Java：为什么这个线程在它已经拥有锁的时候等待？

Question

这是一个来自转储的单线程，用于说明问题。没有其他线程持有锁0x00000007d7f78430但仍然等待。

"main" prio=6 tid=0x000000000033b800 nid=0x2478 in Object.wait() [0x000000000257d000]   java.lang.Thread.State: TIMED_WAITING (on object monitor) 
at java.lang.Object.wait(Native Method) 
- waiting on <0x00000007d7f78430> (a org.osgi.util.tracker.ServiceTracker$AllTracked) 
at org.osgi.util.tracker.ServiceTracker.waitForService(ServiceTracker.java:456) 
- locked <0x00000007d7f78430> (a org.osgi.util.tracker.ServiceTracker$AllTracked) 
at org.apache.camel.test.blueprint.CamelBlueprintHelper.getOsgiService(CamelBlueprintHelper.java:190) 
at org.apache.camel.test.blueprint.CamelBlueprintHelper.getOsgiService(CamelBlueprintHelper.java:165) 
at org.apache.camel.test.blueprint.CamelBlueprintTestSupport.createCamelContext(CamelBlueprintTestSupport.java:116) 
at org.apache.camel.test.junit4.CamelTestSupport.doSetUp(CamelTestSupport.java:247) 
at org.apache.camel.test.junit4.CamelTestSupport.setUp(CamelTestSupport.java:217) 
at org.apache.camel.test.blueprint.CamelBlueprintTestSupport.setUp(CamelBlueprintTestSupport.java:50) 
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) 
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39) 

Answer 1

看看在org.osgi.util.tracker.ServiceTracker.waitForService(long timeout)方法。

你会看到，它的目的是：

等待至少一个服务由该ServiceTracker的跟踪。

所以这不是锁争用 - 底层系统使用锁来等待东西 - 在大多数情况下这实际上是一个非常好的主意。