- HotSpot, the primary reference Java VM implementation
- OpenJDK — Open Java Development Kit is a free and open source implementation licensed under the GNU General Public License (GNU GPL) with a linking exception.
Does OpenJDK contain HotSpot virtual machine?
In wikipedia it says that OpenJDK as (JDK) contains HotSpot virtual machine. But I am looking for somewhat official document or article or anything except wikipedia. Openjdk is the open source foundation for Java. Oracle Java is openjdk rebranded with additional functionality. There are some components which are not licenced for openjdk.
What is OpenJDK?
OpenJDK is an open source implementation of the Java Standard Edition (Java SE) and Java Development Kit (JDK). The OpenJDK source code is the basis for these commercial Java products. OpenJDK is available for free with a GNU General Public license.
What is the difference between hotspot and Oracle JDK?
People occasionally think that HotSpot is synonymous with the Oracle JDK. Whilst the Oracle JDK does indeed include the HotSpot VM; this is only because it is part of OpenJDK. All unmodified builds of OpenJDK, therefore, use the HotSpot VM.
What is Java HotSpot?
From the wikipedia, HotSpot, released as the "Java HotSpot Performance Engine" is a Java virtual machine for desktops and servers, maintained and distributed by Oracle Corporation. I heard OpenJDK also uses HotSpot to execute code?
What Is OpenJDK?
OpenJDK is an open source implementation of the Java Standard Edition (Java SE) and Java Development Kit (JDK). The OpenJDK source code is the basis for these commercial Java products. OpenJDK is available for free with a GNU General Public license.
What is OpenJDK source?
The OpenJDK source forms the basis from which all other version of Java is created. And by extension, the use of OpenJDK brings developers closer to the epicenter of Java development.
What is OpenLogic team?
The team at OpenLogic consists of passionate, experienced Enterprise Architects who provide end-to-end services to organizations so they can confidently gain the freedom and savings that come with using open source software. Learn more about open source Java services and support.
What is the most popular programming language in 2020?
As of 2020, Java is listed as the third most popular programming language in the world, behind Javascript and Python, according to GitHub’s State of the Octoverse report ( Language Popularity ). In 2006, Sun Microsystems made the decision to open source Java.
Is OpenJDK more accessible than Java?
Oracle JDK Environment. OpenJDK itself can be acquired in a number of ways, but for modern environments is arguably more accessible than versions of Java which must be downloaded and manually installed. This shouldn’t be surprising. Since the code is GPL licensed, it is freely redistributable.
Is OpenJDK compatible with Oracle JVM?
This means that built distributions of OpenJDK are feature-compatible with the Oracle JVM. Using OpenJDK looks and feels very much like using Oracle JDK, or other builds of OpenJDK that you may be familiar with. Unlike Oracle JDK, even OpenJDK 8 doesn’t come with license obligations.
Is OpenJDK still the base code for Java?
Today in 2020, the OpenJDK source code is still the base code upon which all other distributions of Java, including the Oracle JDK, are built:
What version of Java is OpenJDK?
OpenJDK was initially based only on the JDK 7 version of the Java platform.
When was OpenJDK 8 released?
The OpenJDK 8 project, which is the basis for JDK 8, was released on 18 March 2014. The OpenJDK 7u project, which is based on JDK 7 and produces updates to the existing Java 7 releases. The OpenJDK 6 project, which is based on JDK 7, retrofitted to provide an open-source version of Java 6.
What is OpenJDK?
OpenJDK ( Open Java Development Kit) is a free and open-source implementation of the Java Platform, Standard Edition (Java SE). It is the result of an effort Sun Microsystems began in 2006. The implementation is licensed under the GPL-2.0-only with a linking exception.
When did Sun move OpenJDK to Mercurial?
In December 2007, Sun moved the revision control of OpenJDK from TeamWare to Mercurial (and later to Git and GitHub ), as part of the process of releasing it to open-source communities.
When did OpenJDK 6 come out?
By the appearance of OpenJDK 6 in May 2008, less than 1% (the SNMP implementation, which is not part of the Java specification) remained, making it possible to build OpenJDK without any binary plugs. The binary plug requirement was later dropped from OpenJDK 7 as part of b53 in April 2009.
When did Red Hat sign Sun Contributor Agreement?
On November 5, 2007, Red Hat signed both the Sun Contributor Agreement and the OpenJDK Community TCK License. One of the first benefits of this agreement is tighter alignment with the IcedTea project, which brings together Fedora, the Linux distribution, and JBoss, the application server, technologies in a Linux environment. IcedTea provided free software alternatives for the few remaining proprietary sections in the OpenJDK project.
