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這篇文章主要講解了“Java多線程的內(nèi)置鎖與顯示鎖介紹”,文中的講解內(nèi)容簡(jiǎn)單清晰,易于學(xué)習(xí)與理解,下面請(qǐng)大家跟著小編的思路慢慢深入,一起來(lái)研究和學(xué)習(xí)“Java多線程的內(nèi)置鎖與顯示鎖介紹”吧!
Synchronized
內(nèi)置鎖獲得鎖和釋放鎖是隱式的,進(jìn)入synchronized修飾的代碼就獲得鎖,走出相應(yīng)的代碼就釋放鎖。
synchronized(list){ //獲得鎖 list.append(); list.count(); }//釋放鎖
通信
與Synchronized配套使用的通信方法通常有wait(),notify()。
wait()方法會(huì)立即釋放當(dāng)前鎖,并進(jìn)入等待狀態(tài),等待到相應(yīng)的notify并重新獲得鎖過(guò)后才能繼續(xù)執(zhí)行;notify()不會(huì)立刻立刻釋放鎖,必須要等notify()所在線程執(zhí)行完synchronized塊中的所有代碼才會(huì)釋放。用如下代碼來(lái)進(jìn)行驗(yàn)證:
public static void main(String[] args){ List list = new LinkedList(); Thread r = new Thread(new ReadList(list)); Thread w = new Thread(new WriteList(list)); r.start(); w.start(); }class ReadList implements Runnable{ private List list; public ReadList(List list){ this.list = list; } @Override public void run(){ System.out.println("ReadList begin at "+System.currentTimeMillis()); synchronized (list){ try { Thread.sleep(1000); System.out.println("list.wait() begin at "+System.currentTimeMillis()); list.wait(); System.out.println("list.wait() end at "+System.currentTimeMillis()); } catch (InterruptedException e) { e.printStackTrace(); } } System.out.println("ReadList end at "+System.currentTimeMillis()); } }class WriteList implements Runnable{ private List list; public WriteList(List list){ this.list = list; } @Override public void run(){ System.out.println("WriteList begin at "+System.currentTimeMillis()); synchronized (list){ System.out.println("get lock at "+System.currentTimeMillis()); list.notify(); System.out.println("list.notify() at "+System.currentTimeMillis()); try { Thread.sleep(2000); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("get out of block at "+System.currentTimeMillis()); } System.out.println("WriteList end at "+System.currentTimeMillis()); } }
運(yùn)行結(jié)果
ReadList begin at 1493650526582WriteList begin at 1493650526582list.wait() begin at 1493650527584get lock at 1493650527584list.notify() at 1493650527584get out of block at 1493650529584WriteList end at 1493650529584list.wait() end at 1493650529584ReadList end at 1493650529584
可見(jiàn)讀線程開(kāi)始運(yùn)行,開(kāi)始wait過(guò)后,寫(xiě)線程才獲得鎖;寫(xiě)線程走出同步塊而不是notify過(guò)后,讀線程才wait結(jié)束,亦即獲得鎖。所以notify不會(huì)釋放鎖,wait會(huì)釋放鎖。值得一提的是,notifyall()會(huì)通知等待隊(duì)列中的所有線程。
編碼
編碼模式比較簡(jiǎn)單,單一,不必顯示的獲得鎖,釋放鎖,能降低因粗心忘記釋放鎖的錯(cuò)誤。使用模式如下:
synchronized(object){ }
靈活性
內(nèi)置鎖在進(jìn)入同步塊時(shí),采取的是***等待的策略,一旦開(kāi)始等待,就既不能中斷也不能取消,容易產(chǎn)生饑餓與死鎖的問(wèn)題
在線程調(diào)用notify方法時(shí),會(huì)隨機(jī)選擇相應(yīng)對(duì)象的等待隊(duì)列的一個(gè)線程將其喚醒,而不是按照FIFO的方式,如果有強(qiáng)烈的公平性要求,比如FIFO就無(wú)法滿足
性能
Synchronized在JDK1.5及之前性能(主要指吞吐率)比較差,擴(kuò)展性也不如ReentrantLock。但是JDK1.6以后,修改了管理內(nèi)置鎖的算法,使得Synchronized和標(biāo)準(zhǔn)的ReentrantLock性能差別不大。
