The OpenJDK project consists of a number of components - HotSpot (the virtual machine), the Java Class Library and the javac Java compiler.For more on it, visit its site.
HotSpot is, as mentioned above, a cross-platform implementation of the Java Virtual Machine (JVM) Concept, and is distributed with the OpenJDK (Java Developer Kit), under the GNU General Public License. HotSpot is written mainly in C++, and was originally developed under Sun Microsystems. It is currently developed under the OpenJDK Project, at www.java.net. The HotSpot JVM was available as an add-on for Java 1.2, and later was used as the default Sun JVM in Java 1.3. The JVM is currently in Version 7, Build b147, and has an active development community behind it.
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There are several HotSpot VM launchers in the Java StandardEdition, the general purpose launcher typically used is the javacommand on Unix and on Windows java and javaw commands, not to beconfused with javaws which is a network based launcher.
The most common reason for class loading is during bytecoderesolution, when a constant pool symbol in the classfile requiresresolution. Java APIs such as Class.forName(), classLoader.loadClass(), reflection APIs, andJNI_FindClass can initiate class loading. The VMitself can initiate class loading. The VM loads core classes suchas java.lang.Object, java.lang.Thread, etc. at JVM startup. Loading aclass requires loading all superclasses and superinterfaces. Andclassfile verification, which is part of the linking phase, canrequire loading additional classes.
The VM and Java SE class loading libraries share theresponsibility for class loading. The VM performs constant poolresolution, linking and initialization for classes and interfaces.The loading phase is a cooperative effort between the VM andspecific class loaders (java.lang.classLoader).
The load class phase takes a class or interface name, finds thebinary in classfile format, defines the class and creates thejava.lang.Class object. The load class phase canthrow a NoClassDefFound error if a binary representationcan not be found. In addition, the load class phase does formatchecking on the syntax of the classfile, which can throw aClassFormatError or UnsupportedClassVersionError. Prior to completingloading of a class, the VM must load all of its superclasses andsuperinterfaces. If the class hierarchy has a problem such thatthis class is its own superclass or superinterface (recursively),then the VM will throw a ClassCircularityError. The VM also throwsIncompatibleClassChangeError if the directsuperinterface is not an interface, or the direct superclass is aninterface.
Class loading creates either an instanceKlassor an arrayKlass in the GC permanent generation. TheinstanceKlass refers to a java mirror, which is the instance ofjava.lang.Classmirroring this class. The VM C++ access to the instanceKlassis via a klassOop.
The basic threading model in Hotspot is a 1:1 mapping betweenJava threads (an instance of java.lang.Thread) and native operating systemthreads. The native thread is created when the Java thread isstarted, and is reclaimed once it terminates. The operating systemis responsible for scheduling all threads and dispatching to anyavailable CPU.
There are two basic ways for a thread to be introduced into theVM: execution of Java code that calls start() on a java.lang.Thread object; or attaching an existingnative thread to the VM using JNI. Other threads created by the VMfor internal purposes are discussed below.
When a java.lang.Threadis started the VM creates the associated JavaThreadand OSThread objects, and ultimately the nativethread. After preparing all of the VM state (such as thread-localstorage and allocation buffers, synchronization objects and soforth) the native thread is started. The native thread completesinitialization and then executes a start-up method that leads tothe execution of the java.lang.Thread object's run() method, and then, upon its return,terminates the thread after dealing with any uncaught exceptions,and interacting with the VM to check if termination of this threadrequires termination of the whole VM. Thread termination releasesall allocated resources, removes the JavaThread from the set of knownthreads, invokes destructors for the OSThread andJavaThread and ultimately ceases execution whenit's initial startup method completes.
A native thread attaches to the VM using the JNI callAttachCurrentThread. In response to this anassociated OSThread and JavaThreadinstance is created and basic initialization is performed. Next ajava.lang.Thread object must be created for theattached thread, which is done by reflectively invoking the Javacode for the Thread class constructor, based on the argumentssupplied when the thread attached. Once attached, a thread caninvoke whatever Java code it needs to via the other JNI methodsavailable. Finally when the native thread no longer wishes to beinvolved with the VM it can call the JNI DetachCurrentThread method to disassociate itfrom the VM (release resources, drop the reference to thejava.lang.Thread instance, destruct theJavaThread and OSThreadobjects and so forth).
