History
HotSpot, first released April 27, 1999, was based on technologies from the Strongtalk implementation of the Smalltalk programming language originally developed by Longview Technologies, LLC which was doing business as Animorphic. Animorphic's virtual machine technology had earlier been successfully used in a Sun research project, the Self programming language. In 1997, Animorphic was purchased by Sun Microsystems.[1]
Shortly after acquiring Animorphic, Sun also hired Dr. Cliff Click to write a new just-in-time (JIT) compiler for the newly developed virtual machine.[2] This new compiler would be the source of the name "HotSpot", which derives from the fact that as it runs Java bytecode, it continually analyzes the program's performance for "hot spots" which are frequently or repeatedly executed. These are then targeted for optimization, leading to high performance execution with a minimum of overhead for less performance-critical code. In some cases, it is possible for adaptive optimization of a JVM to exceed the performance of hand-coded C++ or C code.[3]
Initially available as an add-on for Java 1.2, HotSpot became the default Sun JVM in Java 1.3.[4]
[edit]Features
Sun's JRE features two virtual machines, one called Client and the other Server. The Client version is tuned for quick loading. It makes use of interpretation. The Server version loads more slowly, putting more effort into producing highly optimized JIT compilations, that yield higher performance. Both VMs compile only often-run methods, using a configurable invocation-count-threshold to decide which methods to compile.
The HotSpot Java virtual machine is written in C++. As stated on the HotSpot web page, the source contains approximately 250,000 lines of code.[5] Hotspot provides:
- A class loader,
- A bytecode interpreter,
- Client and Server virtual machines, optimised for their respective uses
- Several garbage collectors,
- A set of supporting runtime libraries.
The version of HotSpot for Microsoft Windows runs a background process jucheck.exe, which continually checks for updates from Oracle.[6]
[edit]JVM flags
HotSpot supports many command-line arguments for options of the virtual machine execution. Some are standard and must be found in any conforming Java virtual machine, others are specific to HotSpot and may not be found in other JVMs (options that begin with -X or -XX are non-standard).[7][8][9][10]
[edit]License
On 13 November 2006, the Sun JVM and JDK were licensed[11] under the GPL version 2 (see Sun's OpenJDK Hotspot page). This is the code that became part of Java 7 (codename Dolphin[12]).
