答复: What is a structured locking in the JVM specification?

[陈雨亭]

The original explanation is given as: "Structured locking is the situation
when, during a method invocation, every exit on a given monitor matches a
preceding entry on that monitor. Since there is no assurance that all code
submitted to the Java Virtual Machine will perform structured locking,
implementations of the Java Virtual Machine are permitted but not required
to enforce both of the following two rules guaranteeing structured locking.
Let T be a thread and M be a monitor. Then: (1) The number of monitor
entries performed by T on M during a method invocation must equal the number
of monitor exits performed by T on M during the method invocation whether
the method invocation completes normally or abruptly. (2) At no point during
a method invocation may the number of monitor exits performed by T on M
since the method invocation exceed the number of monitor entries performed
by T on M since the method invocation."

I retested some programs on HotSpot and IBM's J9. It seems that HotSpot
takes Rule 1 (rule1 is stricter than rule2, and J9 throws out an
IllegalMonitorStateException at runtime) and J9 only takes Rule 2 (accepting
the program). As the exception can be caught (with further exception
handlings), some of my programs run quite differently on the two platforms.

f(){
   monitorenter r0;
}
g(){
   f();
   monitorexit r0;
}

-----邮件原件-----
发件人: csr-discuss [mailto:csr-discuss-bounces at openjdk.java.net] 代表 陈雨
亭
发送时间: 2017年5月17日 14:08
收件人: csr-discuss at openjdk.java.net
主题: What is a structured locking in the JVM specification?

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