Carl Mastrangelo

A programming and hobby blog.

You Probably Want nanoTime

Quick Quiz: What does the following Java code do?

public class Timer {
  public static void main(String [] args) throws Exception {
    Instant start =;
    System.err.println("Starting at " + start);
    Instant end =;
    System.out.println("Slept for " + Duration.between(start, end));

On the surface, it looks correct. The code tries to sleep for 10 seconds, and then prints out how long it actually slept for. However, there is a subtle bug: It’s using calendar time instead of monotonic time is Calendar Time seems like a good API to use. It’s typesafe, modern, and has nanosecond resolution! All good right? The problem is that the time comes from computer’s clock, which can move around unpredictably. To show this, I recorded running this program:

As we can see, the program takes a little over 10 seconds to run. However, what would happen if the system clock were to be adjusted? Let’s look:

Time went backwards and our program didn’t measure the duration correctly! This can happen during daylight savings time switches, users changing their system clock manually, and even when returning from sleep or hibernate power states.

Use System.nanoTime to Measure Duration

To avoid clock drift, we can use System.nanoTime(). This API returns a timestamp that is arbitrary, but is consistent during the run of our program. Here’s how to use it:

public class Timer {
  public static void main(String [] args) throws Exception {
    long start = System.nanoTime();
    System.err.println("Starting at " + start);
    long end = System.nanoTime();
    System.out.println("Slept for " + Duration.ofNanos(end - start));

We don’t get to use the object oriented time APIs, but those weren’t meant for recording duration anyways. It feels a little more raw to use long primitives, but the result is always correct. If you are looking for a typesafe way to do this, consider using Guava’s Stopwatch class.

The nanoTime() call is great in lot’s of situations:

What about System.currentTimeMillis()?

While this function worked well for a long time, it has been superseded by I usually see other programmers use this function because they only care about millisecond granularity. However, this suffer from the same clock drift problem as

Accidental Class Loading in Java

Recently I’ve been working on improving the ergonomics of my tracing library PerfMark. One of the interesting things I noticed is that the JVM was loading some classes I didn’t expect it to. Let’s find out why!

“That’s Weird”

PerfMark works on much older JVM’s, but strives to be as fast as possible. To accomplish this, the code bootstraps itself based on what advanced JVM features are available (MethodHandles, VarHandles, Hidden Classes, etc.). Because of this, the library needs to avoid loading classes that aren’t usable. In the event that no advanced features are available, the library safely disables itself. The code shows how to load optionally available classes safely:

// SecretPerfMarkImpl.PerfMarkImpl

static {
  Generator gen = null;
  Throwable problem = null;
  try {
    Class clz = 
    gen = clz.asSubclass(Generator.class).getConstructor().newInstance();
  } catch (Throwable t) {
    problem = t;
  if (gen != null) {
    generator = gen;
  } else {
    generator = new NoopGenerator();
  // More Initialization code.

As we can see, the static initializer tries to load a special class that requires Java 7 or higher. If it isn’t available, we fall back to the no-op implementation. The actual types of the classes are not as important, except that Generator is an abstract class. We can check to see what is actually loaded by passing the


flag to the JVM. This lets us see what classes are loaded and when:

[1.685s][info][class,load] io.perfmark.PerfMark 
[1.693s][info][class,load] io.perfmark.Impl 
[1.694s][info][class,load] io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl
[1.695s][info][class,load] java.util.concurrent.ConcurrentHashMap$ForwardingNode 
[1.697s][info][class,load] java.lang.AssertionError 
[1.701s][info][class,load] io.perfmark.impl.Generator 
[1.702s][info][class,load] io.perfmark.impl.NoopGenerator
[1.704s][info][class,load] io.perfmark.Tag
[1.706s][info][class,load] io.perfmark.Link
[1.707s][info][class,load] io.perfmark.impl.SecretPerfMarkImpl
[1.712s][info][class,load] java.lang.Long$LongCache
[1.714s][info][class,load] java.lang.invoke.LambdaForm$MH/0x00000008000c5000
  source: __JVM_LookupDefineClass__
[1.718s][info][class,load] java.lang.invoke.LambdaForm$MH/0x00000008000c5400
  source: __JVM_LookupDefineClass__
[1.721s][info][class,load] java.lang.invoke.LambdaForm$BMH/0x00000008000c5800 
  source: __JVM_LookupDefineClass__

This is kind of strange. The MethodHandleGenerator class is definitely available, but it loads after the NoopGenerator. Worse, it seems like both classes end up being loaded. What’s going on?

