Difference between revisions of "Java Virtual Machine"
m (New page: ==Dynamic Compilation== The compilation process for a Java application is different from that of statically compiled languages like C or C++. A static compiler converts source code direct...) |
(No difference)
|
Revision as of 10:04, 14 March 2011
Dynamic Compilation
The compilation process for a Java application is different from that of statically compiled languages like C or C++. A static compiler converts source code directly to machine code that can be directly executed on the target platform, and different hardware platforms require different compilers. The Java compiler converts Java source code into portable JVM bytecodes, which are "virtual machine instructions" for the JVM. Unlike static compilers, javac does very little optimization -- the optimizations that would be done by the compiler in a statically compiled language are performed instead by the runtime when the program is executed.
HotSpot dynamic compilation
The HotSpot execution process combines interpretation, profiling, and dynamic compilation. Rather than convert all bytescodes into machine code before they are executed, HotSpot first runs as an interpreter and only compiles the "hot" code -- the code executed most frequently. As it executes, it gathers profiling data, used to decide which code sections are being executed frequently enough to merit compilation.
HotSpot comes with two compilers: the client compiler and the server compiler. The default is to use the client compiler; you can select the server compiler by specifying the -server switch when starting the JVM. The server compiler has been optimized to maximize peak operating speed, and is intended for long-running server applications. The client compiler has been optimized to reduce application startup time and memory footprint, employing fewer complex optimizations than the server compiler, and accordingly requiring less time for compilation.
After interpreting a code path a certain number of times, it is compiled into machine code. But the JVM continues profiling, and may recompile the code again later with a higher level of optimization if it decides the code path is particularly hot or future profiling data suggests opportunities for additional optimization. The JVM may recompile the same bytecodes many times in a single application execution. Try invoking the JVM with the -XX:+PrintCompilation flag, which causes the compiler (client or server) to print a short message every time it runs.
