在触发应用程序事件时创建 bean 时，如何避免 Spring 中的死锁？

Question

当一个线程对 mongo 数据库执行查询时，我的应用程序中发生死锁，导致触发应用程序事件，而另一个线程尝试使用AutowireCapableBeanFactory.

这发生在 @Service 的构造函数中，并阻止整个应用程序的启动，因为服务是在启动时创建的。

伪代码：

@Service
public class MyService {

    @Autowired
    public MyService(AutowireCapableBeanFactory factory, TaskScheduler scheduler) {
        MyFirstCommand command1 = factory.getBean(MyFirstCommand.class);
        scheduler.schedule(command1, new Date());

        MySecondCommand command2 = factory.getBean(MySecondCommand.class);
        scheduler.schedule(command2, new Date());
    }
}

public class MyFirstCommand extends Runnable {

    @Autowired MongoOperations mongo;

    @Override
    public void run() {
       mongo.findById("some-id", MyData.class); 
    }
}

请注意，我有一些AbstractMongoEventListeners 注册为@Components。

死锁发生在一个线程MyFirstCommand.run()执行时mongo.findById()。另一个线程在MyService执行factory.getBean(MySecondCommand.class)的构造函数中：

Found one Java-level deadlock:
=============================
"pool-1-thread-1":
  waiting to lock monitor 0x00007fd05d5bf5d8 (object 0x00000000816d0970, a java.util.concurrent.ConcurrentHashMap),
  which is held by "main"
"main":
  waiting for ownable synchronizer 0x0000000081ed8468, (a java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync),
  which is held by "pool-1-thread-1"


Java stack information for the threads listed above:
===================================================
"pool-1-thread-1":
at org.springframework.context.event.AbstractApplicationEventMulticaster.getApplicationListeners(AbstractApplicationEventMulticaster.java:179)
- waiting to lock <0x00000000816d0970> (a java.util.concurrent.ConcurrentHashMap)
at org.springframework.context.event.SimpleApplicationEventMulticaster.multicastEvent(SimpleApplicationEventMulticaster.java:127)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:381)
at org.springframework.context.support.AbstractApplicationContext.publishEvent(AbstractApplicationContext.java:335)
at org.springframework.data.mapping.context.AbstractMappingContext.addPersistentEntity(AbstractMappingContext.java:326)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:179)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:139)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:66)
at org.springframework.data.mongodb.core.MongoTemplate.determineCollectionName(MongoTemplate.java:1976)
at org.springframework.data.mongodb.core.MongoTemplate.findById(MongoTemplate.java:607)


"main":
at sun.misc.Unsafe.park(Native Method)
- parking to wait for  <0x0000000081ed8468> (a java.util.concurrent.locks.ReentrantReadWriteLock$NonfairSync)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireShared(AbstractQueuedSynchronizer.java:967)
at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireShared(AbstractQueuedSynchronizer.java:1283)
at java.util.concurrent.locks.ReentrantReadWriteLock$ReadLock.lock(ReentrantReadWriteLock.java:727)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:160)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:139)
at org.springframework.data.mapping.context.AbstractMappingContext.getPersistentEntity(AbstractMappingContext.java:66)
at org.springframework.data.mongodb.repository.support.MongoRepositoryFactory.getEntityInformation(MongoRepositoryFactory.java:118)
at org.springframework.data.mongodb.repository.support.MongoRepositoryFactory.getTargetRepository(MongoRepositoryFactory.java:92)
at org.springframework.data.repository.core.support.RepositoryFactorySupport.getRepository(RepositoryFactorySupport.java:185)
at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.initAndReturn(RepositoryFactoryBeanSupport.java:251)
at org.springframework.data.repository.core.support.RepositoryFactoryBeanSupport.afterPropertiesSet(RepositoryFactoryBeanSupport.java:237)
at org.springframework.data.mongodb.repository.support.MongoRepositoryFactoryBean.afterPropertiesSet(MongoRepositoryFactoryBean.java:108)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1637)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1574)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:545)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482) 

为什么 Spring 这里不是线程安全的？我该怎么做才能避免这种僵局？

Answer 1

这个死锁是由于 Spring 的一个 bug 造成的。我在他们的 JIRA 中创建了一个问题：DATACMNS-901。

作为一种解决方法，我现在在安排它们之前创建所有可运行的 bean：

@Autowired
public MyService(AutowireCapableBeanFactory factory, TaskScheduler scheduler) {
    MyFirstCommand command1 = factory.getBean(MyFirstCommand.class);
    MySecondCommand command2 = factory.getBean(MySecondCommand.class);

    scheduler.schedule(command2, new Date());
    scheduler.schedule(command1, new Date());
}