RFR: 8268829: Provide an optimized way to walk the stack with Class object only [v8]

[Brent Christian]

On Tue, 29 Aug 2023 20:51:56 GMT, Mandy Chung <mchung at openjdk.org> wrote:

>> 8268829: Provide an optimized way to walk the stack with Class object only
>> 
>> `StackWalker::walk` creates one `StackFrame` per frame and the current implementation
>> allocates one `StackFrameInfo` and one `MemberName` objects per frame. Some frameworks
>> like logging may only interest in the Class object but not the method name nor the BCI,
>> for example, filters out its implementation classes to find the caller class.  It's
>> similar to `StackWalker::getCallerClass` but allows a predicate to filter out the element.
>> 
>> This PR proposes to add `Option::DROP_METHOD_INFO` enum that requests to drop the method information.  If no method information is needed, a `StackWalker` with `DROP_METHOD_INFO`
>> can be used instead and such stack walker will save the overhead of extracting the method information
>> and the memory used for the stack walking.
>> 
>> New factory methods to take a parameter to specify the kind of stack walker to be created are defined.
>> This provides a simple way for existing code, for example logging frameworks, to take advantage of
>> this enhancement with the least change as it can keep the existing function for traversing
>> `StackFrame`s.
>> 
>> For example: to find the first caller filtering a known list of implementation class,
>> existing code can create a stack walker instance with `DROP_METHOD_INFO` option:
>> 
>> 
>>      StackWalker walker = StackWalker.getInstance(Option.DROP_METHOD_INFO, Option.RETAIN_CLASS_REFERENCE);
>>      Optional<Class<?>> callerClass = walker.walk(s ->
>>              s.map(StackFrame::getDeclaringClass)
>>               .filter(Predicate.not(implClasses::contains))
>>               .findFirst());
>> 
>> 
>> If method information is accessed on the `StackFrame`s produced by this stack walker such as
>> `StackFrame::getMethodName`, then `UnsupportedOperationException` will be thrown.
>> 
>> #### Javadoc & specdiff
>> 
>> https://cr.openjdk.org/~mchung/api/java.base/java/lang/StackWalker.html
>> https://cr.openjdk.org/~mchung/jdk22/specdiff/overview-summary.html
>> 
>> #### Alternatives Considered
>> One alternative is to provide a new API:
>> `<T> T walkClass(Function<? super Stream<Class<?>, ? extends T> function)`
>> 
>> In this case, the caller would need to pass a function that takes a stream
>> of `Class` object instead of `StackFrame`.  Existing code would have to
>> modify calls to the `walk` method to `walkClass` and the function body.
>> 
>> ### Implementation Details
>> 
>> A `StackWalker` configured with `DROP_METHOD_INFO` ...
>
> Mandy Chung has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - update mode to be int rather than long
>  - update tests
>  - Review feedback on javadoc

src/java.base/share/classes/java/lang/ClassFrameInfo.java line 31:

> 29: import java.lang.StackWalker.StackFrame;
> 30: 
> 31: class ClassFrameInfo implements StackFrame {

Maybe a minimal comment on when/how this class is used?

src/java.base/share/classes/java/lang/ClassFrameInfo.java line 37:

> 35:     int flags;                   // updated by VM to set hidden and caller-sensitive bits
> 36: 
> 37:     ClassFrameInfo(StackWalker walker) {

The StackFrameInfo constructor has comment. Maybe add one here, too?

src/java.base/share/classes/java/lang/StackFrameInfo.java line 32:

> 30: import java.lang.reflect.Modifier;
> 31: 
> 32: class StackFrameInfo extends ClassFrameInfo {

Maybe a minimal comment on when/how this class is used?

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/15370#discussion_r1310866414
PR Review Comment: https://git.openjdk.org/jdk/pull/15370#discussion_r1310876832
PR Review Comment: https://git.openjdk.org/jdk/pull/15370#discussion_r1310866565