Java class loading tips and tricks - Java Colombo Meetup, January, 2014

Java Colombo Meetup
January 16th, 2014

Java Class Loading:
Tips and Tricks
Sameera Jayasoma
Software Architect
WSO2

Java Objects

http://docs.oracle.com/javase/tutorial/java/concepts/object.html

Java objects consist of state and behaviour just like any
real-world object.

Car myCar = new Car();

myCar is an object of the class Car.
Bytecodes of the class Car is stored at Car.class file.

Class myCarClass = myCar.getClass()

Now what is myCarClass?

Class myCarClass = myCar.getClass()

A Class object is an instance of the java.lang.Class.
It is used to describe a class of a Java object.

Java Application
● Can be considered as a set of classes.
○ These classes are called user-defined classes
○ Classpath parameter tells the Java Virtual Machine
(JVM) or the Java compiler where to look for these
user-defined classes.

java -classpath C:my-java-app.jar org.sample.Main

Java Virtual Machine
http://en.wikipedia.org/wiki/Java_virtual_machine

● A virtual machine which can
execute java bytecode.
● JVM is responsible for loading
and executing code/classes on
the Java platform.
● JVM loads these code/classes
only when they are required.
(lazy loading of classes)

What is Classloading?
● Loading classes (Java bytecode) into the JVM.
● Simple Java applications can use the built-in class
loading facility.
● More complex applications usually defines custom class
loading mechanisms.
● JVM uses an entity called ClassLoader to load classes.

(Q) Why should I worry about class loading and
ClassLoaders?
(Q) Can I simply survive with the built-in class loading
facility in Java?

http://alyerks.blogspot.com

Agenda
Basics of Java class loading <-Custom ClassLoaders
Common class loading problems: Diagnosing and resolving
them.
J2EE class loading architectures
Classloading in OSGi

Java Class Loading
● .class file
○ Smallest unit that gets loaded by the ClassLoader.
○ Contains executable bytecodes and links to
dependent classes.
○ CL reads the bytecodes and create java.lang.Class
instance.
○ When JVM starts, only the main class is loaded,
other classes are loaded lazily.
○ This behaviour allows the dynamic extensibility of
the Java platform.

Java Class Loading..

java -classpath C:my-java-app.jar org.sample.Main

● JVM loads Main class and other referenced classes
implicitly by using default ClassLoaders available in
Java.
○ I.e Developer do not need to write code to load
classes.

Java Class Loading..
● Explicit class loading.
○ How a developer writes code to load a class.
○ e.g. Loading an implementation of the Car interface.

// Create a Class instance of the MazdaAxela class.

Class clazz = Class.forName(“org.cars.MazdaAxela”);
// Creates an instance of the Car.

Car mzAxela = (Car) clazz.newInstance();

Phases of Class Loading
Three phase of class loading: Physical loading, linking
and initializing.

Java ClassLoaders
● Responsible for loading
classes.
● Instances of java.lang.
ClassLoader class.
● Usually arranged in a
hierarchical manner with a
parent-child relationship.
● Every classloader has a parent
classloader except for the
Bootstrap classloader.

http://www.cubrid.org/blog/dev-platform/understanding-jvm-internals/

Java ClassLoaders..
● Initiating ClassLoader
○ ClassLoader that received the initial request to the
load the class
● Defining ClassLoader
○ ClassLoader that actually loads the class.

Java ClassLoaders..
● Uses unique namespaces per ClassLoader
○ Class name of the ClassLoader.
○ Loads a class only once.
○ Same class loaded by two different class loaders are not
compatible with each.
■

ClassCastExceptions can occur.

Features of ClassLoaders
1) Hierarchical Structure: Class loaders in Java are organized
into a hierarchy with a parent-child relationship. The Bootstrap Class
Loader is the parent of all class loaders.

http://imtiger.net/blog/2009/11/09/java-classloader/

Features of ClassLoaders..
2) Delegation mode:
○ A class loading request is delegate between classloaders.
This delegation is based on the hierarchical structure.

○ In parent-first delegation mode, the class loading request
is delegated to the parent to determine whether or not the
class is in the parent class loader.

○ If the parent class loader has the class, the class is used.
○ If not, the class loader requested for loading loads the
class.

2) Delegation mode:
● Default is the parentfirst mode.

http://patentimages.storage.googleapis.com/US7603666B2/US07603666-20091013D00002.png

3) Visibility limit:
● A child class loader can find the class in the parent class
loader;
● however, a parent class loader cannot find the class in the
child class loader.

