DevTut

# Stack-Walking API

Prior to Java 9, access to the thread stack frames was limited to an internal class sun.reflect.Reflection. Specifically the method sun.reflect.Reflection::getCallerClass. Some libraries relies on this method which is deprecated.

An alternative standard API is now provided in JDK 9 via the java.lang.StackWalker class, and is designed to be efficient by allowing lazy access to the stack frames. Some applications may use this API to traverse the execution stack and filter on classes.

The following prints all stack frames of the current thread:

1  package test;
2  
3  import java.lang.StackWalker.StackFrame;
4  import java.lang.reflect.InvocationTargetException;
5  import java.lang.reflect.Method;
6  import java.util.List;
7  import java.util.stream.Collectors;
8  
9  public class StackWalkerExample {
10 
11    public static void main(String[] args) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
12        Method fooMethod = FooHelper.class.getDeclaredMethod("foo", (Class<?>[])null);
13        fooMethod.invoke(null, (Object[]) null);
14    }
15 }
16
17 class FooHelper {
18    protected static void foo() {
19        BarHelper.bar();
20    }
21 }
22 
23 class BarHelper {
24    protected static void bar() {
25        List<StackFrame> stack = StackWalker.getInstance()
26                .walk((s) -> s.collect(Collectors.toList()));
27        for(StackFrame frame : stack) {
28            System.out.println(frame.getClassName() + " " + frame.getLineNumber() + " " + frame.getMethodName());
29        }
30    }
31 }

Output:

test.BarHelper 26 bar
test.FooHelper 19 foo
test.StackWalkerExample 13 main

The following prints the current caller class. Note that in this case, the StackWalker (opens new window) needs to be created with the option RETAIN_CLASS_REFERENCE (opens new window), so that Class instances are retained in the StackFrame (opens new window) objects. Otherwise an exception would occur.

public class StackWalkerExample {

    public static void main(String[] args) {
        FooHelper.foo();
    }

}

class FooHelper {
    protected static void foo() {
        BarHelper.bar();
    }
}

class BarHelper {
    protected static void bar() {
        System.out.println(StackWalker.getInstance(Option.RETAIN_CLASS_REFERENCE).getCallerClass());
    }
}

Output:

class test.FooHelper

# Showing reflection and other hidden frames

A couple of other options allow stack traces to include implementation and/or reflection frames. This may be useful for debugging purposes. For instance, we can add the SHOW_REFLECT_FRAMES (opens new window) option to the StackWalker (opens new window) instance upon creation, so that the frames for the reflective methods are printed as well:

package test;

import java.lang.StackWalker.Option;
import java.lang.StackWalker.StackFrame;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.List;
import java.util.stream.Collectors;

public class StackWalkerExample {

    public static void main(String[] args) throws NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException {
        Method fooMethod = FooHelper.class.getDeclaredMethod("foo", (Class<?>[])null);
        fooMethod.invoke(null, (Object[]) null);
    }
}

class FooHelper {
    protected static void foo() {
        BarHelper.bar();
    }
}

class BarHelper {
    protected static void bar() {
        // show reflection methods
        List<StackFrame> stack = StackWalker.getInstance(Option.SHOW_REFLECT_FRAMES)
                .walk((s) -> s.collect(Collectors.toList()));
        for(StackFrame frame : stack) {
            System.out.println(frame.getClassName() + " " + frame.getLineNumber() + " " + frame.getMethodName());
        }
    }
}

Output:

test.BarHelper 27 bar
test.FooHelper 20 foo
jdk.internal.reflect.NativeMethodAccessorImpl -2 invoke0
jdk.internal.reflect.NativeMethodAccessorImpl 62 invoke
jdk.internal.reflect.DelegatingMethodAccessorImpl 43 invoke
java.lang.reflect.Method 563 invoke
test.StackWalkerExample 14 main

Note that line numbers for some reflection methods may not be available so StackFrame.getLineNumber() may return negative values.

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