“When Flags Aren’t Enough”

Let’s ratchet up the verbosity to see what the loader is doing:


Running with this shows that the class loading it caused by the verification step the JVM performs when loading classes:

[1.826s][info ][class,load   ] io.perfmark.Impl 
[1.826s][debug][class,resolve] io.perfmark.Impl java.lang.Object (super)
  java.lang.Class io.perfmark.Impl (explicit)
[1.828s][info ][class,load   ] io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl 
  io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl io.perfmark.Impl (super)
  java.lang.Class io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl (explicit)
[1.828s][debug][class,resolve] io.perfmark.Impl java.lang.Throwable (verification)
[1.832s][debug][class,resolve] io.perfmark.Impl java.lang.AssertionError  (verification)
  io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl java.lang.Throwable
[1.835s][info ][class,load   ] io.perfmark.impl.Generator 
[1.835s][debug][class,resolve] io.perfmark.impl.Generator java.lang.Object (super)
[1.836s][debug][class,resolve] java.lang.Class io.perfmark.impl.Generator (explicit)
  io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl io.perfmark.impl.Generator (verification)
[1.837s][info ][class,load   ] io.perfmark.impl.NoopGenerator 
[1.837s][debug][class,resolve] io.perfmark.impl.NoopGenerator io.perfmark.impl.Generator (super)
[1.837s][debug][class,resolve] java.lang.Class io.perfmark.impl.NoopGenerator (explicit)
[1.838s][info ][class,load   ]
  Found Noop: io.perfmark.impl.NoopGenerator
  io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl io.perfmark.impl.NoopGenerator (verification)
 io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl java.lang.Object (verification)
 io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl java.lang.Object (verification)
 io.perfmark.impl.SecretPerfMarkImpl$PerfMarkImpl java.util.concurrent.atomic.AtomicLong
[1.848s][info ][class,load   ]

While the JVM is well documented, it is hard to penetrate for someone who isn’t making their own implementation. What we want to know is why this class is relevant to verification. Rather than go over the Specification with a fine tooth comb, let’s just put a breakpoint into the JVM itself!

Instrumenting the JVM

First, let’s get a copy:

git clone

After fumbling with the configuration arguments, let’s try out a slowdebug build.

bash configure \
  --with-boot-jdk=/home/carl/.gradle/jdks/adoptium-19-x64-hotspot-linux/jdk-19.0.1+10/ \
  --with-debug-level=slowdebug  \
  --enable-headless-only \

I (ab)used the JDK that Gradle downloaded for me, but the rest of the configuration is pretty regular. I am using slowdebug and with-native-debug-symbols because for some reason GDB was unable to find the function names in the back trace. I used --enable-headless-only because I don’t have all the header files locally. Okay, let’s build!

CONF=linux-x86_64-server-slowdebug make

This takes about 8 minutes on my Skylake processor. Soon enough, we have a fully functional JDK. Because this is a hack, I modified the java command Gradle builds for me manually to call GDB.:

-   exec "$JAVACMD" "$@"
+   exec gdb --args "$JAVACMD" "$@"

Finally, let’s see that it works:

$ JAVA_HOME=~/git/jdk/build/linux-x86_64-server-slowdebug/jdk/ \

GNU gdb (Debian 12.1-4+b1) 12.1
Copyright (C) 2022 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
Find the GDB manual and other documentation resources online at:

For help, type "help".
Type "apropos word" to search for commands related to "word"...
Reading symbols from /home/carl/git/jdk/build/linux-x86_64-server-slowdebug/jdk//bin/java...

Alright, We can boot our JVM with GDB, and we are ready to see what it’s doing. However, we need a way to see when the NoopGenerator class really get’s loaded. After some walking through the code, I find that verification happens in a file called verifier.cpp. Let’s add a print in Verifier::trace_class_resolution:

void Verifier::trace_class_resolution(
    Klass* resolve_class, InstanceKlass* verify_class) {
  assert(verify_class != nullptr, "Unexpected null verify_class");
  ResourceMark rm;
  Symbol* s = verify_class->source_file_name();
  const char* source_file = (s != nullptr ? s->as_C_string() : nullptr);
  const char* verify = verify_class->external_name();
  const char* resolve = resolve_class->external_name();

  if (strstr(resolve, "NoopGenerator") != nullptr) {
    log_info(class, load)("Found NoopGenerator: %s", resolve);

