Skip to main content

Java Virtual Machine - The Method Area

Inside a Java Virtual Machine instance, information about loaded types is stored in a logical area of memory area  called the method area. When the Jave Virtual Machine loads a type, it uses a class loader to locate the appropriate class file. The  class loader reads in the class file -- a linear stream to binary data  and passes it to the virtual machine. Memory for class(static) variables declared in the class is also taken from the method area.

The method area can also be garbage collected. Because Java programs can  be dynamically extended via class loader objects, classes can become "unreferenced" by the application. If a class becomes unreferenced, a Java Virtual Machine can unload the class(garbage collect it) to keep the memory occupied by the method area at a minimum.

Type Information: for each type it loads, a Java Virtual Machine must store the following kinds of information in the method area:
. the fully qualified name of the type
. the fully qualified name of the type's direct superclass
.whether or not the type is a class or an interface
. the type's modifiers(some sumbset of ' public , abstract, final)
.an ordered list of the fully qualified names of any direct superinterfaces.

In addition to basic type information listed above, it must also store for each loaded type:
. the constant pool for the type
. field information
. method information
.all class(static) variables declared in the type, except constants
.a reference to class ClassLoader
. a reference to Class Class

Comments

Post a Comment

Popular posts from this blog

Quicksort implementation by using Java

 source: http://www.algolist.net/Algorithms/Sorting/Quicksort. The divide-and-conquer strategy is used in quicksort. Below the recursion step is described: 1st: Choose a pivot value. We take the value of the middle element as pivot value, but it can be any value(e.g. some people would like to pick the first element and do the exchange in the end) 2nd: Partition. Rearrange elements in such a way, that all elements which are lesser than the pivot go to the left part of the array and all elements greater than the pivot, go to the right part of the array. Values equal to the pivot can stay in any part of the array. Apply quicksort algorithm recursively to the left and the right parts - the previous pivot element excluded! Partition algorithm in detail: There are two indices i and j and at the very beginning of the partition algorithm i points to the first element in the array and j points to the last one. Then algorithm moves i forward, until an element with value greater or equal
Post a Comment
Read more

Live - solving the jasper report out of memory and high cpu usage problems

I still can not find the solution. So I summary all the things and tell my boss about it. If any one knows the solution, please let me know. Symptom: 1.        The JVM became Out of memory when creating big consumption report 2.        Those JRTemplateElement-instances is still there occupied even if I logged out the system Reason:         1. There is a large number of JRTemplateElement-instances cached in the memory 2.     The clearobjects() method in ReportThread class has not been triggered when logging out Action I tried:      About the Virtualizer: 1.     Replacing the JRSwapFileVirtualizer with JRFileVirtualizer 2.     Not use any FileVirtualizer for cache the report in the hard disk Result: The japserreport still creating the a large number of JRTemplateElement-instances in the memory        About the work around below,      I tried: item 3(in below work around list) – result: it helps to reduce  the size of the JRTemplateElement Object        
1 comment
Read more

Stretch a row if data overflows in jasper reports

It is very common that some columns of the report need to stretch to show all the content in that column. But  if you just specify the property " stretch with overflow' to that column(we called text field in jasper report world) , it will just stretch that column and won't change other columns, so the row could be ridiculous. Haven't find the solution from internet yet. So I just review the properties in iReport one by one and find two useful properties(the bold  highlighted in example below) which resolve the problems.   example: <band height="20" splitType="Stretch" > <textField isStretchWithOverflow="true" pattern="" isBlankWhenNull="true"> <reportElement stretchType="RelativeToTallestObject" mode="Opaque" x="192" y="0" width="183" height="20"/> <box leftPadding="2"> <pen lineWidth="0.25"/>
4 comments
Read more