建设网站定制,免费的wordpress怎样提高浏览量,项目进度管理软件app,学校网站的建设在我的上一个博客中#xff0c;我研究了使用Java的传统synchronized关键字和锁排序来修复破碎的#xff0c;死锁的余额转移示例代码。 但是#xff0c;有一种替代方法称为显式锁定。 这里#xff0c;将锁定机制称为显式而非隐式的想法是#xff0c; 显式表示它不是Java语… 在我的上一个博客中我研究了使用Java的传统synchronized关键字和锁排序来修复破碎的死锁的余额转移示例代码。 但是有一种替代方法称为显式锁定。 这里将锁定机制称为显式而非隐式的想法是 显式表示它不是Java语言的一部分并且已编写了一些类来实现锁定功能。 另一方面 隐式锁定可以定义为该语言的一部分并且可以使用语言关键字synchronchized在后台实现锁定。 您可以争论明确锁定是否是一个好主意。 是否应该对Java语言进行改进使其包括显式锁定功能而不是向已经庞大的API中添加另一组类 例如 trysynchronized() 。 显式锁定基于Lock接口及其ReentrantLock实现 。 与传统的synchronized关键字相比 Lock包含许多方法这些方法使您对锁定有更多控制。 它具有您期望的方法例如lock()会在代码的受保护部分中创建入口点而unlock()会在代码中创建出口点。 它还具有tryLock()和tryLock(long time,TimeUnit unit)仅当它可用且尚未被另一个线程获取时才获取tryLock() 而tryLock(long time,TimeUnit unit)将尝试获取一个锁如果不可用则等待指定的计时器。在放弃之前过期。 为了实现显式锁定我首先向本系列以前的博客中使用的Account类添加了Lock接口。 public class Account implements Lock {private final int number;private int balance;private final ReentrantLock lock;public Account(int number, int openingBalance) {this.number number;this.balance openingBalance;this.lock new ReentrantLock();}public void withDrawAmount(int amount) throws OverdrawnException {if (amount balance) {throw new OverdrawnException();}balance - amount;}public void deposit(int amount) {balance amount;}public int getNumber() {return number;}public int getBalance() {return balance;}// ------- Lock interface implementationOverridepublic void lock() {lock.lock();}Overridepublic void lockInterruptibly() throws InterruptedException {lock.lockInterruptibly();}Overridepublic Condition newCondition() {return lock.newCondition();}Overridepublic boolean tryLock() {return lock.tryLock();}Overridepublic boolean tryLock(long arg0, TimeUnit arg1) throws InterruptedException {return lock.tryLock(arg0, arg1);}Overridepublic void unlock() {if (lock.isHeldByCurrentThread()) {lock.unlock();}}} 在上面的代码中您可以看到我赞成通过封装一个ReentrantLock对象来支持聚合 Account类将锁定功能委托给该对象。 唯一需要注意的小型GOTCHA是在unlock()实现中 Overridepublic void unlock() {if (lock.isHeldByCurrentThread()) {lock.unlock();}} 它具有附加的if()语句该语句检查调用线程是否是当前持有锁的线程。 如果错过了这一行代码那么您将获得以下IllegalMonitorStateException Exception in thread Thread-7 java.lang.IllegalMonitorStateExceptionat java.util.concurrent.locks.ReentrantLock$Sync.tryRelease(ReentrantLock.java:155)at java.util.concurrent.locks.AbstractQueuedSynchronizer.release(AbstractQueuedSynchronizer.java:1260)at java.util.concurrent.locks.ReentrantLock.unlock(ReentrantLock.java:460)at threads.lock.Account.unlock(Account.java:76)at threads.lock.TrylockDemo$BadTransferOperation.transfer(TrylockDemo.java:98)at threads.lock.TrylockDemo$BadTransferOperation.run(TrylockDemo.java:67) 那么这是如何实现的呢 下面是基于我的原始DeadLockDemo程序的TryLockDemo示例的列表。 