ReentrantLock
ReentrantLock是顯示鎖,需要顯示進(jìn)行 lock 以及 unlock 操作。
通信
與ReentrantLock搭配的通行方式是Condition,如下:
private Lock lock = new ReentrantLock(); private Condition condition = lock.newCondition(); condition.await();//this.wait(); condition.signal();//this.notify(); condition.signalAll();//this.notifyAll();
Condition是被綁定到Lock上的,必須使用lock.newCondition()才能創(chuàng)建一個(gè)Condition。從上面的代碼可以看出,Synchronized能實(shí)現(xiàn)的通信方式,Condition都可以實(shí)現(xiàn),功能類(lèi)似的代碼寫(xiě)在同一行中。而Condition的優(yōu)秀之處在于它可以為多個(gè)線程間建立不同的Condition,比如對(duì)象的讀/寫(xiě)Condition,隊(duì)列的空/滿Condition,在JDK源碼中的ArrayBlockingQueue中就使用了這個(gè)特性:
public ArrayBlockingQueue(int capacity, boolean fair) { if (capacity <= 0) throw new IllegalArgumentException(); this.items = new Object[capacity]; lock = new ReentrantLock(fair); notEmpty = lock.newCondition(); notFull = lock.newCondition(); }public void put(E e) throws InterruptedException { checkNotNull(e); final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == items.length) notFull.await(); enqueue(e); } finally { lock.unlock(); } }public E take() throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { while (count == 0) notEmpty.await(); return dequeue(); } finally { lock.unlock(); } }private void enqueue(E x) { // assert lock.getHoldCount() == 1; // assert items[putIndex] == null; final Object[] items = this.items; items[putIndex] = x; if (++putIndex == items.length) putIndex = 0; count++; notEmpty.signal(); }private E dequeue() { // assert lock.getHoldCount() == 1; // assert items[takeIndex] != null; final Object[] items = this.items; @SuppressWarnings("unchecked") E x = (E) items[takeIndex]; items[takeIndex] = null; if (++takeIndex == items.length) takeIndex = 0; count--; if (itrs != null) itrs.elementDequeued(); notFull.signal(); return x; }
編碼
Lock lock = new ReentrantLock();lock.lock();try{ }finally{ lock.unlock(); }
相比于Synchronized要復(fù)雜一些,而且一定要記得在finally中釋放鎖而不是其他地方,這樣才能保證即使出了異常也能釋放鎖。
靈活性
lock.lockInterruptibly() 可以使得線程在等待鎖是支持響應(yīng)中斷;lock.tryLock() 可以使得線程在等待一段時(shí)間過(guò)后如果還未獲得鎖就停止等待而非一直等待。有了這兩種機(jī)制就可以更好的制定獲得鎖的重試機(jī)制,而非盲目一直等待,可以更好的避免饑餓和死鎖問(wèn)題
ReentrantLock可以成為公平鎖(非默認(rèn)的),所謂公平鎖就是鎖的等待隊(duì)列的FIFO,不過(guò)公平鎖會(huì)帶來(lái)性能消耗,如果不是必須的不建議使用。這和CPU對(duì)指令進(jìn)行重排序的理由是相似的,如果強(qiáng)行的按照代碼的書(shū)寫(xiě)順序來(lái)執(zhí)行指令,就會(huì)浪費(fèi)許多時(shí)鐘周期,達(dá)不到***利用率
性能
雖然Synchronized和標(biāo)準(zhǔn)的ReentrantLock性能差別不大,但是ReentrantLock還提供了一種非互斥的讀寫(xiě)鎖,
也就是不強(qiáng)制每次最多只有一個(gè)線程能持有鎖,它會(huì)避免“讀/寫(xiě)”沖突,“寫(xiě)/寫(xiě)”沖突,但是不會(huì)排除“讀/讀”沖突,
因?yàn)椤白x/讀”并不影響數(shù)據(jù)的完整性,所以可以多個(gè)讀線程同時(shí)持有鎖,這樣在讀寫(xiě)比較高的情況下,性能會(huì)有很大的提升。
下面用兩種鎖分別實(shí)現(xiàn)的線程安全的linkedlist:
