Sometimes I need to read the JVM source code. The starting procedure is probably where to start. This article is a record of a little research on the JVM startup procedure and the libjvm.so
that got there while experimenting.
In the working environment, use Vagrant 2.2.8 for Ubuntu 18.04. First, prepare this Vagrantfile
.
Vagrantfile
Vagrant.configure("2") do |config|
config.vm.box = "ubuntu/bionic64"
config.vbguest.auto_update = false
config.vm.provider "virtualbox" do |vb|
vb.memory = "4096"
end
end
And register in the virtual machine
$ vagrant up
$ vagrant ssh
Install the required packages
$ apt-get update
$ DEBIAN_FRONTEND=noninteractive ln -fs /usr/share/zoneinfo/Asia/Tokyo /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 gdb -y
Next, download the source code
$ cd /vagrant
$ mkdir jdk && cd jdk
$ git clone https://github.com/openjdk/jdk13u.git
$ wget https://download.java.net/java/GA/jdk12.0.2/e482c34c86bd4bf8b56c0b35558996b9/10/GPL/openjdk-12.0.2_linux-x64_bin.tar.gz
$ tar xvf openjdk-12.0.2_linux-x64_bin.tar.gz
Start with the building
$ cd jdk13u
$ bash configure --enable-debug --with-jvm-variants=server --enable-dtrace --with-boot-jdk=../jdk-12.0.2 --enable-ccache
$ make images
After waiting for about 2 hours, I finally succeeded. And let's examine the building and the results.
$ export JAVA_HOME=$PWD/build/linux-x86_64-server-fastdebug/jdk
$ $JAVA_HOME/bin/java -version
openjdk version "13.0.3-internal" 2020-04-14
OpenJDK Runtime Environment (fastdebug build 13.0.3-internal+0-adhoc.root.jdk13u)
OpenJDK 64-Bit Server VM (fastdebug build 13.0.3-internal+0-adhoc.root.jdk13u, mixed mode)
First, set the environment variable _JAVA_LAUNCHER_DEBUG
to get more detailed startup information.
$ _JAVA_LAUNCHER_DEBUG=1 $JAVA_HOME/bin/java -version
----_JAVA_LAUNCHER_DEBUG----
Launcher state:
First application arg index: -1
debug:on
javargs:off
program name:java
launcher name:openjdk
javaw:off
fullversion:13.0.3-internal+0-adhoc.root.jdk13u
Java args:
Command line args:
argv[0] = /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/bin/java
argv[1] = -version
JRE path is /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk
jvm.cfg[0] = ->-server<-
jvm.cfg[1] = ->-client<-
1 micro seconds to parse jvm.cfg
Default VM: server
Does `/vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/lib/server/libjvm.so' exist ... yes.
mustsetenv: FALSE
JVM path is /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/lib/server/libjvm.so
1 micro seconds to LoadJavaVM
JavaVM args:
version 0x00010002, ignoreUnrecognized is JNI_FALSE, nOptions is 3
option[ 0] = '-Dsun.java.launcher.diag=true'
option[ 1] = '-Dsun.java.launcher=SUN_STANDARD'
option[ 2] = '-Dsun.java.launcher.pid=11021'
openjdk version "13.0.3-internal" 2020-04-14
OpenJDK Runtime Environment (fastdebug build 13.0.3-internal+0-adhoc.root.jdk13u)
OpenJDK 64-Bit Server VM (fastdebug build 13.0.3-internal+0-adhoc.root.jdk13u, mixed mode)
Various functions are printed out, and the one that should be noted is here
JVM path is /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/lib/server/libjvm.so
It seems that you are using a library called libjvm.so
.
libjvm.so
If you look for libjvm.so
in the source code, you will find java_md_solinux.c: 556
. Dlopen (3)
was found in /libjli/java_md_solinux.c#L556).
c:src/java.base/unix/native/libjli/java_md_solinux.c
...
JLI_TraceLauncher("JVM path is %s\n", jvmpath);
libjvm = dlopen(jvmpath, RTLD_NOW + RTLD_GLOBAL);
if (libjvm == NULL) {
...
