从 Singleton EJB 到 Stateless Session Bean 的锁传播答案

【问题标题】：Lock propagation from Singleton EJB to Stateless Session Bean从 Singleton EJB 到 Stateless Session Bean 的锁传播
【发布时间】：2017-04-13 20:27:04
【问题描述】：

我有这个 EJB Singleton (EJB 3.1)：

@Singleton
@Startup
@Lock(LockType.READ)
public class SingletonExample {

@EJB
private StatelessSBExample stlsb;
...
    @Schedule(..........., persistent = false)
    @AccessTimeout(0)
    @Lock(LockType.READ)
    public void call1SB() {
         stlsb.doSomething();
    }

    @Schedule(..........., persistent = false)
    @AccessTimeout(0)
    @Lock(LockType.READ)
    public void call2SB() {
        stlsb.doSomething();
    }
}

我的 bean 是一个传统的 EJB 无状态会话 Bean：

@Stateless
public class StatelessSBExample {
    public void domSomething() {
    ...
    }
}

用visualvm监控，发现有些线程在堆积。该应用程序以 Thread Live Peak = 92 开始，现在为 102。而且还在增加。在 VisualVM Threads 中，我有几个状态为“Park”和“Wait”的线程。在我的线程转储中我有很多：

"Thread-42" - Thread t@190
   java.lang.Thread.State: WAITING
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for <71bfce05> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:1088)
    at java.util.concurrent.ScheduledThreadPoolExecutor$DelayedWorkQueue.take(ScheduledThreadPoolExecutor.java:809)
    at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

和

"__ejb-thread-pool13" - Thread t@130
   java.lang.Thread.State: WAITING
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for <5cfe398e> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:2039)
    at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:442)
    at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:1067)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1127)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at java.lang.Thread.run(Thread.java:745)

   Locked ownable synchronizers:
    - None

我的错在哪里？我只想执行 call1SB()，如果它正在运行，则不再执行此方法（与 call2SB 相同）

附：我不能使用 LockType.WRITE，因为我想同时执行 call1SB() 和 call2SB()（我的 Singleton 中没有属性.. 只有方法）

【问题讨论】：

标签： java multithreading jakarta-ee threadpool ejb-3.1

【解决方案1】：

默认的 EJB 锁定机制在常见用例中运行良好，但它们不是很灵活。在这种情况下，我建议您使用自己的锁定机制，如下所示：

private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();

public void call1SB() {
    if(lock.writeLock().tryLock()) { 
    // Acquires the write lock only if it 
    // is not held by another thread at the time of invocation.
        stlsb.doSomething();
    } // else { return; } // do nothing if already locked
}

与您的第二个 Singleton 方法的第二个锁相同。

【讨论】：

ReentrantReadWriteLock 需要是静态的吗？