Java performance

Java is often perceived as significantly slower and more memory-consuming than natively compiled languages such as C or C++. However, Java programs' execution speed have improved a lot, due to introduction of Just-In Time compilation and mainly optimizations in the Java Virtual Machine itself introduced overtime.

Virtual Machine optimization techniques

=Split bytecode verification

The JVM verifies all bytecode before it is executed (see Bytecode verifier). This verification is lazily performed : classes bytecodes are only loaded and verified when they are used, and not at the beginning of the program. However, as a the Java Class libraries are also regular Java classes, they must also be loaded when they are used, which makes that the start-up time of a Java program is often longer than for C++ programs, for example.

A technique named Split-time verification, first introduced in the J2ME edition of the Java platform, is used in the Java Virtual Machine since the [[Java version history#Java SE 6 (December 11, 2006)|Java 6 version]]. It split the verification of bytecodes in two phases [1]:

• Design-time : during the compilation of the class from source to bytecode
• runtime : when loading the class.

=Register allocation

Prior to [[Java version history#Java SE 6 (December 11, 2006)|Java 6]], allocation of registers was very primitive, which was a problem in architectures which did not have a lot of registers available, as x86 for example. An Optimization of register allocation was introduced in this version [2]. This allegedly leaded to approximately 60% performance gain in some benchmarks [3].

See also

• [[[Java Platform]]]]
• Java Virtual Machine
• HotSpot
• Java Runtime Environment
• Java version history
• virtual machine
• Common Language Runtime

External Links

• Computer language shootout
• Comparison of Java Virtual Machines
• Mustang versus Tiger performance gain thread
• register allocation gain