  // print in a single call to reduce interleaving between threads
  if (source_file != nullptr) {
    log_debug(class, resolve)(
      "%s %s %s (verification)", verify, resolve, source_file);
  } else {
    log_debug(class, resolve)("%s %s (verification)", verify, resolve);

As of this writing, this happens on line 129. Let’s rebuild and re-run:

CONF=linux-x86_64-server-slowdebug make
$ JAVA_HOME=~/git/jdk/build/linux-x86_64-server-slowdebug/jdk/ \


At this point, we need to make GDB not pester us as we are stepping through. The JVM uses Segmentation Faults to implement efficient NullPointerException calls, so we want to avoid being notified of that. It also uses other signals (i.e. SIGUSR2) for thread pausing and resuming, which we aren’t interested in:

Reading symbols from /home/carl/git/jdk/build/linux-x86_64-server-slowdebug/jdk//bin/java...
(gdb) handle SIGSEGV noprint nostop
Signal        Stop  Print   Pass to program Description
SIGSEGV       No    No  Yes     Segmentation fault
(gdb) handle SIGUSR2 noprint nostop
Signal        Stop  Print   Pass to program Description
SIGUSR2       No    No  Yes     User defined signal 2

Okay, let’s insert a breakpoint:

(gdb) break verifier.cpp:129
No source file named verifier.cpp.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (verifier.cpp:129) pending.

Alright, let’s run!

(gdb) run
Starting program: /home/carl/git/jdk/build/linux-x86_64-server-slowdebug/jdk/bin/java
//   .... Lot's of output
Thread 2 "java" hit Breakpoint 1, Verifier::trace_class_resolution 
    (resolve_class=0x8000c44d8, verify_class=0x8000c4000) 
    at /home/carl/git/jdk/src/hotspot/share/classfile/verifier.cpp:129
129     log_info(class, load)("Found NoopGenerator: %s", resolve);

Now that we have hit out break point, let’s see how the class loader actually got here.

(gdb) bt
#0  Verifier::trace_class_resolution (resolve_class=0x8000c44d8, verify_class=0x8000c4000) at /home/carl/git/jdk/src/hotspot/share/classfile/verifier.cpp:129
#1  0x00007ffff6c99e82 in VerificationType::resolve_and_check_assignability (klass=0x8000c4000, name=0x7ffff05705c8, from_name=0x7ffff0570d48, from_field_is_protected=false, from_is_array=false, 
    from_is_object=true, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/classfile/verificationType.cpp:85
#2  0x00007ffff6c9a055 in VerificationType::is_reference_assignable_from (this=0x7ffff571b5a0, from=..., context=0x7ffff571c870, from_field_is_protected=false, __the_thread__=0x7ffff0032830)
    at /home/carl/git/jdk/src/hotspot/share/classfile/verificationType.cpp:122
#3  0x00007ffff6b1827f in VerificationType::is_assignable_from (this=0x7ffff571b5a0, from=..., context=0x7ffff571c870, from_field_is_protected=false, __the_thread__=0x7ffff0032830)
    at /home/carl/git/jdk/src/hotspot/share/classfile/verificationType.hpp:289
#4  0x00007ffff6cad7ad in StackMapFrame::pop_stack (this=0x7ffff571bdf0, type=..., __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/classfile/stackMapFrame.hpp:236
#5  0x00007ffff6ca8321 in ClassVerifier::verify_field_instructions (this=0x7ffff571c870, bcs=0x7ffff571bd80, current_frame=0x7ffff571bdf0, cp=..., allow_arrays=true, __the_thread__=0x7ffff0032830)
    at /home/carl/git/jdk/src/hotspot/share/classfile/verifier.cpp:2367
#6  0x00007ffff6ca439a in ClassVerifier::verify_method (this=0x7ffff571c870, m=..., __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/classfile/verifier.cpp:1693
#7  0x00007ffff6c9d1c6 in ClassVerifier::verify_class (this=0x7ffff571c870, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/classfile/verifier.cpp:645
#8  0x00007ffff6c9b499 in Verifier::verify (klass=0x8000c4000, should_verify_class=true, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/classfile/verifier.cpp:201
#9  0x00007ffff6444437 in InstanceKlass::verify_code (this=0x8000c4000, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/oops/instanceKlass.cpp:752
#10 0x00007ffff6444a91 in InstanceKlass::link_class_impl (this=0x8000c4000, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/oops/instanceKlass.cpp:873
#11 0x00007ffff64444c2 in InstanceKlass::link_class (this=0x8000c4000, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/oops/instanceKlass.cpp:758
#12 0x00007ffff644539d in InstanceKlass::initialize_impl (this=0x8000c4000, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/oops/instanceKlass.cpp:1027
#13 0x00007ffff64443a2 in InstanceKlass::initialize (this=0x8000c4000, __the_thread__=0x7ffff0032830) at /home/carl/git/jdk/src/hotspot/share/oops/instanceKlass.cpp:740
#14 0x00007ffff659bc52 in find_class_from_class_loader (env=0x7ffff0032b50, name=0x7ffff0572b78, init=1 '\001', loader=..., protection_domain=..., throwError=0 '\000', __the_thread__=0x7ffff0032830)
    at /home/carl/git/jdk/src/hotspot/share/prims/jvm.cpp:3537
#15 0x00007ffff658c395 in JVM_FindClassFromCaller (env=0x7ffff0032b50, name=0x7ffff571edd0 "io/perfmark/impl/SecretPerfMarkImpl$PerfMarkImpl", init=1 '\001', loader=0x7ffff571ef08, caller=0x7ffff571ef00)
    at /home/carl/git/jdk/src/hotspot/share/prims/jvm.cpp:825
#16 0x00007ffff54bff29 in Java_java_lang_Class_forName0 (env=0x7ffff0032b50, this=0x7ffff571eef0, classname=0x7ffff571ef18, initialize=1 '\001', loader=0x7ffff571ef08, caller=0x7ffff571ef00)
    at /home/carl/git/jdk/src/java.base/share/native/libjava/Class.c:145
#17 0x00007fffe855aaad in ?? ()
#18 0x0000000000000002 in ?? ()
#19 0x00007fffbc187288 in ?? ()
#20 0x0000000000000000 in ?? ()

