Draft JEP: Efficient Stack Walking API

[Mandy Chung]

Below is a draft JEP we are considering submitting for JDK 9.

Mandy

----------------------------
Title: Efficient API for Stack Walking

Goal
----

Define a standard API for stack walking that will be efficient and
performant.

Non-goal
--------

It is not a goal for this API be easy to use via Reflection for example
use in code that is compiled for an older JDK.

Motivation
----------

There is no standard API to obtain information about the caller's class
and traverse the execution stack in a performant way.  Existing libraries
and frameworks such as Log4j and Groovy have to resort to using the
JDK internal API `sun.reflect.Reflection.getCallerClass(int depth)`.

This JEP proposes to define a standard API for stack walking that will
be efficient and performant and also enable the implementation up
level the stack walk machinery from the VM to Java and replaces
the current mechanism of `Throwable.fillInStackTrace.

Description
-----------

There is no standard API to traverse certain frames on the execution
stack efficiently and access the Class instance of each frame.

There are APIs that allow to access the stack trace information:
   - `Throwable.getStackTrace()` and `Thread.getStackTrace()` that returns
      an array of `StackTraceElement` which contains the classname
      and method name of a stack trace.
   - `SecurityManager.getClassContext()` which is a protected method
      such that only `SecurityManager` subclass can access the class
      context.

These APIs require the VM to eagerly capture a snapshot of the entire
stack trace and returns the information representing the entire stack.
There is no other way to avoid the cost to examine all frames if
the caller is only interested in the top few frames on the stack.
Both `Throwable.getStackTrace()` and `Thread.getStackTrace()` methods
return an array of `StackTraceElement` that contains the classname and
method name of a stack frame but the `Class` instance.

In fact, for applications interested in the entire stack, the specification
allows VM implementation to omit some frames in the stack for performance.
In other words, `Thread.getStackTrace()` may return a partial stack trace.

These APIs do not satisfy the use cases that currently depend on
the `getCallerClass(int depth)` method or its performance overhead
is intolerable.  The use cases include:

   - JDK caller-sensitive APIs look up its immediate caller's class
     which will be used to determine the behavior of the API.  For example
     `Class.forName(String classname)` and
     `ResourceBundle.getBundle(String rbname)` methods use the immediate
     caller's class loader to load a class and a resource bundle respectively.
     `Class.getMethod` etc will use the immediate caller's class loader
     to determine the security checks to be performed.

   - `java.util.logging`, Log4j and Groovy runtime filter the intermediary
     stack frames (typically implementation-specific and reflection frames)
     and find the caller's class to be used by the runtime of such library
     or framework.

   - Traverse the entire stack trace or the stack trace of a `Throwbale`
     and obtain additional information about classes for enhanced
     diagnosibility in addition to the class and method name.

This JEP will define a stack walk API that allows laziness, frame filtering,
supports short reaches to stop at a frame matching some criteria
as well as long reaches to traverse the entire stack trace.  This would
need the JVM to provide a flexible mechanism to traverse and materialize
the specific stack frame information to be used and allow efficient
lazy access to additional stack frames when required.
Native JVM transitions should be minimzed.

The API will define how it works when running with a security manager
that allows access to a `Class` instance
of any frame ensuring that the security is not compromised.

An example API to walk the stack can be like:
    Thread.walkStack(Consumer<StackFrameInfo> action, int depthLimit)

that takes a callback to be invoked for each frame traversed.  A variant
of the walkStack method will take a predicate for stack frame filtering.

    Thread.getCaller(Function<StackFrameInfo, R> function)
    Thread.findCaller(Predicate<StackFrameInfo> predicate,
                      Function<StackFrameInfo, R> function)

finds the caller frame with or without filtering.

Testing
-------

Unit tests and JCK tests for the new SE API will need to be developed.
In addition, the performance of the new API for different use cases
will be assessed.


Impact
------

   - Performance/scalability: performance measurement shall be performed
     using micro-benchmarks as well as real world usage of `getCallerClass`
     replaced with the new API.

   - TCK: New JCK test cases shall be developed.