = 0 Thread-1 doOtherThing, i = 0 Thread-0, i = 1 Thread-1 doOtherThing, i = 1 Thread-0, i = 2 Thread-1 doOtherThing, i = 2 Thread-0, i = 3 Thread-1 doOtherThing, i = 3 Thread-0, i = 4 Thread-1 doOtherThing, i = 4 Thread-0, i = 5 Thread-1 doOtherThing, i = 5
DEMO3:
public class Test { private static class Counter { private ReentrantLock mReentrantLock = new ReentrantLock(); public void count() { for (int i = 0; i < 6; i++) { mReentrantLock.lock(); // 模擬耗時(shí),突出線(xiàn)程是否阻塞 try{ Thread.sleep(100); System.out.println(Thread.currentThread().getName() + ", i = " + i); } catch (InterruptedException e) { e.printStackTrace(); } finally { // 必須在 finally 釋放鎖 mReentrantLock.unlock(); } } } public void doOtherThing(){ mReentrantLock.lock(); try{ for (int i = 0; i < 6; i++) { // 模擬耗時(shí),突出線(xiàn)程是否阻塞 try { Thread.sleep(100); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println(Thread.currentThread().getName() + " doOtherThing, i = " + i); } }finally { mReentrantLock.unlock(); } } } public static void main(String[] var0) { final Counter counter = new Counter(); new Thread(new Runnable() { @Override public void run() { counter.count(); } }).start(); new Thread(new Runnable() { @Override public void run() { counter.doOtherThing(); } }).start(); } }
DEMO3 輸出:
Thread-0, i = 0 Thread-0, i = 1 Thread-0, i = 2 Thread-0, i = 3 Thread-0, i = 4 Thread-0, i = 5 Thread-1 doOtherThing, i = 0 Thread-1 doOtherThing, i = 1 Thread-1 doOtherThing, i = 2 Thread-1 doOtherThing, i = 3 Thread-1 doOtherThing, i = 4 Thread-1 doOtherThing, i = 5
結(jié)合 DEMO2 和 DEMO3 輸出可見(jiàn),鎖的作用域在于 mReentrantLock,因?yàn)樗鶃?lái)自于 mReentrantLock。
可終止等待
DEMO4:
public class Test { static final int TIMEOUT = 300; private static class Counter { private ReentrantLock mReentrantLock = new ReentrantLock(); public void count() { try{ //lock() 不可中斷 mReentrantLock.lock(); // 模擬耗時(shí),突出線(xiàn)程是否阻塞 for (int i = 0; i < 6; i++) { long startTime = System.currentTimeMillis(); while (true) { if (System.currentTimeMillis() - startTime > 100) break; } System.out.println(Thread.currentThread().getName() + ", i = " + i); } } finally { // 必須在 finally 釋放鎖 mReentrantLock.unlock(); } } public void doOtherThing(){ try{ //lockInterruptibly() 可中斷,若線(xiàn)程沒(méi)有中斷,則獲取鎖 mReentrantLock.lockInterruptibly(); for (int i = 0; i < 6; i++) { // 模擬耗時(shí),突出線(xiàn)程是否阻塞 long startTime = System.currentTimeMillis(); while (true) { if (System.currentTimeMillis() - startTime > 100) break; } System.out.println(Thread.currentThread().getName() + " doOtherThing, i = " + i); } } catch (InterruptedException e) { System.out.println(Thread.currentThread().getName() + " 中斷 "); }finally { // 若當(dāng)前線(xiàn)程持有鎖,則釋放 if(mReentrantLock.isHeldByCurrentThread()){ mReentrantLock.unlock(); } } } } public static void main(String[] var0) { final Counter counter = new Counter(); new Thread(new Runnable() { @Override public void run() { counter.count(); } }).start(); Thread thread2 = new Thread(new Runnable() { @Override public void run() { counter.doOtherThing(); } }); thread2.start(); long start = System.currentTimeMillis(); while (true){ if (System.currentTimeMillis() - start > TIMEOUT) { // 若線(xiàn)程還在運(yùn)行,嘗試中斷 if(thread2.isAlive()){ System.out.println(" 不等了,嘗試中斷 "); thread2.interrupt(); } break; } } } }
