Check the behavior of Java Intrinsic Locks with bpftrace


Java has a mechanism called Intrinsic Locks. Each object has its own unique lock (Monitor Lock), and easy multithreaded parallel processing can be realized with just the keyword synchronized. So how do the threads collaborate when the program is running? This time, use bpftrace to check this.

Work environment

The OS should be Ubuntu 20.04.

OpenJDK 14

The JDK's Dtrace feature is required, so build it from source.

$ apt-get update
$ DEBIAN_FRONTEND=noninteractive ln -fs /usr/share/zoneinfo/Asia/Shanghai /etc/localtime && apt-get install -y tzdata && dpkg-reconfigure --frontend noninteractive tzdata
$ apt-get install build-essential autoconf systemtap-sdt-dev libx11-dev libxext-dev libxrender-dev libxrandr-dev libxtst-dev libxt-dev libcups2-dev libfontconfig1-dev libasound2-dev unzip zip ccache -y

## clone source and boot jdk
$ mkdir jdk && cd jdk
$ git clone

$ wget
$ tar xvf openjdk-13.0.2_linux-x64_bin.tar.gz

$ cd jdk14u

$ bash configure --enable-debug --with-jvm-variants=server --enable-dtrace --with-boot-jdk=../jdk-13.0.2 --enable-ccache
$ make images

$ export JAVA_HOME=$PWD/build/linux-x86_64-server-fastdebug/jdk

bcc and bpftrace

## bcc
$ apt-get install -y linux-headers-$(uname -r) bison build-essential cmake flex g++ git libelf-dev zlib1g-dev libfl-dev systemtap-sdt-dev binutils-dev llvm-8-dev llvm-8-runtime libclang-8-dev clang-8 arping netperf iperf3 python3-distutils
$ git clone --recurse-submodules
$ mkdir bcc/build; cd bcc/build
$ cmake -DPYTHON_CMD=python3 ..
$ make -j8 && make install && ldconfig

$ cd ../..

## bpftrace

$ git clone
$ mkdir bpftrace/build; cd bpftrace/build

$ make -j8 && make install

Prepare a sample program

Now, prepare this java program. All you are doing is calling two competing threats.

public class Test {

    private static synchronized void enter(String name) {
        try {
            System.out.println("enter " + name);
            System.out.println("exit " + name);
        } catch (InterruptedException e) {

    public static void main(String[] args) {
        new Thread(() -> enter("foo"), "foo").start();
        new Thread(() -> enter("bar"), "bar").start();


$ $JAVA_HOME/bin/javac

Execution and results

First, start this bpftrace command in another terminal to prepare the trace.

$ bpftrace - <<EOF
BEGIN { printf("%-12s %-6s %-27s %s\n", "TIME", "TID", "ACTION", "DESC"); }

usdt:$JAVA_HOME/lib/server/ { printf("%-12ld %-6d %-27s", elapsed, arg2, "start");printf("name=%s\n", str(arg0)); }
usdt:$JAVA_HOME/lib/server/ { printf("%-12ld %-6d %-27s", elapsed, arg2, "stop");printf("name=%s\n", str(arg0)); }

usdt:$JAVA_HOME/lib/server/ { printf("%-12ld %-6d %-27s", elapsed, arg0, "monitor_contended_enter"); printf("monitor=0x%lx, class=%s\n", arg1, str(arg2)); }
usdt:$JAVA_HOME/lib/server/ { printf("%-12ld %-6d %-27s", elapsed, arg0, "monitor_contended_entered"); printf("monitor=0x%lx, class=%s\n", arg1, str(arg2)); }
usdt:$JAVA_HOME/lib/server/ { printf("%-12ld %-6d %-27s", elapsed, arg0, "monitor_contended_exit"); printf("monitor=0x%lx, class=%s\n", arg1, str(arg2)); }

Attaching 6 probes...

Next, run the previous java program

$ $JAVA_HOME/bin/java -XX:+ExtendedDTraceProbes  Test
enter bar
exit bar
enter foo
exit foo

The two threats ran in sequence. So what about tracing?

Attaching 6 probes...
TIME         TID    ACTION                      DESC
3568183750   2      start                      name=Reference Handler
3568644123   3      start                      name=Finalizer
3581708591   1      monitor_contended_exit     monitor=0x7f7314003100, class=java/lang/Object����
3583984755   4      start                      name=Signal Dispatcher
3584404128   6      start                      name=C2 CompilerThread0
3584475441   5      start                      name=Service Thread
3584812927   7      start                      name=C1 CompilerThread0
3585382491   8      start                      name=Sweeper thread
3618900047   9      start                      name=Common-Cleaner
4170272484   2      monitor_contended_exit     monitor=0x7f7314005100, class=java/lang/ref/ReferenceQueue$Lock���#
4176664886   10     start                      name=Notification Thread
4201863620   11     start                      name=foo
4202328663   12     start                      name=bar
4203255448   11     monitor_contended_enter    monitor=0x7f7314007000, class=java/lang/Class�����
5229989467   12     monitor_contended_exit     monitor=0x7f7314007000, class=java/lang/Class�����
5230226530   11     monitor_contended_entered  monitor=0x7f7314007000, class=java/lang/Class�����
5230593438   12     stop                       name=bar
6242293321   11     stop                       name=foo
6247012424   4      stop                       name=Signal Dispatcher

What you can see here is that after some JVM System threats were started, so were the foo and bar threats. And foo is now in the state of monitor_contended_enter, because of course the bar entered the critical region first. After that, bar becomes monitor_contended_exit and leaves the critical region. Immediately foo also became monitor_contended_entered and execution started.


It's a simple application, but bpftrace or bcc is a very powerful tool, and I'd like to explore more possibilities in the future.

Reference material

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