Do JVMs on Desktops Use JIT Compilation?_问答_开发者

I always come across articles which claim that Java is interpreted. I know that Oracle's HotSpot JRE provides just-in-time compilation, however is this the case for a majority of des开发者_运维技巧ktop users? For example, if I download Java via: http://www.java.com/en/download, will this include a JIT Compiler?

Yes, absolutely. Articles claiming Java is interpreted are typically written by people who either don't understand how Java works or don't understand what interpreted means.

Having said that, HotSpot will interpret code sometimes - and that's a good thing. There are definitely portions of any application (around startup, usually) which are only executed once. If you can interpret that faster than you can JIT compile it, why bother with the overhead? On the other hand, my experience of "Java is interpreted" articles is that this isn't what they mean :)

EDIT: To take T. J. Crowder's point in: yes, the JVM downloaded from java.com will be HotSpot. There are two different JITs for HotSpot, however - server and desktop. To sum up the differences in a single sentence, the desktop JIT is designed to start apps quickly, whereas the server JIT is more focused on high performance over time: server apps typically run for a very long time, so time spent optimising them really heavily pays off in the long run.

There is nothing in the JVM specification that mandates any particular execution strategy. Some JVMs only interpret, they don't even have a compiler. Some JVMs only JIT compile, they don't even have an interpreter. Some JVMs have both an intepreter and a compiler (or even multiple compilers) and statically choose between the two on startup. Some have both and dynamically switch back and forth during runtime. Some aren't even virtual machines in the usual sense of the word at all, they just statically compile JVM bytecode into native machinecode ahead-of-time.

The particular JVM that you are asking about, Oracle's HotSpot JVM, has one interpreter and two compilers, called the C1 and C2 compiler, also colloquially known as the client and server compilers, after their corresponding commandline options. HotSpot dynamically switches back and forth between the interpreter and one of the compilers at runtime ~~(but it will not switch between the two compilers, you have to specify one of them on the commandline and then only that one will be used for the entire runtime of the JVM)~~.

As per document here Starting with some of the later Java SE 7 releases, a new feature called tiered compilation became available. This feature uses the C1 compiler mode at the start to provide better startup performance. Once the application is properly warmed up, the C2 compiler mode takes over to provide more-aggressive optimizations and, usually, better performance

The C1 compiler is an optimizing compiler which is pretty fast and doesn't use a lot of memory. The C2 compiler is much more aggressively optimizing, but is also slower and uses more memory.

You select between the two by specifying the -client and -server commandline options (-client is the default if you don't specify one), which also sets a couple of other JVM parameters like the default JIT threshold (in -client mode, methods will be compiled after they have been interpreted 1500 times, in -server mode after 10000 times, can be set with the -XX:CompileThreshold commandline argument).

Whether or not "the majority of desktop users" actually will run in compiled or interpreted mode depends largely on what code they are running. My guess is that the vast majority of desktop users run the HotSpot JVM from Oracle's JRE/JDK or one of its forks (e.g. SoyLatte on OSX, IcedTea or OpenJDK on Unix/BSD/Linux) and they don't fiddle with the commandline options, so they will probably get the C1 compiler with the default 1500 JIT threshold. (But applications such as IntelliJ, Eclipse or NetBeans have their own launcher scripts that usually supply different commandline arguments.)

In my case, for example, I often run small scripts which never actually reach the JIT threshold, so they are never compiled. (Nor should they be.)

Some of these links about the Hotspot JVM (what you are downloading in the java.com download link above) might help:

Java SE HotSpot at a Glance
The Java HotSpot Performance Engine Architecture
Frequently Asked Questions About the Java HotSpot VM

Neither of the (otherwise-excellent) answers so far seems to have actually answered your last question, so: Yes, the Java runtime you downloaded from www.java.com is Oracle's (Sun's) Hotspot JVM, and so yes, it will do JIT compilation. HotSpot isn't just for servers or anything like that, it runs on desktops and takes full advantage of its (very mature) optimizing JIT compiler.

Jvm spec never claim how to execute the java bytecode, however, you can specify a JIT compiler if you use the JVM from hotspot VM, JIT is just a technique to optimize byte code execution.