Added in API level 34

Summary: Nested Classes | Methods | Inherited Methods

StackWalker

public final class StackWalker
extends Object

java.lang.Object
↳	java.lang.StackWalker

---

A stack walker.

The walk method opens a sequential stream of StackFrames for the current thread and then applies the given function to walk the StackFrame stream. The stream reports stack frame elements in order, from the top most frame that represents the execution point at which the stack was generated to the bottom most frame. The StackFrame stream is closed when the walk method returns. If an attempt is made to reuse the closed stream, IllegalStateException will be thrown.

The stack walking options of a StackWalker determines the information of StackFrame objects to be returned. By default, stack frames of the reflection API and implementation classes are hidden and StackFrames have the class name and method name available but not the Class reference.

StackWalker is thread-safe. Multiple threads can share a single StackWalker object to traverse its own stack. A permission check is performed when a StackWalker is created, according to the options it requests. No further permission check is done at stack walking time.

Summary

Nested classes
interface	StackWalker.StackFrame A StackFrame object represents a method invocation returned by StackWalker.

Public methods
void	forEach(Consumer<? super StackWalker.StackFrame> action) Performs the given action on each element of StackFrame stream of the current thread, traversing from the top frame of the stack, which is the method calling this forEach method.
Class<?>	getCallerClass() Gets the Class object of the caller who invoked the method that invoked getCallerClass.
static StackWalker	getInstance(Set<StackWalker.Option> options) Returns a StackWalker instance with the given options specifying the stack frame information it can access.
static StackWalker	getInstance() Returns a StackWalker instance.
static StackWalker	getInstance(Set<StackWalker.Option> options, int estimateDepth) Returns a StackWalker instance with the given options specifying the stack frame information it can access.
static StackWalker	getInstance(StackWalker.Option option) Returns a StackWalker instance with the given option specifying the stack frame information it can access.
<T> T	walk(Function<? super Stream<StackWalker.StackFrame>, ? extends T> function) Applies the given function to the stream of StackFrames for the current thread, traversing from the top frame of the stack, which is the method calling this walk method.

Inherited methods

From class java.lang.Object Object clone() Creates and returns a copy of this object. boolean equals(Object obj) Indicates whether some other object is "equal to" this one. void finalize() Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. final Class<?> getClass() Returns the runtime class of this Object. int hashCode() Returns a hash code value for the object. final void notify() Wakes up a single thread that is waiting on this object's monitor. final void notifyAll() Wakes up all threads that are waiting on this object's monitor. String toString() Returns a string representation of the object. final void wait(long timeoutMillis, int nanos) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait(long timeoutMillis) Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed. final void wait() Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

Public methods

forEach

Added in API level 34

public void forEach (Consumer<? super StackWalker.StackFrame> action)

Performs the given action on each element of StackFrame stream of the current thread, traversing from the top frame of the stack, which is the method calling this forEach method.

This method is equivalent to calling

walk(s -> { s.forEach(action); return null; });

Parameters
action	Consumer: an action to be performed on each StackFrame of the stack of the current thread

getCallerClass

Added in API level 34

public Class<?> getCallerClass ()

Gets the Class object of the caller who invoked the method that invoked getCallerClass.

This method filters frames, MethodHandle, and hidden frames regardless of the SHOW_REFLECT_FRAMES and SHOW_HIDDEN_FRAMES options this StackWalker has been configured with.

This method should be called when a caller frame is present. If it is called from the bottom most frame on the stack, IllegalCallerException will be thrown.

This method throws UnsupportedOperationException if this StackWalker is not configured with the RETAIN_CLASS_REFERENCE option.

API Note:

• For example, Util::getResourceBundle loads a resource bundle on behalf of the caller. It invokes getCallerClass to identify the class whose method called Util::getResourceBundle. Then, it obtains the class loader of that class, and uses the class loader to load the resource bundle. The caller class in this example is MyTool.

  class Util {
       private final StackWalker walker = StackWalker.getInstance(Option.RETAIN_CLASS_REFERENCE);
       public ResourceBundle getResourceBundle(String bundleName) {
           Class<?> caller = walker.getCallerClass();
           return ResourceBundle.getBundle(bundleName, Locale.getDefault(), caller.getClassLoader());
       }
   }
  
   class MyTool {
       private final Util util = new Util();
       private void init() {
           ResourceBundle rb = util.getResourceBundle("mybundle");
       }
   }
   

  An equivalent way to find the caller class using the walk method is as follows (filtering the reflection frames, MethodHandle and hidden frames not shown below):

  Optional<Class<?>> caller = walker.walk(s ->
           s.map(StackFrame::getDeclaringClass)
            .skip(2)
            .findFirst());
   

  When the getCallerClass method is called from a method that is the bottom most frame on the stack, for example, static public void main method launched by the java launcher, or a method invoked from a JNI attached thread, IllegalCallerException is thrown.

