What is a structured locking in the JVM specification?

[陈雨亭]

According to the definition, it seems that a structured locking appears
like: Monitorenter r0; …monitorenter r0; …； monitorexit r0; …monitorexit
r0; 

  * The definition is given as follows: Structured locking is the situation
when, during a method invocation, every exit on a given monitor matches a
preceding entry on that monitor.
(https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-2.html#jvms-2.11.10
)

 

I’m a little curious whether a structured locking can be composed by the
following instructions:

Monitorenter r0; monitorenter r2; …； monitorexit r0; monitorexit r2; 

 

In addition, should this sequence be accepted or rejected? At runtime or
during the verification time?

 

Yuting