And then in that java_md_solinux.c: 609
JNI_CreateJavaVM I read the symbol
.
c:src/java.base/unix/native/libjli/java_md_solinux.c
...
ifn->CreateJavaVM = (CreateJavaVM_t)
dlsym(libjvm, "JNI_CreateJavaVM");
...
This JNI_CreateJavaVM
is probably the entrance to the JVM.
JNI_CreateJavaVM
Actually [src / java.base / share / native / launcher / main.c
](https://github.com/openjdk/jdk13u/blob/master/src/java.base/share/native/launcher/main If you read a little from .c), you can see the general procedure.
main.c::main
java.c::JLI_Launch
java_md_solinux.c::LoadJavaVM
java_md_solinux.c::JVMInit
java_md_solinux.c::CallJavaMainInNewThread
java.c::JavaMain
java.c::InitializeJVM
jni.cpp::JNI_CreateJavaVM
Let's verify with gdb.
$ gdb -q --args $JAVA_HOME/bin/java -version <<EOF
set print thread-events off
set print frame-arguments none
set breakpoint pending on
handle SIGSEGV nostop noprint pass
break JNI_CreateJavaVM
commands
silent
thread apply all backtrace
continue
end
run
EOF
Reading symbols from /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/bin/java...(no debugging symbols found)...done.
(gdb) (gdb) (gdb) (gdb) Signal Stop Print Pass to program Description
SIGSEGV No No Yes Segmentation fault
(gdb) Signal Stop Print Pass to program Description
SIGSEGV No No Yes Segmentation fault
(gdb) Function "JNI_CreateJavaVM" not defined.
Breakpoint 1 (JNI_CreateJavaVM) pending.
(gdb) >>>>(gdb) (gdb) Starting program: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/bin/java -version
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[Switching to Thread 0x7ffff7fea700 (LWP 11128)]
Thread 2 (Thread 0x7ffff7fea700 (LWP 11128)):
#0 JNI_CreateJavaVM (vm=..., penv=..., args=...) at ../../src/hotspot/share/prims/jni.cpp:4012
#1 0x00007ffff7bca1e6 in InitializeJVM (ifn=..., penv=..., pvm=...) at ../src/java.base/share/native/libjli/java.c:1539
#2 JavaMain (_args=...) at ../src/java.base/share/native/libjli/java.c:417
#3 0x00007ffff7bce239 in ThreadJavaMain (args=...) at ../src/java.base/unix/native/libjli/java_md_solinux.c:740
#4 0x00007ffff719c6db in start_thread (arg=...) at pthread_create.c:463
#5 0x00007ffff78f688f in clone () at ../sysdeps/unix/sysv/linux/x86_64/clone.S:95
Thread 1 (Thread 0x7ffff7fec380 (LWP 11124)):
#0 0x00007ffff719dd2d in __GI___pthread_timedjoin_ex (threadid=..., thread_return=..., abstime=..., block=...) at pthread_join_common.c:89
#1 0x00007ffff719db5c in __pthread_join (threadid=..., thread_return=...) at pthread_join.c:24
#2 0x00007ffff7bcee2d in CallJavaMainInNewThread (stack_size=..., args=...) at ../src/java.base/unix/native/libjli/java_md_solinux.c:762
#3 0x00007ffff7bcb70d in ContinueInNewThread (ifn=..., threadStackSize=..., argc=..., argv=..., mode=..., what=..., ret=...) at ../src/java.base/share/native/libjli/java.c:2367
#4 0x00007ffff7bceefb in JVMInit (ifn=..., threadStackSize=..., argc=..., argv=..., mode=..., what=..., ret=...) at ../src/java.base/unix/native/libjli/java_md_solinux.c:809
#5 0x00007ffff7bccd1b in JLI_Launch (argc=..., argv=..., jargc=..., jargv=..., appclassc=..., appclassv=..., fullversion=..., dotversion=..., pname=..., lname=..., javaargs=..., cpwildcard=..., javaw=..., ergo=...) at ../src/java.base/share/native/libjli/java.c:344
#6 0x0000555555554ad3 in main ()
openjdk version "13.0.3-internal" 2020-04-14
OpenJDK Runtime Environment (fastdebug build 13.0.3-internal+0-adhoc.root.jdk13u)
OpenJDK 64-Bit Server VM (fastdebug build 13.0.3-internal+0-adhoc.root.jdk13u, mixed mode)
[Inferior 1 (process 11124) exited normally]
(gdb) quit
libjvm.so
Finally, let's take a look at libjvm.so
. The format of Shared Library is elf, so use readelf
and nm
.