4) Unload is not allowed:
● A class loader can load a class but cannot unload it.
● Instead of unloading, the current class loader can be deleted,
and a new class loader can be created.

Agenda
Basics of Java class loading.
Custom ClassLoaders <-Common class loading problems: Diagnosing and resolving
them.

Custom ClassLoaders
● Through a custom ClassLoader, a programmer can
customize the class loading behaviour.
● Custom ClassLoaders are implemented by extending
java.lang.ClassLoader abstract class.
● Common approach to creating a custom classloader is
to override the loadClass() method in the java.lang.
ClassLoader.

Java 2 ClassLoader API
public abstract class ClassLoader extends Object {
protected ClassLoader(ClassLoader parent);
// Converts an array of bytes into an instance of class
protected final Class defineClass(String name,byte[] b,int off,int len)
throws ClassFormatError;
// Finds the class with the specified binary name.
protected Class findClass(String className) throws
ClassNotFoundException;
// Returns the class with the name, if it has already been loaded.
protected final Class findLoadedClass(String name);

// Loads the class with the specified binary name.
public class loadClass(String className) throws ClassNotFoundException;
}

URLClassLoader
● Load classes and resources from a search path of
URLs referring to both JAR files and directories.
○ Any URL that ends with a '/' is assumed to refer to a directory.
○ Otherwise, the URL is assumed to refer to a JAR file which will
be opened as needed.

Creating a Custom ClassLoader
public class loadClass(String name) throws ClassNotFoundException {
// Check whether the class is already loaded
Class c = findLoadedClass(name);
if( c == null) {
try {
// Delegation happens here
c.getParent().loadClass(name)
} catch (ClassNotFoundException e) { // Ignored }

if (c == null) {
// Load the class from the local repositories
c = findClass(name);
}
}
return c;
}

Applications of ClassLoaders
● Hot deployment of code
○

updating running software w/o a restart

● Modifying class files
○

to inject extra debugging information.

● Classloaders and security.
○
○
○

Classloaders define namespaces for the classes loaded by them.
A class is uniquely identified by the package name and the
classloader.
Different trust levels can be assigned to namespaces defined by
classloaders.

Agenda
Basics of Java class loading
Custom ClassLoaders
Common class loading problems: Diagnosing and
resolving them. <-J2EE class loading architectures

Retrieving Class Loading Info.
> Thread.currentThread().getContextClassLoader();
> this.getClass().getClassLoader();
> ClassLoader.getParent();
> Class.getClassLoader();

JVM Tools
-verbose - argument gives you a comprehensive output of the class
loading process.

ClassNotFoundException
Occurs when the following conditions exists.
● The class is not visible in the logical classpath of the
classloader.
○ Reference to the class is too high in the class loading
hierarchy

● The application incorrectly uses a class loader API
● A dependent class is not visible.

ClassCastException
Occurs when the following conditions exists.
● The source object is not an instance of the target class.
Car myCar = (Car) myDog

● The classloader that loaded the source class is different
from the classloader that loaded the target class.
Car myCar = (Car) yourCar

NoClassDefFoundError
● Class existed at compile time but no longer available in
the runtime.
○ Basically class does not exist in the logical class path.
● The class cannot be loaded. This can occur due to
various reasons.
○ failure to load the dependent class,
○ the dependent class has a bad format,
○ or the version number of a class.

LinkageErrors

Subclasses:
●
●
●
●
●

AbstractMethodError
ClassCircularityError
ClassFormatError
NoSuchMethodError
..

Agenda
Custom ClassLoaders
them.
J2EE class loading architectures <-Classloading in OSGi

Tomcat

http://tomcat.apache.org/tomcat-7.0-doc/class-loader-howto.html

JBOSS 4.0.x

http://www.infoq.com/presentations/java-classloading-architectures-ernie-svehla

Agenda
Custom ClassLoaders
them.
Classloading in OSGi <--

Class loading in OSGi

Classloader
Network.
Delegate each
other for
classloading
requests.

References
● http://www.infoq.com/presentations/java-classloading-architecturesernie-svehla
● http://www2.sys-con.
com/itsg/virtualcd/java/archives/0808/chaudhri/index.html
● http://www.techjava.de/topics/2008/01/java-class-loading/

http://www.smileysymbol.com/2012/05/smiley-face-collection-10-pics.html

Thank You!!!

Java class loading tips and tricks - Java Colombo Meetup, January, 2014

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Java class loading tips and tricks - Java Colombo Meetup, January, 2014