JVM Compilers & Version

There are 2 compiler options available in JVM. These are essentially two different compilers interfacing the same runtime system.

The types of Hot Spot JVM flavors are

classic: disables the Hotspot JIT compiler.

Client (default): activates the Hotspot JIT for "client" applications. The client system is optimal for applications which need fast startup and small footprints.

Server: activates the "server" Hotspot JIT: it requires a fair amount of time to warm up, but delivers best performance for server. The server JVM uses a larger default size for the heap, a parallel GC and optimizes code aggressively at runtime.

Don't forget that, if you use them, -server or -client must be the first argument passed to Java.

Version

It is very important to know the java installation and which type of JVM is used when our code is being executed. This can be obtained using the “java –version” command but there is much more to learn from the output of the command,

$ java -version java version "1.6.0_24" Java(TM) SE Runtime Environment (build 1.6.0_24-b07) Java HotSpot(TM) Client VM (build 19.1-b02, mixed mode)

The output shows the Java version number (1.6.0_24) and the exact build ID of the JRE used (1.6.0_24-b07). We also see the name (HotSpot), the type (Client) and the build ID (19.1-b02) of the JVM.

$ java –version java version "1.6.0_24" Java(TM) SE Runtime Environment (build 1.6.0_24-b07) Java HotSpot(TM) Client VM (build 19.1-b02, mixed mode, sharing)

In the above about the version is same but we can see another field call “sharing”.

The JVM in the above outputs runs in mixed mode. This mode of execution is the default mode of HotSpot and means that the JVM dynamically compiles byte code into native code at run time.

The Sharing element tells us that class data sharing is enabled. Class data sharing is an approach that stores the system classes of the JRE in a readonly cache (in a jsa file, “Java Shared Archive”) which is used as a shared resource by the classloader of all Java processes. Class data sharing may be beneficial to performance when compared to reading all the class data from jar archives over and over again.