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new o b j e c t ( ) j
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Mon itor Enter ( monitorObj ) j t ry
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C h a pter 6: D a t a a n d C o n t ro l Syn c h ro n i z a t i o n
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I I D o some c ri t i c a l operat i o n s } finally { Mon i t o r E xit ( monitorObj ) ;
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This ensures that, so long as the call to E nt e r succeeds, the call to E x i t will always be made, no matter what happens in the critical region Asyn chronous exceptions threaten the reliability of even this code, because an exception can theoretically arise between the call to E n t e r and the entrance into the try block We'll examine this situation in more detail just a little bit later Because this pattern is so common, the C# and VB languages offer keywords to encapsulate this pattern In C#, we can use the l o c k keyword
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o b j e c t mon itorObj II
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loc k ( monitorObj ) { II Do some c r it i c a l o p e r a t i o n s }
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This example is functionally equivalent to the previous one In fact, the same IL is emitted by the C# compiler in both cases In Visual Basic, you can use the Syn c Lo c k keyword
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Dim mon itorObj As Obj e c t el sewhere
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Sync Loc k mon itorObj Do some c ri t i c a l operat i o n s E n d S y n c Loc k
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To support the synchronized keyword in Java (for J#), which is used as a method modifier indicating callers of the method implicitly acquire / release the target monitor, there is a method-level attribute that can be used In S y s t e m R u n t ime Com p i l e rS e r v i c e s you'll find the
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Met hod l m p lAtt r i b u t e type You can annotate any method definition with
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it, passing the Met h o d l m p l O pt i o n s Syn c h ro n i z e d flag to its constructor, and the CLR will automatically acquire and release a monitor when calls are made to it Note that this method of synchronization is effectively dep recated and only described for educational purposes-that is, in case you run across code that is already using it For example, in J# we might write some function f to be s y n c h ro n i z ed
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syn c h ronized void f ( )
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II Do some c rit i c a l operat ions
This is simply translated into the following
[ Method lmplAtt ri but e ( Met hod lmplOption s Sy n c h ro n i zed ) ] void f O
II Do some c rit i c a l o p e r a t i o n s
Note that this attribute is usable from any CLR language, not just J#, although most languages do not support the sy n c h ro n i z e d keyword itself The next question is, what monitor is acquired and released For instance methods, the monitor is the instance on which the call was made Thus, the preceding code is effectively equivalent to wrapping f's body in l oc k ( t h i s ) { } For static methods, the monitor is the Type object on which the method is defined Thus, if f were marked static and was on some type T, it would be equivalent to wrapping the method body in
loc k ( typeof ( T ) { } While this might look nice at first glance, both
instance and static methods use dangerous practices Locking on t h i s is discouraged because it exposes synchronization details; and locking on a CLR Ty pe object can cause some surprisingly strange behavior because
Types can be shared across AppDomains (more on that later)
Avoiding Blocking: TryEnter and Spin Waiting The Mon i tor class also offers a TryEnter method to avoid blocking, or to block for only a certain period of time before giving up Two of the three overloads accept a timeout-either
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with a n integer count o f the milliseconds o r a TimeS pan value-and all return
t r u e or f a l s e to indicate whether the lock was acquired
p u b l i c s t a t i c bool TryEnt e r ( ob j e c t obj ) j p u b l i c s t a t i c bool T ry E nt e r ( obj ect obj , int m i l l i s e c ond sTimeout ) j p u b l i c s t a t i c bool T r y E nt e r ( o b j e c t obj , T imeS p a n t imeout ) j
If the T ry E n t e r overload without a timeout is called, or the timeout argument is e or n ew TimeSpa n ( e ) , then the method will test if the monitor is available and, if not, return fa l s e immediately without waiting Other wise, the method will block for approximately the timeout specified as an argument (Timer resolutions vary across platforms, and, because the thread must be placed back into the OS thread scheduler to run after the timeout has expired, precisely when the thread is rescheduled for execution depends heavily on the current load of the machine) Using T ry E n t e r is a good approach to test locks for availability, choosing to spend time on some other activity instead of blocking and periodically checking back to dis cover when it has become available Note that T ry E n t e r is generally not good as a deadlock prevention technique, although this is perhaps its most popular (mis)use To use a nonblocking or timeout acquire, you have to throw out the lan guage keywords and go back to using the Mo n it o r class directly