DEMO4 輸出:
Thread-0, i = 0 Thread-0, i = 1 Thread-0, i = 2 不等了,嘗試中斷 Thread-1 中斷 Thread-0, i = 3 Thread-0, i = 4 Thread-0, i = 5
線(xiàn)程 thread2 等待 300ms 后 timeout,中斷等待成功。
若把 TIMEOUT 改成 3000ms,輸出結(jié)果:(正常運(yùn)行)
Thread-0, i = 0 Thread-0, i = 1 Thread-0, i = 2 Thread-0, i = 3 Thread-0, i = 4 Thread-0, i = 5 Thread-1 doOtherThing, i = 0 Thread-1 doOtherThing, i = 1 Thread-1 doOtherThing, i = 2 Thread-1 doOtherThing, i = 3 Thread-1 doOtherThing, i = 4 Thread-1 doOtherThing, i = 5
定時(shí)鎖
DEMO5:
public class Test { static final int TIMEOUT = 3000; private static class Counter { private ReentrantLock mReentrantLock = new ReentrantLock(); public void count() { try{ //lock() 不可中斷 mReentrantLock.lock(); // 模擬耗時(shí),突出線(xiàn)程是否阻塞 for (int i = 0; i < 6; i++) { long startTime = System.currentTimeMillis(); while (true) { if (System.currentTimeMillis() - startTime > 100) break; } System.out.println(Thread.currentThread().getName() + ", i = " + i); } } finally { // 必須在 finally 釋放鎖 mReentrantLock.unlock(); } } public void doOtherThing(){ try{ //tryLock(long timeout, TimeUnit unit) 嘗試獲得鎖 boolean isLock = mReentrantLock.tryLock(300, TimeUnit.MILLISECONDS); System.out.println(Thread.currentThread().getName() + " isLock:" + isLock); if(isLock){ for (int i = 0; i < 6; i++) { // 模擬耗時(shí),突出線(xiàn)程是否阻塞 long startTime = System.currentTimeMillis(); while (true) { if (System.currentTimeMillis() - startTime > 100) break; } System.out.println(Thread.currentThread().getName() + " doOtherThing, i = " + i); } }else{ System.out.println(Thread.currentThread().getName() + " timeout"); } } catch (InterruptedException e) { System.out.println(Thread.currentThread().getName() + " 中斷 "); }finally { // 若當(dāng)前線(xiàn)程持有鎖,則釋放 if(mReentrantLock.isHeldByCurrentThread()){ mReentrantLock.unlock(); } } } } public static void main(String[] var0) { final Counter counter = new Counter(); new Thread(new Runnable() { @Override public void run() { counter.count(); } }).start(); Thread thread2 = new Thread(new Runnable() { @Override public void run() { counter.doOtherThing(); } }); thread2.start(); } }
DEMO5 輸出:
Thread-0, i = 0 Thread-0, i = 1 Thread-0, i = 2 Thread-1 isLock:false Thread-1 timeout Thread-0, i = 3 Thread-0, i = 4 Thread-0, i = 5
tryLock() 嘗試獲得鎖,tryLock(long timeout, TimeUnit unit) 在給定的 timeout 時(shí)間內(nèi)嘗試獲得鎖,若超時(shí),則不帶鎖往下走,所以必須加以判斷。
ReentrantLock or synchronized
ReentrantLock 、synchronized 之間如何選擇?
ReentrantLock 在性能上 比 synchronized 更勝一籌。
ReentrantLock 需格外小心,因?yàn)樾枰@式釋放鎖,lock() 后記得 unlock(),而且必須在 finally 里面,否則容易造成死鎖。
synchronized 隱式自動(dòng)釋放鎖,使用方便。
ReentrantLock 擴(kuò)展性好,可中斷鎖,定時(shí)鎖,自由控制。
synchronized 一但進(jìn)入阻塞等待,則無(wú)法中斷等待。
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