Skip to main content

Tomcat and JVM

JVM is Java Virtual Machine, a memory space where classes (code) are loaded and objects (data) are shared. JVM is equivalent to an Operating System process.
When you type java... in you command line you are executing an independent process that loads Java classes in memory, the base classes from Java and yours (from the .class files or .jar you indicate).
Another java... command will load a different process with its own memory and will load classes by itself.
Instance word confusion: when you say 'two instances of the same JVM'. It's usual to say instance of a JVM to a separate process, it is, to a loaded independent JVM. If you are saying: two process are running JVM 1.5, OK, it's the same JVM in the sense it's the same version but they are different processes, different 'instances', independent in all sense.
Webapp confusion: A webapp (by example) is simply a bunch of classes and objects instantiated, attending some URL in a web server. You can start Tomcat with 10 different apps -it is, 10 different bunches of classes and objects each of them attending different request, but in fact they share the same memory space (OS process). A webapp cannot touch other webapp's objects because nobody gives it a reference to the other objects (and classes are in some way hidden but that's another story called: class-loading). Tomcat runs in a single JVM, so every app deployed to a single tomcat instance runs in the same VM as every other application. They get different classloaders, so they're isolated from each other in that sense, but the JVM is the same. Tomcat uses a hierarchy of classloaders: bootstrap, server, and application. Every application gets their own classloader, but they share the higher ones

Comments

Post a Comment

Popular posts from this blog

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" pa...
4 comments
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 c...
1 comment
Read more

JasperReports - Configuration Reference

Data Source / Query Executer net.sf.jasperreports.csv.column.names.{arbitrary_name} net.sf.jasperreports.csv.date.pattern net.sf.jasperreports.csv.encoding net.sf.jasperreports.csv.field.delimiter net.sf.jasperreports.csv.locale.code net.sf.jasperreports.csv.number.pattern net.sf.jasperreports.csv.record.delimiter net.sf.jasperreports.csv.source net.sf.jasperreports.csv.timezone.id net.sf.jasperreports.ejbql.query.hint.{hint} net.sf.jasperreports.ejbql.query.page.size net.sf.jasperreports.hql.clear.cache net.sf.jasperreports.hql.field.mapping.descriptions net.sf.jasperreports.hql.query.list.page.size net.sf.jasperreports.hql.query.run.type net.sf.jasperreports.jdbc.concurrency net.sf.jasperreports.jdbc.fetch.size net.sf.jasperreports.jdbc.holdability net.sf.jasperreports.jdbc.max.field.size net.sf.jasperreports.jdbc.result.set.type net.sf.jasperreports.query.chunk.token.separators net.sf.jasperreports.query.executer.factory.{language} net.sf.jasperreports.xpath....
Post a Comment
Read more