$ readelf -h $JAVA_HOME/lib/server/libjvm.so
ELF Header:
Magic: 7f 45 4c 46 02 01 01 03 00 00 00 00 00 00 00 00
Class: ELF64
Data: 2's complement, little endian
Version: 1 (current)
OS/ABI: UNIX - GNU
ABI Version: 0
Type: DYN (Shared object file)
Machine: Advanced Micro Devices X86-64
Version: 0x1
Entry point address: 0x303bf0
Start of program headers: 64 (bytes into file)
Start of section headers: 38283720 (bytes into file)
Flags: 0x0
Size of this header: 64 (bytes)
Size of program headers: 56 (bytes)
Number of program headers: 8
Size of section headers: 64 (bytes)
Number of section headers: 37
Section header string table index: 36
There are 37 sections.
$ readelf -S $JAVA_HOME/lib/server/libjvm.so
There are 37 section headers, starting at offset 0x24829c8:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .note.gnu.build-i NOTE 0000000000000200 00000200
0000000000000024 0000000000000000 A 0 0 4
[ 2] .hash HASH 0000000000000228 00000228
0000000000001028 0000000000000004 A 4 0 8
[ 3] .gnu.hash GNU_HASH 0000000000001250 00001250
0000000000000a44 0000000000000000 A 4 0 8
[ 4] .dynsym DYNSYM 0000000000001c98 00001c98
0000000000002c70 0000000000000018 A 5 1 8
[ 5] .dynstr STRTAB 0000000000004908 00004908
0000000000001e60 0000000000000000 A 0 0 1
[ 6] .gnu.version VERSYM 0000000000006768 00006768
00000000000003b4 0000000000000002 A 4 0 2
[ 7] .gnu.version_d VERDEF 0000000000006b20 00006b20
0000000000000038 0000000000000000 A 5 2 8
[ 8] .gnu.version_r VERNEED 0000000000006b58 00006b58
0000000000000130 0000000000000000 A 5 5 8
[ 9] .rela.dyn RELA 0000000000006c88 00006c88
00000000002fa6d8 0000000000000018 A 4 0 8
[10] .rela.plt RELA 0000000000301360 00301360
0000000000001830 0000000000000018 AI 4 28 8
[11] .init PROGBITS 0000000000302b90 00302b90
0000000000000017 0000000000000000 AX 0 0 4
[12] .plt PROGBITS 0000000000302bb0 00302bb0
0000000000001030 0000000000000010 AX 0 0 16
[13] .plt.got PROGBITS 0000000000303be0 00303be0
0000000000000010 0000000000000008 AX 0 0 8
[14] .text PROGBITS 0000000000303bf0 00303bf0
00000000016df6be 0000000000000000 AX 0 0 16
[15] .fini PROGBITS 00000000019e32b0 019e32b0
0000000000000009 0000000000000000 AX 0 0 4
[16] .rodata PROGBITS 00000000019e32c0 019e32c0
0000000000204358 0000000000000000 A 0 0 32
[17] .stapsdt.base PROGBITS 0000000001be7618 01be7618
0000000000000001 0000000000000000 A 0 0 1
[18] .eh_frame_hdr PROGBITS 0000000001be761c 01be761c
000000000006646c 0000000000000000 A 0 0 4
[19] .eh_frame PROGBITS 0000000001c4da88 01c4da88
0000000000209730 0000000000000000 A 0 0 8
[20] .gcc_except_table PROGBITS 0000000001e571b8 01e571b8
00000000000000bc 0000000000000000 A 0 0 4
[21] .tdata PROGBITS 0000000002057f50 01e57f50
0000000000000008 0000000000000000 WAT 0 0 8
[22] .tbss NOBITS 0000000002057f58 01e57f58
0000000000000038 0000000000000000 WAT 0 0 8
[23] .init_array INIT_ARRAY 0000000002057f58 01e57f58
0000000000001620 0000000000000008 WA 0 0 8
[24] .fini_array FINI_ARRAY 0000000002059578 01e59578
0000000000000008 0000000000000008 WA 0 0 8
[25] .data.rel.ro PROGBITS 0000000002059580 01e59580
00000000000ef5d8 0000000000000000 WA 0 0 32
[26] .dynamic DYNAMIC 0000000002148b58 01f48b58
0000000000000240 0000000000000010 WA 5 0 8
