Module java.base

Package java.lang

Class StackWalker

java.lang.Object

java.lang.StackWalker

public final class StackWalker
extends Object

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.

Stack walker options configure the stack frame information obtained by a StackWalker. By default, the class name and method information are collected but not the Class reference. The method information can be dropped via the DROP_METHOD_INFO option. The Class object can be retained for access via the RETAIN_CLASS_REFERENCE option. Stack frames of the reflection API and implementation classes are hidden by default.

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.

API Note:

Examples

1. To find the first caller filtering out a known list of implementation class:

    StackWalker walker = StackWalker.getInstance(Set.of(Option.DROP_METHOD_INFO, Option.RETAIN_CLASS_REFERENCE));
    Optional<Class<?>> callerClass = walker.walk(s ->
            s.map(StackFrame::getDeclaringClass)
             .filter(Predicate.not(implClasses::contains))
             .findFirst());

2. To snapshot the top 10 stack frames of the current thread,

    List<StackFrame> stack = StackWalker.getInstance().walk(s -> s.limit(10).toList());

Unless otherwise noted, passing a null argument to a constructor or method in this StackWalker class will cause a NullPointerException to be thrown.

Since:

9

Nested Class Summary

Nested Classes

Modifier and Type	Class	Description
static enum	StackWalker.Option	Stack walker option to configure the stack frame information obtained by a StackWalker.
static interface	StackWalker.StackFrame	A StackFrame object represents a method invocation returned by StackWalker.

Method Summary

Modifier and Type	Method	Description
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()	Returns a StackWalker instance.
static StackWalker	getInstance(StackWalker.Option option)	Returns a StackWalker instance with the given option specifying the stack frame information it can access.
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(Set<StackWalker.Option> options, int estimateDepth)	Returns a StackWalker instance with the given options 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.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Details

getInstance

public static StackWalker getInstance()

Returns a StackWalker instance.

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

Returns:

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

getInstance

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 - stack walking option

Returns:

a StackWalker configured with the given option

Throws:

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

getInstance

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 - stack walking options

Returns:

a StackWalker configured with the given options

Throws:

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

getInstance

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 - stack walking options

estimateDepth - Estimate number of stack frames to be traversed.

Returns:

a StackWalker configured with the given options

Throws:

IllegalArgumentException - if estimateDepth <= 0

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

walk

public <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.

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)
         .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.

Type Parameters:

T - The type of the result of applying the function to the stream of stack frame.

Parameters:

function - a function that takes a stream of stack frames and returns a result.

Returns:

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

forEach

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 - an action to be performed on each StackFrame of the stack of the current thread

getCallerClass

public Class<?> getCallerClass()

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

This method filters reflection 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(Set.of(Option.DROP_METHOD_INFO, 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 object of the caller's caller invoking this method.

Throws:

UnsupportedOperationException - if this StackWalker is not configured with Option.RETAIN_CLASS_REFERENCE.

IllegalCallerException - 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.