For your local program (for example - say you have written a standalone DOM xml parser or any memory intensive program on which you want to check GC* usage).
In RAD (for IBM JVM) --> right click on the java class --> goto "Run As" --> goto "Run configurations" --> goto "Arguments" tab in the pop-up --> set the below under VM arguments
-Xverbosegclog:D:\Your_Path\Your_Sub_Path\jvm.log On a Sun JVM:-Xloggc:C:\Your_Path\jvm.log -verbose:gc -XX:+PrintGCDateStamps (if there are spaces in folder or file names, it will be interpreted as another VM argument) Then Run your program. GC details will be logged in that file (jvm.log in this case) * GC- Garbage Collector Referene:
The Java Virtual Machine (JVM) is an abstract computing machine. The JVM is a program that looks like a machine to the programs written to execute in it. This way, Java programs are written to the same set of interfaces and libraries. Each JVM implementation for a specific operating system, translates the Java programming instructions into instructions and commands that run on the local operating system. This way, Java programs achieve platform independence.
The Java virtual machine knows nothing of the Java programming language, only of a particular binary format, the class file format. A class file contains Java virtual machine instructions (or bytecodes) and a symbol table, as well as other ancillary information.
For the sake of security, the Java virtual machine imposes strong syntactic and structural constraints on the code in a class file. However, any language with functionality that can be expressed in terms of a valid class file can be hosted by the Java virtual machine. Attracted by a generally available, machine-independent platform, implementors of other languages can turn to the Java virtual machine as a delivery vehicle for their languages.
Exploring the JVM Architecture
Hotspot Architecture
The HotSpot JVM possesses an architecture that supports a strong foundation of features and capabilities and supports the ability to realize high performance and massive scalability. For example, the HotSpot JVM JIT compilers generate dynamic optimizations. In other words, they make optimization decisions while the Java application is running and generate high-performing native machine instructions targeted for the underlying system architecture. In addition, through the maturing evolution and continuous engineering of its runtime environment and multithreaded garbage collector, the HotSpot JVM yields high scalability on even the largest available computer systems.
The main components of the JVM include the classloader, the runtime data areas, and the execution engine.
Key Hotspot Components
The key components of the JVM that relate to performance are highlighted in the following image.
There are three components of the JVM that are focused on when tuning performance. The heap is where your object data is stored. This area is then managed by the garbage collector selected at startup. Most tuning options relate to sizing the heap and choosing the most appropriate garbage collector for your situation. The JIT compiler also has a big impact on performance but rarely requires tuning with the newer versions of the JVM.
•Eden Space (heap): The pool from which memory is initially allocated for most objects. •Survivor Space (heap): The pool containing objects that have survived the garbage collection of the Eden space. •Tenured Space (heap): The pool containing objects that have existed for some time in the survivor space. •Perm Space (non-heap): The pool containing all the reflective data of the virtual machine itself, such as class and method objects. With Java VMs that use class data sharing, this generation is divided into read-only and read-write areas. •Code Cache (non-heap): The HotSpot Java VM also includes a code cache, containing memory that is used for compilation and storage of native code.
All the four access specifiers, Java supports, can be applied to variables – public,protected, default and private. If the specifier is not mentioned, it takes default access. A local variable must be default only.The two access specifiers a class can accept are public or default. Default means the specifier is not mentioned at all.
The list of modifiers are : public, protected,default,private,abstract,final, static.