I highlighted the relevant methods. It seems like the JVM is keen on making sure that NoopGenerator is assignable to Generator, in the event that line is executed. With this is mind, we can guess how to fix it.

Cheating the Verifier

So far, the class hierarchy looks something like:

public abstract class Generator {
  // ...
final class NoopGenerator extends Generator {
  // ...

Recall the original function that triggers this behavior:

// SecretPerfMarkImpl.PerfMarkImpl

static {
  Generator gen = ...
  if (gen != null) {
    generator = gen;
  } else {
    generator = new NoopGenerator();
  // More Initialization code.

The JVM wants to be sure that in case NoopGenerator is loaded, it will actually be Generator. The problem is that it is eagerly doing the verification of the type, even if it is seldom loaded. With this in mind, we can come up with a solution.

Warning: Hacks

Do not write the following code without a pretty beefy comment:

static {
  Generator gen = ...
  if (gen != null) {
    generator = gen;
  } else {
    generator = (Generator) (Object) new NoopGenerator();
  // More Initialization code.

This double cast (Object masking?) modifies the byte code to doubt itself about it’s assignment. Note the change in the byte code:

       176: aload_0
       177: ifnonnull     196
       180: new           #3                  // class io/perfmark/impl/NoopGenerator
       183: dup
       184: invokespecial #4                  // Method io/perfmark/impl/NoopGenerator."":()V
       187: checkcast     #2                  // class io/perfmark/impl/Generator
       190: putstatic     #26                 // Field generator:Lio/perfmark/impl/Generator;
       193: goto          200
       196: aload_0
       197: putstatic     #26                 // Field generator:Lio/perfmark/impl/Generator;
       200: iconst_0
       201: istore_2
       202: iconst_0
       203: istore_3
       204: ldc           #108                // String io.perfmark.PerfMark.startEnabled
       206: invokestatic  #109                // Method java/lang/Boolean.getBoolean:(Ljava/lang/String;)Z

This minor change causes the verifier to not double check the type until runtime. I won’t re-paste all the debug output, but let’s see that the change had an effect. We will use JMH with the classloader profiler to measure the changes:

Benchmark                                             Mode  Cnt     Score     Error        Units
forName_init                         ss  400  8730.958 ± 142.833        us/op
forName_init:·class.load             ss  400     1.166 ±   0.015  classes/sec
forName_init:·class.load.norm        ss  400    72.105 ±   0.112   classes/op
forName_init:·class.unload           ss  400       ≈ 0            classes/sec
forName_init:·class.unload.norm      ss  400       ≈ 0             classes/op

Benchmark                                             Mode  Cnt     Score     Error        Units
forName_init                         ss  400  8411.036 ± 148.305        us/op
forName_init:·class.load             ss  400     1.136 ±   0.016  classes/sec
forName_init:·class.load.norm        ss  400    71.040 ±   0.113   classes/op
forName_init:·class.unload           ss  400       ≈ 0            classes/sec
forName_init:·class.unload.norm      ss  400       ≈ 0             classes/op

As we can see the number of classes loaded drops by about one. We won’t read too much into the speed up, since the error bars are pretty high already.