A special case of attaching a native thread is the initialcreation of the VM via the JNI CreateJavaVMcall, which can be done by a native application or by the launcher(java.c). This causes arange of initialization operations to take place and then actseffectively as if a call to AttachCurrentThread was made. The thread can theninvoke Java code as needed, such as reflective invocation of themain method of an application. See the JNIsection for further details.
Other subsystems andlibraries impose their own state information, such as the JVMTIsystem and the ThreadStateexposed by thejava.lang.Threadclass itself. Such informationis generally not accessible to, nor relevant to, the management ofthreads inside the VM.
Usually when JVM crashes on a fatal error, it will dump ahotspot error log file called hs_err_pidtag_hash_109.log, (where tag_hash_110 is replaced bythe crashed java process id) to the Windows desktop or the currentapplication directory on Solaris/Linux. Several enhancements havebeen made to improve the diagnosability of this file since JDK 6and many of them have been back ported to the JDK-1.4.2_09 release.Here are some highlights of these improvements:
Another important feature is you can specify -XX:OnError="cmd1 args...;com2..." to the java command so that whenever VMcrashes, it will execute a list of commands you specified withinthe quotes shown above. A typical usage of this feature is you caninvoke the debugger such as dbx or Windbg to look into the crashwhen that happens. For the earlier releases, you can specify
-XX:+ShowMessageBoxOnError as aruntime option so that when VM crashes, you can attach the runningJava process to your favorite debugger.
"C:\Program Files\Eclipse Adoptium\jdk-11.0.15.10-hotspot/bin/java.exe" "-Xmx64m" "-Xms64m" "-Dorg.gradle.appname=gradlew" -classpath "D:\dev\starling2\game\2022\androidExportTest\android\build\\gradle\wrapper\gradle-wrapper.jar" org.gradle.wrapper.GradleWrapperMain
One way I want to try and show you that classloading is a little bit magical, is actually by writing a custom classloader. This is something that you can do. There's various different implementations that people have of this. This is something I've got on GitHub. It does a couple of weird things, actually. This is deliberate because I used to use this to teach Java classloaders. What the functionality is, is you take a Java file and you drag it into the classes' folder. Inside the classloader, it uses some of the tooling to actually run Javac from inside the Java process. It takes the .java file. It compiles it to a class file. Then it loads it into your system. You cannot know anything about the class. That's the thing I'll try to get across. You can't know anything about it.
Let's have a look. We'll hit play. I've set up this watcher here. It's watching the file. I've put some reflection code in here, of course, because it wouldn't be a Java demo without a little bit of reflection. We do demo and ClassNotFound. What I'm going to do is I'm going to take this demo.javafile, which is just effectively getA returns 42 and a setter. I'm going to drag that into this classes' folder here. I've attempted to load demo.java. What is this thing? Then it's compiled it to a class file, and now the class is loaded. If I go in here and type in demo, I can now see that I've got getA and setA. I showed this as an example in a presentation and somebody said, "That means I can write Java code and put it into the JVM at runtime, and nobody will know anything about it from production controls." It's like, maybe you want to step away from that slightly. It's really just created the .class file. Javac is not that smart. You can use it in lots of smart ways. There's more to the story. Actually, the biggest part of the Java story starts after classloaders.
A Java-based JIT compiler takes .class files as input rather than Java code, which is consumed by javac. In this way, a JIT compiler differs from a compiler like GCC, which directly consumes the code that you produce. The JIT compiler's role is to turn class files (composed of bytecode, which is the JVM's instruction set) into machine code that the CPU executes directly. Another way a Java JIT compiler differs from an offline compiler is that the JVM verifies class files at load time. When it's time to compile, there's little need for parsing or verification.
Other than performance variations, the JIT compiler's execution is transparent to the end user. It can, however, be observed by running the java command with diagnostic options. As an example, take this simple HelloWorld program:
Hunting down code hotspots is probably the most common task for Java profilers. JDK Flight Recorder (JFR) and Mission Control (MC) are free and open source performance/profiling products available with OpenJDK 11. They have a few powerful tools for code execution profiling. 2351a5e196
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