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java9-stack-stackwalking

preface

StackWalker

StackWalker.Option

  • enum
  • configures StackWalker
  • values
    • RETAIN_CLASS_REFERENCE - StackFrame will contain reference to the Class object of the method represented by this frame
    • SHOW_HIDDEN_FRAMES - show all hidden frames (for example - implementation specific and reflection frames are not included by default)
    • SHOW_REFLECT_FRAMES - show only reflection frames ( specific implementation frames could still be hidden)
  • note that there is also enum StackWalker.ExtendedOption with only one, self-explaining value LOCALS_AND_OPERANDS

StackWalker.StackFrame

  • interface
  • we don't have direct access to concrete implementation
  • useful methods:
    • String getClassName()
    • String getMethodName()
    • int getLineNumber()
    • public Class<?> getDeclaringClass()
      • UnsupportedOperationException if not configured with RETAIN_CLASS_REFERENCE

methods

  • creating instance of StackWalker
    • static StackWalker getInstance()
    • static StackWalker getInstance(Option option)
    • static StackWalker getInstance(Set<Option> options)
    • static StackWalker getInstance(Set<Option> options, int estimateDepth)
      • estimateDepth specifies the estimate number of stack frames this StackWalker will traverse that the StackWalker could use as a hint for the buffer size
  • traverse the stack
    • void forEach(Consumer<? super StackFrame> action)
      • traversing from the top frame of the stack, which is the method calling this forEach method
    • <T> T walk(Function<? super Stream<StackFrame>, ? extends T> function)
      • returning a Stream would be unsafe, as the stream could be used to access the stack frames in an uncontrolled manner
      • applies the given function to the stream of StackFrames for the current thread, traversing from the top frame of the stack
      • stream will be closed when this method returns
      • 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

project description

  1. for test purposes
    class StackWalking {
        void m1() {
            m2();
        }
    
        private void m2() {
            m3();
        }
    
        private void m3() {
            System.out.println("---");
            System.out.println("stack from m3():");
            StackWalker.getInstance(Set.of(RETAIN_CLASS_REFERENCE, SHOW_HIDDEN_FRAMES), 4)
                    .walk(frames -> {
                        frames.limit(4).forEach(System.out::println);
                        return null;
                    });
            System.out.println("---");
    
            m4();
        }
    
        private void m4() {
            System.out.println("---");
            System.out.println("stack from m4():");
            StackWalker.getInstance(Set.of(RETAIN_CLASS_REFERENCE, SHOW_HIDDEN_FRAMES), 5)
                    .walk(frames -> {
                        frames.limit(5).forEach(System.out::println);
                        return null;
                    });
            System.out.println("---");
        }
    }
    
  2. simulation
    @Test
    public void simulation() {
        new StackWalking().m1();
    }
    
    produces
    ---
    stack from m3():
    StackWalking.m3(StackWalking.java:22)
    StackWalking.m2(StackWalking.java:15)
    StackWalking.m1(StackWalking.java:11)
    StackWalkingTest.simulation(StackWalkingTest.java:10)
    ---
    ---
    stack from m4():
    StackWalking.m4(StackWalking.java:35)
    StackWalking.m3(StackWalking.java:28)
    StackWalking.m2(StackWalking.java:15)
    StackWalking.m1(StackWalking.java:11)
    StackWalkingTest.simulation(StackWalkingTest.java:10)
    ---
    

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Overview of StackWalker API in java 9.

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