[27] .got PROGBITS 0000000002148d98 01f48d98
0000000000000268 0000000000000008 WA 0 0 8
[28] .got.plt PROGBITS 0000000002149000 01f49000
0000000000000828 0000000000000008 WA 0 0 8
[29] .data PROGBITS 0000000002149840 01f49840
000000000003d950 0000000000000000 WA 0 0 64
[30] .bss NOBITS 00000000021871c0 01f87190
00000000000d1300 0000000000000000 WA 0 0 64
[31] .comment PROGBITS 0000000000000000 01f87190
0000000000000029 0000000000000001 MS 0 0 1
[32] .note.stapsdt NOTE 0000000000000000 01f871bc
000000000000d504 0000000000000000 0 0 4
[33] .gnu_debuglink PROGBITS 0000000000000000 01f946c0
0000000000000018 0000000000000000 0 0 4
[34] .symtab SYMTAB 0000000000000000 01f946d8
00000000001aede8 0000000000000018 35 73063 8
[35] .strtab STRTAB 0000000000000000 021434c0
000000000033f3b1 0000000000000000 0 0 1
[36] .shstrtab STRTAB 0000000000000000 02482871
0000000000000157 0000000000000000 0 0 1
Key to Flags:
W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
L (link order), O (extra OS processing required), G (group), T (TLS),
C (compressed), x (unknown), o (OS specific), E (exclude),
l (large), p (processor specific)
The center of interest is the .dynsym
section, which is where the so-called dynamic symbols are stored. The previous JNI_CreateJavaVM
is also here.
$ readelf --dyn-syms $JAVA_HOME/lib/server/libjvm.so | grep JNI_CreateJavaVM
353: 0000000000f0b260 1396 FUNC GLOBAL DEFAULT 14 JNI_CreateJavaVM@@SUNWprivate_1.1
With nm
, you can also get the line number.
$ nm -gl --defined-only $JAVA_HOME/lib/server/libjvm.so | grep JNI_CreateJavaVM
0000000000f0b260 T JNI_CreateJavaVM /vagrant/jdk/jdk13u/make/hotspot/../../src/hotspot/share/prims/jni.cpp:4012
Other requested libraries can be found using ldd
.
$ ldd $JAVA_HOME/lib/server/libjvm.so
linux-vdso.so.1 (0x00007fffeef52000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f92d1fab000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f92d1d8c000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f92d19ee000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f92d15fd000)
/lib64/ld-linux-x86-64.so.2 (0x00007f92d4408000)
Finally, when I built OpenJDK earlier, I put in a variable called --enable-dtrace
, so the information on those DTrace Probes (USDT) is stored in the .note.stapsdt
section.
$ readelf -n $JAVA_HOME/lib/server/libjvm.so | head -n 18
Displaying notes found in: .note.gnu.build-id
Owner Data size Description
GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring)
Build ID: 8fc95ac20704f218afd980ae1f303abfdf0b0586
Displaying notes found in: .note.stapsdt
Owner Data size Description
stapsdt 0x00000050 NT_STAPSDT (SystemTap probe descriptors)
Provider: hotspot
Name: class__unloaded
Location: 0x000000000092a226, Base: 0x0000000001be7618, Semaphore: 0x0000000000000000
Arguments: 8@%rdx -4@%eax 8@152(%rdi) 1@$0
stapsdt 0x00000050 NT_STAPSDT (SystemTap probe descriptors)
Provider: hotspot
Name: class__loaded
Location: 0x000000000092a31b, Base: 0x0000000001be7618, Semaphore: 0x0000000000000000
Arguments: 8@%rdx -4@%eax 8@152(%rdi) 1@%sil
What I learned so far
java
works like Launcher, after doing some initial processing go to libjvm.so
._JAVA_LAUNCHER_DEBUG
.libjvm.so
is JNI_CreateJavaVM
, and everything starts here.Recommended Posts