public class TrylockDemo {private static final int NUM_ACCOUNTS 10;private static final int NUM_THREADS 20;private static final int NUM_ITERATIONS 100000;private static final int LOCK_ATTEMPTS 10000;static final Random rnd new Random();ListAccount accounts new ArrayListAccount();public static void main(String args[]) {TrylockDemo demo new TrylockDemo();demo.setUp();demo.run();}void setUp() {for (int i 0; i NUM_ACCOUNTS; i) {Account account new Account(i, 1000);accounts.add(account);}}void run() {for (int i 0; i NUM_THREADS; i) {new BadTransferOperation(i).start();}}class BadTransferOperation extends Thread {int threadNum;BadTransferOperation(int threadNum) {this.threadNum threadNum;}Overridepublic void run() {int transactionCount 0;for (int i 0; i NUM_ITERATIONS; i) {Account toAccount accounts.get(rnd.nextInt(NUM_ACCOUNTS));Account fromAccount accounts.get(rnd.nextInt(NUM_ACCOUNTS));int amount rnd.nextInt(1000);if (!toAccount.equals(fromAccount)) {boolean successfulTransfer false;try {successfulTransfer transfer(fromAccount, toAccount, amount);} catch (OverdrawnException e) {successfulTransfer true;}if (successfulTransfer) {transactionCount;}}}System.out.println(Thread Complete: threadNum Successfully made transactionCount out of NUM_ITERATIONS);}private boolean transfer(Account fromAccount, Account toAccount, int transferAmount) throws OverdrawnException {boolean success false;for (int i 0; i LOCK_ATTEMPTS; i) {try {if (fromAccount.tryLock()) {try {if (toAccount.tryLock()) {success true;fromAccount.withDrawAmount(transferAmount);toAccount.deposit(transferAmount);break;}} finally {toAccount.unlock();}}} finally {fromAccount.unlock();}}return success;}}
} 想法是一样的我有一个银行帐户列表我将随机选择两个帐户并从一个帐户中随机转移一个金额。 问题的核心是我更新的transfer(...)方法如下所示。 private boolean transfer(Account fromAccount, Account toAccount, int transferAmount) throws OverdrawnException {boolean success false;for (int i 0; i LOCK_ATTEMPTS; i) {try {if (fromAccount.tryLock()) {try {if (toAccount.tryLock()) {success true;fromAccount.withDrawAmount(transferAmount);toAccount.deposit(transferAmount);break;}} finally {toAccount.unlock();}}} finally {fromAccount.unlock();}}return success;} 这里的想法是我尝试锁定fromAccount然后锁定toAccount 。 如果可行那么我先进行转移然后再记得解锁两个帐户。 如果那时帐户已经被锁定那么我的tryLock()方法将失败整个过程将循环并再次尝试。 尝试10000次锁定后线程将放弃并忽略传输。 我猜想在现实世界的应用程序中您希望将此故障放入某种队列中以便以后进行调查。 在使用显式锁定时您必须考虑它的工作原理因此请看下面的结果… Thread Complete: 17 Successfully made 58142 out of 100000
Thread Complete: 12 Successfully made 57627 out of 100000
Thread Complete: 9 Successfully made 57901 out of 100000
Thread Complete: 16 Successfully made 56754 out of 100000
Thread Complete: 3 Successfully made 56914 out of 100000
Thread Complete: 14 Successfully made 57048 out of 100000
Thread Complete: 8 Successfully made 56817 out of 100000
Thread Complete: 4 Successfully made 57134 out of 100000
Thread Complete: 15 Successfully made 56636 out of 100000
Thread Complete: 19 Successfully made 56399 out of 100000
Thread Complete: 2 Successfully made 56603 out of 100000
Thread Complete: 13 Successfully made 56889 out of 100000
