Tracking excessive garbage collection in Hotspot JVM

Quite frequently due to memory leaks or other memory problems applications freeze leaving only the garbage collector (GC) process running  unsuccessfully trying to free some space. This happens until watchdog (or frustrated administrator) restarts the application and the problem is never solved.

The goal of this article is to show how to identify excessive GC and to get a heap dump whenever it occurs. It assumes Hotspot JVM version 1.6 or higher.

Introduction to BTrace

The aim of this article is to learn how to dynamically trace/observe running Java applications (JDK 6+) using BTrace without changing the code and configuration params of the applications.

What is BTrace?

BTrace is an open source project that was started in 2007 and was originally owned by two people – A.Sundararajan and K. Balasubramanian. It gained fame thanks to Java One 2008 conference.

BTrace helps us to pinpoint complicated code problems in the application. The problems that can be solved using BTrace include code bugs, unpredictable flows, concurrency problems and performance problems that happen under specific and usually hard to reproduce circumstances. 

BTrace dynamically (without restarting application) instruments (changes) bytecode of an application in the way that programmer defines. The goal of the instrumentation is to see what happens in a specific area of the code. If used beyond this scope it may harm applications' flow and therefore is forbidden by a validator.

For example let's try to solve following problem – an important file gets deleted occasionally once a day. We want to find the code that does this. Therefore we would like to change "delete" method of java.io.File and print a stack trace of the calling thread if the file name fits. With BTrace we can do this by writing a short and straightforward Java code.

Just in Time Compiler (JIT) in Hotspot

What is JIT Compiler?

The Just In Time Compiler (JIT) concept and more generally adaptive optimization is well known concept in many languages besides Java (.Net, Lua, JRuby).

In order to explain what is JIT Compiler I want to start with a definition of compiler concept. According to wikipedia compiler is "a computer program that transforms the source language into another computer language (the target language)".

We are all familiar with static java compiler (javac) that compiles human readable .java files to a byte code that can be interpreted by JVM - .class files. Then what does JIT compile? The answer will given a moment later after explanation of what is "Just in Time".

Storing and delivering large files (binary data)

The regular way to store configuration is to persist it to database. In my case Hibernate is used for that purpose and the configuration is sent to client computers in XML format that is created by JAXB (Metro implementation) via HTTP.  In this post I describe what are the pros and cons of different ways to persist and to send large files.

Introduction

Good day. My name is Artiom.

During my work as a Java developer I often encounter interesting problems that I want to share with colleagues and people, interested in Java or programming in general. This blog will contain both theoretical and practical issues.

 Any comments are always welcome.