连续锁定2个不同的锁会死锁

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，有另一种方法称为显式锁定。

在这里，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，这些方法可以使您对锁定进行更多控制。 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) ，只有在有可用但尚未被另一个线程获取的情况下才会获取锁，而tryLock(long time,TimeUnit unit)则将尝试获取锁定，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 implementation@Overridepublic 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在unlock()实现中：

@Overridepublic void unlock() {if (lock.isHeldByCurrentThread()) {lock.unlock();}}

它有一个附加的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，那么您将得到以下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();List<Account> accounts = new ArrayList<Account>();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，并将一个随机数从一个帐户转移到另一个帐户。 事情的核心是我更新的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，线程放弃并忽略传输。 我猜想在现实世界中的应用程序中，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，但它仅设法使余额中的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-完全不是很有效。 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，那么您会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，从而选择可以调整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，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

连续锁定2个不同的锁会死锁