Overview

The HotSpot Runtime Overview says:

The VM loads core classes such as java.lang.Object, java.lang.Thread, etc. at JVM startup. Loading a class requires loading all superclasses and superinterfaces. And classfile verification, which is part of the linking phase, can require loading additional classes.

The JVM loads Classes such as java.lang.Object, java.lang.Thread as soon as it starts.
The requested super and interface will load automatically
Load other Classes only when needed

So what kind of Class was loaded and when? Let's take a look at Unified JVM Logging.

Work environment

Ubuntu 18.04 as well as previous article And I am using OpenJDK 13, please refer to that for the specific construction method.

Take a look at `startup time`

First, let's see the rough startup time of the JVM with the variable -Xlog: startuptime.

$ $JAVA_HOME/bin/java -Xlog:startuptime -version
[0.168s][info][startuptime] StubRoutines generation 1, 0.0257810 secs
[0.229s][info][startuptime] Genesis, 0.0605841 secs
[0.232s][info][startuptime] TemplateTable initialization, 0.0010887 secs
[0.263s][info][startuptime] Interpreter generation, 0.0302811 secs
[0.693s][info][startuptime] StubRoutines generation 2, 0.0152926 secs
[0.697s][info][startuptime] MethodHandles adapters generation, 0.0017809 secs
[0.703s][info][startuptime] Start VMThread, 0.0010136 secs
[0.997s][info][startuptime] Initialize java.lang classes, 0.2923160 secs
[1.018s][info][startuptime] Initialize java.lang.invoke classes, 0.0159241 secs
[3.161s][info][startuptime] Initialize module system, 2.1431533 secs
[3.173s][info][startuptime] Create VM, 3.1052374 secs
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)

What you can see here is how many classes and modules have been initialized in the Start VM Thread throat.

[0.997s][info][startuptime] Initialize java.lang classes, 0.2923160 secs
[1.018s][info][startuptime] Initialize java.lang.invoke classes, 0.0159241 secs
[3.161s][info][startuptime] Initialize module system, 2.1431533 secs

See Class `load` and ʻinit`

Next, set the variables -Xlog: startuptime, class + init, class + load and try again.

$ $JAVA_HOME/bin/java -Xlog:startuptime,class+init,class+load -version
[0.096s][info][class,load] path: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[0.159s][info][startuptime] StubRoutines generation 1, 0.0283598 secs
[0.220s][info][startuptime] Genesis, 0.0597463 secs
[0.223s][info][startuptime] TemplateTable initialization, 0.0012748 secs
[0.252s][info][startuptime] Interpreter generation, 0.0291667 secs
[0.284s][info][class,load ] path: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[0.285s][info][class,load ] path: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[0.314s][info][class,load ] java.lang.Object source: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[0.321s][info][class,load ] java.io.Serializable source: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
...
[0.553s][info][class,load ] java.util.Iterator source: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[0.578s][info][class,load ] java.lang.NullPointerException source: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[0.579s][info][class,load ] java.lang.ArithmeticException source: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[0.619s][info][startuptime] StubRoutines generation 2, 0.0132195 secs
[0.622s][info][startuptime] MethodHandles adapters generation, 0.0018546 secs
[0.627s][info][startuptime] Start VMThread, 0.0007446 secs
[0.629s][info][class,init ] 0 Initializing 'java/lang/Object' (0x0000000100001080)
[0.644s][info][class,init ] 1 Initializing 'java/lang/CharSequence'(no method) (0x00000001000016b8)
...
[0.898s][info][class,init ] 135 Initializing 'java/lang/IllegalArgumentException'(no method) (0x000000010002b0e8)
[0.898s][info][startuptime] Initialize java.lang classes, 0.2690178 secs
[0.902s][info][class,init ] 136 Initializing 'java/lang/invoke/MethodHandle' (0x000000010000c560)
...
[0.925s][info][class,init ] 144 Initializing 'java/lang/invoke/MethodHandleNatives' (0x000000010000d2e8)
[0.925s][info][startuptime] Initialize java.lang.invoke classes, 0.0232579 secs
[0.935s][info][class,load ] jdk.internal.module.ModuleBootstrap source: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
...
[3.112s][info][class,load ] jdk.internal.module.ModuleBootstrap$SafeModuleFinder source: /vagrant/jdk/jdk13u/build/linux-x86_64-server-fastdebug/jdk/modules/java.base
[3.113s][info][class,init ] 662 Initializing 'jdk/internal/module/ModuleBootstrap$SafeModuleFinder'(no method) (0x000000010008e930)
[3.113s][info][startuptime] Initialize module system, 2.1876125 secs
[3.124s][info][startuptime] Create VM, 3.0688743 secs
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)

What I found here is actually two stages

Several Classes have been loaded between ʻInterpreter generation and StubRoutines generation 2`.
Start VMThread 663 (0-662) Classes have been initialized.
java.lang 136 pieces (0-135)
java.lang.invoke 9 (136-144)
module system 518 pieces (145-622)

Check with source code

If you search the source code for the above text, you'll find where to inspire these two steps.

SystemDictionary :: resolve_well_known_classes: SystemDictionary is almost JVM Class management It's working. When this SystemDictionary is initialized, the so-called ** Well Know Classes ** core Classes are also initialized. A list of Well Known Classes can be found here [https://github.com/openjdk/jdk13u/blob/master/src/hotspot/share/classfile/systemDictionary.hpp#L103), the order is important It seems like.
Threads :: create_vm:java in the throat ofStart VM Thread This is where .lang classes, java.lang.invoke classes and module system are initialized at once. Specifically, with these three functions

Summary

Initializes these core Classes when the JVM boots

Well Know Classes (with System Dictionary initialization)
java.lang Class
java.lang.invoke Class
Class of $ JAVA_HOME / modules

About Class loading and initialization when the JVM starts