Skip to main content

Java Custom ClassLoader


Problem

I want my previously written java application to be run multiple times by a shell script with calculated parameters. However, target platforms may not contain bash. Thus I decied to write another class which acts like a bash script file and launch my application with generated parameters.

My application has static variables and static classes. Before every launch I want the state information be cleared. But since I work in the same JVM, static objects will not be removed. It comes out using static fileds and initializers may not be a good approach. There should be a way to solve this.

 The solution is to use a custom classloader. Classloader will load all classes. At the end of the execution, the classloader and all the classes loaded will be garbage collected. At next execution, all classes will be reloaded.


Solution : Custom Class Loader

Class loaders can be considered a container to launch an application. Servlet containers like Tomcat uses a custom classloader to launch servlet applications.

Writing a custom classloader is a sensitive concept. Multiple loading of the same with different classes should be avoided. It is possible that multiple threads may use the same classloader, thus it should be thread safe.


Note the synchronized keyword. Allowing multiple threads to access the method, may result in unexpected behaviour. System classes are delegated to parent class loader. Only project classes are loaded. This can be extended to use other framework classes. However, standard java classess needs to be loaded by the system class loader.

When reference to class loader is lost, the class loader and all the classes it loaded is garbage collected.

Launching Main Class


Now that we have class loader we can use reflection to launch application's main method.


Conclusion

You should avoid creating class loaders if possible. When used with caution, they become quiet usefull.

Labels:

Comments

Post a Comment

Popular posts from this blog

Obfuscating Spring Boot Projects Using Maven Proguard Plugin

Introduction Obfuscation is the act of reorganizing bytecode such that it becomes hard to decompile. Many developers rely on obfuscation to save their sensitive code from undesired eyes. Publishing jars without obfuscation may hinder competitiveness because rivals may take advantage of easily decompilable nature of java binaries. Objective Spring Boot applications make use of public interfaces, annotations which makes applications harder to obfuscate. Additionally, maven Spring Boot plugin creates a fat jar which contains all dependent jars. It is not viable to obfuscate the whole fat jar. Thus obfuscating Spring Boot applications is different than obfuscating regular java applications and requires a suitable strategy. Audience Those who use Spring Boot and Maven and wish to obfuscate their application using Proguard are the target audience for this article. Sample Application As the sample application, I will use elastic search synch application from my GitHub repository.
4 comments
Read more

Hadoop Installation Document - Standalone Mode

This document shows my experience on following apache document titled “Hadoop:Setting up a Single Node Cluster”[1] which is for Hadoop version 3.0.0-Alpha2 [2]. A. Prepare the guest environment Install VirtualBox. Create a virtual 64 bit Linux machine. Name it “ubuntul_hadoop_master”. Give it 500MB memory. Create a VMDK disc which is dynamically allocated up to 30GB. In network settings in first tab you should see Adapter 1 enabled and attached to “NAT”. In second table enable adapter 2 and attach to “Host Only Adaptor”. First adapter is required for internet connection. Second one is required for letting outside connect to a guest service. In storage settings, attach a Linux iso file to IDE channel. Use any distribution you like. Because of small installation size, I choose minimal Ubuntu iso [1]. In package selection menu, I only left standard packages selected.  Login to system.  Setup JDK. $ sudo apt-get install openjdk-8-jdk Install ssh and pdsh, if not already i
3 comments
Read more

Java: Cost of Volatile Variables

Introduction Use of volatile variables is common among Java developers as a way of implicit synchronization. JIT compilers may reorder program execution to increase performance. Java memory model[1] constraints reordering of volatile variables. Thus volatile variable access should has a cost which is different than a non-volatile variable access. This article will not discuss technical details on use of volatile variables. Performance impact of volatile variables is explored by using a test application. Objective Exploring volatile variable costs and comparing with alternative approaches. Audience This article is written for developers who seek to have a view about cost of volatile variables. Test Configuration Test application runs read and write actions on java variables. A non volatile primitive integer, a volatile primitive integer and an AtomicInteger is tested. Non-volatile primitive integer access is controlled with ReentrantLock and ReentrantReadWriteLock  to compa
Post a Comment
Read more