The Java Language Environment

 
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8.1 Performance

Java has been ported to and run on a variety of hardware platforms executing a variety of operating system software. Test measurement of some simple Java programs on current high-end computer systems such as workstations and high-performance personal computers show results roughly as follows:

new Object 119,000 per second
new C() (class with several methods) 89,000 per second
o.f() (method f invoked on object o) 590,000 per second
o.sf() (synchronized method sf invoked on object o) 61,500 per second

Thus, we see that creating a new object requires approximately 8.4 µsec, creating a new class containing several methods consumes about 11 µsec, and invoking a method on an object requires roughly 1.7 µsec.

While these performance numbers for interpreted bytecodes are usually more than adequate to run interactive graphical end-user applications, situations may arise where higher performance is required. In such cases, Java bytecodes can be translated on the fly (at run time) into machine code for the particular CPU on which the application is executing. This process is performed by the Just In Time (JIT) compiler. For those accustomed to the normal design of a compiler and dynamic loader, the Just In Time compiler is somewhat like putting the final machine code generator in the dynamic loader.

The bytecode format was designed with generating machine codes in mind, so the actual process of generating machine code is generally simple. Reasonably good code is produced: it does automatic register allocation and the compiler does some optimization when it produces the bytecodes. Performance of bytecodes converted to machine code is roughly the same as native C or C++.


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