Returns
Class<?>	Class object of the caller's caller invoking this method.

Throws
UnsupportedOperationException	if this StackWalker is not configured with Option.RETAIN_CLASS_REFERENCE.
	if there is no caller frame, i.e. when this getCallerClass method is called from a method which is the last frame on the stack.

getInstance

Added in API level 34

public static StackWalker getInstance (Set<StackWalker.Option> options)

Returns a StackWalker instance with the given options specifying the stack frame information it can access. If the given options is empty, this StackWalker is configured to skip all hidden frames and no class reference is retained.

If a security manager is present and the given options contains Option.RETAIN_CLASS_REFERENCE, it calls its checkPermission method for RuntimePermission("getStackWalkerWithClassReference").

Parameters
options	Set: stack walking option

Returns
StackWalker	a StackWalker configured with the given options

Throws
SecurityException	if a security manager exists and its checkPermission method denies access.

getInstance

Added in API level 34

public static StackWalker getInstance ()

Returns a StackWalker instance.

This StackWalker is configured to skip all hidden frames and no class reference is retained.

Returns
StackWalker	a StackWalker configured to skip all hidden frames and no class reference is retained.

getInstance

Added in API level 34

public static StackWalker getInstance (Set<StackWalker.Option> options, 
                int estimateDepth)

Returns a StackWalker instance with the given options specifying the stack frame information it can access. If the given options is empty, this StackWalker is configured to skip all hidden frames and no class reference is retained.

If a security manager is present and the given options contains Option.RETAIN_CLASS_REFERENCE, it calls its checkPermission method for RuntimePermission("getStackWalkerWithClassReference").

The estimateDepth specifies the estimate number of stack frames this StackWalker will traverse that the StackWalker could use as a hint for the buffer size.

Parameters
options	Set: stack walking options
estimateDepth	int: Estimate number of stack frames to be traversed.

Returns
StackWalker	a StackWalker configured with the given options

Throws
IllegalArgumentException	if estimateDepth <= 0
SecurityException	if a security manager exists and its checkPermission method denies access.

getInstance

Added in API level 34

public static StackWalker getInstance (StackWalker.Option option)

Returns a StackWalker instance with the given option specifying the stack frame information it can access.

If a security manager is present and the given option is Option.RETAIN_CLASS_REFERENCE, it calls its checkPermission method for RuntimePermission("getStackWalkerWithClassReference").

Parameters
option	StackWalker.Option: stack walking option

Returns
StackWalker	a StackWalker configured with the given option

Throws
SecurityException	if a security manager exists and its checkPermission method denies access.

walk

Added in API level 34

public T walk (Function<? super Stream<StackWalker.StackFrame>, ? extends T> function)

Applies the given function to the stream of StackFrames for the current thread, traversing from the top frame of the stack, which is the method calling this walk method.

The StackFrame stream will be closed when this method returns. When a closed Stream<StackFrame> object is reused, IllegalStateException will be thrown.

API Note:

• For example, to find the first 10 calling frames, first skipping those frames whose declaring class is in package com.foo:

  List<StackFrame> frames = StackWalker.getInstance().walk(s ->
       s.dropWhile(f -> f.getClassName().startsWith("com.foo."))
        .limit(10)
        .collect(Collectors.toList()));
   

  This method takes a Function accepting a Stream<StackFrame>, rather than returning a Stream<StackFrame> and allowing the caller to directly manipulate the stream. The Java virtual machine is free to reorganize a thread's control stack, for example, via deoptimization. By taking a Function parameter, this method allows access to stack frames through a stable view of a thread's control stack.

  Parallel execution is effectively disabled and stream pipeline execution will only occur on the current thread.

Implementation Note:

• The implementation stabilizes the stack by anchoring a frame specific to the stack walking and ensures that the stack walking is performed above the anchored frame. When the stream object is closed or being reused, IllegalStateException will be thrown.

Parameters
function	Function: a function that takes a stream of stack frames and returns a result.

Returns
T	the result of applying the function to the stream of stack frame.