The Java HotSpotTM Client Virtual Machine

Documentation Contents

The Java HotSpot Client Virtual Machine* serves as a replacement for both the "classic" virtual machine and the Just-in-time (JIT) compilers used by previous versions of the Java 2 SDK to offer improved runtime performance for applications and applets. The Java HotSpot Client VM employs Java HotSpot^TM technology to achieve its superior performance. In many ways, therefore, the new Client VM is similar to the server version of the Java HotSpot Performance Engine, which is available as a separate product. The SDK's Client VM, however, has been specially tuned to reduce application start-up time, making it particulary well suited for client environments.

Some features of Java HotSpot technology 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 Client VM compiles 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 Client VM also uses 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.

• Improved memory allocation and garbage collection - The Java HotSpot Client VM allocates memory for objects more quickly than the classic virtual machine. The Client VM also has a state-of-the-art garbage collector that is faster and more efficient than the garbage collector of the classic VM.

• Thread synchronization - The Java programming language allows for use of multiple, concurrent paths of program execution (called "threads"). The Java HotSpot Client VM provides a leaner, speedier thread-handling capability that is designed to scale readily for use in large, shared-memory multiprocessor servers.

More information about Java HotSpot technology can be found in the Java HotSpot Architeture white paper.

The Java application launcher of the Java 2 SDK and Java 2 Runtime Environment will use Client VM by default when launching applications. The classic virtual machine may be invoked by using the -classic command-line option when launching an application or applet with the Java application launcher and Appletviewer utitilies. For more information, see

• Java application launcher reference page
• Appletviewer reference page

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