provides one or more implementations of
the Java™ virtual machine (VM):
On platforms typically used for client applications, the JDK comes with
a VM implementation called the Java HotSpot™
Client VM (client
VM). The client VM is tuned for reducing start-up time and memory
footprint. It can be invoked by using the
option when launching an application.
On all platforms, the JDK comes with an implementation of the Java virtual
machine called the Java HotSpot Server VM (server VM). The
server VM is designed for maximum program execution speed. It can be invoked
by using the
-server command-line option when launching an
Some features of Java HotSpot technology, common to both VM implementations,
are the following.
- Adaptive compiler - Applications are launched using a standard
interpreter, but the code is then analyzed as it runs to detect performance
bottlenecks, or "hot spots". The Java HotSpot VMs compile those performance-critical
portions of the code for a boost in performance, while avoiding unnecessary
compilation of seldom-used code (most of the program). The Java HotSpot
VMs also use the adaptive compiler to decide, on the fly, how best to optimize
compiled code with techniques such as in-lining. The runtime analysis performed
by the compiler allows it to eliminate guesswork in determining which optimizations
will yield the largest performance benefit.
- Rapid memory allocation and garbage collection - Java HotSpot
technology provides for rapid memory allocation for objects, and it offers
a choice of fast, efficient, state-of-the-art garbage collectors.
- Thread synchronization - The Java programming language allows
for use of multiple, concurrent paths of program execution (called "threads").
Java HotSpot technology provides a thread-handling capability that is designed
to scale readily for use in large, shared-memory multiprocessor servers.
Some operating characteristics of the Java HotSpot VM can be
controlled by the use of command-line flags.
Enhancements in JDK 6:
Enhancements in JDK 5.0:
Enhancements in J2SE 1.4.x: