理解和避免Java多线程并发问题案例

原创ゝ一世哀愁。 2024-11-26 14:39 20阅读 0赞

Java多线程并发问题，主要涉及到线程安全、同步机制、死锁等问题。以下是一些具体案例：

1. **竞态条件**：假设两个线程读写同一个共享变量，但由于执行顺序的不同，可能导致结果不一致。
   ```java
   int sharedVar = 0;
   Thread t1 = new Thread(() -> sharedVar++));
   Thread t2 = new Thread(() -> sharedVar--));

t1.start();
   t2.start();

t1.join();
   t2.join();

System.out.println("Final value: " + sharedVar); // May print 0 or 1
   ```

2. **死锁**：当两个或更多的线程因争夺资源而造成的一种相互等待的现象时，就会发生死锁。
   ```java
   class Bank {
       int account[10];
       int status[10] = {0};

void openAccount(int number) {
           if (status[number]) {
               System.out.println("Account already exists");
               return;
           }
           status[number] = 1;
           account[number] = number;
           System.out.println("Account opened successfully with number " + number);
       }

void closeAccount(int number) {
           if (!status[number]) {
               System.out.println("Account does not exist");
               return;
           }
           status[number] = 0;
           account[number] = -1; // Indicate the account has been closed
           System.out.println("Account successfully closed with number " + number);
       }
   }

class AccountThread extends Thread {
       Bank bank;

public AccountThread(Bank bank) {
           this.bank = bank;
       }

@Override
       public void run() {
           bank.openAccount(100); // Simulating a user opening an account
           try {
               Thread.sleep(5000); // Pausing for 5 seconds to simulate user inactivity
           } catch (InterruptedException e) {
               e.printStackTrace();
           }
           bank.closeAccount(100); // Simulating a user closing their account
       }
   }

public class Main {
       public static void main(String[] args) {
           Bank bank = new Bank();
           AccountThread accountThread = new AccountThread(bank);

accountThread.start();

// This is just an example to show how the threads interact with the shared resource
           // You can replace this part with your specific use case
       }
   }
   ```

在上述代码中，`Bank`类模拟一个银行，有两个方法：`openAccount()`用于打开新账户，`closeAccount()`用于关闭已存在的账户。这两个方法都与一个共享的数组`account[number]`关联。

`AccountThread`类代表一个用户操作账户的线程。它在`run()`方法中调用银行的方法来模拟用户行为。

通过这个例子，你可以看到多线程并发可能导致的资源争抢和死锁问题。