The JVM checks our safety by making sure our classes are sound, but sometimes we want to defer those checks until later. This post shows how to diagnose such cases, and how to avoid doing unnecssary class loads.

Becoming an Extrovert

In 2022, I made the painful transition from introvert to extrovert. It has greatly improved my life by making me feel more confident and comfortable in social settings. While the fear of talking to new people still lingers on, I can now override that little voice inside. Several people have expressed interest in my conversion, and I wanted to document how I changed. If you are looking for an avenue for self improvement that is challenging and rewarding, I’d recommend giving this a try.


As the old joke goes, introverts look at their shoes when talking, but extroverts look at your shoes. I originally thought the truth was along these lines, where introverts don’t talk to other people, but extroverts do. When I talked to people about it, a lot of people claimed to be introverts, even though they’re social and engaging people.

Introverts do not get energy talking to other people. It’s not black and white but this is the litmus test for if you are introverted. Prior to 2022, I was like this. However, it manifested in less obvious ways. For example:

A superficial understanding would be that these sentiments are about not wanting to talk with others. However, a more introspective survey would show (at least for me) that these insecurities and confidence issues. The surface level logic makes sense, and no one can really challenge that (e.g. you might actually be interrupting someone) at face value. I would rebut that it’s more about fear of rejection.

If you want to:

Then, being an introvert will not serve you. This is the conclusion I came to at the beginning of 2022. I decided that the awkward and cringy experiences would ultimately be worth it for my own personal development. I needed to venture from one side of the social chasm to the other.


Extroverts absorb energy talking to other people. Not only do they get ramped up talking and learning about others, every moment in the conversation makes them want it more. Have you ever felt drained after attending a social obligation? Have you ever thought “Uggh I’ve been out talking to people all day, I need a break”? Extroverts do not feel this way.

I can see the internal changes inside myself. It’s a mindset thing; the first thought when seeing other people changes. Notice the difference in narrative:

If that last one bothers you, don’t let it! My own pendulum swung so far into the introversion side before I started my journey. In order to push myself to feel comfortable, I had to override my own internal monologue to be bold enough to talk to others. Remember, it’s a free country! It’s not illegal to talk to the human beings around you. The extrovert thinks: “if they don’t want to talk to me, they will let me know [via verbal or body language cues]“.

One of the major downsides I noticed of being extroverted is that we (they) feel a desire to talk to others. Some people don’t want to talk, but we have an unfulfilled need. As a result, extroverts are under constant rejection every day. They have to talk to others; their nature compels them to initiate. Would you rather live in a world where you never talk to anyone, or a world where no one wants to talk to you? I think the extrovert scenario is the more painful one!

How to Make the Leap

Several mindset changes were needed for me to make the change. I won’t sugar-coat it, it’s going to be a painful and possibly embarrassing experience. Here is what I internalized:

I had (and maybe still have?) a bad habit of trying to followup on everything someone else says. It left no room in the dialogue for the other person to open up: to be heard. It’s painful to have that silence, but I’ve noticed other people will reveal themselves more if you leave them room. And remember, they hate that silence too!


I took a Myers Briggs test at the beginning of 2022, and then again around August. I answered honestly how I really felt and what I would do. The test picked up the change from IXXX to EXXX which was kind of amazing to me.

Additionally, my confidence in myself has risen dramatically. Other people have noticed as well. It’s not fully complete, but it’s far enough long that I can see what is needed to finish.

Confidence and comfort to talk to others is held inside like a sieve. It gets refilled constantly by talking to others, which is how extroverts rally. I feel like I can last for longer and longer in conversation, and at some point I won’t be tired by it. A friend of mine said he could talk all day long (like 8 hours) and hunger for more at the end of it. This is the level I am going for and the changes in attitude I listed above are will what will get me there.

More Thoughts:

You can find me on Twitter @CarlMastrangelo