class RWLockList {//讀寫(xiě)鎖 private List list; private final ReadWriteLock lock = new ReentrantReadWriteLock(); private final Lock readLock = lock.readLock(); private final Lock writeLock = lock.writeLock(); public RWLockList(List list){this.list = list;} public int get(int k) { readLock.lock(); try { return (int)list.get(k); } finally { readLock.unlock(); } } public void put(int value) { writeLock.lock(); try { list.add(value); } finally { writeLock.unlock(); } } }class SyncList { private List list; public SyncList(List list){this.list = list;} public synchronized int get(int k){ return (int)list.get(k); } public synchronized void put(int value){ list.add(value); } }
讀寫(xiě)鎖測(cè)試代碼:
List list = new LinkedList();for (int i=0;i<10000;i++){ list.add(i); } RWLockList rwLockList = new RWLockList(list);//初始化數(shù)據(jù)Thread writer = new Thread(new Runnable() { @Override public void run() { for (int i=0;i<10000;i++){ rwLockList.put(i); } } }); Thread reader1 = new Thread(new Runnable() { @Override public void run() { for (int i=0;i<10000;i++){ rwLockList.get(i); } } }); Thread reader2 = new Thread(new Runnable() { @Override public void run() { for (int i=0;i<10000;i++){ rwLockList.get(i); } } });long begin = System.currentTimeMillis(); writer.start();reader1.start();reader2.start();try { writer.join(); reader1.join(); reader2.join(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("RWLockList take "+(System.currentTimeMillis()-begin) + "ms");
同步鎖測(cè)試代碼:
List list = new LinkedList();for (int i=0;i<10000;i++){ list.add(i); } SyncList syncList = new SyncList(list);//初始化數(shù)據(jù)Thread writerS = new Thread(new Runnable() { @Override public void run() { for (int i=0;i<10000;i++){ syncList.put(i); } } }); Thread reader1S = new Thread(new Runnable() { @Override public void run() { for (int i=0;i<10000;i++){ syncList.get(i); } } }); Thread reader2S = new Thread(new Runnable() { @Override public void run() { for (int i=0;i<10000;i++){ syncList.get(i); } } });long begin1 = System.currentTimeMillis(); writerS.start();reader1S.start();reader2S.start();try { writerS.join(); reader1S.join(); reader2S.join(); } catch (InterruptedException e) { e.printStackTrace(); } System.out.println("SyncList take "+(System.currentTimeMillis()-begin1) + "ms");
結(jié)果:
RWLockList take 248msRWLockList take 255msRWLockList take 249msRWLockList take 224msSyncList take 351msSyncList take 367msSyncList take 315msSyncList take 323ms
可見(jiàn)讀寫(xiě)鎖的確是優(yōu)于純碎的互斥鎖
總結(jié)
內(nèi)置鎖***優(yōu)點(diǎn)是簡(jiǎn)潔易用,顯示鎖***優(yōu)點(diǎn)是功能豐富,所以能用內(nèi)置鎖就用內(nèi)置鎖,在內(nèi)置鎖功能不能滿足之時(shí)在考慮顯示鎖。
感謝各位的閱讀,以上就是“Java多線程的內(nèi)置鎖與顯示鎖介紹”的內(nèi)容了,經(jīng)過(guò)本文的學(xué)習(xí)后,相信大家對(duì)Java多線程的內(nèi)置鎖與顯示鎖介紹這一問(wèn)題有了更深刻的體會(huì),具體使用情況還需要大家實(shí)踐驗(yàn)證。這里是億速云,小編將為大家推送更多相關(guān)知識(shí)點(diǎn)的文章,歡迎關(guān)注!
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