Is OpenJDK 7 proprietary?
When initially released in May 2007, 4% of the OpenJDK class library remained proprietary. By the appearance of OpenJDK 6 in May 2008, less than 1% (the SNMP implementation, which is not part of the Java specification) remained, making it possible to build OpenJDK without any binary plugs. The binary plug requirement was later dropped from OpenJDK 7 as part of b53 in April 2009.
What is Java runtime environment?
The Java runtime environment as provided by OpenJDK consists of the HotSpot JVM combined with class libraries (which are largely bundled up into rt.jar). As Java is a portable environment, anything that requires a call into the operating system is ultimately handled by a native method.
What is an OOP in Java?
An OOP is a genuine pointer in the C / C++ sense - a machine word which points to a memory location inside the Java heap. The Java heap is allocated as a single continuous address range in terms of the JVM process’s virtual address space, and then memory is managed purely from within user space by the JVM process itself, unless the JVM needs to resize the heap for any reason.
How many methods are mapped to JVM entry points?
These five methods are mapped to JVM entry points (which are designated by the JVM_ prefix on the C method name) - using the registerNatives () mechanism (which allows the developer to change the mapping of Java native methods to C function names).
Where does klassoop live?
It is not possible to obtain a reference to a klassOop directly from Java code - and they live in the Permgen area of the heap.
Where are the fields laid out in Java?
The fields of the instance are laid out in a very specific order in the bytes following the OOP header. For the precise details read Nitsan Wakart’s excellent blog post " Know thy Java Object Memory Layout ".
Where is OpenJDK stored?
Both approaches are useful, but it’s important to be comfortable with the second as well as the first. The OpenJDK source code is stored in Mercurial (a Distributed Version Control System similar to the ubiquitous Git version control system.) If you aren’t familiar with Mercurial, there’s a free book called “ Version Control By Example ” which covers the basics.
How many native methods are there in Java?
JNI has another way of loading native methods, which is used by the remaining five native methods of java.lang.Object:
What is tiered compilation?
With the introduction of tiered compilation, OpenJDK HotSpot VM users can benefit from improved startup times with the server compiler.#N#Tiered compilation has five tiers of optimization. It starts in tier-0, the interpreter tier, where instrumentation provides information on the performance critical methods. Soon enough the tier 1 level, the simple C1 (client) compiler, optimizes the code. At tier 1, there is no profiling information. Next comes tier 2, where only a few methods are compiled (again by the client compiler). At tier 2, for those few methods, profiling information is gathered for entry-counters and loop-back branches. Tier 3 would then see all the methods getting compiled by the client compiler with full profiling information, and finally tier 4 would avail itself of C2, the server compiler.
How does OpenJDK Hotspot VM work?
OpenJDK HotSpot VM converts bytecode into machine executable code by “mixed-mode” execution. With “mixed-mode”, the first step is interpretation, which converts bytecode into assembly code using a description table. This pre-defined table, also known as the “template table”, has assembly code for each bytecode instruction.
What is a JVM switch?
JVM switches provide diagnostic information about the JIT compilation.
How many compilers does OpenJDK Hotspot have?
OpenJDK HotSpot VM has two different compilers, each with its own compilation thresholds:
What is the command line option for OpenJDK Hotspot?
In order to visualize what methods get compiled and when, OpenJDK HotSpot VM provides a very useful command line option called -XX:+PrintCompilation that reports when the code cache becomes full and when the compilation stops.
What are the advantages of adaptive optimization?
There are a lot of “tuning” options for inlining, based on size and invocation thresholds, and Inlining has been thoroughly studied and optimized to very near its maximum potential.
What is method entry count?
Method entry counts - assigns a call counter to every method.
Overview
History
Sun announced in JavaOne 2006 that Java would become open-source software, and on October 25, 2006, at the Oracle OpenWorld conference, Jonathan Schwartz said that the company intended to announce the open-sourcing of the core Java Platform within 30 to 60 days.
Sun released the Java HotSpot virtual machine and compiler as free software under the GNU General Public License on November 13, 2006, with a promise that the rest of the JDK (which inc…
Components
The OpenJDK project produces a number of components: most importantly the virtual machine (HotSpot), the Java Class Library and the Java compiler (javac).
