RFR (12): 8191053: Provide a mechanism to make system's security manager immutable

[mandy chung]

On 9/13/18 1:02 PM, Sean Mullan wrote:
> This is a SecurityManager related change which warrants some 
> additional details for its motivation.
>
> The current System.setSecurityManager() API allows a SecurityManager 
> to be set at run-time. However, because of this mutability, it incurs 
> a performance overhead even for applications that never call it and do 
> not enable a SecurityManager dynamically, which is probably the 
> majority of applications.
>
> :
> webrev: http://cr.openjdk.java.net/~mullan/webrevs/8191053/webrev.00/
> CSR: https://bugs.openjdk.java.net/browse/JDK-8203316
> JBS: https://bugs.openjdk.java.net/browse/JDK-8191053

This is a welcoming change as many applications run without security 
manager and they will benefit the performance improvement.   This patch 
makes the private System::security field @Stable hich is the first 
installment of perf optimization.   I hope to see further optimization 
can be done for example speed up of doPrivileged block in the 
no-security manager case.

This patch looks good.  It may be good to add one test case to launch 
with -Djava.security.manager and -Djdk.allowSecurityManager=false to 
show that no security manager is installed; essential 
-Djava.security.manager is ignored.  Maybe @implNote should mention such 
case where -Djava.security.manager is ignored if 
-Djdk.allowSecurityManager=false.

Mandy
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