研究僵局–第4部分：修复代码

在这个简短的博客系列的最后BadTransferOperation中，我一直在讨论分析死锁，我将修复BadTransferOperation代码。如果您看过本系列的其他博客，那么您将知道，为了达到这一点，我创建了死锁的演示代码，展示了如何掌握线程转储，然后分析了线程转储，弄清楚发生僵局的位置和方式。为了节省空间，下面的讨论同时引用了本系列第1部分中的Account和DeadlockDemo类，其中包含完整的代码清单。

教科书中有关死锁的描述通常是这样的：“线程A将获得对象1的锁定，并等待对象2的锁定，而线程B将获得对象2的锁定，同时等待对象1的锁定”。我以前的博客中显示的堆积，并在下面突出显示，是一个真实的死锁，其他线程，锁和对象陷入了直接，简单，理论上的死锁情况。

Found one Java-level deadlock:
=============================
'Thread-21':waiting to lock monitor 7f97118bd560 (object 7f3366f58, a threads.deadlock.Account),which is held by 'Thread-20'
'Thread-20':waiting to lock monitor 7f97118bc108 (object 7f3366e98, a threads.deadlock.Account),which is held by 'Thread-4'
'Thread-4':waiting to lock monitor 7f9711834360 (object 7f3366e80, a threads.deadlock.Account),which is held by 'Thread-7'
'Thread-7':waiting to lock monitor 7f97118b9708 (object 7f3366eb0, a threads.deadlock.Account),which is held by 'Thread-11'
'Thread-11':waiting to lock monitor 7f97118bd560 (object 7f3366f58, a threads.deadlock.Account),which is held by 'Thread-20'

如果将上面的文本和图像与以下代码相关联，则可以看到Thread-20已锁定其fromAccount对象（ fromAccount ），正在等待锁定其toAccount对象（e98）

private void transfer(Account fromAccount, Account toAccount, int transferAmount) throws OverdrawnException {synchronized (fromAccount) {synchronized (toAccount) {fromAccount.withdraw(transferAmount);toAccount.deposit(transferAmount);}}}

不幸的是，由于时序问题， Thread-20无法获得对对象e98的锁定，因为它正在等待Thread-4释放对该对象的锁定。 Thread-4无法释放锁，因为它正在等待Thread-7 ， Thread-7正在等待Thread-11而Thread-11正在等待Thread-20释放对对象f58的锁。这个现实世界的僵局只是教科书描述的一个更复杂的版本。

这段代码的问题是，从下面的代码片段中，您可以看到我正在从Accounts数组中随机选择两个Account对象作为fromAccount和toAccount并将它们锁定。由于fromAccount和toAccount可以引用accounts数组中的任何对象，这意味着它们以随机顺序被锁定。

Account toAccount = accounts.get(rnd.nextInt(NUM_ACCOUNTS));Account fromAccount = accounts.get(rnd.nextInt(NUM_ACCOUNTS));

因此， 解决方法是对Account对象的锁定方式施加顺序，并且只要顺序一致，任何顺序都可以执行。

private void transfer(Account fromAccount, Account toAccount, int transferAmount) throws OverdrawnException {if (fromAccount.getNumber() > toAccount.getNumber()) {synchronized (fromAccount) {synchronized (toAccount) {fromAccount.withdraw(transferAmount);toAccount.deposit(transferAmount);}}} else {synchronized (toAccount) {synchronized (fromAccount) {fromAccount.withdraw(transferAmount);toAccount.deposit(transferAmount);}}}}

上面的代码显示了此修复程序。在此代码中，我使用帐号来确保首先锁定具有最高帐号的Account对象，以便永远不会出现以上的死锁情况。

以下代码是此修复程序的完整列表：

public class AvoidsDeadlockDemo {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 MAX_COLUMNS = 60;static final Random rnd = new Random();List<Account> accounts = new ArrayList<Account>();public static void main(String args[]) {AvoidsDeadlockDemo demo = new AvoidsDeadlockDemo();demo.setUp();demo.run();}void setUp() {for (int i = 0; i < NUM_ACCOUNTS; i++) {Account account = new Account(i, rnd.nextInt(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() {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)) {try {transfer(fromAccount, toAccount, amount);System.out.print(".");} catch (OverdrawnException e) {System.out.print("-");}printNewLine(i);}}System.out.println("Thread Complete: " + threadNum);}private void printNewLine(int columnNumber) {if (columnNumber % MAX_COLUMNS == 0) {System.out.print("\n");}}/*** This is the crucial point here. The idea is that to avoid deadlock you need to ensure that threads can't try* to lock the same two accounts in the same order*/private void transfer(Account fromAccount, Account toAccount, int transferAmount) throws OverdrawnException {if (fromAccount.getNumber() > toAccount.getNumber()) {synchronized (fromAccount) {synchronized (toAccount) {fromAccount.withdraw(transferAmount);toAccount.deposit(transferAmount);}}} else {synchronized (toAccount) {synchronized (fromAccount) {fromAccount.withdraw(transferAmount);toAccount.deposit(transferAmount);}}}}}
}

在我的示例代码，死锁的发生是因为时机问题，嵌套的synchronized在我的关键字BadTransferOperation类。在此代码中， synchronized关键字位于相邻的行上；但是，最后一点是，值得注意的是， synchronized关键字在代码中的什么位置都没关系（它们不必相邻）。只要您使用同一线程锁定两个（或更多）不同的监视对象，就会发生排序和死锁。

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4部分：来自Captain Debug博客博客的JCG合作伙伴 Roger Hughes 修复代码。