Discussion: Interpretation of system property flags

[Eirik Bjørsnøs]

On Wed, Dec 4, 2024 at 3:47 PM Eirik Bjørsnøs <eirbjo at gmail.com> wrote:


> The OpenJDK includes many boolean flags in the form of system properties.
> These toggle different behavior such as debug logging, verification,
> caching, compatibility and conditional features.
>

To learn more about how the JDK uses boolean system properties, I conducted
a survey of the ones I could identify in java.base and thought it could be
useful to share the observations:

First, some definitions:

a) A *boolean system property *(BSP) is one whose value is evaluated to
either true or false.
b) A BSP *defaults* to either true or false*.*The default value is used
when a system property has no configured value.
c) A system property is either *case sensitive* or *case insensitive*. A
case insensitive property ignores the case when comparing configured values
with "true or "false".
d) The *normalized value* of a BSP is one of TRUE, FALSE, EMPTY or OTHER.
e) The normalized value is evaluated to true or false using a *definition
of truth*. This is the set of normalized values which are considered as
true.

Theoretically, there are 14 possible definitions of truth (sets of size
1-3). However, given that we want TRUE and FALSE be stable, there are not
many practical ones, and in fact, the JDK uses only three:

isTrue: {TRUE}
isTrueOrEmpty: {TRUE, EMPTY}
isNotFalse: {TRUE, EMPTY, OTHER}

Additionally, there is the "isSet" definition which considers any set value
as true, including "false".

*Some stats:*

Total number of BSPs found in java.base: 83

*Default values:*

Default false: 66
Default true: 17

*Case sensitivity:*

Case sensitive: 5
Case insensitive: 76

*Definitions of truth:*

isTrue: 61
isTrueOrEmpty: 15
isNotFalse: 5
isSet: 2

*Positive or negative name:*

Additionally, I looked into whether a system property name has a positive
or negative name. Names such as "allow" or "enable" are considered
positive, while names like "disable" or "inhibit" are considered negative:

Positive: 65
Negative: 8

Hope this was useful!

Thanks,
Eirik.
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