The web-browser plugin and Web Start, which form part of Oracle Java, are not included in OpenJDK. Sun previously indicated that they would try to open-source these components, but neither Sun nor Oracle have done so. The only currently available free plugin and Web Start imple…
OpenJDK versions
OpenJDK was initially based only on the JDK 7 version of the Java platform.
Since JDK 10, the effort to produce an open-source reference implementation of the Java SE Platform was moved over to the JDK Project. Unlike past JDK Release Projects, which produced just one feature release and then terminated, this long-running project will produce all future JDK feature releases and will ship a feature release every six months according to a strict, time-base…
IcedTea and inclusion in software distributions
In order to bundle OpenJDK in Fedora and other free Linux distributions, OpenJDK needed to be buildable using only free software components. Due to the encumbered components in the class library and implicit assumptions within the build system that the JDK being used to build OpenJDK was a Sun JDK, this was not possible. To achieve openness, Red Hat started the IcedTea project in June 2007. It began life as an OpenJDK/GNU Classpath hybrid that could be used to bootstrap Op…
See also
• Free Java implementations
External links
• Main site
• OpenJDK builds from Oracle
• OpenJDK builds from Adoptium
• OpenJDK builds from Red Hat
Java VM’s Execution Engine and Runtime
- The execution engine consists of two major components: the garbage collector, (which reclaims garbage objects and provides automatic memory/heap management) and the JIT compiler (which converts bytecode to executable machine code). In OpenJDK 8, the “tiered compiler” is the default server compiler. HotSpot users can still select the non-tiered serv...
Adaptive Jit and Runtime Optimizations
- The JVM system is the backend helper for Java’s write once, run anywhere capability. Once a Java program is compiled into bytecode, it can be executed by a JVM instance. OpenJDK HotSpot VM converts bytecode into machine executable code by “mixed-mode” execution. With “mixed-mode”, the first step is interpretation, which converts bytecode into assembly code using a desc…
Five Levels of Tiered Compilation
- With the introduction of tiered compilation, OpenJDK HotSpot VM users can benefit from improved startup times with the server compiler. Tiered compilation has five tiers of optimization. It starts in tier-0, the interpreter tier, where instrumentation provides information on the performance critical methods. Soon enough the tier 1 level, the simple C1 (client) compiler, opti…
Tiered Compilation and Effects on Code Cache
- When compiling with the client compiler (tier 2 onwards), the code is profiled by the client compiler during startup, when the critical execution paths are still warming up. This helps produce better profiled information than interpreted code. The compiled code resides in a cache known as the “code cache”. A code cache has a fixed size, and when full, the Java VM will cease method c…
Understanding Compilation
- In order to visualize what methods get compiled and when, OpenJDK HotSpot VM provides a very useful command line option called -XX:+PrintCompilationthat reports when the code cache becomes full and when the compilation stops. Let’s look at some examples: The above output is formatted as: timestamp compilation-id flags tiered-compilation-level class:method <@ osr_bci…
Dynamic De-Optimization
- We know that Java does dynamic class loading, and the Java VM checks the inter-dependencies at every dynamic class load. When a previously optimized method is no longer relevant, OpenJDK HotSpot VM will perform dynamic de-optimization of that method. Adaptive optimization aids in dynamic de-optimization; in other words, a dynamically de-optimized code would revert/move to …
Understanding Inlining
- One of the biggest advantages of adaptive optimization is the ability to inline performance critical methods. This helps in avoiding the method invocation overhead for these critical methods, by replacing the invocations by actual method bodies. There are a lot of “tuning” options for inlining, based on size and invocation thresholds, and Inlining has been thoroughly studied and optimize…
Intrinsics
- Usually the OpenJDK HotSpot VM JIT compiler would execute generated code for performance critical methods, but at times some methods have a very common pattern e.g. java.lang.System::arraycopy as shown in the PrintInliningoutput in the previous section. These methods can be hand-optimized to generate more performant, optimized code similar to having …
Vectorization
- When talking about intrinsics, I would like to highlight a common compiler optimization called vectorization. Vectorization can be applied wherever the underlying platform (processor) can handle special parallel computation/vector instructions known as “SIMD” instructions (single instruction, multiple data). SIMD instructions and “vectorization” help with data-level parallelism …
Escape Analysis
- Escape analysis is another perk of adaptive optimization. Escape analysis (EA in short) takes the entire intermediate representation graph into consideration in order to determine if any allocations are ”:escaping”. That is, if any allocations are not one of the following: 1. stored to a static field or a nonstatic field of an external object; 2. returned from method; 3. passed as parameter to anoth…