Thread Complete: 0 Successfully made 56904 out of 100000
Thread Complete: 5 Successfully made 57119 out of 100000
Thread Complete: 7 Successfully made 56776 out of 100000
Thread Complete: 6 Successfully made 57076 out of 100000
Thread Complete: 10 Successfully made 56871 out of 100000
Thread Complete: 11 Successfully made 56863 out of 100000
Thread Complete: 18 Successfully made 56916 out of 100000
Thread Complete: 1 Successfully made 57304 out of 100000 这些表明尽管该程序没有死锁并无限期地挂起但它仅设法使余额转移只超过转移请求的一半。 这意味着它正在消耗大量的处理能力包括循环循环和循环-总体上不是很有效。 另外我刚才说过该程序“没有死锁并无限期地挂起”这不是真的。 如果您考虑发生了什么那么您将意识到程序陷入僵局然后退出这种情况。 我的显式锁定演示代码的第二个版本使用上面提到的tryLock(long time,TimeUnit unit) 。 private boolean transfer(Account fromAccount, Account toAccount, int transferAmount) throws OverdrawnException {boolean success false;try {if (fromAccount.tryLock(LOCK_TIMEOUT, TimeUnit.MILLISECONDS)) {try {if (toAccount.tryLock(LOCK_TIMEOUT, TimeUnit.MILLISECONDS)) {success true;fromAccount.withDrawAmount(transferAmount);toAccount.deposit(transferAmount);}} finally {toAccount.unlock();}}} catch (InterruptedException e) {e.printStackTrace();} finally {fromAccount.unlock();}return success;} 在这段代码中我用1毫秒的tryLock(...)超时替换了for循环。 这意味着当tryLock(...)被调用并且无法获取锁定时它将等待1 ms然后回滚并放弃。 Thread Complete: 0 Successfully made 26637 out of 100000
Thread Complete: 14 Successfully made 26516 out of 100000
Thread Complete: 3 Successfully made 26552 out of 100000
Thread Complete: 11 Successfully made 26653 out of 100000
Thread Complete: 7 Successfully made 26399 out of 100000
Thread Complete: 1 Successfully made 26602 out of 100000
Thread Complete: 18 Successfully made 26606 out of 100000
Thread Complete: 17 Successfully made 26358 out of 100000
Thread Complete: 19 Successfully made 26407 out of 100000
Thread Complete: 16 Successfully made 26312 out of 100000
Thread Complete: 15 Successfully made 26449 out of 100000
Thread Complete: 5 Successfully made 26388 out of 100000
Thread Complete: 8 Successfully made 26613 out of 100000
Thread Complete: 2 Successfully made 26504 out of 100000
Thread Complete: 6 Successfully made 26420 out of 100000
Thread Complete: 4 Successfully made 26452 out of 100000
Thread Complete: 9 Successfully made 26287 out of 100000
Thread Complete: 12 Successfully made 26507 out of 100000
Thread Complete: 10 Successfully made 26660 out of 100000
Thread Complete: 13 Successfully made 26523 out of 100000 上面的结果表明使用计时器时余额转移成功率甚至下降到25以上。 尽管现在还没有消耗大量的处理器时间但效率仍然很低。 在相当长的一段时间内我可能会花时间处理这两个代码示例从而选择可以优化应用程序并提高性能的变量但最终没有什么真正的选择可以正确地进行锁排序。 我个人更愿意在可能的情况下使用老式的synchronized关键字隐式锁定并为死锁代码过时陈旧难以理解的少数情况保留显式锁定我已经尝试了其他所有方法该应用需要上线已经很晚了该回家了…… 有关更多信息请参阅本系列中的其他博客 。 该系列以及其他博客的所有源代码都可以在Github上找到网址为git//github.com/roghughe/captaindebug.git 参考 调查死锁–第5部分使用来自Captain Debug博客博客的JCG合作伙伴 Roger Hughes的显式锁定 。 翻译自: https://www.javacodegeeks.com/2012/11/investigating-deadlocks-part-5-using-explicit-locking.html
